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Posted By Topic: Switch board loops

josh007
Jul 04 2015 12:09

what is that correct size cable to loop to your rcds and mcbs in a domestic 63 amp switch board, some one told me 16mm but I've always seen 6mm2
If your rcds 63amps then 16mm
If your rcds 40amps then 6mm
All jobs I've been on inspectors pass 6mm on 63amp rcd
Cheers
   

Waikato1
Jul 04 2015 12:41

why is the inspector inspecting this ....when high risk inspection is mains work ... Active conductor to the point of isolatoin at the board , neutral conductor , men link and MEC , earth electrode
   

josh007
Jul 04 2015 13:30

Sorry Waikato in the situation when you replace a switchboard from old to new and do you switch gear upgrades
   

josh007
Jul 04 2015 13:32

Surly if your loops are in 6mm and It should be 16mm they shouldn't pass it ?
   

herby
Jul 04 2015 16:20

I've always used 6mm conduit wire.

Not that the inspector is there to look at the wire from Iso to RCD...
   

Sarmajor
Jul 04 2015 16:38

This has been discussed before.
Here is a selection from a previous discussion on this issue.

there's more than one way to skin a cat - or to get a suitably rated conductor.
eg You can use 2 x 4 mm instead of that 10 mm (conductors in parallel).
Ring-looping used to be common; but while common-sense says a ring-loop should be very similar rating to a simple parallel-conductor set-up, it's hard to show compliance for every point on the ring.
And since the parallel 4 mm@ gives 92 A you have some margin for de-rating-for-grouping even without completing the ring. Even if some of the mcb groups are higher than "Someone's" 50 A. nce you go past 60 A you're limited and protected by the supply fuse, usually 63A.
Only a fool would struggle to loop 10 m2 conductors in the confines of a typical domestic swbd, when 2 x 4m2 or 2 x 6 mm2 are cheaper, easier, quicker, and have less risk of a bad connection.
   

Sarmajor
Jul 04 2015 16:42

For those of you who question the inspectors rights or role think on and read ESR 59 and ESR 70(3).



   

pluto
Jul 04 2015 16:44

I usually use a 6 mm2 in a ring main type circuit; from the main switch terminal to the 1st fitting, then to the 2nd fitting, and so on until the last fitting is reached, and then back to main switch terminal from which you started from.

This effectively gives the 6 mm2 cable in parallel and when in free air the current rating of a single 6 mm2 is vary close to the rating of the 16 mm2 consumers mains, so when in parallel it will exceed the rating of the consumers mains.

When you are using MCBs, RCCBs, RCBOs and main switches, the terminal size are such to allow the termination of single 25 mm2 conductor, so this terminal size easily allows you to terminate 2 x 6 mm2 cables in a common terminal holes and thus is never a problem with respect to maximum current rating.
   

Waikato1
Jul 04 2015 18:46

Sarmajor. Both those regs are for the high risk part of prescribed electrical work ,which from the load side of the main switch on a men board isn't , it's sub mains not mains and comes under general which is not requiring inspection
   

Sarmajor
Jul 04 2015 19:30

Suggest you actually read ESR 70 (3) and actuallyy think about what it says.
If in the opinion of the inspector (obviously backed up by the ESR's and AS/NZS3000:2007) the installation is not safe then it will not be livened.
This does not just apply to the High Risk Work that he is there to inspect.

For example if the inspector arrived on site and found that the earthing system was complete but the sparky has not run a bond to the meter box should the inspector just accept the COC do his mandated tests and then liven the installation? I think not as the installation is unsafe. The COC is incorrect and if the sparky missed the unbonded meter box what else did he miss? Did he even test?

In theory an incomplete or incorrectly filled in COC could result in the installation not being livened.

ESR 73A also applies to connecting installations.

We are not just there to rubber stamp the electricians paper work and move on. We should be using our eyes at least to assess the level of workmanship even if we can only access the meter box.
Is the mains cable bushed where it enters and leaves the meter box. Is there a bond onto the meter box if it is metal. Does the crimp on the neutral lug look ok.
Is there a label on the MEC at the earth electrode.
If we can acces the house what does the rest of the installation look like. Are there any wire hanging out of the walls. Is the switchboard closed up. All masking blanks in place. Quick check in the bathroom to make sure there is not a power point in the shower / bath zones.
And finally have you inspected other jobs done by this electrician / company. Were you happy with their workmanship and application of the rules. Basically do you trust the electricians work enough to accept his COC at face value.

   

BrianW
Jul 05 2015 17:08

@ Sarmajor. Perhaps it's you that needs to re-read esr 70(3).

The opening line of that clause states "A person inspecting high-risk prescribed electrical work"
Clearly the only thing this applies to is High Risk work, and to deny sign off for any other work is beyond the inspectors jurisdiction.

73(A) Applies to ANYONE doing a final connection on an installation, or part installation on which PEW has been performed. This isnt the exclusive domain of an inspector, and if no High Risk work has been performed, has nothing to do with an inspector. I could wire a house extension, with a sub board connecting to an existing board, where NO high risk work in involved. Then i become the person completing the final connection allowing electricity to flow into the Part Installation. None of an inspectors business.

And esr 59 is simply a set of statements that the work must comply with AS/NZS 3000.
Once again, if no High Risk work involved, Nothing to do with an inspector.

You're not one of the problem people in our industry that makes up their own rules as they go, are you?
   

Sarmajor
Jul 05 2015 19:10

If I was as the person inspecting the high risk work to come across something in the installation or part installation that was not compliant should I then just look the other way and carry on.

Think about it because regardless of the regulation quoted doing nothing is wrong!!!

ESR 73A covers it quite nicely. If I (either as an electrician or inspector) am not satisfied that the installation or part installation meets all of the requirements of 73A before connection then no connection.
ESR 73A applies to anyone making a connection to a power supply.
   

Sarmajor
Jul 05 2015 19:20

And BrianW regulation 70(3)(b) uses the words installation or part installation will be electrically safe when enlivened.
The definition in the Act defines an installation as all fittings past the point of supply.

   

BrianW
Jul 05 2015 20:49

You've completely missed the point. If NO High Risk, then NO inspection. How are you going to stand on your soapbox if you're not involved in the job?
   

Waikato1
Jul 05 2015 21:45

Under the risk categories , electricians will issue the ESC and COC for the general prescribed work where no inspection required , the inspector can take that on sighting the Cerrtificate and accept on 'good faith' the work has been carried out safely and lawfully , then inspect the high risk part which is MAINS only



   

Sarmajor
Jul 05 2015 22:04

If no high risk then I am not there and don't care as my name and number are not associated with the job.

If high risk and I am there then every aspect of the installation is important to me as I will be making the final connection and livening the installation.

If the work does not meet the requirements of Reg 73A then the installation is not getting livened. End of story.


   

BrianW
Jul 06 2015 07:03

As I suspected, another one making his own rules up as he goes, despite what the rule book says.

Down this neck of the woods you'd have no soapbox. Its the liney that makes the final connection, and all he wants to see is the completed paperwork, and yours is ONLY for the High Risk work.

End Of Story.
   

Sarmajor
Jul 06 2015 18:35

Seems pointless to argue with you because you are not willing to accept that all electrical practitioners have a responsibility to ensure that the work that they do is safe and compliant.

Reg73A(1) covers all the things that any person making the final connection is required to consider including your linesman. If they don't and something later goes wrong the investigation will drag them in.

Just this week I have been to several jobs where I have been presented with COC's that looked like the work of primary school children. Only one had any actual test results on it.

By your logic I should just accept these COC's acting in good faith and complete the inspection and liven the site as long as the high risk elements are ok.

Not good enough and the situation is not going to get better until people actually start to do their jobs properly and test according to section 8.

I have never been presented with a COC containing an EFLI test result. Given that most of the wiring isrotected by RCD's the electrician should only have to test the range circuit and the hot water circuit using the method described in 3000 section 8.3.9.3 (1).
   

BrianW
Jul 06 2015 18:49

Ummm... where is the requirement to include Test Results? or is that another one of Your rules.

   

wireman
Jul 06 2015 20:03

Wouldn't imagine a hot water circuit to be installed with a socket-outlet.
   

Sarmajor
Jul 06 2015 20:42

BrianW, there is no requirement to include test results as you well know, but at least a tick or cross in the test box to indicate that you thought about testing would be better than a blank space don't you think.



   

DougP
Jul 06 2015 21:00

So Sarmajor.
You go to inspect a new installation. The Electrician gives you a COC for the mains work. Nothing is connected to the load side of the main switch. Main earth, N & E bars & link are fine. Meter box is earthed.
Do you carry out your required testing and liven the installation?

Sure, we all want to improve compliance by our practitioners, but there needs to be some deference in the delivery of the message.
   

AlecK
Jul 06 2015 21:07

"but at least a tick or cross in the test box to indicate that you thought about testing would be better than a blank space don't you think"

well actually: No,I don't think that at all.
As BW has indicated, and you acknowledge, there is NO requirement to include test results.
And on anything more than the smallest / simplest (certifiable) jobs, the sort of simplistic "test results" info (as provided for on eg the EWRB CoC form) is TOTALLY USELESS.
Fact is that we certify that our work has been done "lawfully and safely", and that INCLUDES that it has been tested as required.
Which is generally WAY more than simply one value per test type, especially for "mains work" !
Ticking (or crossing) a box proves absolutely NOTHING. And even recording actual test values, per circuit and / or per fitting (as appropriate) doesn't prove anything either, except that we now what the values should be.
And therefore most of the "tick boxes" on forms such as the EWRB one are just a waste of space; which could be better used for a proper description of work done.

recording test results requires a decent dedicated form, which can be produced as evidence (but not conclusive proof) when and if required to defend allegations of bad work.

And even the best forms I have seen still only record worst case for each type of test for each circuit. For example; an average lighting circuit will have something like 20 ECC results, one per "point" plus one for any earthed associated equipment.
What use is a single ECC value on the CoC. absolutely NONE!
What use is a tick indicating EC tests carried out? Less than none.

http://uiring TPI.
   

cassiachatte
Jul 07 2015 15:23

Haven't these Inspectors realised we got rid of full inspections in 1992. If an inspector finds, what they believe to be non-compliant work that has been Certified they have a duty to report the person signing the CoC to the EWRB.

If the Inspector refuses to liven they could be sued for damages should they be proved wrong. So if they want to avoid this just keep to what you are being paid to do by the customer.

In the Regs it says you are entitled to rely on the veracity of the CoC's provided. (veracity = truthfulness, accuracy etc)
   

Sarmajor
Jul 07 2015 17:55

DougP, sure we do. Did 3 jobs exactly as you have written above. No problems with that. The electrician had completed his switchboard leaving the tails feeding the RCD's out except for the one completed outside power point. The switchboard cover was fitted and the COC was in the meter box.

cassiachatte,
Full inspections are not what I was referring to in this instance. Just highlighting the fact that as an inspector there is an obligation to do something about observed non compliant work especially if livening the installation will cause an immediate danger to life or property.

Even if it is as simple as bringing it to the attention of the electrician in the nicest possible way.

Refusal to liven is of course the ultimate action but only as a last resort if the installation represents an immediate danger to life or property if livened.

When the Network Inspector turns up to do the high risk inspection the customer is not the electrician or the homeowner, it is either the metering service provider or the retailer and they are very risk averse.
   

AlecK
Jul 08 2015 09:11

"When the Network Inspector turns up to do the high risk inspection the customer is not the electrician or the homeowner, it is either the metering service provider or the retailer and they are very risk averse."

Wrong on several major points.

1
An Inspector is not a "Network Inspector". We haven't had those since 1993.
These days an Inspector is just an "Inspector".
The fact that some are employed by or contracted to network operators, in order for the network to be ale to offer inspection services, is entirely incidental and NOT relevant.

2
A metering installer works for a retailer or a metering service provider. Their work sometimes needs to be certified (eg new install) but only needs inspection where it counts as "mains work" (eg direct-connected Ns). In which case they will arrange their own inspection - which may be by the same Inspector and at closely similar time; but is a separate inspection contract.

3
work on the network never needs inspection, because it is not work on an installation. So the customer is NEVER the network.

Not even for cases where the network makes it a condition of connection (in their contract with the consumer) that some kind of inspection be done additional to what's req'd by ESRs. For those, the Inspector's customer is the consumer, or maybe the consumer's agent the electrician.

4
The responsibility to have HRPEW inspected lies with the person(s) who did the HRPEW. Therefore the Inspector is working for the sparky. IE the inspector is a subcontractor.
If employed by a network o y any other organisation), then that organisation is the subby and the Inspector is an employee of the subby.

So neither network, nor retailer, nor metering service provider is the "customer". The electrician is the customer!
They are paying the Inspector to do a particular type of PEW.
No different in principle from hiring someone to dig a trench; just a different skill-set req'd by the subby.

And if we don't perform, ie if we don't do an inspection strictly "by the book", they don't have to pay us because we have not performed the contract.

similarly they don't have to pay for extra time spent looking at things that are not part of the HRPEW, which is what we are there to inspect.

------
we may not like this free-market, user-pays world, but we have to work in it.
And a key part of that is correctly identifying the commercial relationships.
   

nalla
Jul 08 2015 14:36

sarmajor

I know this is a bit delayed. But Im with you for inspections. from your remarks on the visual part im taking it like some of us in this office Energy Saftey / Work Safe have had a wee look into you.
AS a result of our audit and what they have poined out. We are of the same opion as you. WE are to look for more than we are there to test. I use the word LOOK. they say visual If we see something not safe even though its not part of the inspection. NO POWER. Or its on our head for livening.
Also heres a bit from their last news letter on the role of an inspector.
It is not acceptable to rely on test results obtained by the installer or other information from the installers COC. Interesting the piece of other information. (Ive asked energy safety to explain this)
Considering im there for the High risk and I don't get to see how the mains are installed.
And how the switchboard is left when I leave.
Weve had a remark back from energy safety on what happens if after an inspection someone damages an underground cable and they investigate. and who they are going to involve in their investigation. Guess who has signed the HRPEW off.
Shame I wont see the comments this will bring as im away bush for a few days
   

AlecK
Jul 08 2015 15:38

"WE are to look for more than we are there to test. I use the word LOOK. they say visual If we see something not safe even though its not part of the inspection. NO POWER. Or its on our head for livening."

Newsflash: Inspectors don't liven.
Inspectors only inspect (including testing).
Once the RoI is issued , and a copy of CoC is attached to it, the inspector's job is over.

And that MUST happen before connection, so it CAN'T be livened at that stage.
Anything the person who inspected may do by way of connecting and / or livening after inspection is completed is NOT done as an "inspector".

So any comeback for livening must land first on whoever connected, not on the inspector.



"Also heres a bit from their last news letter on the role of an inspector.
It is not acceptable to rely on test results obtained by the installer or other information from the installers COC"

Of course not.
ESR 70(30 clearly says we must do our own testing (of the High risk work).

And a person about to connect must do all the checks in ESR 73A; meaning some things get checked three time, by at least 2 different people.



"Considering im there for the High risk and I don't get to see how the mains are installed."

If you are not sure the installer did it right, then don't issue RoI stating "work complies and / or "is and will be when enlivened, electrically safe.". Simple.
Being sure may mean making more than one visit. Or maybe getting photographs showing depth of trench, installation of signal strip, able protection, etc. That's all part of inspection, and only relates to HRPEW.

But no matter how it happens, an inspector simply cannot be held legally responsible for anything other than HRPEW.


Of course we can act as mentors and point out nicely things that are outside the strict scope of inspection of HRPEW. ES may want us to do more, and I know they expect us to check CoCs. But unless / until that gets written into the ESRs any such expectations cannot be enforced.

Right now the only legally enforceable obligations we have as inspectors are to issue an RoI (regardless of pass or fail), and to report any "immediate hazard" as per ESR 19. At time of inspection of "mains", the work being inspected is not permitted to be connected to a supply (other than temporarily for purposes of testing), so there can't be an "immediate hazard" either on the HRPEW or on anything downstream of it.
Yes there an be inspection of "mains work" on the earthing system, with supply already connected; but even then any "immediate hazard" noticed only needs to be reported, not acted on. After that it becomes the owner's reponsibility.

Maybe if ES want inspectors to become policemen, they should pay us for the extra work. But wearing my other hat as an electrician, I am certainly NOT going put up with inspectors charging for "extra" checks & tests that are not related to the HRPEW I have asked them to inspect. And thanks to the free market in inspections, I don't have to.

------------
When it comes to what someone employed by ES says, best to get it officially in writing.
Some of them (like EWRB) have some very strange ideas. Anything not published officially is just a personal opinion, and is worth no more than yours or mine.

-----------------
Enjoy the bush Nalla. Odd choice though; most of us would prefer somewhere warmer this time of year.
   

Andrew
Jul 08 2015 16:04

I was busy writing the same thing when your reply appeared AlecK. The only thing I'd add is that the inspector must come to a definite conclusion one way or the other before issuing a pass/fail RoI because the RoI must definitively state whether or not the part installation is and will be electrically safe and whether or not it is compliant.

If the part installation is safe and the RoI states that it is unsafe just because the inspector is not sure then the RoI is materially false and the inspector has broken the law.
   

AlecK
Jul 08 2015 16:15

Good point. Level 2 penalty for that.

as to pass / fail, I know some Inspectors who avoid issuing a "fail" by simply saying their inspection is not yet complete.

The problem with "fails" is the person connecting doesn't check. In fact doesn't have to check; all they have to do is "sight" an RoI for the work.
Of course, if the brown stuff hits the whirly thing a "fail" RoI would count against them having been "satisfied safe to connect".

Interesting that the HR database doesn't record whether "pass" or "fail", it just seems to assume "pass".

Connection's where the buck really stops, nothing to do with Inspectors at all
   

nalla
Jul 08 2015 16:27

Not Gone Yet but will after typing this
It will be cold but love it there (no phone)

Aleck

Im expecting a few replys.
Quick answer to your points. as theres lots of instances I can put down when I ill put my ROI in my left hand as this hand wont be the inspector part of me. And not enliven.
Heres a recent but regular situation
New House no sparky on site. both hands installed metering. Right hand did inspection. but will inspecting noted switchboard was complete even phase wires connected to circuit breakers (busbar there AS well), but while entering house also noticed not fittings on wiring except one plug below board.write this concern on ROI. Right hand now gives ROI to left hand and reads ROI. left hand dosnt put fuse in as it thinks unsafe and whos testing outlet.
You might now realise these 3 guys work well together and are an unsplitable team.

   

dlink
Jul 08 2015 17:11

oh i love this pidgeon engrish and the great battle thats going on, keep it up guys. AlecK i'm with you on this one ;)
   

DougP
Jul 08 2015 17:12

The point of my carefully worded example and question to Sarmajor, was that the responsibility of the HRPEW inspector technically stops at the line side of the main switch.

The examples that are being given, of incomplete work or no COC or test results for work on the installation, shouldn't be the concern the inspector if the electrician has left the installation safe. i.e. locked off the main switch or nothing connected to the load side of the main switch.

Even in Nalla's last example, if the main switch was locked off and the switchboard cover available to him, he shouldn't have a problem livening the installation with his "network" hand.

Part of the problems is that many electricians consider that a piece of insulation tape over the switch handle constitutes "isolated".
   

Sarmajor
Jul 08 2015 17:57

DoupP,
I totally agree with you about supply to a compliant installation with the main switch locked off or with no cables connected to the load side of the main switch as long as the switchboard cover is on.
As discussed earlier all the responsibility for the installation wiring is with the issuer of the COC from that point on.
   

BubbaB
May 28 2017 16:30

I was sure I had this discussion with an inspector back in the 70's... We decided then 4mm2 conduit wire was fine for linking fuses/CB's in a switchboard. When you take into account the length of the linking wire and the size at 4mm2 it is capable of carrying more than enough current without melting to trip any mains protective fuse/device.

So am I to stand corrected and this is now not so. If not please show me the regs that state so.
   

bakinthegame
May 28 2017 18:25

If it's just an switchboard upgrade where you are just replacing what's there already with new, you don't even need an inspection anyway
   

BrianW
May 28 2017 19:02

Incorrect.

If that switchboard replacement involves a new earth/neutral bar with the MEN link, then as i understand it, inspection IS required.

But only for the link and main earth and neutral connections.

Nothing else.
   

ShaneR
May 28 2017 20:05

"If that switchboard replacement involves a new earth/neutral bar with the MEN link, then as i understand it, inspection IS required."



Have you got a reference for this?
   

BrianW
May 28 2017 22:25

Yep. Its called the Regs Book.

Main Earth and MEN link is High Risk.

I'm sure if you look hard enough and read it thoroughly you'll find it.
   

bakinthegame
May 29 2017 06:35

You said it yourself though Brian - it's merely replacing what's there already so no inspection required, unless you replace the Main Earth or alter the mains.
I've attended multiple refresher courses where this always comes up & answer is always if you are re-using everything but merely upgrading the switchboard & switchgear - no inspection required.
Always a topic for debate but technically no - although it is always recommended.
   

AlecK
May 29 2017 08:29

BubbaB:

Things have indeed changed since the '70s.
New Regs 1993
another lot 1997
and again 2010
All three of which have had amendments in between the full new sets of Regs.

And over that 40 -odd years, we've moved from (almost) everything being in the Regs, to having most of the technical stuff in first ECPs and now Standards
And those Standards have moved on as well, mostly for the better.
We now (generally) have a reason for things, instead of some local inspector's gut feeling about what's good enough.

On the particular point of whether 4 mm2 is "fine" for feeding circuit protective devices in a switchboard, it isn't... and it never was.

Sure it can usually ope with the maximum demand, but max demand is about "normal" and "usual", and the requirement is to protect against the unusual (after all overloading a conductor is - or should be - by definition an unusual circumstance). You can't protect against the abnormal by reference to what's normal; you can only protect against abnormal by ensuring that actual protection is in place. In this case, either a bigger conductor or some combination of protective devices that gives actual protection.

   

BubbaB
May 29 2017 22:18

AlecK I do understand things change. And unlike the dinosaurs I too have changed and being licenced in The USA and Australia at different times the electrical trade is similar all over the world in that the regs are more or less for the protection of insurance companies much more than actual human life.

In my opinion.

If an overload is not a normal situation then why aren't CB's made to trip accordingly and RCD's forced on the public for their own safety? Retrospectively.

We have had fire detectors for some time now and still not compulsory?

And to be clear what actually is the current carrying capacity of a 50cm length of 4mm2 conduit wire? 100's of amps at least? Definitely large enough to carry any short circuit currents long enough for the CB to trip....
   

AlecK
May 30 2017 08:30

Now you appear to be confusing overload with short circuit. The CCC of 4 mm2 single core Cu / PVC installed "spaced" (ie best case for a swbd) is 46 A. The short circuit rating will be much higher (the actual calculation is a bit complicated); and given the probability of the supply fuse being a 63 A HRC yes the fuse is likely to blow before the cable is damaged.

But that does NOT mean the cable is protected against overload by the supply fuse - it isn't.

Which is why 4 mm2 is NOT automatically OK for the job of feeding a bunch of subcircuit fuses / mcbs. A moment's thought will show that if the subcircuit protection is (say) 3 x 20 A, then each circuit can draw 20 A, total 60 A; and we've already seen the CCC is only 46 A at best.

This is NOT about protecting the insurance company; it's about avoiding a fire caused by shoddy work practices. For the record, there are NO insurance representatives on any of the committees that write Wiring Rules.
   

pluto
May 30 2017 10:41

The 1 second current rating of 4 mm2 PVC cable is 444 amps or for 5 seconds 198 amps.

These figures from a cable manufacturers data.

A 63 A HRC fuse will take approx 63 x 1.6 (fusing factor) = 100 amps to blow on a hard short circuit.
   

AlecK
May 30 2017 11:14

So short circuit protection is in place; and probably fault protection also (different requirement, different calculation - but if in place for the mains entering the cabinet will be OK for downstream conductors as well).
But overload protection isn't in place - unless the sum of ratings of mcbs supplied is less than CCC (46A at best, as above. Less if bunched)
   

Flashman
May 30 2017 16:24

So looks like the main issue is O/L protection of the loops and all I can see in 3000 para 2.9.6 is "switchboard wiring shall be designed and installed to withstand any thermal and magnetic effects on the conductors" Awesome.
AlecK, a question on your 20A MCB example, would the conventional tripping current of 1.45 x In not apply? (3000 B3.2.2.2)This could potentially take each one up to 29A. Under that abnormal situation of 3 x overloaded 20A MCB's even 10mm at 79A wouldn't be big enough.
How far do we have to go when anticipating what a future abnormal problem might be? I'd be interested to hear any examples of damage caused by undersized loops.
   

AlecK
May 30 2017 17:47

3.4.1 every conductor shall have a CCC, i.a.w. "3008.1, not less than the current to be carried by the conductor"
That's where you can apply max demand, and work out that not every circuit will be fully loaded at same time.

2.5.1 active conductors shall be protected by one or more devices that automatically disconnect the supply in the event of overcurrent, before such overcurrent attains a magnitude or duration that could cause injury to persons or livestock or damage because of excessive temperatures or electromechanical stresses... ... Protection against overcurrent shall consist of protection against (a) overload current, i.a.w. 2.5.2 & 2.5.3; and (b) short circuit current, i.a.w.2.5.2 & 2.5.4 "

2.5.3.1 "the operating characteristics of a device protecting a conductor against overload shall satisfy the following two conditions:
Re-phrased):
1. max demand <= nominal / design current <= CCC
2. operating current <= 1.45 x CCC

So yes, the 1.45 is relevant, but it's taken care of so all we need to do is make sure the nominal rating of the mcb is not more than the CCC.
For (HRC) fuses; rating not exceed 90% CCC.

Or for the loop-feeding situation, if we have upstream protection by 63A HRC, we need to either use a loop conductor with 70 A CCC (to meet the 90% rule); or downstream mcb protection not > CCC of the conductor we've chosen.

It's simply not good enough to say "it'll never happen"; we can use what's likely to happen for establishing max demand 9any of several methods is acceptable); but then we protect that cable just in case the unlikely does happen.

Point being, we select for the expected load, then protect against the possibility of excess current.



   

BubbaB
May 30 2017 21:15

AlecK you state, "The CCC of 4 mm2 single core Cu / PVC installed "spaced" (ie best case for a swbd) is 46 A." I assume it is relevant to a specific maximum length of 4mm2 conduit wire? What is that length?

Still not confused...
   

pluto
May 30 2017 21:42


I often have usually used a 4 mm2 in a ring main type circuit; from the main switch terminal to the 1st fitting, then to the 2nd fitting, and so on until the last fitting is reached, and then back to main switch terminal from which you started from.

This effectively gives the 4 mm2 cable in parallel and when in free air the current rating of a single 4 mm2 is vary close to the rating of the 16 mm2 consumers mains, so when in parallel it will exceed the rating of the consumers mains.

When you are using MCBs, RCCBs and main switches, the terminal size are such to allow the termination of single 25 mm2 conductor, so this terminal size easily allows you to terminate 2 x 4 mm2 cables in a common terminal holes and thus is never a problem with respect to maximum current rating.

RCBOs are a problem with small terminal holes size with a 20 amp RCBO only able to terminate a single 6 mm2 cable only, so the use of a connection bus bar as made by the RCBO manufacturer then makes it more practical to connect a number of RCBOs and keep within the maximum current rating required.

   

AlecK
May 31 2017 08:16

length has no bearing on CCC; which is governed by cross-sectional area, stranding, conductor ,material, and insulation material (the last due to temperature rise).


Length increases voltage drop, but that's an (almost) entirely different issue; and barely relevant to switchboard loops.
   

Flashman
May 31 2017 08:58

Thanks AlecK, re-reading those paras with Swbd loops in mind makes more sense. (a case of not seeing the wood for the trees)
Your comment on MCB protection > CCC rating is covered by 2.5.3.3(b)which in a practical sense would be the easiest way to work it out. While a 4mm might be suitable in a two bedroom flat it probably wont be in a commercial SwBd.
   

AlecK
May 31 2017 10:20

The key being that if it isn't adequately protected by the supply fuse, then it has to be protected by sum of mcbs it feeds; for 4 mm2 46 A tops.
So 3 x 16 A mcb = 48; too high.
But 2 x 16 A + 1 x 10 A = 42 A; OK

For ring-looped 4mm2, (as per Pluto) double the CCC. Remembering that you are NOT allowed to ring-loop 2.5, 'cos 4 mm2 is smallest for paralleling.

And of course the CCC reduces if bunched.


Given the 3-subcircuit per RCD rule, maybe ring-loop the RCD feeds and then either 6 mm2 looped or individual feed to each mcb
   

BubbaB
May 31 2017 15:50

Aleck Surprised you believe length of cable isn't relevant? Always was as far as I was aware.

And you finally see what I have been saying all along. 4mm2 conduit wire from main switch to C/B's.

I never said how many tails to use but if it helps 1 to 2 C/B's power - each, 1 to stove C/B, 1 to 4 lighting c/b's and 1 to hot water C/B if uncontrolled and maybe 1 to the garage C/B...




   

AlecK
May 31 2017 16:47

Have a read of 3008.1.2 which is the reference for selection of cables.
It covers selection for CCC, selection for short circuit temp, selection for volt drop, and latest (not cited) edition has selection for reduction of losses over life of installation.
   

BubbaB
May 31 2017 20:58

AlecK

I read it back in the 80's in Aus and a few times since. Not sure why your suggesting I do again.

How about V=IR
if V=240 divided by
R= 0 (resistance over 1 meter of 4mm2 approx.)
Therefore I=240 amps

Obviously if we are talking greater lengths of cable or different enclosures or temperatures this would change but for behind a fuseboard I think you will see that common sense prevails...

I hope this answers your question josh007

Now if electricity has started reading the standards and EWRB regs and decided to work differently than say it has FOREVER then I may have all this wrong... But I do not think so...


   

AlecK
May 31 2017 23:12

By that reasoning, one strand from a flex also has (almost) zero resistance and can take 240A; and so we can carry any amount of current via the tiniest of wires.
The problem is that "very low resistance" is NOT the same as "zero resistance".
Even short lengths of conductor get hot, the smaller the hotter and the more current the hotter.
But clearly you already know absolutely everything, so i won't try to teach you; I have better things to do with my time.
Just accept that the RULES say that 4 mm2 - or any other size is NOT automatically OK but needs to be protected from being overheated by excess current. Those rules are based on science, not your gut feelings / common sense.

   

AlecK
Jun 01 2017 08:50

Having slept on it (one of those "better things to do); this post is not aimed at BubbaB, but at others who might think BubbaB knows what he;s talking about.

First, 240 divided by zero isn't 240.
240 divided by 1 is 240.
240 divided by 0.5 is 480.
240 divided by 0.25 is 960.
240 divided by zero is infinity.

Note that these current figures are well above the short circuit ratings (as posted by Pluto), let alone CCC ratings, for 4 mm2.

The calculations that arrive at CCC are well beyond me. They include knowing how fast copper heats up (remember doing specific heat capacity calculations at night tech?), and how fast a particular cable loses heat. But it's easy enough to do the experiment of running any cable at or close to it's CCC - it gets very hot.
If heated too much, the insulation is damaged (75 deg C is max for normal TPS). Hence higher CCCs for situations where heat can be dissipated more easily.
   

AlecK
Jun 01 2017 08:51

2nd, for the record, standard low voltage is 230 V, not 240 V.

3rd, regardless of the nominal voltage, it's applied across the entire circuit, with most being across the load at the end (assuming we've done our job correctly). So the voltage across our short length of feed conductor is actually very small. From tables, for 4 mm2, it's 10 mV per amp per metre. Assume the total length is 1m (keep the maths simple), and we run 50 A through it (more than the CCC, but again, keep the maths simple).
V = 10 mV x 50 x 1 + 500 mV (o.5 V).

R varies with temperature (increasing for copper); but from Table 34 of "3008.1.2" 5.06 ohms per km @ 45 degC; so for 1 m = 0.00506.

If we pass 50 A through this resistance energy consumed (power) = I2R = 12.65 W.
If we pass 100 A through it, = 50.6 W.

Which is why the cable can carry 198 a for 5 sec without damage, but at 444A only for 1 sec (Pluto's figures).

444 A will be 997.5 W.

   

AlecK
Jun 01 2017 08:52

Being a short length doesn't make much difference to volt drop over an entire circuit, but the resistance is never zero and no matter how large and short it generates a LOT of heat when passing high current.

The CCC is what has been found, by (very complicated)calculation and by experiment, as keeping the conductor temp below the level where damage will occur for constant current. In other words, by selecting a protective device rated less than CCC, we ensure that the current is never high enough that damage will occur, regardless of how long the current lasts.

Now all of this stuff is in 3008, which BubbaB claims to have read. may have read,; certainly didn't understand.
   

AlecK
Jun 01 2017 09:04

"Forbidden" error prevents final post; but the important stuff is above.
   

BubbaB
Jun 01 2017 10:10

"Josh007
Jul 04 2015 12:09


what is that correct size cable to loop to your rcds and mcbs in a domestic 63 amp switch board, some one told me 16mm but I've always seen 6mm2
If your rcds 63amps then 16mm
If your rcds 40amps then 6mm
All jobs I've been on inspectors pass 6mm on 63amp rcd
Cheers "

Aleck you seemed to have missed the point of this post so I have repeated it above to help you. I never asked for your help but you just butted in as you seem to do on many posts. I never read where you were a self proclaimed expert on everything. Leave your need to be 'RIGHT' and the 'ONLY ONE' that knows anything in your own head.

This post is aimed at Aleck and those that think they know it all. They do not. If someone else comes up with an assumption based on what they know or have observed does not make it a fact or true, just their observation.

I gave my answer to this post with what I had observed in the past.

4mm2 conduit wire works if used correctly. Period.

I understand that in quantum mechanics if a particle is observed it changes purely because of the fact that is is being observed. WOW what a mind spin.

I am not saying here Aleck isn't rather intelligent or helpful to some but he really is....

"Forbidden" error prevents final post; but the important stuff is above.
   

Andrew
Jun 01 2017 10:45

To add a little more science in to explain why length doesn't matter for thermal calculations even though it does matter for resistance calculation:

For resistance, the formula is resistivity x length / cross-sectional area. Resistivity is dependent on temperature and material, but the important thing is that for the same material at the same temperature, resistance increases as length increases and decreases as area increases.

For thermal calculation the amount of heat generated is given by P=I^2xR, and from above R is dependent on length and area. The amount of heat dissipated is dependent on the temperature difference between the wire and its surroundings, the heat transfer coefficient of the wire, and the area exposed for cooling (Q=hA(T1-T0)). The area exposed for cooling is the perimeter (diameter) of the wire x the length of the wire, so the longer the wire is the more heat it dissipates. Since both the heat generated and the heat dissipated are both equally dependent on the length of the wire, they cancel out and the maximum current a wire can carry is independent of its length.
   

Andrew
Jun 01 2017 11:09

BubbaB, you're missing the point. 4mm cable is rated for 46A max. If you wire it to a single CB rated at less than 46A then yes the wire will be fine. If you run ten tails from the main switch to ten 20A MCBs this is also fine. But if you wire it to a 20A MCB, then loop to a second and a third MCB also rated at 20A (all on the supply side), then the wire from the 3rd MCB to the second must carry 20A, from the 2nd to the 1st it must carry 40A, and from the 1st to the main switch it must carry 60A, which is more than 4mm is capable of, but less than the 63A supply fuse will protect at. 60A in a wire rated for 46A means it heats up and the insulation catches fire.

And to accuse someone of "butting in" on a forum designed for people to ask questions and provide answers is odd to say the least - especially when you have "butted in" yourself to comment on a thread that was dead for almost 2 years.
   

BubbaB
Jun 01 2017 11:27

Andrew do you read what you write?

1st you say wire length has nothing to do with the maximum current it can carry???

Then you state, " BubbaB, you're missing the point. 4mm cable is rated for 46A max." BUT AGAIN I SAY, AT WHAT LENGTH...........BECAUSE LENGTH IS RELEVANT.

Then you go ahead and confirm what I have been saying that in many cases 4mm2 cable is fine for looping.....

Not sure why you even butted in here other than your 'friend' asked you to....?

This post is obviously answered so further comments are purely for someone's self gratification.
   

Andrew
Jun 01 2017 13:47

Did YOU read what I wrote? My first post explains why length is irrelevant. If you care to, you can confirm this by getting two current controlled power supplies and applying 60A to a 10cm length of 4mm2 wire from one, and the same to a 10m length from the same reel from the second supply. Leave them for a while and you'll find they both start melting their insulation at roughly the same time.
I've now given you the science behind why length is irrelevant and a way to confirm this for yourself practically (and you also have the standard demanding that you don't put more current through the wire than its CCC for the situation even if you do think it's fine). You're welcome to keep claiming length is relevant in the face of evidence to the contrary, but that will just make you look like a troll.

Your original comment suggested that whether 4mm2 is ok depends on length. It does not. It depends on the maximum current it can draw without tripping any protective devices, and whether this is above or below the CCC of the wire (46A best case for PVC).
   

AlecK
Jun 01 2017 14:15

As I said, this morning's posts were not aimed at BubbaB ; but at avoiding anyone else being misled by his repetition of incorrect "facts".

Bubba; your first post on this thread (28 May, after it had lain dormant nearly 2 years) was factually incorrect.
You stated: "When you take into account the length of the linking wire and the size at 4 mm2 it is more than capable of carrying more than enough current without melting to trip any mains protective fuse/device." That's not saying 4 mm2 is often OK, or in many cases OK, it's saying it's always OK - regardless of rating of protection. It's simply balderdash.

A 63 A fuse (by far the most common type of "mains protective fuse/device) will carry 63 A indefinitely. A 4 mm2 single Cu PVC will carry it for a considerable time before melting, but it(or rather its insulation) will melt without ever blowing the fuse.

The latest from BubbaB repeats his unfounded assertion that length is relevant to CCC. It isn't; because CCC is a function of CSA, material, insulation, ambient temp, and installation conditions. The only time length comes anywhere near affecting CCC is for variation of installation conditions along a run, where a short length of worse condition can sometimes be ignored. [3008.1.2; clause 3.4.6 lead-in for u/g cables; also 3.5.5.2(b) limited length of grouping; and maybe a couple of other places]
A look at ANY of the Tables for CCC will show that NONE of them vary the CCC due to length.

I certainly don't know it all (apparently unlike BubbaB); but I do know
a) a fair bit about what the ESRs & Wiring Rules require; and
b) some about why they require it.
I have no idea who Andrew is (or BubbaB for that matter), but I certainly endorse Andrew's view that this forum is intended for people in the trade to ask and / or answer questions (thanks Ron). I'll continue to contribute whenever I feel my opinion may be useful / relevant.
And - as long-term users will know - when I am proven wrong; I try to acknowledge it and apologise, instead of resorting to bluster and personal abuse.



   

BubbaB
Jun 01 2017 16:13

This post is obviously answered so further comments are purely for someone's self gratification.

Thank you for proving me correct.
   

AlecK
Jun 01 2017 16:24

I'll leave other users to judge the relative value of our respective contributions.

   

JonoQC
Jun 01 2017 21:45

One thing that's worth noting through all of this is that, to expand on AlecK's point about the 63A fuse, the 1.6x 'blow' point of the fuse isn't really a standard, just a shorthand for the lowest current that will be guaranteed to clear the fuse 'eventually'. This is normally the 1000s current.

So something that is only protected from overload by a 63A HRC fuse can potentially be subjected to 100Arms for a little over 15 minutes.

That's why fuses are so difficult to use properly for anything other than short circuit protection. It takes a lot of work to make sure they get hotter and blow faster than the things downstream under all circumstances/fault impedances, etc.

(It looks like this is already fizzling out, but I couldn't get my account recovered so had to make a new one)
   

BrianW
Jun 02 2017 21:12

And if anyone is taking bets on the person with the most accurate knowledge, I'll put a tenner on Aleck.....