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Posted By Topic: Shifting loads to same phase

OctaneOutlaw
Dec 27 2018 07:44

Morning

Just wanting some advice on what process you would use/the best way to go about it if you were asked to move all or as many items as you can to a single phase when the switchboard was originally set up as three phase?

I'm just looking for an actual proper process as I've had a little bit of doubts and uncertainty with my supervision and has felt more like a "she'll be right"/"haven't had any issues before" rather than a set process of proper calculation

Also as a side question I'm unsure of what CCC table to use for conductors inside the SB and mainly the feeds from the main switch to the groups of submains? Do I also derate these cables when it's an old messy board with cables everywhere?

And lastly what do people commonly do in situations when they have more feeds of cables than can fit into the main switch? What is common practice for making this work when they won't all fit in the terminal?

Thanks
   

DougP
Dec 27 2018 10:12

Have you done a maximum demand for each phase and the new arrangement?

As for the switchboard internal wiring, in a switchboard, the conductors are classed as "unenclosed in air". So 3008 table 4 column 2 is where you get your rating.

As for cables fitting in the main switch, I presume that the cable selected are too big. Replace them with the correct size cables.

From 3008, 6mm2 is rated at 58A. So two would easily be able to feed a standard installation. Either feeding the circuit protection in a ring arrangement, or separated into two or three section. Or use 10mm2 at 79A.

Or feed sections of busbar for MCBs.

Obviously it depends on the type of switchboard.
   

OctaneOutlaw
Dec 28 2018 09:43

Just a little unsure of the procedure being used so that's why I'm wanting to develop a method of my own that I know the correct precautions are being taken, now the following is from my own research so please correct me if I'm wrong or provide any additional guidance

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1)

From what I've been reading and from my experience about grouping in a SB is that the feeding conductor needs to either have overload protection at origin and any point of CCC reduction (2.5.3.2) or it's protection is downstream by the sum of downstream protection (2.5.3.3 (b))

So each conductor used to feed them has to either be capable of carrying above the value of the main switch which it originates (or wherever it originates from) from (provided it's a CB and not just an isolating switch) or by the sum of the downstream MCBs?

For example a 63a MCB Main Switch feeding let's say, 2 16A plug subcircuits and a 10a lighting subcircuit. The cable feeding said circuits would have to be capable of (16*2 + 10) 42A to comply with 2.5.3.3 (b)

If we changed this to let's say 5 16A plug circuits then (16*5) then we get 80A but because the main switch is a 63A MCB the cable would only ever have to be higher than 63A as this is protecting it in accordance with 2.5.3.2

Basically if the CCC of cables feeding them can carry above 63A (10mm2 or 2 6mm2 together for example) I'm never going to have issues as it's always protected upstream but if it's CCC is below that of the upstream protection I just need to make sure that the sum of the MCBs never exceeds it

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2) Just want to confirm that it's table 4 of 3008 I should be using for SBs? With Table 22 for derating?

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I think I understand those bits but it's more what considerations do I need to be taking when moving all 3 phases to one? Are there certain things I need to watch for?




   

rarrar
Dec 28 2018 16:59

run a cable from the mains switch to a clipsal l4t 35 series unit, have you got an on sight tradesman helping you as that's simple stuff?
   

Someone
Dec 28 2018 21:06

I'm assuming that you've checked there are no multi-phase loads (for obvious reasons), but you might also want to check for anything else shared between phases.

For example, using a 3C+E with two phases plus neutral to run two banks of heaters/lights, as the neutral currents cancel out. If you put both active conductors on the same phase, the neutral current could be twice the current in each active.
   

DougP
Dec 28 2018 21:35

What you're saying about the protection of conductors in the switchboard is basically correct.

You do need to keep in mind that all services in NZ have protection on the network side, and that usually provides overload protection as well.

Where I am, overload protection is not normally provided on the installation side for the mains unless there's a reason to limit the supply below the service capacity. So we don't usually use an MCB as a main switch for standalone installations.

Yes within a switchboard is classed as "in air", so it's table 4.
You don't have to allow for derating for short lengths of grouping such as entering or within a switchboard. Just do a keyword search for switchboard in 3008 and you'll find the details.

Personally, I don't tightly bundle cables in a switchboard anyway. But the usual rule of thumb is 1 conductor diameter spacing, so as to not have to derating the cable. But as I said, it doesn't apply to short distances (<1m).

Usually final subcircuit conductors are greatly oversized anyway due to voltage drop (compared to their actual current rating), so generally it's only the switchboard wiring itself that you have to be careful with.

As for changing from 3ph to 1ph, that's not exactly what you said originally. But if that's what you're doing, then just link them together. If you need to provide current limiting, then that will be on the single phase side, not at the three individual phase connections.
   

OctaneOutlaw
Dec 29 2018 08:18

Rannar

Thank you, I figured there was items that could be used in those situations but I've never seen anyone I've worked along side ever use them

Yes, I have a tradesman on-site with me but not always watching me directly but I do ask him to check my work, expecially if it's something that I'm not 100% confident with and also due to the fact his name goes on it so I want him to be happy signing it off, not just for his sake but more so for the customers sake

I've just been going by his direction and he says he's happy with it but my reason for coming here is because I ask certain things and don't always receive the greatest explanation or confidence at times around the actual process

So this is what has led me here because I've learnt a lot here and normally get good info and explanations

Simply trying to learn and further better my skills through my own research so please excuse any stupid questions or basic stuff it because as many people have seen even some of the qualified people seem to be lacking some of this knowledge

So I appreciate people's time, replies and patience

-----

Someone

Yes I do look at any loads requiring the three phases and leave them as is

As for the second part, something less obvious like that I'm sure could quite easily go under the radar and cause problems like you said, is there an easy way to identify such loads? Most the boards we have dealt with haven't been the newest and have been absolute birds nests

Is this the same case with some hobs for example that have dual phase wiring options?

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DougP

Awesome, always nice to have a little confirmation

Yes I am aware of this, I know here in Christchurch it depends on the service provider as some provide bother short and overload but some are only providing short but I was mostly using just using those figures for the examples to make sure my thinking was correct

So you believe derating is not required? I have read 3.5.2.2 (b) in 3008 and I did find the 1m switchboard entry rule (i) but when I read half the length of the conductor (iii) it made me question that because I figured if I'd used a conductor as a feed in a SB normally there is that many other wires around almost always for an old messy board more than half of the short length would be in contact with other wires or am I missing something? Do you see it differently?

Lastly sorry if I've caused any confusion basically the goal for myself and what I have been assigned to do on these jobs is due to a single phase solar inverter being installed and to get the most out of it is to get every load (or as many as we can) on to the same phase as the inverter

The reason I brought up the feed conductors is because sometimes we replace/change some of the existing ones so I just wanted to confirm a few points in regards to that

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Thanks for all your replies and time, hope I'm not making things too confusing
   

rarrar
Dec 29 2018 14:09

you'll often see me posting what looks like dumb questions, when i find dodgy stuff i tell the customer and drop it on here so they can see other peoples answers too, either way, there's no such thing as a dumb questions, everybody should have a right to ask anything pretty much
   

DougP
Dec 29 2018 15:08

Sure.. post any questions you like... But try and keep them concise if possible..
It's not always easy to cover more than 1 or 2 topics at a time.
   

Satobsat
Dec 29 2018 15:13

OctaneOutlaw Don't worry about asking what you think is a dumb question, there are no dumb questions, just dumb people that don't question what they are doing and why they are doing things that way.
I too was trained by a tradesman that had a lackadaisical approach to the work we were doing and my education, don't worry you seem conscientious enough to think for yourself about what you are doing and seek answers where you can, either on here or in the standards.
We all learn from your questions and other questions posted on here, even those that are involved in drafting the standards, as it can be hard for them to see how their explanations in standards are sometimes interpreted from another perspective.
Keep it up, from your questions and the way you ask them I'm confident you will make an excellent Electrician. All the best for the new year.

   

Phazor
Dec 31 2018 12:57

The subject is happening a lot with regard to solar installations... The non technical salesman from solar marketing companies tell the home owner with a multi phase supply no worries we will shift all the load to the solar phase.
These a lot of mis info out there with regard to this