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Posted By Topic: Cable size to use

Jun 05 2015 11:36

I am in dispute with another sparky over what cable size should be used for the following circuit: 1ph 230V; resistive load 13.5amps; cable length 25mtr; 2c+E flat TPS cable; laying in a confined space unenclosed.
Using the GC calculator I believe 2.5mm is required. He beleives 1.5mm can be used - just.
Am I missing something or is he wrong.


Jun 05 2015 12:03

Installation conditions? (other than unenclosed)
eg BTI? (remembering that in residential BTI must be assumed)

VD on 1.5 mm2 will be in region of 10 V, so unless mains are very large or very short that alone will force an upgrade to 2.5 mm2.


Jun 05 2015 12:40

Thanks AlecK. I am not aware of mains size or length of.
To further this discussion - the 25mtr run terminates in a receiver that controls this load via a remote tstat. From the receiver (relay), a further 10mtr run to the load termination point.
We definitely fall into 2.5mm category now.


Jun 05 2015 13:45

What ever happened to erring on the side of caution and having some redundancy in the cabling calculations? For the price point between 1.5 and 2.5 what real difference does it make.

Might entertain the argument if it was the difference between 300 and 400 Cu for mains over 50 odd meters.

Jun 05 2015 18:14

and how can you size a cable for a subcircuit without taking account of the mains?

Jun 05 2015 19:14

You could make the assumption that 2.5% was allowed for the mains and then allow the other 2.5% for the sub circuit.
But checking out the size of the mains shouldn\'t present to much of a problem and would perhaps allow a better decision to be made.

Jun 05 2015 19:22

Are we actually permitted to assume that we have 2.5% to play with on the final sub circuit? What is the regulatory reference please that supports that?

Jun 05 2015 20:07

There is NO rules in ESR 2010 or AS/NZS 3000 which covers the distribution of the permitted 5% max. voltage drop between the point of supply at the start of the consumers mains and the remote end of the final sub circuits.

It seems that a 2.5% split for the consumers mains and the final sub circuit has become an industry \"Rule of thumb\" and it does NOT work properly when the installation contains a sub main.

Jun 05 2015 20:26

thanks pluto... i wasnt aware of such a mechanism of compliance... and on checking myself couldnt find anything supporting it... it\'s just another urban myth for the lazy and cant be bothered clan

Jun 05 2015 21:18

Not quite a myth.

Time was when Regs of the day allowed us 2.5% from main switch.
These days it\'s 5 % from point of supply

Jun 05 2015 21:30

I never said it was a mechanism for compliance. Just something to consider. It is something that I would only consider for a domestic installation and then only at the design stage.

Unless the volt drops for the various sections of the installation are known this whole discussion will continue to chase its tail

Jun 05 2015 22:21

This forms part of a discussion I\'ve had before. Unless we have been the designer for the system as whole, we just don\'t know the Vd for any section of cable. At the least the Vd for the mains should also be written on the MSB.

Jun 05 2015 22:29

i wasnt suggesting anyone had said it was a mechanism for compliance sarmajor :o)

and at mralarms.. if you dont how can you know the ones youre installing fit the bill ?

Jun 06 2015 10:31

This is a subject that always amuzes me. Technically 2.5 is correct. However I think back to all the jobs we used to do. Long runs of power point circuits and 27a breakers on the board and insulation in walls and ceilings. Funny how despite or may be in spite of the technically correct sizing all those 2.5 cables refused to malfunction and are still in use today.

Worse still. I can remember one old sparkie who used to wire the whole house plugs in 1.5 and put 20a breakers on them. I am aware of at least a couple that I have been to over recent times that are still working fine. I guess you can defy the law of gravity :)

Jun 06 2015 16:24

In the old days, one never did earth fault loop impedance measurements so how did you known the maximum fault times and the shock voltage from earthed metal to the mass of ground until the protective device operated?

It was never considered, perhaps due to lower fault levels it was just lucky too many people were killed or badly injured.

Jun 06 2015 16:28

One further point, the old 2-7/029 +e TPS was larger (2.9 mm2 in today\'s size) that the current 2.5 mm2 TPS and thus will have less voltage drop for the same current.

Jun 06 2015 16:57


7/029 fair comment. However I am referring to long runs of 2.5 on 27a breakers. And that does not explain the sparkie I knew who wired houses in 1.5 on 20a breakers with insulation in the roof and walls, and they are still going, or at least the few I have encountered in recent years.

As for good luck. I think you are pushing your luck with that comment. I do not belive that good luck could run that long without failing. That said I do believe that improvements were needed. But how far is too far? What lengths do we go to? Where do we stop? Where do we draw the line? At what cost? And how much is not really necessary, but is a conspiricy?

Perhaps we should all just scrap everything we know and all build ourselves a hendershot generator.

Jun 06 2015 22:10

bee, something to cheer you up -

Jun 06 2015 22:11

hmm, links ...

Jun 06 2015 23:28

@ Rule12, you say this is for a wireless t/stat receiver, might pay to check the relay contact rating, as most are only 10amp rated that i have seen.

Jun 07 2015 08:56

If those circuits wired in 2.5 mm2 and protected at 27 A, or 1.5 protected at 20 A, have never had enough loaded onto them loaded onto them to trip; that\'s luck.
Being charitable, and assuming the installation conditions allowed protection at that level; otherwise the assumption (not fact, since the evidence hasn\'t been looked for) that no damage has occurred as a result of excess current is also down to luck.

The long runs will have had increased volt drop, and if it hasn\'t been obvious that\'s also luck; either there has been enough \"spare\" in the mains and / or the loads haven\'t been at the ends of the runs. Or maybe it has, and the users have blamed the power company for the symptoms of undervoltage.

This sort of \"evidence\" doesn\'t show that such installation work is OK. A leaky building is still a leaky building even when it\'s not raining, and dodgy wiring is still dodgy wiring even if it hasn\'t burned the house down (yet).

In fact given the right installation conditions, it\'s still OK to protect at those levels. But the long runs were never OK; they just happened when people were too lazy to work out volt drop.

How far do we go? That\'s easy.
Remember the underlying rule is to protect the cable from being damaged by overcurrent.
So we go that far and no more.

The most common form of damage is thermal, breakdown of insulation because the conductor heats up. The rules we work to today are based on results of experiments that tell us exactly what temp various types and sizes of conductors reach under different loads.
Different insulation types react differently, hence different CCCs for XLPE than for PVC.
This data also lets us keep accessible cables below a safe temp to touch.
And ensures that there are enough volts left at the end of the run.

The science hasn\'t changed, but the way houses are built has. True BTI is supposed to make houses warmer, but the wiring is not supposed to be contributing.

Jun 07 2015 10:47


Or perhaps its just one big conspiricy.

Jun 07 2015 10:51


The other thing that just sprang to mind, had to be after I had already hit the enter key on the last post did\'nt it, I can remember whole subdivisions of 3 bedroom homes being wired with 2 PP circuits and both on 1 27a breaker.


Jun 07 2015 11:42

and experience has taught us that that was a really bad idea and now we do it differently.

Jun 07 2015 12:36


At the time it probably was not a bad idea.

But as more and more demand was required we did need to separate these 2 circuits and put each on its own 27a breaker.

And yes Brian, now we do things differently. As per my previous posts, I aknowledged that some improvements were needed. But I also ask how far is too far and how far is unecessary.

Of course in another hundred years they will be looking back at all of us in this industry and saying what a bunch of F---wits. How is it they could not see how to produce electricity for free out of this air, or something to that effect.

Remember that Edison discovvered 10,000 ways how not to invent the light bulb and everyone said he was nuts.

And man will never fly to the moon, or will they, or is this another conspiricy, just like the hendershot generator.