# Forum Home Renovation Electrical  cable size, from main meter to sub board

## madmuso

hey guys, 
I am turning my garage into a man cave (2 rooms). A sub board will be installed but im wondering if anyone can offer some advice on what size cable I need to run from the house main meter to the sub board. I am going to do most of the roughing in and then get my sparky to inspect and fit everything off. 
There will be two 2.5kw split aircons installed (one in each room), a small bar fridge, 5 power points, some music equipment such as a guitar amp, etc, and 4 lights. 
After consulting a sparky a while back he advised to run 4mm tps but this was BEFORE I added air cons to the design. I cant get a hold of him now cause he's overseas which is why im asking for some help here. 
Will the 4mm still suffice, or do I have to go 6mm or bigger? 
Thanks,

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## JB1

You may as well go 6mm. 
Price difference is small. 
Better to have more capacity than less. 
But I would say the Aircon will need it's own circuit. 
So 2 runs of 6mm. 
But I'm not a sparky  :Smilie:   
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## Bros

Yes 6mm will be fine under normal conditions BUT and unfortunately there is always a BUT fault loop impedance will have to be done so I would say wait for the electrician to come home and discuss it with him.

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## koshari

keep in mind with a load increase such as that its quite probable that your mains may require upgrading as well.

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## Brian7886

a sub board can have whatever size submain you want run to it.  
You could run a 2.5 to it if you want. But you cant have circuit protection that doesnt suit said cable.  
You need to consider a few things in running a cable, which determines its size.  
1. Length of run. Cables drop voltage over length, the maximum length of a cable is then determined by the conditions its run
2. How its run. Clipped under a house as opposed to in the ceiling amongst the insulation etc changes how long the cable can be.  
You could run a 16mm SDI (2 of) and a 6mm earth wire from your MSB to the Sub. Put a 63A circuit breaker in the MSB and 1 in the Sub, then what happens on the other side of the subboard bears now problems for the submain cable. Circuit breakers are there to protect your cables.  
As for the 2 air cons. Anything drawing under 10A can legally be put on a power circuit, it needs its own isolation (power point or isolator), but if you are going to have 2 of them run from this board, run 1 circuit for the 2 a/cs and install isolators for each.  
If you only plan on having say a power circuit, and the air cons (assuming you make the power circuit a mixed circuit with lights etc) id have a 32A 6mm circuit.  
You really need to consider your load. Air cons will have running currents listed in their specs, do the maximum demand calc. Off the top of my head, air cons are 75% of the connected load (assuming something like 5A for 1 air con, you would be 3.75A each i think) plus the load of your lights, power points etc (all in the maximum demand table.) This will give you a maximum demand for your sub main. This then allows you to determine circuit protection size, and with that, a suitable cable size.  
Or just get ya sparky to do it

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## greenhj

> a sub board can have whatever size submain you want run to it.

  Really?

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## barney118

I understand it will be a function of length of run to board from meter, and amps/voltage on sub board.
I had to derate my cable from board to swimming pool for these reasons.  
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## Brian7886

> Really?

  depending on circuit protection and purpose

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## Moondog55

> Really?

  Well up to the size and amperage of the mains in I guess. Not much point in a 100amp sub board if the supply in is fused at 63A is there.

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## chalkyt

Just be aware if you start taking maximum demand into account, that the tables in AS3000 Appendix C assume that not everything will be running at once, hence the allowance to "derate" the loads so that you don't have to assume full rated load when selecting cables CBs etc. (an example of this is that it is unlikely that all elements of a stove will be on at the same time i.e. oven top and bottom, four hotplates, griller, so the maximum demand for the stove is taken to be 50% of the rated load) 
However, (yes, another however) if you know that everything WILL often be on at the same time, you need to take the actual load created into account or you might end up with under-rated CBs and cable. 
Yet another case of talk to the electrician before you spend some time and money on something that turns out not to be quite right.

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## Brian7886

> Just be aware if you start taking maximum demand into account, that the tables in AS3000 Appendix C assume that not everything will be running at once, hence the allowance to "derate" the loads so that you don't have to assume full rated load when selecting cables CBs etc. (an example of this is that it is unlikely that all elements of a stove will be on at the same time i.e. oven top and bottom, four hotplates, griller, so the maximum demand for the stove is taken to be 50% of the rated load) 
> However, (yes, another however) if you know that everything WILL often be on at the same time, you need to take the actual load created into account or you might end up with under-rated CBs and cable. 
> Yet another case of talk to the electrician before you spend some time and money on something that turns out not to be quite right.

  Well thats a case by case thing isnt it. But you will not be breaking any laws if your cable is sized and circuit breakers rated if you have correct Maximum demand calculations. Its like electricians come and count 3 power circuits (20A each) 2 light (10A each) air con (20A) Stove (32A) and hot water (20A) and say you have a load of 152A so your mains are too small.  
If you are undersizing your cable to what a full load may be, meaning all power points loaded up, stove on, air cons on full load (inverter loads vary) etc, the worst case scenario is a circuit breaker trip (assuming you select the correct CB). If you put a 63A breaker on a 6 or 10mm cable and then started loading it up, that cable is running the risk of over heating due to not having the correct trip times and properties.  
All circuit breakers do is protect your cable (Im talking the submain cb) so if you start overloading, it trips to avoid any fire or damage to cables etc. On top of that each circuit will have its own protection in form of either RCD protection, or just a circuit breaker. These sub-circuits are protected by themselves from their respective breakers.  
Like i said, if you ran a 6mm and a 32A submain circuit breaker, and had a power circuit, air con circuit and a light circuit, the worst possible scenario for it would be tripping the mainswitch of the sub board. Every size cable you go up, the dearer its going to be. So make sure you size it correctly, and also consider future options too.

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## oldtrack123

HI Brian 
A lot of words there ,BUT the whole purpose of calulating max demand is to Minimize false tripping due to loads 
It also allows caculating the miminum size cable that can be used, but taking into consideration voltage drop & loop impedance[loads are not the only consideration] 
Not a perfect system & in some cases special circumstances may require some adjustment to the figures  
PeterQ

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## Brian7886

Thats why i wrote in my first post, first you work out your maximum demand, which gives you a circuit current to work with, then start determining the conditions which will de-rate your cable run ie Length, Route (through roof, under ground, clipped to timbers, in conduit etc).  
To do this you would need AS3000 and AS3008.  
And like i said, circuit breakers protect your cable, if can have any larger size cable than required provided the circuit protection doesnt exceed said cables limits

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