# Australian Residential New Installation Switchboard Cabling



## New_Spark (Dec 14, 2011)

Hi all, I am a newly qualified electrician but have no experience in domestic other than installing GPO's and SB testing; however I have scored myself a big job; a brand new domestic installation. I am confident with all aspect's of the job, even purchased the only tool I didn't have to legally complete the installation, a Megger. I however have one question, according to AS3000 all consumer mains, submains, final subcircuits, and all parts of electrical accessories must be fitted with cable with a current carrying capacity exceeding the maximum demand of that part of the circuit, and the cable's current carrying capacity must not exceed the rated Amperage of the protective device feeding the cable. 

ie. (Max Demand < Current Capacity > Protective Device Rating)

So why can no electricain explain to me why when a 63A Main breaker is installed in a domestic switchboard, it often has only 6mm or of the likes feeding out to the subcircuit breaker's. 63A should be around 16mm to fulfill the above equation regardless of total maximum demand. 
6mm cable could burn out if all the subcircuits are drwaing close to full current but not enough to trip any of their breakers.

Any help appreciated and references to AS300 would be great, thanks.


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## HARRY304E (Sep 15, 2010)

New_Spark said:


> Hi all, I am a newly qualified electrician but have no experience in domestic other than installing GPO's and SB testing; however I have scored myself a big job; a brand new domestic installation. I am confident with all aspect's of the job, even purchased the only tool I didn't have to legally complete the installation, a Megger. I however have one question, according to AS3000 all consumer mains, submains, final subcircuits, and all parts of electrical accessories must be fitted with cable with a current carrying capacity exceeding the maximum demand of that part of the circuit, and the cable's current carrying capacity must not exceed the rated Amperage of the protective device feeding the cable.
> 
> ie. (Max Demand < Current Capacity > Protective Device Rating)
> 
> ...


There are a few Australian guys here that can answer that question,.

Normally the current carrying capacity of the conductor exceeds the overcurrent protection around here at least.

Welcome to the forum...............:thumbup::thumbup:


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## New_Spark (Dec 14, 2011)

So if you were using a 63A circuit breaker as the main switch, would you use minimum of 16mm feeds in you're board, so that current carrying capasity is greater than CB? I have never seen this in residential even tho all use 63A or even 80A for main switch. I'm thinking people just go off the max demand of all the circuit's and dissregard the rating of the main switch.

For example people run 40A RCD's straight after the 63A main breaker, so 40A internals as it exceeds the max demand, yet protected by a larger 63A protective device.


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## Dennis Alwon (May 9, 2009)

New_Spark said:


> So if you were using a 63A circuit breaker as the main switch, would you use minimum of 16mm feeds in you're board, so that current carrying capasity is greater than CB? I have never seen this in residential even tho all use 63A or even 80A for main switch. I'm thinking people just go off the max demand of all the circuit's and dissregard the rating of the main switch.
> 
> For example people run 40A RCD's straight after the 63A main breaker, so 40A internals as it exceeds the max demand, yet protected by a larger 63A protective device.


The problem is we have no idea what the capacity of your wires are in relation to ours. We use american wire gauge- awg not mm.


Hopefully someone from the other side will respond. :thumbsup:


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## New_Spark (Dec 14, 2011)

Sorry mate that's a fair point, but the principal is the same. But anyway's I've basically been saying I constantly see 6mm feed's to the breaker's, fed from the 63A main breaker. well breaker size 63A - 6mm cable is rated 40A in this case - should it not be 16mm which is rated at 63A in an enclosure. Also as I stated, 40A RCD's are often fed from the 63A breaker with no protection in between, now we know an RCD does not trip on overcurrent (It is not RCDMCB, or RCD and Breaker in one), it is purely RCD (Safety Switch)

So a 63A breaker is protecting 40A cable and a 40A RCD. The cable is not 40A because of the 40A RCD, this is just an example, the RCD could even be less in some cases and the cable still 40A, the RCD has nothing to do with protecting the cable. the 63A Breaker is there for that.

This cable size still exceed's the maximum demand tho in all cases, so is this legal? It still defies the formula I first stated.
Breaker Size<Cable Capacity>Max Demand.

Thanks


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## New_Spark (Dec 14, 2011)

OK after a few day's of pulling my hair out, this is the best I can come up with.

AS3000:

2.5.3.1 
IB<IN<IZ 
max demand < protective device rating < current carrying capacity

However: clause 2.5.3.3
A device providing protection for overload current may be ommitted if the conductor has no branch circuits or socket outlets between origin and further protection device.

What is a branch circuit?
"Each of the circuit breakers in the service panel controls electricity on a branch circuit. A branch circuit is typically a loop of wire that runs from the service panel, out to receptacles, light fixtures, appliances, etc. and back again"
Wiki...

So basically unless it's a final sub circuit, ie. has branch circuits or GPO's 2.5.3.1 does not really apply, which is where my confusion starts.

If you don't follow me sorry, but if ur a bright spark, wanna dust off ur AS3000, then get back to me with ur thaught's, especially if ur a domestic sparky, it would be appreciated.


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## Frank Mc (Nov 7, 2010)

New_Spark said:


> OK after a few day's of pulling my hair out, this is the best I can come up with.
> 
> AS3000:
> 
> ...


Hey New spark

Head over to http://www.phased.com.au/index.php one of the domestic guys will be able to help you....

Frank


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## Meadow (Jan 14, 2011)

heat dissipation


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