Online assistance for electrical trade people in New Zealand and Australia Login  |  Register  |   Forgot Password
Assistance for electrical trade people




Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: Earthing conductor to a Submain Board in Garage

Nov 19 2018 13:23

I have an installation adding a sub-main board to a garage connected directly to the house. The submain includes curcuits to the garage and a linked sleepout connected to the garage. 1 lighting curcuit (6A) and 2 radial socket curcuits (both 20A) for the garage (rcd protected)(25A) and 1 lighting curcuit (6A) and 1 radial socket curcuit (16A) for the sleepout (also rcd protected)(25A).
The main board at the house is old with the ceramic fuses and has a 60A isolation switch, it has the MEN link. The submain board I put a 40A twin pole isolation switch.
Here's my question..
I installed an earthing rod in the ground outside of the garage wall and installed a 6mm earth cable (with tag) to the earth bar on the submain. Do I need to bring across an earthing conductor from the main board or can just bring over the neutral and line to the sub board and just use the earth rod outside the garage as my protection? Cheers


Nov 19 2018 14:04

If the garage is attached to the house, you need phase(s)/neutral/earth to the subboard, no need for a second earth stake or MEN link.

You don't need a dual pole switch unless you have multiple phases.

If the garage was separate, you could take just a phase and neutral and treat it like a separate installation with its own MEN link... but that's not really recommended.

Nov 19 2018 14:14

Read You need to access the particular situation as to the best solution.

Also think about the cost of the solutions, if in fact there is a choice of acceptable solutions.

Think how separate the garage and sleep out actually are, plumbing etc?

Nov 19 2018 15:26

Here is a picture of the Sub-main

Nov 19 2018 15:27

Here is a picture of the main board

Nov 19 2018 15:34

As you can see from my picture I didn't install an MEN link to the Submain Board as I believe the wiring rules state that you can only run no more that one MEN link to a domestic dwelling where more that one Board is being installed.
Please correct me if I'm wrong. Cheers

Nov 19 2018 15:52

If you do not have an earth conductor back to the supply, you MUST have an MEN link. Consider where the fault current will flow if you get a phase to earth fault. It doesn't have a path back to the transformer, except via shitty earth stakes. So the whole installation now floats at ~230V above true ground.

Nov 19 2018 15:54


Firstly you need to consider the definition of an OUTBUILDING (clause 1.4.69) in AS/NZS 3000:2007 +A1+A2 to see if the outbuilding provision of clause is permitted to be used.

Secondly and only if the outcomes for an outbuilding can be complied with, clause (and also shown in Figure 5.1) detail the connections for neutral and earth required.

Thirdly, if there is any changes that an Electric vehicle is going to be charged in the outbuilding area, the use of the outbuilding provision is NOT permitted by the electric vehicle charging guidelines. The guidelines will form part of the ESR 2010 and AS/NZS 3000 in the near future (1 to 2 years away).

Nov 19 2018 16:52

There's so many things wrong or simply non-standard with what you've done, I would have to ask if you are actually an electrician or just a DIY'r?

If the garage is connected to the house, you can't use the "outbuilding" provisions. You have to run an earth from the main board.

If it was separate from the house and you were able to connect as an outbuilding without an earth from the main switchboard, then you must have a MEN link in the sub board and you can NOT have a double pole main switch.

The use of 25A RCDs for the arrangement of your circuits is possibly too small. What is the size of the circuit breaker on the main switchboard? And the size of the cable?

Nov 19 2018 17:43

Thank you all for your comments. There is a 60A switch at the main switch board, I have installed a 40A switch to the sub-main board. I'm only using 10mm cable which has to travel approx 15 meters to get from the main board to the sub-main board.

Nov 19 2018 17:53

Ok, so you're not an electrician.

Let me just be clear - the way you've wired it is dangerous and should not be livened.

You have two option, find an electrician who is prepared to rectify all the faults and certify the work - Doing it this way is not legal, but you might find someone prepared to do it.

Or, find an electrical inspector who will work with you to get the work up to standard and certify your work as homeowner work.

Nov 19 2018 19:14

Hi All,

I've had a chance to have a good indepth look at section 5 of the wiring rules on the earthing arrangement and earthing conductors of the job I've got going on. And also the help from you guys.
I'm going to do the following:
I'm not going to treat this installation as and Outbuilding and therefore run the phase/ neutral and earth from the main board on a 16mm 2 core neutral screened pvc sheath cable to the sub-main board (no MEN at the sub-main).
I will just use the earth off the mains board and lose the connection to the earthing stack.
As I already installed the 2 pole isolation switch in the sub-main, I'll just use one side for the phase and leave the other side empty.
Cheers for your help.


Nov 19 2018 20:46

There are issues with the way you have wired up your sub board. Get an Inspector in or an electrician before you go any further.