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Posted By Topic: shed wiring

Oct 31 2012 15:50

Hi there, customer wants a couple of power points and lights in her shed,can i run a 2.5mm tps protected by a 20a RCBO to power this without the need for a separate small subboard.

The Don
Oct 31 2012 16:59

Yes thats potentially possible , but would be checking installtion situation of cable , e.g. lenght of run , pressence of insulation etc to confirm that you can infact use a 20A RCBO , I would tend to suspect that you would need to use a 16A.
I would also tend to stick to the same cable size for the complete circuit as well . Although possible to drop down in cable size for the lights its not really good practise to do that .


Oct 31 2012 21:15

If the shed is steel, I\'d expect that to be bonded... which would require a 4mm from the switch board.

Nov 01 2012 10:54

Why would you expect either (equipotential) bonding or earthing, provided all the cabling is double-insulated ?

Nov 02 2012 08:03

Alex. I was under the impression that a steel portal building (which is typically 90% of all the modern sheds these days) require equil bonding irrelevant if the cable is double insulated or not. I was trying to find something in 3000 to say otherwise, but I guess when these were written back then, there wasnt so many steel portal buildings around in the domestic market.


Nov 02 2012 14:03

Until 1 April 2010, Regs dictated what had to be earthed and what had to be bonded.

Going way back R 151 (1976) required bonding of accessible structural steel, but only where segregation of wiring was impracticable. The Notes included waste pipes, bench tops, etc.
Trouble is, many inspectors didn\'t understand, and insisted on bonding regardless of whether there was segregation.

1993 Regs had a list of things that need not be earthed [R 75 (1993), R 85(1997)], and many assumed that everything else had to be; but that wasn\'t the case. These items were exceptions to the basic requiremnt for earthing of \"metal forming part of electrical installations\". Since structural framing isn\'t part of the electrical installation, the case for a steel-clad shed is no different from a steel-clad roof. If it\'s segregated from the wiring, it doesn\'t need to be earthed.
And steel framing is no different.

Since the 1999 amendment cited NZS 3000; 1997, the distinction between earthing (designed to carry fault current) and equipotential bonding has been clear.

The list for bonding included sink benches, shower trays, and urinals, and exposed structural steel work.

The list was modified in AS/NZS 3000: 2000, cited in 2002; and since then (over 10 years)bonding has only been required for pipes, pools, wiring enclosures, and telecoms.

So the requirement to bond structural steel was only in force from 1999 to 2002; by which time the rule writers ahd realised that in most cases it simply doesn\'t serve any useful purpose.

Same applies to cladding, and to under-floor foil; the answer is NOT to try - and fail - to get an effective high-current non-corroding connection to each sheet. The answer is to maintain segregation by ensuring it\'s not connected to wiring.

Nov 02 2012 14:27

So 2.5mm with rcbo protection at the board powering 2 points and 1 light in timber framed steel clad shed is good to go? Also i need to dig a small trench about a meter long for the feed ,can i use grey conduit underground or does it need to be the more visible orange.Don\'t normally do this type of work.Thanks.

Nov 02 2012 15:10

In comment to what AleK has stated, what is your take on a situation in which a light fitting that did not have a earth wire, attached to a steel shed (maybe by a chain)and the possibility of it developing a fault? Would having the bond have any benefit to the installation making it MORE safe or not??

Grey conduit is fine, as long as it marked appropriately at the correct depth as per reg with marking tape.


Nov 02 2012 16:32

To not have a PEC and still be compliant, it would have had to be either a double insulated fitting or (pre 1993) one which was out of arms reach from ground and from any earthed metal.
With metal suspension chains / brackets from a steel structure, it wouldn\'t have qualified for the 1976 exemption from earthing.

Under today\'s rules, if there\'s double insulation there\'s nothing to be gained from earthing or bonding the shed. So that\'s going to depend on the fitting, as many fluos etc don\'t maintain double insulation if on chains, but some do. But then they pretty much all require a PEC (typically onto the chassis, and as long as there\'s continuity from gear tray (eg via the tray clips) to chains the PEC takes care of everything.

Problems arise with some plastic-bodied fittings that don\'t have continuity from gear tray to the mounting brackets, or if chains are attached only to the plastic body; but that\'s the same for any DI fitting / enclosure if the installer uses conductive fixings to mount it to conductive structure (note the DI mounting blocks have the fixings in a separate compartment from the electrics).

For your example fitting, it needs a PEC, after that bonding the shed won\'t add any safety.

Nov 02 2012 18:46

AlecK Thanks for clearing that up. I think I have been thinking the earth strap you tie to a steel shed as equipotential bonding rather than as a PEC which is in accordance to
requirement in the regs.

Nov 04 2012 00:10

@ Alexk, Your comment
So the requirement to bond structural steel was only in force from 1999 to 2002; by which time the rule writers ahd realised that in most cases it simply doesn\'t serve any useful purpose.

So if that is correct, then can you explain clause, That seems pretty clear to me , I understand the original post was talking about a shed but there are plenty of steel framed houses out there.


Nov 05 2012 08:38 is about earthing, not bonding; same as
It\'s also restricted to framing, not cladding.

The way to keep them clear is to think: \"is this cable designed for fault current?\".
If the answer is \"yes\"; then it\'s earthing , and you size the cable according to the size of the largest active conductor. Ie for a normal house on 63A supply, 6 mm earthwire nor 4 mm bond..

And earthing the frame is about the risk of short circuit from damage to the cable; either from incorrectly installed (eg unbushed cable apertures etc) or from screws etc being put in later.

Ask yourself what the electrical difference is between a steel-framed dwelling and a steel-framed other building, and there is no answer; so have to conclude that this clause exists because of an increased likelihood of damage from the occupier.

The original post was about bonding of conductive cladding of a timber-framed shed.
And even a steel-framed shed escapes, because it\'s not a dwelling. Which I suppose is something to watch if the garage is ever converted into a sleep-out.

Next question is whether steel ceiling battens are part of the \"frame\" of a dwelling... and I reckon they technically are, but were not intended to be covered (given that the only cables that ever go through these battens are for individual light fittings).

It\'s a relatively new thing to use steel battens on a timber-frame house. There\'s also a fairly low chance of occupants screwing into ceiling battens and striking a cable. Maybe time for an FAQ to the EL 001 Committee?

Jan 16 2019 08:14

Sorry to drag up this old thread. But it has the original answers which are relevant.

In 3000:2018, has been updated. It has eliminated the old, and now specifically includes buildings that are not considered "dwellings".

But my question right now, is regarding the first two paragraphs of (3000:2018).

In the first paragraph, it refers to earthing the same as we knew previously, with the earthing conductor size being related to the size of the largest active.

But in the second paragraph regarding combined outbuildings, it then refers to it as a "bonding connection to the conductive frames".

I think this is confusing and could lead people to believe that a 4mm2 bond is what's required.

Could the word "bonding" in the second paragraph be a mistake?


Jan 16 2019 11:50

I believe the word should be "earthing" rather than "bonding".
There's a lot of confusion between these two terms, and this doesn't help avoid it continuing.
The overall heading of 5.4 is "Earthing of equipment", so the only proper interpretation is that this is an earthig connection, and therfore should be sized i.a.w. size of largest active; as is clearly stated in 1st para.
The essential point of the new para is that for a "combined outbuilding", ie one with 2 or more supplies to it, the structure must be connected (ie earthed) to EACH switchboard. If the structure were earthed only to one swbd, and that supply was later removed, the structure would lose it's connection to earth.

I suspect the word "bonding" got her only because of the - completely separate ' requirements for bonding of reinforced concrete associated with bathrooms etc; where again [] a (bonding) connection is required for each part; or alternatively a single bonding conductor may be run all the way back to "the switchboard that supplies all of the combined outbuilding"; which will be external to the outbuilding, being at the upstream end of the several submains for each part. Same underlying reason; to avoid future alterations removing a protective measure.

A number of errors have been identified in 2018 edition, and are being dealt with by way of an urgent Amendment due to be published late 2019. Not sure whether this is on the list. You should really address the question to SNZ for consideration.

Jan 16 2019 12:16

Thanks Alec. That's what I thought.
I was wondering if you were just adding these to the "list"? ;)

Jan 16 2019 12:54

No, the list was closed off some time ago; 'cos if stuff keeps getting added the Amendment will never be finished. And as this is a fairly minor point; not an urgent safety matter but simply trying to avoid people reaching a conclusion that could only be arrived at by taking the paragraph out of context; I can't see it being allowed in late.

However the list for Amendment 2 is open; and as I indicated the proper channel is to put it to SNZ. However I will make a note of it and discuss with others.