SWA 2 or 3 core to garage?

[Mr Sandman]

hi all,

should i go for 2 or 3 core swa down to my garage? its about 40 meters away and i am going for 16mm3

cheers

steve

[John Rumm]

Two, unless you are exporting a PME earth as well (and hence the main bonding conductor).

[John Rumm]

Pardon me talking to myself, but its just been pointed out to me that you were talking about 16mm^2 SWA. The armour on that has enough copper equivalent CSA to be used as a main bonding conductor as well - so you will only need two core regardless.

[ARWadsworth]

Sorry John, I was working when I pointed that out to you and I needed to check before posting to the newsgroup.

I would say a 16mm two core is fine for a 40m run using a 40A MCB. This should cover voltage drop for a lighting circuit in the garage and allow any mains bonding in the garage to meet the regs.

[Bruce Burnett]

16mm2 armour is fine in copper equivalence terms to act as a cpc for fault protection, but NOT to act as a main protective bonding conductor. See note at bottom of 544.1.1. For bonding it is the equivalent conductance that matters. Regards Bruce

[ARWadsworth]

The CSA of the armour of a 2 core 16mm SWA cable is 42mm^2. That gives it the copper equivalent of 18.6mm^2.

For most domestic installations that will be fine.

[BruceB]

I agree it gives a copper equivalence in K terms which includes specific heat. That is relevant for temperature rise in the adiabatic equation in relation to sizing a cpc. But that is the wrong equivalence for a bonding conducter which has to be done in pure conductance terms. I said above it was at the bottom of 544.1.1, it is actually at the bottom of Table 54.8 immediately below that regulation.

You need to be around 70mm2 cable before the armour reaches an equivalence of 10mm2 copper in conductance terms.

Regards Bruce

[John Rumm]

Yup, I th "Note however that the calculated copper equivalent area is only for use in the adiabatic equation described above. If you need to compute the actual armour resistance then use the resistance figures quoted instead."

and

"Also note that if one is exporting the equipotential zone, that probably means the CPC of the submain is also being used as a main bonding conductor, and so it will will have to meet the minimum CSA size requirements for a main equipotential bonding conductor. Since this is often 10mm² of copper (or an appropriate CSA of another metal offering equal conductance), it will preclude the sole use of the armour wires as a combined CPC / Bonding Conductor, since the armour resistance is typically too large (this is generally true for all SWA cables less than

70mm²). In these cases a separate bonding conductor operating in parallel with the armour can be used. "

[ARWadsworth]

I get you.

With an 8 times greater resistivity for the steel than the copper then yes it will need to be 70mm^2 2 core SWA (as that is where the armourings reaches 80mm CSA)

[John Rumm]

I am no fan of exporting PME earths and equipotential zones anyway in most cases. So you would be better off making the far end TT and sticking with two core SWA and isolating the earth at the far end IMHO.

[Andy Wade]

Quite right - as debated previously at length both here and in the IET forum. I do wonder whether it's what the committee intended to say though, or whether it's just badly worded.

After all there's no upper limit allowed on the resistance of a copper main bonding conductor, so why limit the conductance of a steel one? Logically it should be either the adiabatic (I^2*t) capacity of the conductor or its continuous current carrying capacity that should matter for main bonding.

[BruceB]

I was wondering the same as neither maximum resistance nor maximum length is specified. But I think it was probably deliberate because in addition to adiabatic capacity and current withstand there is another factor. The bonding is there to hold the touch voltage down during a fault and hence the conductance/resistance of the bonding conductor compared to say the cable introducing a fault is relevant. Regards Bruce

[Andy Wade]

Agreed, so why don't they stipulate a maximum resistance for copper main bonding? There is a resistance limit for supplementary bonding of course, fist introduced in the 16th ed., IIRC.

I can only imagine that it's a non-issue in practice, with the length in practice unlikely to be enough to lead to an excessive resistance. Large buildings will have larger capacity supplies, mandating larger main bonding conductors. Supplementary bonding, OTOH, can be as small as 1 mm^2 if incorporated in a cable or otherwise mechanically protected, so the possibility of excessive resistance is more likely.

[Tim Watts]

It's a tricky issue. That solution is clean if you are doing a single submain to a remote shed.

But what would be the best solution for running a lighting circuit round the garden (gatepost lamps, couple of lamps in the garden sort of thing? The same circuit might supply a small lamp in a bike shed and other odds and sods. This would be a single supply circuit with local switching.

I suppose you could make everything class II and worry less (we'll assume an RCD at the house end of the circuit source).

Or you could TT it - but if it goes to a variety of distant loads would you have to stake it in several places?

What if it was desirable to use this circuit in the vicinity of another circuit, eg in the workshop[1] so that if a power tool tripped the socket circuit, the lights stayed on - you'd have to have the same earthing system and cross bond them.

I'm still undecided on mine. My original plan was:

20A radial to a few waterproof sockets on the house wall (one on each corner) - In theory a class I appliance *could* be plugged in outside from this. 32A supply to workshop for sockets only (re point about tripping and lights not going out as a result) 10A general lighting supply going all over the place including the workshop (which would have a pair of DP isolators locally on point of entry)

This seemed like a good idea when I believed I had a TN-S earth, but got a bit unravelled when the bloke proved EDF's records were wrong by opening up the cutout housing (I actually have TN-C-S).

Thoughts appreciated :)

[BruceB]

I know I'll be at odds with some here, but I see very little problem exporting TN-C-S earth. Or as I describe it extending the equipotential zone. You do have to watch out on the bonding issue, but the likelihood of a broken neutral coming into a house are vanishingly small UNLESS the final few metres of cable is coming in overhead through trees into your house. I watched my PME supply go in and the final earth electrode was buried with the cable within a few metres of my house. So virtually zero chance opf broken neutral in my case. Many believe there is a general prohibition on exporting a TN-C-S earth. But that is a myth - no such rule exists.

Regards Bruce

[John Rumm]

I have no problem with exporting it in some circumstances. To a shed or garage etc might be fine. I would be less keen where there is easy access to an independent earth such as in a building with its own incoming services or a greenhouse. Since in these cases one has to take care to maintain the EQ zone. In the case of a greenhouse this may be impossible.

All the properties round here are wired overhead from a transformer on a pole. They recently replaced all the LV wiring from the transformer to the houses and along the road. Its interesting to note that they changed the wiring style from four independent wires spaced vertically a few inches apart, to an arrangement with all the wires twisted into a bundle around a support wire. I presume the latter arrangement also makes disconnection of the PEN conductor in isolation somewhat less likely.

Probably not that much even on an overhead supply these days... I note they certified ours for PME when they replaced the drop cable, even though the house is setup for TT.

Indeed.

[BruceB]

known I think as ABC - aerial bunched conductor

[John Rumm]

A quick google would seem to confer.

Probably a better solution where you must use overhead. Only downside is aesthetic... the cables carry far more visual weight than before even if there are fewer of them.