Wiring Up Garages

[Drumtochty]

I have a 100 Amp supply into the house and at peak use we maybe get to 25 Amps in the house. No electric showers or hob.

I want to take a 40 amp supply to the garage via a 10 sqmm three core SWA cable around 30m long buried for 20 mtrs.

Please advise if I am getting this correct.

As I will have a car lift it needs a 32amp type D mcb in the garage.

At the house consumer unit the swa is earthed to the consumer unit as well as using one of the conductors to run the earth. The house consumer unit mcb for the garage will be 50 amp type D. It will be on the earth leakage side of the house consumer unit.

At the main garage consumer unit, SWA will not be connected. The three conductors will be connected phase, neutral and earth.

The lights in the main garage will be on the non earth leakage side of the garage consumer unit. The following will be connected to the earth leakage side of the main garage consumer unit.

Sockets 32A mcb type B with max load of a compressor 10 amps.

Car Lift 32A mcb type D, 25 amp at start up but load while lifting 15 amps. So only used for a two minutes at a time.

Heating 32A mcb type B with at max a 20 amp load but very well insulated garage so hopefully 10 amps most of the time.

Garage 2

This is half way between the house and main garage. At most lighting and a compressor. Max load 12 amps usually nearer 0.5 amps.

Lights on garage 2 non earth leakage side of garage 2 consumer unit. Sockets in garage 2 on the earth leakage side of garage 2 consumer unit.

The 15mtr long 2.5 sqmm SWA three core cable will not have the SWA connected to earth at garage 2 consumer unit but will have at the other side. It will be buried

It suits me better to take the garage 2 supply from the main garage rather than the house consumer unit. I assume it should be fed from the earth leakage side of the main garage consumer unit. A 32 Amp type B mcb.

Comments please.

[John Rumm]

This is a detached garage I take it?

What earthing system does the house have [1]?

What type of CU do you currently have (e.g. split load with single RCD etc?)

Why are you using a conductor as well as the armour? (i.e. is it intended to be a "belt & braces" or are you planning to export a PME equipotential zone and hence need the effective CSA?)

Do you mean in will be on the RCD protected side? If so then that would not be a good plan, since any trip at the head end will isolate the garage completely - potentially creating a hazardous situation with loss of lighting in the garage.

Also if you want a 40A supply, why are you using a 50A MCB?

(at 50A, the voltage drop on an 30m XLPE SWA would be over your allowable 3% for lighting circuits - so you would need to go to 16mm^2)

I can't see the logic for not connecting the armour, if you are exporting the earth from the house down a core in the cable.

This sort of arrangement makes sense when you are not exporting the an earth, and making the garage a TT earthed installation in its own right.

Note that if the feed itself is RCD protected (and the RCD is not a time delay RCD) then any further segregating into RCD and non RCD circuits makes little sense. Since any trip would be equally likely to take out the head end RCD.

Sounds like a 16A type D would be more in keeping here? Presumably the lift motor will be an induction motor. Their inrush will normally be in the range of 5 to 9x the nominal load. A type D breaker will permit an inrush of up to at least 10x the nominal current.

ok

Could I suggest having a look at:

and note the sections on exporting earths and equipotential zones.

What you describe here sounds like a classic scenario for not exporting the house earth, and going TT in the garage(s) [2]

Thus you would typically use a 2 core SWA with the armour connected at the head end. The SWA fed from a non RCD protected MCB (assuming the house is not already TT, or a type S time delay RCD if it is). The garage can then be configured as a TT installation, with an insulated (i.e. plastic) 17th edition style split load CU (or better a high integrity type, or all RCBO CU [3]) connected to just the L&N feeds, and a separate connection to an earth stake.

(Note also I have not checked all your proposed cable sizes for suitability).

[1] [2] [3]

[Stewith]

Its very likely that the the short circuit fault current will not be suffucient to trip the A D50 mcb at the house end quickly enough, and also unlikely to be enough for the D32 at the other end. It also sounds like you are going to have 2 RCD's in series so unless you are going to TT the garage,the garage lights will still take out the RCD in the house.

[Drumtochty]

The house earthing is TN-S The main switch on the consumer unit is a double pole 100 Amp. The fuse rail is split into two, one side having no RCD protection, the other side is though a Votex VR63 63 A capacity 30 mA RCD I thought using both the swa and the conductor for earth would be belts and braces.

I will read the linked documents tomorrow and come back tomorrow evening. Thanks so far guys.

[John Rumm]

That makes life a bit easier, since if you did choose to export the earth, you would not also need to export the equipotential zone with it (and hence main bonding conductor etc).

side is though a Votex VR63 63 A capacity 30 mA RCD

In which case you can take the SWA feed from the non RCD side, and that saves problems with having any cascaded RCDs. If exporting the earth, then a 2 core SWA, using the armour for earth, connected to a metal CU in the garage. If going the TT route, then similar except an insulated CU in the garage so that the armour is in effect only earthed at one end.

Thanks so far guys.