Cable size - garage/workshop

[tec67]

I plan to wire a new garage/workshop beside an existing dwelling house. The extent of the electrical work within the garage will be lights and sockets. Possibly 1 or 2 convector heaters from sockets.

The distance from the existing house CU to the position of a new CU within the garage is approx. 14M. I plan to use SWA cable as the garage supply cable and it will be buried for most of it's length. I also plan to install a 40A RCD protected CU within the garage with individual MCB circuits.

Can someone please verify the following:

1. What size of SWA would be recommended?
2. The existing house CU is of the old type pull-out fuses - would it be best to replace this at the same time and install a split load CU and take the garage supply from the RCD protected side via MCB (40A)?

Any help would be much appreciated, thanks.

 

[lewy]

1) You want at least 6mm cable, 10mm if you can get it.

2) put the new CU in your house then off of a 40A MCB on the NON RCD protected side put the feed to the garage CU, then feed sockets from that through an RCD.

 

[brown-nought]

Your garage supply will need to run from a separate fuse/breaker in your house's CU and could be protected by a RCD prior to leaving the house. Consider using 6mm or 10 mm SWA cable buried below 450mm deep. SWA cable needs correctly terminating at both ends.

In the garage you can run the lights off a 6A Mcb in 1.5 mm T&E and put the sockets on a radial circuit rated at 20A in 2.5mm T&E. (This will be enough for a convector heater). I’d look for a garage CU rated at IP44 or better. Consider protecting the garage cables in conduit/trunking if they are surface mounted.

You can protect the cable with a 30A fuse or 32A MCB.

Are you aware of Part P, especially if the new garage is subject to Building Control inspections?

 

[ban-all-sheds]

Your garage supply will need to run from a separate fuse/breaker in your house's CU and be protected by a RCD prior to leaving the house.

You do NOT need the supply cable to be RCD protected, and if you have an RCD in the garage CU you don't want one at the house either.....

 

[securespark]

As FWL and myself have argued in the past, you could protect the supply cable at the house end with a 100mA RCD, and fit a 30mA RCD to protect the final circuits in the garage.

10mm SWA will take you much further than 14m at 40A...

As far as VD is concerned, 4mm2 will do it, but I would go for 6.

Brown, you recommend a 32A breaker to supply the garage, and a 20A radial, but he wanted TWO heaters.

Also, is there discrimination between a 20A and a 32A breaker? IE if there is a fault, will the 20A breaker always trip before the 32A one?

 

[ban-all-sheds]

As FWL and myself have argued in the past, you could protect the supply cable at the house end with a 100mA RCD, and fit a 30mA RCD to protect the final circuits in the garage.

You could, but.....

 

[plugwash]

FWL has expensive tastes

i agree thats the ideal soloution but it just doesn't seem worth it SWA is pretty safe even without rcd protection because the armour gives a stong earth to whatever cuts it (and it takes a pretty shapr object or a lot of force to go through it in the first place) and having to walk to the house to reset power to your shed whilst annoying is unlikely to be too much of a problem

it also might motivate you to do something to stop yourself tripping it in the first place

 

[Lectrician]

Umm.....so a 240mm/sq 4core is best protected by a 100mA RCD when used a s a submain to an office block, trenched under the main carpark??

Every instal is different, and that is the difficulty with advice via these sites. Not every underground service needs, or warrents RCD protection.

 

[brown-nought]

Ah the perils of writing answers too quickly and reviewing too little. (I’ve edited the original post)

You do NOT need the supply cable to be RCD protected, and if you have an RCD in the garage CU you don't want one at the house either.....

Yes, point taken.

I meant to say could be.

One concern I have with no RCD is if a SWA cable is terminated at a shed/garage in a non insulated enclosure it will rely solely on the fault loop for disconnection unless an RCD was provided further upstream.

Brown, you recommend a 32A breaker to supply the garage, and a 20A radial, but he wanted TWO heaters.
Also, is there discrimination between a 20A and a 32A breaker? IE if there is a fault, will the 20A breaker always trip before the 32A one?

20A gives 4.6Kw, most convectors are 2Kw. If you don’t mind the extra cable expense 32A would be fine too.

20A MCB’s should have diversity with 32A from the same manufacturer.

Apologies for the confusion.