Our existing 3 phase electric layout is as shown in the attached diagram.

 

I am planning to demolish the existing garage and build a new one elsewhere on the plot and am digging a trench for water and electric.

 

I'm trying to decide what size of armoured cable to run.

 

I don't need 3 phase (for any tools etc), but I am wondering about future ev chargers 

Mains Supply Design.pdf

A duct with a rope through it would be better. 

Cable size depends on distance and load required.  Plus acceptable voltage drop.

 

If you don't know get advise from your electrician.

 

Charging car batteries and a fair distance equals a very big cable

What distances are involved between equipment?

Its 50ft from the house incoming mains to the proposed garage

6 hours ago, Mattg4321 said:

A duct with a rope through it would be better. 

 

If it was a fairly straight line I would put a duct with a rope in, but I don't want to risk it with the route the cable will go

Best consult an electrician which I'm not. But the Cable calculator here..

 

https://www.elandcables.com/cable-calculator

 

If you enter 100A, 20m, single phase, buried in the ground it suggests either..

 

Armoured thermoplastic insulated cables - Multicore 70°C

Cable size: 35mm

Cables include: BS5467 SWA, BS6724 SWA (equipment connected rated 70⁰C)

 

Or

Armoured thermoset insulated cables - Multicore 90°C

Cable size: 25mm

Cables include: BS5467 SWA, BS6724 SWA

 

35mm^2 SWA is  perhaps £9 to £10 a meter. 20m would £200, some places offer free delivery.

 

 

 

You can’t use the 90 degree temp unless all accessories connected are also rated to that, which invariably they won’t be. 
 

It’s highly likely that 35mm2 will be overkill. It’s also likely you’ll want to run data out there I would think. Especially if you’re fitting EV charging. 
 

I’d be allowing for a TPN EV charger just to future proof it as much as possible. At a guess, 4C 16mm2 and at least one separate cat 6. 
 

It really would be best to discuss with the person who is doing the job though. 

I looked at a few online calculators and 16mm² kept coming up.

 

I unearthed the incoming mains and it looks like 16mm² swa.

 

At 2pm on a Saturday most electrical wholesalers are closed, except one which had 19m 16mm² 4 core SWA, so that was just long enough and has gone in!

That will give you the flexibility to add another phase if ever necessary.

4 core will be suitable for 3 phase, either by using the SWA as earth - if calculated to be suitable, or by making the garage TT - again if suitable. If you want belt and braces, chuck a 16mm2 earth through the duct with it. This was always the accepted way of doing it. Seeing 5 core installed is rare. 
 

The incoming DNO supply will very likely be aluminium concentric, so external appearance won’t be much clue. Not that you really need to know! Only what size your DNO fuse(s) are. 

Edited August 6, 20233 yr by Mattg4321