What size SWA for main 80A supply?

[d000hg]

The new electrical supply for our new building is installed on a wall next to the electrical pole, leaving us to get the cable about 15m across our garden to the building, where the consumer unit will be. (This was recommended by the powergrid team for cost reasons not to pay them for the trenching.)

We've got a trench dug and conduits laid but what size SWA will be suitable for the full 80A our supply fuse is rated for, since this cable is basically just extending the supply to the building?

I am thinking maybe 10mm2? We don't expect the demand to be anything like 80A but it seems to me the cable should not be rated lower than the fuse!?

 

[flameport]

10 is too small.
What will it be connected to at each end?

What exactly do you mean by this:

the distributor unit

 

[d000hg]

10 is too small.
What will it be connected to at each end?

What exactly do you mean by this:

Sorry, CONSUMER unit. Brain must've been elsewhere
At one end is the electricity meter. At the other is the main building consumer unit.

So this cable is just like you'd have between the two in your house, but they are separated 15m

 

[bernardgreen]

The cable from the pole to the cutout ( the DNO's fuse ) will normally be supplied by the DNO engineers

 

[d000hg]

The cable from the pole to the cutout ( the DNO's fuse ) will normally be supplied by the DNO engineers

Yes, it was. We have the supply, meter and cut out already installed. The meter is active but nothing is connected

 

[AndyPRK]

at least 16mm I would think. Try and go for bigger

Do you have any electric showers in the building?
electric heating only?

you need a switch fuse by the meter

 

[d000hg]

at least 16mm I would think. Try and go for bigger

Do you have any electric showers in the building?
electric heating only?

you need a switch fuse by the meter

Since my consumer unit is on the other end of the cable I want the cable able to take whatever the supply is able to deliver, this is the main cable to the whole building hence 80A to match the main supply fuse. Like your house, the energy demands could change over time so I want this main cable never to be the weak link

When you say a switch fuse, you mean the primary supply fuse isn't sufficient? How come - or is this to make sure I never trip their fuse since I can't touch it?

 

[flameport]

When you say a switch fuse, you mean the primary supply fuse isn't sufficient?

Electrically it's sufficient, but DNO rules restrict the length of wiring between the meter and consumer unit to 3 metres.

25mm² would be a sensible choice, even though it's technically oversized for 80A.
3 core would also be a better option than 2 core in this instance, as although the armour as the CPC is probably compliant, having the extra copper core will avoid arguments and wrangling in the future.

 

[d000hg]

Ah, thanks. So we want to make sure we have our own fuse on our side of the meter... essentially we have a 1-fuse consumer unit in the box with a single cable running to the actual consumer unit on the building?

And this might as well be a switched fuse so it can double as an isolator?

 

[flameport]

This, or something similar:
https://www.screwfix.com/p/wylex-100a-domestic-switched-fused-unit/5290j

 

[AndyPRK]

Not seen one of them before.
The earth terminals look tiny in the pic

 

[JohnW2]

Not seen one of them before. The earth terminals look tiny in the pic

Kind Regards, John

 

[bsr]

If you are not doing volt drop and fault current calulations it would seem seem sensible to use 25mm2 three core.

Instead of a switch fuse you could use a mini enclosure with a 100A DP MCB such as this one:
https://www.se.com/ww/en/product/A9N60734/c120n---circuit-breaker---2p---100a---c-curve/

Have you considered adding an RCD? Particularly if you have a TT supply.

 

[d000hg]

I didn't understand the point about 3 core, can someone explain?

For the length of run I don't particularly have a problem fitting oversized cable, the cost increase is not going to be that big and if it avoids needing to replace it later...

 

[JohnW2]

I didn't understand the point about 3 core, can someone explain?

You theoretically can use two core, and thus rely as the armour as the earth/CPC. Some people say that's how it 'should' be done, whereas others favour three core (so that a cores, as well as the armour, is being used as earth/CPC), partially to ensure that there are no arguments abouy the adequacy of the the armour alone, and partially because, in the event of the outer sheath being penetrated/broken, it's not unknown for the armour to corrode and 'fall apart'.

Kind Regards, John