SWA sizing for sub boards

[FreddieC]

Hi guys,

I'm working in a new role which requires me to size SWA cable to feed sub boards which in turn supply EV chargers. I noticed on the jobs that have been done already they have used 50mm 4c and 70mm 4c to feed 3ph 250A main switches on the sub boards. Going by tables and calculators this should be a 95mm 4c swa minimum (to have enough CCC incase it reaches 250A). Is there something obvious I'm missing? Sorry if this seems a stupid question, I just don't have experience in this area and would like to see if anyone can clear it up for me.

 

[Wilko]

Hi - sorry if I’ve not understood correctly, but the protective device (fuse?) needs to be less than the cable CCC, not the cable sized up to an isolation switch rating.

 

[FreddieC]

Hi - sorry if I’ve not understood correctly, but the protective device (fuse?) needs to be less than the cable CCC, not the cable sized up to an isolation switch rating.

Yes. Basically I entered 250A (30m buried/ducted) into a manufacturers calculator and it told me I needed a 95mm cable minimum, as anything less and the cable would reach it's maximum CCC before the protective device kicked in.

When I'm looking at the installs done already they have used 50mm and 70mm 4c swa which seems to me to be too small if the main switch is 250A?

Sorry if I'm not wording it clearly.

 

[Wilko]

3ph 250A main switch

Don’t suppose you’ve the make/model of this?

 

[James]

You need to look at the board that supplies the circuits feeding the 250A distribution boards.

for instance, it is entirely possible that the feed to the board is in fact 200A for example.

In some case they may be adjustable breakers, so even if the breaker is 200A or even 250A, it may have been adjusted down to match the cable characteristics.

 

[Darkwood]

As above the SWA is protected by the device at its origin of that cable, if its 100amp fused the main switch can be 100amp or more on the distribution board it feeds...

I suspect been buried and been a sub-main and as long as the supply it comes off is large enough then at a minimum it should be fed by 160amp MCCB/ HRC fuses. This will allow for full discrimination for any mcb fitted in the Sub.
I think the cable size may be undersized here anyway been buried and the derating factors, so either it is on a lesser front end like a 125 or 100 amp or it may not comply.
I would check it carefully with calcs to see it complies, I may be wrong but 70mm would be OK if it was surface clipped direct or tray, I think it would be pushing it if its on 160's and buried.