current carrying capacities of cables.

[Ross Trician]

Evening all,
I have been asked to quote for some remedial work and one of the jobs is to replace an over rated circuit breaker.

whilst doing the "paperwork stuff" I am trying to make a desicion on the size of circuit breaker required.
the cable is an SWA,70mm, and is covered by a 400A c.b.

Whilst looking in the regs book for correct ratings of cable etc I have raised a few quetsions to myself and can't come up with an obvious answer so thought I'd post on here. so here goes.

1, with reference to 3 phase cables it only states ratings for 3 or 4 core cable, is that because they dis regard the earth (or cpc should that be)

2, why can't i find a rating for cables that are buried in the fabric of the building. only ref mothods C,E,D.

3, what is the difference between thermoplastic and thermosetting??

Thanks for the help.

 

[Sintra]

Evening all,
I have been asked to quote for some remedial work and one of the jobs is to replace an over rated circuit breaker.

whilst doing the "paperwork stuff" I am trying to make a desicion on the size of circuit breaker required.
the cable is an SWA,70mm, and is covered by a 400A c.b.

Whilst looking in the regs book for correct ratings of cable etc I have raised a few quetsions to myself and can't come up with an obvious answer so thought I'd post on here. so here goes.

1, with reference to 3 phase cables it only states ratings for 3 or 4 core cable, is that because they dis regard the earth (or cpc should that be)

2, why can't i find a rating for cables that are buried in the fabric of the building. only ref mothods C,E,D.

3, what is the difference between thermoplastic and thermosetting??

Thanks for the help.

1. the cpc of swa is generally external. 3 core is 3 phase. 4 core is 3 phase and neutral.

2. swa cables are generally not in the building fabric especially 70mm.

3. thermoplastic 1s 70 degree pvc swa and thermosetting is 90 degree xlpe swa.

hope this helps.

how many cores in your cable?

 

[Ross Trician]

Hello Sintra and thanks for the reply.

I assumed the earth was disregarded and therefore left just 3 phase or 3 phase and neutral.
as for the cable, it is in a school and from what info i can get it is a mixture of clipped direct and also in the fabric of the building, thru and in walls, very tight voids in flat roofs etc.

Also I am not sure of the number of cores it has, not been able yet to have power of to acess the board (and going on the state of some of the work i have seen there dont want to mess with it live...lol)

 

[Sintra]

Hello Sintra and thanks for the reply.

I assumed the earth was disregarded and therefore left just 3 phase or 3 phase and neutral.
as for the cable, it is in a school and from what info i can get it is a mixture of clipped direct and also in the fabric of the building, thru and in walls, very tight voids in flat roofs etc.

Also I am not sure of the number of cores it has, not been able yet to have power of to acess the board (and going on the state of some of the work i have seen there dont want to mess with it live...lol)

Don't blame you mate but 400A overcurrent protection is definatley very high. I would say that a 70mm would be about 160A max. Don't have brb near me to check.

 

[scotsparky]

from BRb tables only not including any correction factors tabulated NO only
70mm thermoplasic 3 phase ccc ranges from 143 (ref D) to 207 (ref E).. P280
70mm thermosetting 3 phase cc ranges from 167 (ref D) to 358 (ref F).. P288,290

 

[benji]

is the 400a cb adjustable?


yours benji