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# Sizing mains tails

Discuss Sizing mains tails in the Electrical Forum area at ElectriciansForums.net

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#### DanBrown

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What and how do you calculate the sizing of the mains tails?
For example a three phase + neutral incoming supply rated at 250 Amps serving several distribution boards.
Do you just put mains tails in that are big enough CSA to carry 250Amps each
or less CSA that are just big enough for each board eg 100A?

Also is their a table in any of the 17th books to give current carrying capacity for mains tails?

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#### Guest123

Hey.

Not sure if I understand your question.

If you have a 250A supply this would probably terminate into a 250A main switch in a panel board.

From there you would have individual MCCB's (moulded case circuit breakers) protecting each sub main to each DB sized according to load characteristics at each DB.

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#### DanBrown

Hey.

Not sure if I understand your question.

If you have a 250A supply this would probably terminate into a 250A main switch in a panel board.

From there you would have individual MCCB's (moulded case circuit breakers) protecting each sub main to each DB sized according to load characteristics at each DB.
Thanks for your quick response. Try and word it slightly different.
What we have is a 250A supply feeding a bussbar chamber and from bussbar to several dis-boards. Do we size the cable to 250A as their isnt any additional fuse protection?

And is their any sort of sizing guide in the 17th edition books to refer to mains tails?

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#### Guest123

I havn't got books to hand but it's basically just current carrying capacity of cables allowing for the various factors,

depending on wether you use tails, T+E, SWA etc, once phase size is determined you can then work out CPC size accordingly.

Cheers.

M

What and how do you calculate the sizing of the mains tails?
For example a three phase + neutral incoming supply rated at 250 Amps serving several distribution boards.
Do you just put mains tails in that are big enough CSA to carry 250Amps each
or less CSA that are just big enough for each board eg 100A?

Also is their a table in any of the 17th books to give current carrying capacity for mains tails?

hi there

Woah just a minute it is not just look at the size of the meter tails.

You have to consider every circuit from every board back to the incoming start point not only to allow the existing demand but the future demand of the installation.

other considerations are things like volt drop, current cable capacities , fusing, etc are needed.

Where i think you may be coming from is that domestic installations are all ready designed ie we all use 2.5mm for ring final circuits etc. this i think is for Mr DIY to put it all together without burning out his cables lololol
Diversity etc.

Larger installation need to be designed with consideration to the use to which they are intended for so putting a 250 cable say a 70mmm etc may be OK for its current use but may not be for its future use or upgrade etc

Cables have to withstand fault conditions up to the protective device current disconnection times to that stated in the red book etc (final circuits)

distribution circuits much a lennie the loon says

do not forget the earth fault loop is affect by cable sizing R1+R2 etc that all has to be correct so that the small fuses etc pop in a fault before the fuse behind that one etc.

cheers

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#### Guest123

Hey mate.

The other thing that strikes me as not quite right is that all the sub mains come directly off the busbar with no switchfuse or overcurrent device between the chamber and the sub main DB so are all the sub's in 70mm Cable??? if not then you need to alter it because should a fault occur then it could rise up to 250A as that is the rating of the main cut out fuse. If subs are 16 or 25mm then it wont be good for them!!!!!

Cheers

Sorry just re-read your post. Ideally there would be isolation for each sub main that way anyone can be isolated by itself without lnocking everything off at the same time.

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#### DanBrown

You should look for single cables table in regs, wether they are double insulated won't effect their current carrying capacity. 25mm will take a 100 amps.
When you say look in the regs for single cable,
Is it Table 4D1A - single core 70 degrees thermoplastic insulated cables, non armoured, with or without sheath (copper conductors)?

Thanks

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