Cable calcs 3 phase

thomasparry- · Jan 6, 2021

Its been long since I was in college doing cable calcs and obeing onsite as I've studied a completely different course at university, I understand de-rating factors but need some clarifying up to brush up.

If I need a 600A supply which can be split 200A each phase. and being supplied via SWA underground over a distance of 200m at a depth of 0.8 to a sub dis board.

When doing cable calcs do I use 200A or 600A?
And when reading cable datasheets for 3 phase reference method d - 1|thee or 1 four-core cable three-phase AC , does the data in the table refer to each phase ie. as 1mm2 - 21A per phase or 7A.
Bit of a stupid question

Thanks for your help.

westward10 · Jan 6, 2021

You have 200A from each phase what does that tell you with regard to your protective device/s.

Spoon · Jan 6, 2021

Another thing to consider is the voltage drop over 200m.

Wilko · Jan 6, 2021

Hi - there are no stupid questions, but my thought is you should be able to work this one out from first principles. Have you got BS7671 to hand? In your example what is single phase current carrying capacity for 1mm? Then for 3 phases at 1mm?

Lucien Nunes · Jan 7, 2021

Cable calculations and data tables are based on the actual current in each conductor. You happen to have single-phase loads spread over the three phases of a 200A 3-phase, 4-wire circuit. If instead you chose to supply the loads from a single-phase circuit, the conductor current would then be 600A and that would be the figure to use in the calculations.

thomasparry- · Jan 7, 2021

Lucien Nunes said:
Cable calculations and data tables are based on the actual current in each conductor. You happen to have single-phase loads spread over the three phases of a 200A 3-phase, 4-wire circuit. If instead you chose to supply the loads from a single-phase circuit, the conductor current would then be 600A and that would be the figure to use in the calculations.

Thanks , think i just had a brain meltdown yesterday