Hi, I haven't looked at cable calculations for a few years, and wondering if anyone can point me in the right direction. I want to size the neutral cable on a 3600kW three-phase generator. Preliminary calculations from a supplier have indicated that each phase circuit would require 4x1000mm2 cables and the neutral would be 1x800mm2 cable. I would like to know the formula required to arrive at this conclusion. As background, the generator operates at 725V, with the neutral solidly earthed at the star point. Any help would be appreciated! Thanks.
Looking at the relative cross sectional areas and approximate current carrying capacity, this looks vaguely American in origin to me - someone is potentially (and wrongly) applying either a 12.5% rule based on cross section topped up with some form of assessment of allowing for 200A plus a percentage of the unbalanced loading (either with or without harmonic assessments).
It may also be the case that there is an assumption that the set is impedance earthed (which is credible at 3.5MVA) although you say its solidly earthed
The relationship doesn't fit in with any European codes I could point at.
If it's all pretty industrial with lots of balanced motor loads, it may well be OK as Broadgage points out as the neutral current is quite small - and 800mm2 may well survive a fault to earth with reasonable disconnection times.
Looking at the relative cross sectional areas and approximate current carrying capacity, this looks vaguely American in origin to me - someone is potentially (and wrongly) applying either a 12.5% rule based on cross section topped up with some form of assessment of allowing for 200A plus a percentage of the unbalanced loading (either with or without harmonic assessments).
It may also be the case that there is an assumption that the set is impedance earthed (which is credible at 3.5MVA) although you say its solidly earthed
The relationship doesn't fit in with any European codes I could point at.
If it's all pretty industrial with lots of balanced motor loads, it may well be OK as Broadgage points out as the neutral current is quite small - and 800mm2 may well survive a fault to earth with reasonable disconnection times.
