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.
I am not aware of any formula to calculate the size of neutral conductor from either the size of the phase conductors, or from the total capacity of the generator.
It depends on what is the largest current likely to flow in the neutral, and that in turn depends on the nature of the load. A load consisting primarily of three phase motors will have little neutral current. 720 volts does perhaps suggest a nominal 400/690 volt system, with the generator voltage a little higher so as to allow for voltage drop. 400/690 volt systems are widely used for medium sized induction motors. The neutral current will normally be very low on such a system.
Alternatively, utilisation may be at some lower voltage via a transformer. If such a transformer has a delta primary, then no neutral current will result from the transformer.
It is common practice to make the neutral half the cross section of the phase conductors. This is based not on detailed calculation, but on empirical experience.
Consider also the fault current and confirm that any undersized neutral can carry the current resulting form a phase to neutral short circuit, until either the OCPD operates or the engine shuts down.
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.
3.6MW on 3 phase is 1.2MW phase, so each line needs to be rated for 1.2M/750 or about 1600A.
If you really did mean a 690/400 installation then 1.2M/400 means line current of 300A
If divided over 4 cores each carrying 400A each then either the run is long and you are voltage drop limited, or something closer to 300mm2 cores would be OK for the 690 to ground case, or perhaps 400-500mm2 if we mean 440 to ground. You do not mention ducting, grouping or or spacings so there is scope for some variation.
