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.
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.
