Sizing neutral conductors for worst-case neutral current from phase-angle controlled dimmers on 3 phase power supplies
3 phase dimming systems do not draw conventional levels of neutral current in all situations. Full, or even partial, cancellation of neutral current should not be assumed.
Although the maximum phase current is 250A RMS in example given below, the neutral current is 311A RMS; approximately 25% higher. This occurs because chopped waveforms on each phase do not cancel in the neutral conductor. This condition is easily produced by simply loading a dimmer system equally on each phase and then driving one phase at full conduction angle (0deg), one at zero conduction (180deg) and the third phase at half conduction (90deg).
Depending on the applied diversity factor and the anticipated usage, it is recommended to use a neutral conductor of one standard size larger than any of the phase conductors. At the very least, the neutral conductor must be the same size as the phase conductors and should be protected against over-current at an appropriate level, e.g. by the use of a 4 pole circuit breaker.
Under-sized neutrals should not be used for supplying 3 phase lighting dimming systems. Their use could cause malfunction due to excessive line impedance and could be dangerous if not protected by an over-current device, in the neutral line, rated for the cable section.
Where a 3 pole breaker is employed, with a permanently connected or switched neutral conductor, the neutral conductor must be capable of carrying at least 25% more current than the rating of the breaker.
Technical Director, ETC Ltd