Neutral conductor sizing for commercial loads

BSRIA Power quality guide (AG 2/2000) was written by C C Pearson and V Uthayanan and published by BSRIA in July 2000. It states:

The required rating for the neutral conductor can be estimated using the typical waveforms. For a SMPS, the third harmonic is approximately 70% of the fundamental. If we assume that the loads on the three phases are balanced, as shown in Figure 30 and all have this same characteristic, then the rms phase current and rms neutral current can be approximated by taking only the fundamental, I1, and third harmonic,I3, components as follows:

The neutral current in this case will be 172% of the rms phase current magnitude. The conclusion from this calculation is that neutral conductors in circuits supplying electronic loads should have almost twice the capacity of the phase conductors. An alternative method to wire these circuits is to provide a neutral conductor with each phase conductor.

An approximate formula for calculating the neutral current magnitude as a percentage of the rms phase current is given below. The formula is based on the assumption that the circuit loading is balanced, that the nonlinear load watts are a fraction pnl of the total load, and that the load current has a third harmonic component equal to 70% of the fundamental.

This relationship is illustrated in graphical form in Figure 31.

--BSRIA

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