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Assume that you have a wye-wye 3 phase circuit. Each of the 3 phase wires can take a max of X Amps. What is the max current the neutral of the circuit could have?

Is it possible for the neutral current to be greater than X?

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  • \$\begingroup\$ Imagine connecting a single phase load to the cable, a resistor between one phase and neutral. What current flows in neutral? What happens to the neutral current if you introduce another, much larger resistor between another phase and neutral? \$\endgroup\$ – tomnexus Jan 13 '16 at 5:34
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  1. If the three-phase currents are unbalanced, the neutral current can exceed the phase currents.

  2. If there are 3rd, 6th, 9th, ... harmonics present - the "triple-N harmonics" - then these harmonics are in phase between all three phases. Therefore they all add up in the neutral. These harmonics are drawn by things like switched-mode power supplies (i.e. personal computers) and fluorescent lights.

Problem No.2 is common enough in modern office buildings, with a high concentration of desktop PC's and fluorescent lights, that the neutral conductor often needs to be larger than the phase conductors.

The Australian Wiring Rules, AS 3000, section 3.5.2 Neutral Conductor, refer:

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