Quote Originally Posted by lom View Post
One point of caution for systems where 3-phase is used as 3 x 1-phase (ie , no true 3-phase equipment installed) :
The current flowing in the neutral line is the sum of the current in the 3 phase lines.
So the diameter of the neutral cable should be quite a bit bigger than the phase cables.
Nope! You can't just add the currents to get the neutral current, remember the phases are 120 degrees apart so you'll be needing to do some phasor calculations

Actually, the neutral current will never be greater than the largest phase current even in the most hideously out of balance systems. A quick and dirty calculation for your neutral current is:-

There is a quick way which is not exact but very close;
Example;
L1 = 7.5A
L2 = 5.5A
L3 = 4.0A
Subtract the smallest current from the other two:
L1 - L3
L2 - L3
7.5A - 4.0A = 3.5A
5.5A - 4.0A = 1.5A
The square root of (3.5(squared) - 1.5(squared)) = 3.162A

Works for power-factors better than about 0.8.