Is the neutral in 240v circuit a CCC?

[mivey]

And is no different than a MWBC with similar unbalanced current. L2 runs cooler, N runs a little warmer. Net effect of all three conductors in the raceway or cable is about the same - close enough NEC doesn't think we need to count all three as CCC's for ampacity adjustment reasons. If it were two conductors plus neutral of a wye system however, that current on the N is not unbalance current like it is with split single phase system. Maybe for a dryer it sort of wouldn't really make that much difference, but NEC doesn't single out this sort of thing as an exception to the rule.

But not to ignore the fact that there can be multiple current phases present and there may not be a true balancing out of currents. The NEC assumes loads are evenly split/balanced such that we get similar loading across lines. Any deviations are assumed to be ignorable.

Loading L1N and L2N with similar pf loads makes this assumption close. However, you could have something like a 30d lag (86.6% p.f.) on one line and unity on the other and that would give you 25.9% more current in the pipe than expected. Not such a small number considering that they count the neutral as a CCC for a 50% increase in current. The consequences of assumptions.

 

[mbrooke]

But not to ignore the fact that there can be multiple current phases present and there may not be a true balancing out of currents. The NEC assumes loads are evenly split/balanced such that we get similar loading across lines. Any deviations are assumed to be ignorable.

Loading L1N and L2N with similar pf loads makes this assumption close. However, you could have something like a 30d lag (86.6% p.f.) on one line and unity on the other and that would give you 25.9% more current in the pipe than expected. Not such a small number considering that they count the neutral as a CCC for a 50% increase in current. The consequences of assumptions.

I'm confused by this... Wouldn't both the hot and neut see the same lagging current in magnitude? I can't see the phase angles creating more I2R heating with all conductors thermally summed, but must admit my imagination is very limited on this matter.

 

[mivey]

I'm confused by this... Wouldn't both the hot and neut see the same lagging current in magnitude? I can't see the phase angles creating more I2R heating with all conductors thermally summed, but must admit my imagination is very limited on this matter.

I_L1N = 20<0d, I_L2N = 20<(180d-30d)

I_N = I_L1N + I_L2N = 10.35<75d

I_thermal_assumed = 20 + 20 = 40

I_thermal_actual = 20 + 20 + 10.35 = 50.35 (+25.9%)



For a network system it depends on the phase:

I_L1N = 20<0d, I_L2N = 20<(240d-30d)

I_N = I_L1N + I_L2N = 10.35<-75d

I_thermal_assumed = 20 + 20 +20 = 60

I_thermal_actual = 20 + 20 + 10.35 = 50.35 (-16.1%)

vs.

I_L1N = 20<0d, I_L2N = 20<(120d-30d)

I_N = I_L1N + I_L2N = 28.28<-45d

I_thermal_assumed = 20 + 20 +20 = 60

I_thermal_actual = 20 + 20 + 28.28 = 68.28 (+13.8%)

 

[mbrooke]

I_L1N = 20<0d, I_L2N = 20<(180d-30d)

I_N = I_L1N + I_L2N = 10.35<75d

I_thermal_assumed = 20 + 20 = 40

I_thermal_actual = 20 + 20 + 10.35 = 50.35 (+25.9%)



For a network system it depends on the phase:

I_L1N = 20<0d, I_L2N = 20<(240d-30d)

I_N = I_L1N + I_L2N = 10.35<-75d

I_thermal_assumed = 20 + 20 +20 = 60

I_thermal_actual = 20 + 20 + 10.35 = 50.35 (-16.1%)

vs.

I_L1N = 20<0d, I_L2N = 20<(120d-30d)

I_N = I_L1N + I_L2N = 28.28<-45d

I_thermal_assumed = 20 + 20 +20 = 60

I_thermal_actual = 20 + 20 + 28.28 = 68.28 (+13.8%)

Big thanks.

Can you plot these? Or do an oscilliograph demonstration?


What you are saying is somewhat profound, as many of us do not think about less then unity power factor in NEC equations other then volts x amps = VA After all its the square root of (A^2 + B^2 + C^2 - AB - AC - BC) which is far off.

 

[Coppersmith]

In dryer and range circuits, is the neutral considered a current carrying conductor for derating purposes?

Thanks for all the replies. I must say though that I like Electrofelon's reply the best.

Nope! Free lunch!