Neutral as a Current Carrying Conductor

[winnie]

If you have a wye source, and a MWBC supplying an A-B load and a C-N load, and all phases are loaded to the same current, then the N will carry as much current as the phases.

Similarly if you have a wye source and an MWBC supplying loads which have different power factors, you could have a situation where N will carry as much current as the phases.

In these situations the physics suggests that the N should count as a CCC for purposes of derating.

However I am not sure if a strict reading of the code would require this.

-Jon

 

[winnie]

I don't follow.

In the circumstances where code permits us to not count the neutral as a CCC, the _assumption_ is that currents balanced out in such a way that even though you have a total of 4 conductors with current flowing on them, the maximum heating situation never exceeds that of 3 conductors carrying current.

This might be because all 3 phase conductors are fully loaded and balanced so that there is zero current on the neutral, or it might be that 2 of the phase conductors are carrying full current and one is off, so that the neutral is carrying full current. In any other situation the heating is less. For example if 2 phase conductors are carrying full current and the third is carrying half current, then the neutral is carrying half current and the net heating is 2.5 fully loaded conductors.

wwhitney and I are describing corner cases that break this assumption. In the situation where you are feeding a single A-B load and a single C-N load, you have 4 conductors, you don't get balancing on the neutral, and if A, B, and C are fully loaded, so is N.

-Jon

 

[LarryFine]

I thought we were talking about the 4-wire, high-leg delta.

 

[winnie]

I thought we were talking about the 4-wire, high-leg delta.

We were, then someone brought in an analogous 4 wire wye situation, where you don't get balance on the neutral and thus you have 4 conductors carrying full current (in the worst case scenario).

-Jon

 

[wwhitney]

We were, then someone brought in an analogous 4 wire wye situation, where you don't get balance on the neutral and thus you have 4 conductors carrying full current (in the worst case scenario).

Ah, when scanning quickly I mistakenly thought your post #8 had already expanded the discussion beyond high-leg delta. But then in post #11 you did bring up wye systems for comparison. : - )

Cheers, Wayne