Neutral conductors

[joeyww12000]

Ive read the code on the neutral conductor when and when not to count it as current carrying, but I would like some expert opinion to make sure I understand completely. My reason for needing to know is derating for conduit fill.

 

[kspifldorf]

Try this thread. Search 120/240 MWBChttp://forums.mikeholt.com/showthread.php?t=104130

 

[Mr. Bill]

If you're wiring something single phase and use a hot, neutral, ground, then there are two current carrying conductors in the conduit. The hot conductor which supplies the current and the neutral which is the return path for the current.

If you're wiring something with multiple single phase loads using all 3 phases and a common neutral then there are three current carrying conductors. The 3 hot conductors. The neutral is not considered current carrying when it only carries the unballanced load.

If you oversize this neutral because you're concerned about harmonics then this neutral should then be considered a current carrying conductor. A total of 4 current carrying conductors now. 3 Hot + 1 neutral. Now all of the conductors are derated to 80%.

If you install dedicated neutrals for each single phase circuit then each circuit would have 2 current carrying conductors. With 3 phases pulled in one conduit it would be 6 current carrying conductors. 3 Hot + 3 neutral. All conductors are derated to 80%.

 

[tryinghard]

Ive read the code on the neutral conductor when and when not to count it as current carrying, but I would like some expert opinion to make sure I understand completely. My reason for needing to know is derating for conduit fill.

Check out 120/240 1Phase MWBC

 

[infinity]

Mr. Bill summed it up nicely. :smile:

I would add that the neutral of a MWBC which shares two circuits of the 3 phase WYE system would also be a current carrying conductor.

 

[joeyww12000]

By 3 wire mwbc 3ph Wye, you mean a 208v circuit right?

 

[iwire]

By 3 wire mwbc 3ph Wye, you mean a 208v circuit right?

In the USA typically a 3 phase Wye system will be 208Y/120 or 480Y/277.

Of course there are other possibilities and the power company has Wyes of much higher voltages.

 

[infinity]

By 3 wire mwbc 3ph Wye, you mean a 208v circuit right?

Wye system, two phase conductors on opposite phases sharing a neutral. The neutral is also a CCC.

 

[joeyww12000]

So if I used all 3 phases and 1 neutral, considering 50% of the load was not non linear, I would not count the neutral as CC?

 

[infinity]

So if I used all 3 phases and 1 neutral, considering 50% of the load was not non linear, I would not count the neutral as CC?

Yes, that's correct.