neutral derating help

[-=PEAKABOO=-]

I hate asking questions like this, I should know how to calculate this but I am unsure about my calculations.

I have a 30 circuit 277/480 sub fed panel board 225 amp buss MLO. It is fed with a 200 amp breaker about 60 feet away. There is 4 3 pole breakers, 40, 40, 30, 30 feeing AC units. There is 3 277 volt lighting circuits. This is the only neutral load in the panel. How do I determine what size the neutral can be derated to?? What size neutral do I need??

Thanks in advance.

 

[infinity]

I wouldn't us the word derating to describe the decreased in size neutral conductor. Derating has a whole other connotation. I'm guess but I would think that you neutral conductor would be required to carry the largest unbalanced load.

 

[-=PEAKABOO=-]

The lighting circuits will be protected by 1 pole 20 amp breakers, 3 of them total.

 

[iwire]

Check out 220.61(A) 2005 NEC

Basically that says sized for the maximum possible load on the neutral.

Assuming the lighting circuits are all on separate phases and 20 amp breakers the maximum current would be 20 amps on the neutral.

If they are all on the same phase it would be 60 amps etc.

However there is also 215.2(A) which requires that the neutral with a feeder be not smaller than 250.122 requires for an EGC.

So IMO the smallest legal neutral to this panel served by a 200 amp feeder is a 6 copper or 4 AL.

 

[infinity]

The lighting circuits will be protected by 1 pole 20 amp breakers, 3 of them total.

Normally I would say that the maximum unbalanced neutral load would be 20 amps, but since the loads are likely non-linear the neutral load could be up to 1.73 X 20 amps.

 

[infinity]

Check out 220.61(A) 2005 NEC

Basically that says sized for the maximum possible load on the neutral.

Assuming the lighting circuits are all on separate phases and 20 amp breakers the maximum current would be 20 amps on the neutral.

If they are all on the same phase it would be 60 amps etc.

However there is also 215.2(A) which requires that the neutral with a feeder be not smaller than 250.122 requires for an EGC.

So IMO the smallest legal neutral to this panel served by a 200 amp feeder is a 6 copper or 4 AL.

So you're saying the neutral would be sized, in this case, according to 250.122 according to 215.2(A).

 

[iwire]

So you're saying the neutral would be sized, in this case, according to 250.122 according to 215.2(A).

Yes, as no matter how he landed the breakers on the phases the neutral current will not exceed 60 amps (forgetting possible harmonics) a 6 AWG will handle the 60 amps and it also happens to be the size required in 250.122.

I imagine they want to make sure the neutral, if you run one at all, could handle a phase to neutral fault in the raceway between OCPD and the load.

I may be mistaken, this is not something I have every really done.

 

[augie47]

opps

opps

.......................
However there is also 215.2(A) which requires that the neutral with a feeder be not smaller than 250.122 requires for an EGC...................

.

"There you go again" Every time I think a have an inkling of understanding of the Code, I read one of Bob's post and have one more thing I have to remember.
215.2(A) definitely has snuck in under my radar prior or this evening.

Thanks Bob