Derating ?'s

[celtic]

Here's the scenario...

2" riser containing 19 current carrying conductors(includes neutral count), 1 EGC, and a 4 spare condutors.

This is for a lighting load, the longest run is probably 200'.

All circuits are single pole 20A @ 277V.

CBs have a 75 degree rating.

What would you say the MINIMUM wire size (THHN) would be?

 

[iwire]

Re: Derating ?'s

If you have 19 current carrying conductors in a single raceway you would need 10 AWG 90 C wire minimum for 20 amp circuits.

It happens that Table 310.15(B)(2)(a) shows 10 - 20 conductors gets a 50% reduction in ampacity, so you would need to select a conductor with twice the ampacity as the circuit size. Table 310.16 shows 90 C 10 AWG as 40 amps reduce that 50% and you are left with a 20 amp rated conductor.

If they are 12 AWG you would need to use 15 amp breakers.

I am not sure what you mean by "includes neutral count". Some of the neutrals may not really be current carrying conductors.

 

[infinity]

Re: Derating ?'s

I agree with Bob, #10 THHN. Maybe Celtic is figuring the neutrals would be current carrying if the lighting fixtures contained ballasts that would produce harmonics in the neutrals.

 

[celtic]

Re: Derating ?'s

I counted the neutrals because all the circuits are for lighting...hi-bay type lighting in a parking garage...the neutrals would be counted, correct? Each set of conductors (ABC) has one neutral.

If "someone" were to use the spares that were pulled, technically, the riser is not compliant, correct?

The CB rating has no bearing on derating? This is actually what the question was about - I didn't come right out and say that, but I included the CB rating in the post to see where that information would lead.


Thanks.

 

[wannabe]

Re: Derating ?'s

That 277 is nice is isnt it!

 

[iwire]

Re: Derating ?'s

Originally posted by celtic:
The CB rating has no bearing on derating? This is actually what the question was about

No the circuit breaker rating does not effect the derating but the derating directly affects the rating of the circuit breaker you can use.

You have never said what was in the raceway as far as size and insulation type.

I assumed they where 90 C rated conductors and if they are not the derating will be more restrictive.

 

[George Stolz]

Re: Derating ?'s

Originally posted by celtic:
I counted the neutrals because all the circuits are for lighting

If the phases were balanced, he wouldn't need to count them, would he?

 

[don_resqcapt19]

Re: Derating ?'s

George,

If the phases were balanced, he wouldn't need to count them, would he?

That really doesn't have anything to do with the issue. The grounded conductor of a multiwire branch circuit is not counted unless a majority of the load is non-linear. Even with exactly balanced loads, the grounded conductor must be counted if the majority of the load is non-linear.
Don

 

[celtic]

Re: Derating ?'s

Originally posted by iwire:


You have never said what was in the raceway as far as size and insulation type.

I assumed they where 90 C rated conductors and if they are not the derating will be more restrictive.

I did NOT mention size - as that part of the question - but I did mention type.

Originally posted by celtic:

What would you say the MINIMUM wire size (THHN) would be?