Derating Parallel conductors in a raceway

[StanRob]

Raceway:
I have a 3? raceway (part pvc and part aluminum) that has (8) 3/0 thhn copper conductors. (2 sets of a,b,c and 2 neutrals).

I?m looking at table 310.15(B)(2)(a) and wondering if I should adjust by 80 (6 conductors (the 2 sets of a,b,c)) or if I should adjust by 70 (all 8 wires(the 2 sets of a,b,c and the 2 neutrals)).

Assuming that?s the correct table, I guess it boils down to whether or not the neutrals are considered ?current-carrying conductors? ? are they?

Setup:
GA Power Transformer is feeding a 480/277 3/0, 4-wire, 1200amp main breaker via (3) of the raceways described above.

What would be the adjusted rating of the main breaker in this scenario?

 

[jumper]

This section defines when it it is a CCC or not.


310.15(B)(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit
shall not be required to be counted when applying the provisions
of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wyeconnected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic currents
are present in the neutral conductor; the neutral conductor
shall therefore be considered a current-carrying conductor.

 

[augie47]

If I did my math correctly, you are in trouble either way.
If you are supplying a 1200 main breaker, your total conductor ampacity must be at least 1200 amps.
Parallel 3/0 (225 x 2 x .8) in 3 conduits comes up to 1080 amps

In direct answer to your question, the neutral being a ccc depends on 310.15(B)(4) and if a majority of your load is linear or non-linear. We would need to know a little more about the load to determine. (Normally the neutral does not count on services)

(jumper answered that part as i was typing, sorry)

 

[Smart $]

...

What would be the adjusted rating of the main breaker in this scenario?

Well hopefully the main breaker is adjustable.

3 sets of 4x3/0 THHN Cu with a 75?C terminal temperature limitation [110.14(C)] would put you at 400A max per conduit x 3 = 1200A. That's before ampacity adjustments. Using full ampacity rating, you have 225A x 6 = 1350A. At the very least you have to derate 80% for ccc's. That puts you at 1080A...

 

[StanRob]

Thank you all very much for the quick and thorough replies!