# Thread: Derating Parallel conductors in a raceway

1. Junior Member
Join Date
Apr 2012
Location
Gainesville,GA,USA
Posts
2

## Derating Parallel conductors in a raceway

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?

2. 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
are present in the neutral conductor; the neutral conductor
shall therefore be considered a current-carrying conductor.

3. 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)

4. Originally Posted by StanRob
...

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...

5. Junior Member
Join Date
Apr 2012
Location
Gainesville,GA,USA
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Thank you all very much for the quick and thorough replies!

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