Neutral derating?

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Kaddas

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Project Superintendent
Hello,
I had an electrical inspection yesterday and failed because I did not consider the “conductor derating” code. We ran #12 wires 6 hot and 6 neutral in a 3/4” Emt conduit from the electrical panel to the pull box, the length is 28”. The inspector’s argument is that we have more than three current carriers, which derates the conductors. From my understanding is that the inspector’s concern was the excessive heat that will be created when having more than three current carriers. ALTHOUGH, my electrician’s counter argument is that there won’t be any heat, because each hot wire is balanced with one neutral; 6 hot and 6 neutral. Does that make any sense or not?
 
Each conductor will carry an equal amount of current so you have 12 current carrying conductors (CCC's) in that raceway. The inspector is correct. If you could get the raceway down to 24" then derating wouldn't apply. This may help:
Neutral Conductors:

Here's some examples of when to count and not count the neutral as a current
carrying conductor or CCC:

3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the
ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between
the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100
definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two
ungrounded conductors so the neutral is not counted as a CCC.

Nonlinear Load. A load where the wave shape of the steady-state current does
not follow the wave shape of the applied voltage.

Informational Note: Electronic equipment, electronic/electric-discharge lighting,
adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Click to expand...
 
The adjustment factors of 310.15(B)(3)(a) would apply and the inspector is correct. In two wire circuits the neutral carries the same current as the hot conductor and generates the same amount of heat.

Roger
 
Your electrician has a clue but he needs to study a bit more. The hot and the neutral in the same pipe balances out the magnetic fields but it won't get rid of heat caused by electrons moving through the wire. Tell him about this sight, I'll bet he will like it.
 
What is being supplied can make a difference as well. If supplying general use receptacles, you must figure the conductors can carry the full current equating to the overcurrent device. If on 20 amp devices ampacity after adjustment would need to be 20 or more. If serving fixed loads ampacity after adjustment would only need to be minimum ampacity required for those loads.

Motor loads a good example and could even have 12 AWG conductor with more than 20 amp OCPD but minimum conductor ampacity needed of say 14 amps.
 
Kaddas said:
ALTHOUGH, my electrician’s counter argument is that there won’t be any heat, because each hot wire is balanced with one neutral; 6 hot and 6 neutral. Does that make any sense or not?
Click to expand...
No, but if you can use multi-wire branch circuits by sharing neutrals you could use two (if 1ph) or three (if 3ph) hots per neutral, and reduce the conductor count to nine or eight wires, respectively.

Either would allow you to keep the #12 wires on 20a breakers. Another option is to use 15a breakers if the loads will allow.
 
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