De-rating 8 current carrying conductors. HELP!

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hurlee555

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Scenario: 6 general purpose receptacles in a commercial application. Each conductor will be protected by a 20A circuit breaker. The 6 circuits + 2 neutrals (all 12 AWG THHN) are installed in one (1) 3/4" EMT.

Question 1: Are the neutrals considered current carrying conductors in this application?

Question 2: If yes to Q-1, would this be a code violation?

From my calculations, this would de-rate the #12's to 14 Amps which cannot be protected by a 20 A CB. Correct?
 
kwired

kwired

Electron manager
Location
NE Nebraska
six circuits with 2 neutrals. This must mean that you have 2 multiwire branch circuits using three phases per neutral.

310.15(B)(4) tells you whether the neutral(s) count as a current carrying conductor.

(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, wye-connected 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.
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If you have what is described in (a) your raceway only has six current carrying conductors for deration purposes. If it supplies loads mentioned in (c) you have 8 current carrying conductors.

Even if you have 8 CCC as long as they are 90deg 12 AWG conductors that should still only derate to 21 amps. Not sure how you came up with 14 amps.
 
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hurlee555

Member
Yes, two multi-wire branch circuits with one neutral per. These are general purpose receptacles in an office area not knowing exactly what will be plugged in.

Assuming the #12's are good for 20 Amps, and using the table for more than 3 ccc in a raceway or cable (somewhere before table 310.16). 8 ccc would de-rate 70%. Or am I using this table wrong?
 
kwired

kwired

Electron manager
Location
NE Nebraska
hurlee555 said:
Yes, two multi-wire branch circuits with one neutral per. These are general purpose receptacles in an office area not knowing exactly what will be plugged in.

Assuming the #12's are good for 20 Amps, and using the table for more than 3 ccc in a raceway or cable (somewhere before table 310.16). 8 ccc would de-rate 70%. Or am I using this table wrong?
Click to expand...

You can use the 90 degree column for starting point for deration (if you have 90 deg insulation on the conductor). Your final conductor size can not be smaller than 60 or 75 degree column size - which ever applies.

So 30 x .7 = 21 amps is your derated value for 8 - 90 deg 12 AWG conductors.
 
S

stew

Senior Member
Location
federal way,washington
And I had a heck of a time with an inspector once on the 90 deg starting point. Never backed off either til I showed him how it was done. He finally relented grudgingly after I gently proved him wrong. Theses inspector guys have real thin skin I have found over the years .
 
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