De-Rating when multiple conductors in raceway -- conduit size?

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paulcbrowne

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NEC Table 316(B)(2)(a) requires de-rating the ampacity of the conductors when more than three current-carrying conductors are in a raceway. I had a specific situation that sparked a discussion. I had #12 THHN in 1" conduit with a 20 amp. breaker. 40% fill says you could have 26 wires but the above table seems to limit you to 9 current carrying conductors, meaning four single-phase branch circuits.

Everyone seems in agreement on the 40% fill, but I've had some pushback on the conduit size not enetring into the equation.

I could find no place in the NEC that modifies this de-rating in any way due to the size of the conduit.

In other words, I'd be limited to 4 single-phase circuits regardless of whether I put them in 3/4" or 4" conduit. Is that correct??
 
paulcbrowne said:
In other words, I'd be limited to 4 single-phase circuits regardless of whether I put them in 3/4" or 4" conduit. Is that correct??
Click to expand...


Correct, the derating percentage would be the same for the number of CCC's in any size conduit.


Roger
 
If you use some MWBC's you could get up to 9 circuits before derating would put you below 20 amps depending on the system they're connected to.
 
Derating!

Derating!

That last post said ERROR forum, and boy it THAT right!
No one responded to the mistakes(S)!? Two exist in this post.

First one: Use the 90? C chart to DERATE!
Second one: A MWBC has 4 Current carrying Conductors. CCC.
(Three Phase, of course) Therefore only 6 circuits are available
without derating. 6 hots, 2 grounded Neutral conductors. The Neutral
in a 3 Phase system carrys the unbalanced load and is CCC!
9 WIRES max to derate point at 70%. Ya just cannot squeeze in
that last 7th Circuit, let alone NINE of them!

You Could Pull the Wires into a 4 Inch Raceway and install Cooling
fans on each end of the Conduit! (Just kidding!)

Byron L. Head
Master Electrician
Certified Electrical Inspector IAEI
US Navy Submarine Sonar Technician V.N. era
Father of Four!
Cotton Pickin Perfectionist
Radio Control Helicopter Pilot - Bluetooth Now!
Bible Student - Looks like I AM getting out ALIVE!
2 Thessalonians 3:18 Peace to those who know.
 
Last edited:
HelicopterHead said:
That last post said ERROR forum, and boy it THAT right!
No one responded to the mistakes(S)!? Two exist in this post..
Click to expand...
easy there :)
HelicopterHead said:
First one: Use the 90? C chart to DERATE!.
Click to expand...
ONLY if you are using 90? conductors
HelicopterHead said:
Second one: A MWBC has 4 Current carrying Conductors. CCC.
(Three Phase, of course) Therefore only 6 circuits are available
without derating. 6 hots, 2 grounded Neutral conductors. The Neutral
in a 3 Phase system carrys the unbalanced load and is CCC!.
Click to expand...
CHeck 310.15(B)(4) carefully:
(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.
HelicopterHead said:
9 WIRES max to derate point at 70%. Ya just cannot squeeze in
that last 7th Circuit, let alone NINE of them!

You Could Pull the Wires into a 4 Inch Raceway and install Cooling
fans on each end of the Conduit! (Just kidding!)

Byron L. Head
Master Electrician
Certified Electrical Inspector IAEI
US Navy Submarine Sonar Technician V.N. era
Father of Four!
Cotton Pickin Perfectionist
Radio Control Helicopter Pilot - Bluetooth Now!
Bible Student - Looks like I AM getting out ALIVE!
2 Thessalonians 3:18 Peace to those who know.
Click to expand...
 
I think 334.80 clears that up for us:
334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
 
HelicopterHead said:
That last post said ERROR forum, and boy it THAT right!
No one responded to the mistakes(S)!? Two exist in this post.

First one: Use the 90? C chart to DERATE!
Second one: A MWBC has 4 Current carrying Conductors. CCC.
(Three Phase, of course) Therefore only 6 circuits are available
without derating. 6 hots, 2 grounded Neutral conductors. The Neutral
in a 3 Phase system carrys the unbalanced load and is CCC!
9 WIRES max to derate point at 70%. Ya just cannot squeeze in
that last 7th Circuit, let alone NINE of them!

You Could Pull the Wires into a 4 Inch Raceway and install Cooling
fans on each end of the Conduit! (Just kidding!)
Click to expand...

HH,

Let's get on the same page. First of all this entire thing is called a thread which is made up by individual posts.

Regarding errors in this thread, don't be so quick to judge. There were two different insulations types mentioned in this thread, THHN and THWN. One would be derated from the 90 degree column the other from the 75 degree column. You are correct in saying that the OP asked about THHN so we can use the 90 degree column. You are incorrect about the neutral counting as a CCC. It can (non-linear loads) but usually doesn't count. Augie provided you with the applicable code section for when neutrals are and are not CCC's. If you re-read post #4 you'll see that it says "If you use some MWBC's you could get up to 9 circuits".
 
augie47 said:
I think 334.80 clears that up for us:
334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
Click to expand...

OK but what if its 90 c wire in emt terminated on 60 c breaker THEN what section validates the order of derating (again I agree with your answers)
 
ibew501ed said:
OK but what if its 90 c wire in emt terminated on 60 c breaker THEN what section validates the order of derating (again I agree with your answers)
Click to expand...


Take a look at this:

110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Click to expand...
 
ibew501ed said:
OK but what if its 90 c wire in emt terminated on 60 c breaker THEN what section validates the order of derating (again I agree with your answers)
Click to expand...
It doesn't matter since NM is limited to 60C. We use the 90C to derate from but can never use overcurrent protection that would equate over the weakest link which is , in this case, 60C for the NM.
 
HelicopterHead said:
Second one: A MWBC has 4 Current carrying Conductors.
Click to expand...
Only in very few instances.
HelicopterHead said:
(Three Phase, of course) Therefore only 6 circuits are available
without derating.
Click to expand...
Three circuits would require derating, don't misinterpret that to mean a change in wire size.

HelicopterHead said:
The Neutral in a 3 Phase system carrys the unbalanced load and is CCC!
Click to expand...
Not true, a neutral is not a CCC unless the majority of the load is non linear, see 310.15(B)(4)

Roger
 
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