Derating and Termination Ratings

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Robert808

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Hello All,

Long time user of the forum, but first time posting.

Question:
In a single 3/4" conduit of 4 current-carrying conductors rated at 90 degrees, do we need to derate using the 60 degree column of Table 310.15(B)(16) if the terminations are assumed to be rated for 60 degrees? We believe we are allowed to use the 90 degree column per the first paragraph of NEC 110.14(C).

Background:
Currently, we have a design where we have a total of four duplex NEMA 5-20R receptacles in a space with two receptacles on one circuit and the other two receptacles on another circuit. We show one 3/4" GRC conduit connecting all receptacles and one homerun to the panelboard. Each receptacle is wired with 2#12 wire and a #12 ground (90 degree rated). Each circuit is protected by a 1P20A circuit breaker. In essence, the 3/4"C homerun to the panel contains 4 current-carrying conductors.

Since there are 4 current-carrying conductors, the code requires us to derate the conductors [Table 310.15(B)(3)(a)]. Here is where the disconnect happens between us and the reviewer. In our interpretation of the code, we are allowed to derate from the 90 degree column of Table 310.15(B)(16) since our conductors are rated for 90 degrees. The reviewer is stating that since we are dealing with loads less than 100A, we need derate from the 60 degree column per NEC 110.14(A)(1). Our stance is that per the first paragraph in NEC 110.14(A) which states that the "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both." we can use the 90 degree column to derate in lieu of the 60 degree column. We believe we are also still in compliance with NEC 110.14(C)(1) since #12 wire is still good for a 60 degree termination since #12 at 60 degrees is rated for 20A. The lower of the two values should be used.

We are being asked to either homerun each circuit or upsize the wire to #10AWG. We don't believe this is necessary, but wanted to get your thoughts.

Thank you for the help!
 
If your conductors are 90C then you start to de-rate from the 90C column. The final overcurrent protective device cannot be larger than the 60C rating of the wire.
 
Robert808 said:
Hello All,


Since there are 4 current-carrying conductors, the code requires us to derate the conductors [Table 310.15(B)(3)(a)]. Here is where the disconnect happens between us and the reviewer. In our interpretation of the code, we are allowed to derate from the 90 degree column of Table 310.15(B)(16) since our conductors are rated for 90 degrees. The reviewer is stating that since we are dealing with loads less than 100A, we need derate from the 60 degree column per NEC 110.14(A)(1). Our stance is that per the first paragraph in NEC 110.14(A) which states that the "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both." we can use the 90 degree column to derate in lieu of the 60 degree column. We believe we are also still in compliance with NEC 110.14(C)(1) since #12 wire is still good for a 60 degree termination since #12 at 60 degrees is rated for 20A. The lower of the two values should be used.
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The reviewer is wrong. You do as you've stated start with the 90° C conductor ampacity which is 30 amps {T310.15(B)(16)} and then multiply by 80% to get the adjusted ampacity of the 4 CCC's in the raceway. (30*80%=24 amps). So now you have 24 amp conductors which are permitted on a 20 amp circuit. The terminal rating has nothing to do with the adjusted ampacity of the conductors.

By simple math you'll see that you can have up to 9-#12 AWG CCC's in the raceway (30*70%=21 amps) before you drop below the standard 20 amp ampacity for #12 AWG conductors. 10-20 CCC's would get you down to 15 amps. (30*50%=15 amps)
 
Might not be allowed for what you are doing but I'll throw it out there in case reviewer is being a pain.

You could run two different phases and share a neutral. The you only have three current carrying conductors and therefore no need to derate. Might need a handle tie on the breakers.
 
Russs57 said:
Might not be allowed for what you are doing but I'll throw it out there in case reviewer is being a pain.

You could run two different phases and share a neutral. The you only have three current carrying conductors and therefore no need to derate. Might need a handle tie on the breakers.
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Yes a MWBC would work but you might be right in stating that in this installation it may be less than desirable. The person who rejected the installation needs to learn something about how to properly apply derating factors so giving in when the original design is code complaint for me would not be an option.
 
there are two values determined when selecting minimum conductor size necessary, one ampacity based on termination temperature rating, the other is ampacity based on insulation temperature rating. Whichever calculated value gives you the larger conductor is the smallest conductor you can use for the application. Termination temp rating gives you larger conductor always until you start to have to make adjustments for temperature or number of conductors in raceway.

Then when you have 60 C termination and 90 C conductor you still can make adjustments based on 90C ampacity tables, but in no case can you run a smaller conductor to the 60C terminal than what 60C ampacity table calls for.
 
There are two different factors in the OP, 1) the conductor ampacity after derating and 2) the ampacity of the circuit. He's asking about the conductor ampacity which is 24 amps. The terminal rating is irrelevant when making that determination. There are other factors involved when determining the circuit size such as the terminals or some other limitation like if the wiring method must be sized at 60° C.
 
Yes, the reviewer is absolutely wrong and should be called on it. I would just on principle. Sadly these kinds of things are getting worse as the world moves more and more toward to plan reviewers and inspectors that are not trained electricians. I blame ICC and their powerful lobbying for this. Real tradesman are a dying breed.
 
Dennis Alwon said:
If your conductors are 90C then you start to de-rate from the 90C column. The final overcurrent protective device cannot be larger than the 60C rating of the wire.
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I agree, that's how it's done. I remembered reading this thread and thinking the plan reviewer was wrong. But today, i was catching up on PE hours on a teaching website saying to do it exactly the way the OP's plan reviewer said to do it, WHICH SEEMS WRONG STILL. Did something change, or are the new generation of electricians being taught wrong? In this Jade learning CE course,

www.jadelearning.com

Conduit and Cables Exposed to Sunlight on Rooftops – JADE Learning

Anyone who has been on top of a roof on a hot summer day knows what it’s like to stand in the middle of an oven. Extreme temperatures from the sun can bake everything in sight, from shingles to appliances, and even electrical conductors and conduit. Conductor insulation is meant to provide long...
www.jadelearning.com

scroll down to the "example" section it says since the termination lugs are rated 75°, one must start there. I would start derating at 90° and only use the lowest temp rating of the system to verify my OCPD selection is good. It teaches otherwise. Can anyone help clarify this, please?
 
poopypants said:
I agree, that's how it's done. I remembered reading this thread and thinking the plan reviewer was wrong. But today, i was catching up on PE hours on a teaching website saying to do it exactly the way the OP's plan reviewer said to do it, WHICH SEEMS WRONG STILL. Did something change, or are the new generation of electricians being taught wrong? In this Jade learning CE course,

www.jadelearning.com

Conduit and Cables Exposed to Sunlight on Rooftops – JADE Learning

Anyone who has been on top of a roof on a hot summer day knows what it’s like to stand in the middle of an oven. Extreme temperatures from the sun can bake everything in sight, from shingles to appliances, and even electrical conductors and conduit. Conductor insulation is meant to provide long...
www.jadelearning.com

scroll down to the "example" section it says since the termination lugs are rated 75°, one must start there. I would start derating at 90° and only use the lowest temp rating of the system to verify my OCPD selection is good. It teaches otherwise. Can anyone help clarify this, please?
Click to expand...
I'm not sure they're saying that you would use the terminal rating for derating purposes but you would use it to determine the final ampacity of the conductor. Terminal rating are not used for derating a conductor ampacity.

Termination Ratings
Termination points have their own temperature ratings. If the termination ratings are different than conductor ratings, then you must use the lower of the two ratings. For instance, if a conductor is rated for 75oC, but the terminations are only rated for 60oC, you must use the 60oC rating in your calculations. Alternately, if the termination is rated for 75oC, but the conductor is only rated for 60oC, you still must use the 60oC rating when calculating permitted conductor ampacities in Table 310.15(B)(16).
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Rob if you look at the example he is using the 75C as the starting point. Imo, this is incorrect otherwise there is no benefit to using 90C conductors
 
Dennis Alwon said:
Rob if you look at the example he is using the 75C as the starting point. Imo, this is incorrect otherwise there is no benefit to using 90C conductors
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Now I see it, yes he's incorrect the conductor chosen THHN (cannot be in a raceway on a roof but that is another issue) is a 90° C conductor so that ampacity would be the starting point for derating. The terminal term rating does not matter at that point in the calculation.
 
Thanks for the reassurance! It is very concerning that this wrong method of de-rating is obviously being taught to the next generation. I guess seeing it in print from a training program makes it seem credible, even if it is on the internet! Thanks for the help, Dennis and Rob!
Good catch on the the THHN in a wet location, Rob. On a side note, does THHN exist that isn't dual rated as THWN though? I have never seen it but haven't really looked for it either...
 
poopypants said:
On a side note, does THHN exist that isn't dual rated as THWN though? I have never seen it but haven't really looked for it either...
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Most that I've seen carries the THWN-2 rating, which is inclusive of both of the above, and is rated 90C in wet, damp and dry locations. The applications requiring a rating for gasoline and oil resistance would be an exception.

I have seen datasheets that state that this "omni-rating" isn't available in all sizes. The small sizes may be rated THHN/THWN, while the large sizes are rated as THWN-2. This means that the small size used in a wet location, would have to start with the 75C rating for derate calculations, and would only be able to take credit for the 90C rating in a dry location. Below is an example from Southwire, with a footnote that reads "marked as THWN-2 in sizes 8 AWG and larger or marked as THWN in sizes 14, 12, and 10 AWG ":
 
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Awesome info, thanks for the reference! Its odd that the "magic plastic" they use to get a THWN-2 rating is reserved only for the 8 and larger lines. Thanks, guys!
 
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