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Discussion Starter · #1 ·
When derating the wire ampacity for a conductor you use the appropriate ampacities of the conductor from Article 310 (e.g. table 310.16 for 3 conductors in a conduit). If you're using #10 THHN, for example, you use the 90 degree C rating (40 amps), regardless of the temperature rating of the equipment , circuit breaker, etc. (usually 75 degrees). My question is, do you use the same logic for Minimum Circuit Ampacity (MCA) determination? I have an AC unit with an MCA of 36.2 amps. I say I can use 3 #10 THHN's for this; others I have discussed this with say I have to use the 75 degree rating (which would require #8 THHN).
 

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When derating the wire ampacity for a conductor you use the appropriate ampacities of the conductor from Article 310 (e.g. table 310.16 for 3 conductors in a conduit). If you're using #10 THHN, for example, you use the 90 degree C rating (40 amps), regardless of the temperature rating of the equipment , circuit breaker, etc. (usually 75 degrees). My question is, do you use the same logic for Minimum Circuit Ampacity (MCA) determination? I have an AC unit with an MCA of 36.2 amps. I say I can use 3 #10 THHN's for this; others I have discussed this with say I have to use the 75 degree rating (which would require #8 THHN).
You are correct...IF: The terminals where terminating are rated for 90 degrees C. If they are only rated at 60 degrees C - which many are, then you would require #8 THHN.

You are limited to the temperature rating of the terminals. For example:

The allowable load on a #10 THHN. 90C conductor is 40 Amps. This same conductor is limited to 30 Amps when connected to a terminal rated 60C and 35 Amps if rated for 75C..

Edited to add: See 110.14(C) a lot of folks are unaware of this and many don't understand it.
 

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Pharon said:
I don't see how you get around the 60 degree C limitations of 110.14(C)(1). Derating for voltage drop is one thing, but load ampacity sizing seems pretty clear to me.
If your terminations are listed and labeled 75or90 c, then you are not limited by 110.14c1.
 

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When derating the wire ampacity for a conductor you use the appropriate ampacities of the conductor from Article 310 (e.g. table 310.16 for 3 conductors in a conduit). If you're using #10 THHN, for example, you use the 90 degree C rating (40 amps), regardless of the temperature rating of the equipment , circuit breaker, etc. (usually 75 degrees). My question is, do you use the same logic for Minimum Circuit Ampacity (MCA) determination? I have an AC unit with an MCA of 36.2 amps. I say I can use 3 #10 THHN's for this; others I have discussed this with say I have to use the 75 degree rating (which would require #8 THHN).
You're not derating here. You're selecting conductors for a piece of equipment. In this case an AC unit, which will almost certainly have 75 degree rated terminals, which means you will be selecting #8 THHN from the 75 degree column
 

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If your terminations are listed and labeled 75or90 c, then you are not limited by 110.14c1.
I think you'll be hard pressed to find 40A CB terminations rated for 90 degrees C. And most of the older ones aren't rated for 75 degrees C, either.

Not to nitpick, you don't derate for voltage drop. Voltage drop calculations and derating are entirely different things.
Right -- my bad. Not sure how I got those two things confused. I meant derating for current carrying conductors in a raceway. Jesus, I'm losing it.
 

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Pharon said:
I think you'll be hard pressed to find 40A CB terminations rated for 90 degrees C. And most of the older ones aren't rated for 75 degrees C, either. Right -- my bad. Not sure how I got those two things confused. I meant derating for current carrying conductors in a raceway. Jesus, I'm losing it.[/QUOT


I agree you will never find a lug rated for 90c, that's just what the code reads. People also have to make sure everything in the circuit is sized for the lowest temperature terminal in the circuit. Our breaker lugs are rated 75c but the Hubble 480v recepts we use are only 60c rated, this makes the whole circuit 60c rated. The terminals have to be listed and labeled in order be used for a temperature other than 60c.
 

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Discussion Starter · #9 ·
I tend to agree with 2dog wrt being able to use the 90 degree wire rating. Furthermore, the AC unit has an RLA of 23.8 amps and a LRA of 27 amps. A 30 amp circuit breaker is OK for these requirements (e.g. max permissible load 80% of 30 amps = 24 amps), and is the size breaker we use (understanding that the breaker is for wire protection and not equipment overload protection). Also, I don't know if the code permits it or if it makes sense, but if a 30 amp breaker is used, the 75 degree wire temperature limit from 110.14 due to the breaker and AC connections are not exceeded.
 

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I thought it was mentioned that the minimum circuit amps specified 36.2.
Most lugs these days are dual-rated 60/75C, so according to the 75C column, #10 at 75C is 35A.
 

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You may use the 90 degree column for derating, but that's not what you are doing here. The condenser is hermetically sealed and the overcurrent is inherent, you are only providing ground fault/short circuit protection with the breaker/fuse. If the minimum circuit ampacity was 34 or so then you would be fine with #10
 

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Discussion Starter · #12 ·
I understand by code that if the MCA is 34 (or 35) amps, use #10, and if its 36.2 amps, use #8. But why does the manufacturer spec such a high MCA if the unit has an RLA of 23.8 amps and the wire protection is satisfied with a 30 amp breaker? Using the published 1.9 amps for the fan RLA, the MCA calculates to 31.65 amps (RLA x 1.25 + fan FLA). Even if you add in another 3 amps for the fan currents of 4 air handlers (which I believe is already calculated in the RLA), the current is under 35 amps. The newer split units (this one included) ramp the compressor up to speed and keep the compressor running for the entire time its on (variable speed compressor) so there aren't any current surges during operation and the 23.8 amps empirically is accurate. These units also operate efficiently over a wide voltage range, so the voltage drop associated with the #10 vs #8 wire is negligible (approx. 1.4 volts for a 50' run).
 

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I understand by code that if the MCA is 34 (or 35) amps, use #10, and if its 36.2 amps, use #8. But why does the manufacturer spec such a high MCA if the unit has an RLA of 23.8 amps and the wire protection is satisfied with a 30 amp breaker? Using the published 1.9 amps for the fan RLA, the MCA calculates to 31.65 amps (RLA x 1.25 + fan FLA). Even if you add in another 3 amps for the fan currents of 4 air handlers (which I believe is already calculated in the RLA), the current is under 35 amps. The newer split units (this one included) ramp the compressor up to speed and keep the compressor running for the entire time its on (variable speed compressor) so there aren't any current surges during operation and the 23.8 amps empirically is accurate. These units also operate efficiently over a wide voltage range, so the voltage drop associated with the #10 vs #8 wire is negligible (approx. 1.4 volts for a 50' run).
Where are you getting the 30 amp breaker from? The compressor may not even start with such a small breaker. With an MCA of 36, I imagine the max ocpd would probably be at least 50 amps
 
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