Conductor ampacity for hermetic motor compressors

C

cppoly

Senior Member
Location
New York
I watched a Mike Holt video where they were going over nameplate info for hermetic motor compressors.

In the example, the MCA was 31.4 amps and the running amps was 25.1 amps.

They sized the wired at 75 degree C and 10 AWG was good at 35 amps.

But a comment was made that if you're using LFMC rated 60 degrees C, you're allowed to use a 10 AWG wire, which is only rated for 30 amps at 60 degrees C. Wouldn't you be required to meet the MCA using the temperature rating of the conductor/terminals and have to go with 8 AWG?
 
cppoly said:
But a comment was made that if you're using LFMC rated 60 degrees C, you're allowed to use a 10 AWG wire, which is only rated for 30 amps at 60 degrees C. Wouldn't you be required to meet the MCA using the temperature rating of the conductor/terminals and have to go with 8 AWG?
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There is some disagreement on this point. Note the tension in your last question--if we are just using the temperature rating of the conductor/terminals, then those are likely both 75C, and #10 would be fine. It is the LFMC that is only rated 60C (in wet locations, or in dry locations if not marked higher), not the conductor/terminals.

So there is a school of thought that 350.10(4) imposes a limit on operating current on a conductor that is completely separate from its ampacity rating. An alternative interpretation is that the operating current limit means that the "maximum current" in the definition of ampacity is reduced, and hence ampacity is reduced. I think it comes down to whether the term "conditions of use" in the definition of ampacity includes the type of conduit used and the restriction of 350.10(4).

I favor the latter interpretation, as apparently does the commenter you referenced.

Cheers, Wayne
 
wwhitney said:
There is some disagreement on this point. Note the tension in your last question--if we are just using the temperature rating of the conductor/terminals, then those are likely both 75C, and #10 would be fine. It is the LFMC that is only rated 60C (in wet locations, or in dry locations if not marked higher), not the conductor/terminals.

So there is a school of thought that 350.10(4) imposes a limit on operating current on a conductor that is completely separate from its ampacity rating. An alternative interpretation is that the operating current limit means that the "maximum current" in the definition of ampacity is reduced, and hence ampacity is reduced. I think it comes down to whether the term "conditions of use" in the definition of ampacity includes the type of conduit used and the restriction of 350.10(4).

I favor the latter interpretation, as apparently does the commenter you referenced.

Cheers, Wayne
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I read that comment exactly opposite. The comment says you can use the 10AWG in the 60°C rated raceway. It came from Mike Holt, and I am 100% sure he says the operating current is the full load current and not the fictitious current found in the MCA.
 
don_resqcapt19 said:
I read that comment exactly opposite. The comment says you can use the 10AWG in the 60°C rated raceway. It came from Mike Holt, and I am 100% sure he says the operating current is the full load current and not the fictitious current found in the MCA.
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OK, then I misunderstood the wording in the OP, I haven't watched the video.

Your and Mike's interpretation is certainly reasonable. The issue I have with it is that in all other cases, the result of the 125% factor in the branch circuit ampacity for motors/HVAC is that you end up with a circuit that would be compliant if, say, the motor started failing and started drawing 125% of its FLA indefinitely (or more likely until the motor burned up). It's a very clear margin of safety (although the NEC doesn't specify why we use a 125% factor, so I'm not sure if that's the reason for the 125% factor).

With this interpretation of 350.10(4) that same margin of safety is not present in the install discussed in the OP. If the motor started drawing 125% of FLA, the LFMC could overheat (and this condition would be a 350.10(4) violation). So that's a lower level of performance for the branch circuit wiring.

Does this matter? Beats me, I just find it inconsistent.

Cheers, Wayne
 
wwhitney said:
OK, then I misunderstood the wording in the OP, I haven't watched the video.

Your and Mike's interpretation is certainly reasonable. The issue I have with it is that in all other cases, the result of the 125% factor in the branch circuit ampacity for motors/HVAC is that you end up with a circuit that would be compliant if, say, the motor started failing and started drawing 125% of its FLA indefinitely (or more likely until the motor burned up). It's a very clear margin of safety (although the NEC doesn't specify why we use a 125% factor, so I'm not sure if that's the reason for the 125% factor).

With this interpretation of 350.10(4) that same margin of safety is not present in the install discussed in the OP. If the motor started drawing 125% of FLA, the LFMC could overheat (and this condition would be a 350.10(4) violation). So that's a lower level of performance for the branch circuit wiring.

Does this matter? Beats me, I just find it inconsistent.

Cheers, Wayne
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Do you do the voltage drop calculation on the real current of the fictitious current found in the MCA?
 
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