Ampacities for type NM-B cable

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chief38

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I have recently ran into a problem with an A/C service tech. The problem is that an existing residential A/C condensor is starting to draw more current during the day and tripping the 2 pole 40 amp breaker. As I monitered the condensor with an Amp Probe the amps slowly started to rise as time went on. (a few hours) Eventually the breaker tripped. I figure the unit is just getting older and needs to be replaced. The A/C service tech's solution is to just put a 2 pole 50 amp breaker on the unit. (the unit is rated for 33 amp. minimum and 50 amp max fuse or circuit breaker) The problem is that the feeder for the unit is 8-2 RX cable. As I under stand it you can not exceed the 60 deg. C column for amperage on type NM-B cable. (which is 40 amps for #8 wire). The A/C service tech has told the Homeowner that because the unit is rated for 50 amp max, that is ok because it is A/C equipment and is not a problem. Is there am exception in the NEC that allowes for this change specifically for A/C equipment? If there is, I can't find it
 
wow. words I rarely speak, The HVAC tech is right. The branch circuit must have an ampacity not less than what the mfg. namepalte rquires (in this case 33 amps) and the over-current protection not over the maximum the mfg list 440.4, 440.35, 440.21, 240.4(G)
 
FWIW, it's fairly normal for an AC compressor unit to draw more on startup as it ages. For this reason, I will often put in the max breaker size noted on the dataplate right from the get-go. Wire to the min, breaker to the max. A hard start kit is another popular solution, most normally used when the startup current is also winking the lights.
 
Remember, guys, starting is not the problem; the current creeps up over time. The hard-start cap wouldn't help.
 
chief38 said:
(the unit is rated for 33 amp. minimum and 50 amp max fuse or circuit breaker)
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I recall someone's quote here (whose exactly, I do not recall):
Wire for minimum, protect at maximum
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The #8 NM-b on a 50A CB is fine.

As further reference, check 240.4(d) ...which will direct the user to Article 440 Parts III & IV
 
I wonder if its over charged. I worked for an HVAC company and one method they would use to quickly check charge is measure compressor current and compare to the nameplate.
 
mikeames said:
I wonder if its over charged. I worked for an HVAC company and one method they would use to quickly check charge is measure compressor current and compare to the nameplate.
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It takes the compressor a while to build up full pressure on the low side so as the pressure rises so does the running amps. I would look into the fact that that the compresser might be over charged. The advice everyone is giving you about protect to the max is fine when the tripping is occuring during start up , which is usually the case. I think you are getting into a borderline continuous load and the breaker is getting hot . It would sure be better if the wire was #6 but I wouldnt worry about a 50 amp on the a c even though the start up isnt the problem.
 
Look at 240.4(D) & (G) for the exception for oversizing the A/C branch circuit breaker over what 310.16 would normally require.

A dirty condenser coil will cause the pressures (and therefore the amps) to rise. Also, if the freon is contaminated with air, the head pressure will rise as you described.
 
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LarryFine said:
Remember, guys, starting is not the problem; the current creeps up over time. The hard-start cap wouldn't help.
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And for this reason, I would go against up-sizing to a 50amp breaker. It's the motor start-up load you're trying to cover by using a 50a, which the 40a seems to cover. Something else is going on and a band-aid fix here isn't the best solution. You will be running the A/C over the #8's ampacity for a sustained time. This is not good.
 
paul said:
And for this reason, I would go against up-sizing to a 50amp breaker. It's the motor start-up load you're trying to cover by using a 50a, which the 40a seems to cover. Something else is going on and a band-aid fix here isn't the best solution. You will be running the A/C over the #8's ampacity for a sustained time. This is not good.
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I thought that as well but if the units overload protection is functioning then I suspec,t if there is an issue with the unit, then the OLP will kick out.

If it is a ground fault or short circuit fault then the 50 amp will trip.
 
Dennis Alwon said:
I thought that as well but if the units overload protection is functioning then I suspec,t if there is an issue with the unit, then the OLP will kick out.

If it is a ground fault or short circuit fault then the 50 amp will trip.
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Aren't those OL's internal on the compressor motor? It sure seems to me that the motor is the problem here. I would at the very least run it with my amprobe on the motor leads and see if it exceeds the nameplate for the motor. I would also check the fan motor to see what it's issue is, too.

With the symptoms as listed, I would not up-size the breaker for the reason I listed earlier.
 
paul said:
Aren't those OL's internal on the compressor motor? It sure seems to me that the motor is the problem here. I would at the very least run it with my amprobe on the motor leads and see if it exceeds the nameplate for the motor. I would also check the fan motor to see what it's issue is, too.

With the symptoms as listed, I would not up-size the breaker for the reason I listed earlier.
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Yes the OL are internal.
What would be the difference if the 50 amp breaker was there from the start? It is perfectly legal to install the 50 so I don't see the problem with doing it.
 
My issue is that it's not the start-up of the motor that's tripping the breaker. It's after it runs for awhile. This unit should not be pulling more than 33 amps after the initial startup, and it is. So there is a problem. Since the OL's aren't kicking in at 40+ amps (8 amps over the rated), who says they will kick in at 50+ amps? Now we would not only be exceeding the nameplate by 18 amps, we are exceeding the ampacity of the #8 NM-B by 10 amps.
 
It is interesting that the op never said what the unit was drawing when the unit tripped. I suspect the unit will be replaced soon. I do see your point, however, if that 50 was installed from the onset the situation would be no different.
 
paul said:
Aren't those OL's internal on the compressor motor? It sure seems to me that the motor is the problem here. I would at the very least run it with my amprobe on the motor leads and see if it exceeds the nameplate for the motor. I would also check the fan motor to see what it's issue is, too.

With the symptoms as listed, I would not up-size the breaker for the reason I listed earlier.
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I agree with you on this Paul. Under normal circumstance, a minimum amp rating of 33 and max of 50, the #8's on a 50 amp breaker would be fine. The amperage would peak at start and drop off a little. Might even run around 30 amps or less. But this unit has a problem. Might be the charge, might be dirty, or both. Or even a discharge valve that is sticking.

Here is the rub. If I wired to that unit when it was new, I probably would have used 8's on a 50 amp. Then unit would have had an unknown problem, until it started tripping the 50 amp, or even cycling on the the overload. It may have done this for a long while, and the only symptom would have been poor efficiency due to extra energy used. There is no telling how many units are installed just like that. 50 amp running continually on #8 romex (not just at start) is another issue.

Point is, what I don't know, may not hurt me. But, if I'm the one who noticed that the baby don't smell right, than it's my job to make sure momma changes it.
 
This could also be all for naught if it's the 40 amp breaker that's faulty. Again, without knowing what the current draw is on the unit and it's individual components, it's difficult to give an accurate answer.

Good luck
 
Two posts by mdshunk and this wasn't talked about- Try megging the motor. Your motor might not be overloading, it might be short circuiting once the motor windings run current for a while.
 
If the overloads are doing their job then installing a 50 amp CB is not a problem. The 33 amp MCA already has an additional 25% added to it for the compressor FLA. My guess is that the compressor normally draws about 24 amps. When all of the components of the system are working the #8 NM should never be overloaded therefore changing the CB to 50 amps should be fine.
 
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