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My brand new HVAC unit, the condensor died. The unit has a sticker that says the following:

Minimum Circuit Ampacity 32 amps
Max Fuse / Breaker 50

The electrician ran a 8 gauge wire with a 40 amp breaker to the ac. The HVAC guy is telling me it should have been a 50 amp circuit with a 6 gauge wire.

The HVAC guy has replaced the condensor, the electrician has replaced the 40 amp breaker with a 50 amp, but did not run a 6 gauge line.

The HVAC guy wants $750 for his work on replacing the condensor on the brand new unit he installed.

So I guess my questions are:

  1. Is this even possible for the condensor to burn out due to the lack of electric to it?

  2. Was the electrician clearly at fault, or should the unit have worked on a 40AMP breaker?

  3. Is the HVAC guy at any fault for not checking the electrical runs before installing the unit?

  4. Is the run of 50amp breaker on a 8 gauge wire safe/sufficient/proper?

I'm in the middle of a finger pointing game and I am the one get stuck with the bills here. Thanks for any help!

  • 1
    6 ga is a bigger wire ( carries more current , ie, is "better") than an 8 ga wire. If the 40 amp breaker did not kick out , I can't imagine that it contributed to a problem. For comparison , I have 50 A breakers for a 5 ton AC. – blacksmith37 Jul 26 '18 at 17:06
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    Apologies, I mixed up the wire numbering. The run is the smaller wire, 8 gauge and the hvac guys says it should have been 6 gauge. I have edited my original post for any new visitors to be fed the correct info. The 40 amp breaker did not kick out, but the person who shut it off manually by switching the breaker said it was making noises and that's why they killed the power to the breaker. The HVAC only replaced the condensor and left the contacts off. The electrician replaced the 40amp breaker to a 50 amp breaker. To @dandavis what's the reasoning behind your comment? – CD Brian Jul 26 '18 at 18:05
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    Your edit changes the picture somewhat. I still think the 8AWG is within spec limits of the A/C unit, unless the cable run is very long. As to whether a 50A breaker is allowed on 8 AWG wire, that very sharply depends on the specs of the motor and we have an expert in that part of Code who will hopefully chime in. How long is the cable run? – Harper - Reinstate Monica Jul 26 '18 at 18:11
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    Your hvac guy dosent know code on hvac systems a larger breaker can be used than the wiresize to allow starting since the minimum ampacity is 32 amps number 8 wire is fine and if the 40 did not trip it is legal, even number 8 with a 50 amp breaker unless the nameplate specifies otherwise your electrician did it corectly. Note I am an electrician and have a universal EPA 608 license. – Ed Beal Jul 26 '18 at 18:30
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    If by "condenser" you mean "the whole outside unit", can you tell us what part exactly failed? Electronics? Compressor? Fan? – Agent_L Jul 26 '18 at 18:34
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Wire and breakers are current-limiting. That means that the wire heats up and eventually fails (burns) if the load (your condensor) draws too much current, or your breaker trips when it senses too much current flow, assuming the breaker is functional.

A device draws whatever current it requires for a given input voltage. If the breaker never tripped and the wire never failed, then the condensor was almost certainly faulty, since there is no mechanism by which the wire or breaker could contribute more current to the fault. Current generates heat, and heat destroys electrical things.

The condensor was either faulty out of the factory or mishandled/miswired in such a way that it failed after installation but without drawing enough current to trip the breaker or melt your wiring.

All fingers should be pointing at a faulty condensor.

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    Too much voltage drop conceivably could have caused a problem. – Loren Pechtel Jul 26 '18 at 20:27
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    @Loren - My wheelhouse is protective relays and industrial voltage regulators... Is consumer gear so poorly engineered that the line drop difference between similar cable gauges causes misoperation or failure? – schadjo Jul 26 '18 at 20:45
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    @LorenPechtel voltage drop on up to a 50 ft run of 8AWG, even at 50A, is a smidge over 3V... if the condensor popped due to a 3V drop (even if it's only 120VAC that's only 2.6%) I'd still call it faulty. I'm pretty sure power variances alone (brownouts, etc) cause more variation than that, and yet you don't have entire swaths of cities that suffer blown condensors when the power sags. – Doktor J Jul 27 '18 at 20:49
  • Especially as most AC units are rated for 208-240V operation; you would need to have a lot of drop from a 240V source to go outside whatever the US tolerance (6%?) for a 208V supply is. – Someone Somewhere Jul 28 '18 at 5:49
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The sticker on the condensing unit stated MAXIMUM 50-A breaker. A 40-A breaker satisfies this requirement, but it seems to me that the electrician should have installed a 50-A breaker, if the #8 wire on this compressor allows that. The sticker on my 27-year-old Carrier 42000 kBTU/h R-22 condensing unit (3.5 ton) states MAX HACR CKT-BKR 40 and the breaker is a 40-A.

Did the HVAC tech know there was a 40-A breaker on the circuit and proceed with the hookup despite reservations? It seems to me that the HVAC tech cannot reasonably claim the electric service is at fault.

  • I do not know if the hvac tech knew there was a 40amp before the problem occurred. The 40-a breaker did not trip but someone in my office said it was buzzing which is why they tripped it by using the switch to the off position and alerted me to the issue. – CD Brian Jul 26 '18 at 18:11
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    So the breaker was buzzing or was it the a/c condensing unit buzzing? If the former, would that mean the breaker was defective or would it mean that the current draw was just below the level to trip the breaker and the breaker was in some sort of oscillation? – Jim Stewart Jul 27 '18 at 1:02
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    Is this breaker in the dedicated shut-off sub-panel outside near the condensing unit or is it in a large panel with other breakers? – Jim Stewart Jul 27 '18 at 10:16
  • Breaker in main panel. – CD Brian Jul 27 '18 at 16:16
  • If the breaker in the main panel was buzzing, then I would think the breaker was the wrong type or the wrong size. Does the sticker on your condensing unit specify a special type of breaker? Mine specifies an HACR breaker. – Jim Stewart Jul 27 '18 at 20:31
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Per the National Electric Code (assuming you are in the US), #8 is for up to 40A, the Minimum Circuit Ampacity (MCA) was 32A, so there was no issue with the wire size he initially selected. The MCA rating is based on the 125% factors already, so you do NOT need to do them again if you already have that number available to you on the nameplate.

Bottom line, the HVAC guy is just trying to dodge responsibility for a defective product.

Also, a "buzzing" sound would not be coming from the breaker unless it was loose. That sound was likely the result of a failed starting capacitor.

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    Might want to clarify that 8 gauge wire for a 50 amp circuit breaker (as was later installed) is not okay (as that's an explicit part of the question); I read that in your answer but it's not clearly separated from the main answer (was the HVAC guy lying). – Joe Jul 26 '18 at 20:51
  • Probably should add that for #1, it is possible to burn out a motor by not providing enough juice, but it seems that there should have been enough given the spec. – JimmyJames Jul 26 '18 at 21:07
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    @Joe -- actually, it is OK as this is an Art. 440 situation -- what the nameplate says trumps normal ampacity rules here. – ThreePhaseEel Jul 27 '18 at 3:18

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