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Just had a home inspection completed and one of the things advised to be corrected is the wiring running to an AC unit. In the breaker panel it's hooked up to a 30-amp double pole breaker with #12 wire.

Inspector notes that #10 wire should be used for 30-amp circuits so that the wire does not melt before the breaker can do its job in an over-current situation. This makes sense to me, but I'm thrown off by the double-pole configuration. Since the load is split over 2 conductors, does it really need to use such heavy gauge wire?

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    The wire needs to be sized for the OCPD or breaker. With this said the minimum size of the circuit required should be listed on the name plate of the compressor unit. If it is listed for 20 amp change the breaker to a 20 amp if 30 upgrade the wiring to 10 awg. – Ed Beal Jan 19 '17 at 18:59
  • @EdBeal you should make that an answer. – Tyson Jan 19 '17 at 19:24
  • If it's an air con unit it is perfectly feasible to have #12 protected by a 30A breaker. I've seen (and installed) #10 on a 50A breaker and was completely within code and safety requirements. – Speedy Petey Jan 19 '17 at 19:37
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    It's not a 30A circuit with a wire shrink. It's a 20A circuit with a breaker bump. – Harper Jan 19 '17 at 21:13
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    static.schneider-electric.us/docs/Circuit%20Protection/… states "Section 7.8 of the 11th edition of UL 489 grants HACR listing to all UL 489 Listed circuit breakers. There is no longer a requirement for special testing. These circuit breakers are suitable for group motor applications requiring HACR designation." – Jim Stewart Jan 19 '17 at 22:54
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It is perfectly within reason to have #12 wire protected by a 30A breaker when the circuit feeds an A/C unit. Depends on the specs of the unit. I would not expect the average home inspector to know this.

A/C units, welders and electric motors have different rules than the "standard" breaker sizing rules. Things are NOT always as simple as a chart at the home center or a simple electrical book.

For an air con unit you would typically size the conductors to the minimum circuit ampacity (MCA), and size the breaker to the maximum overcurrent protection (MOP). This is not a hard and fast rule, but it is typical and works for most installations.

It's kind of complicated, but it's all in the NEC, Art.440, Section III.

Here is an excellent overview on Art440 by one of the most respected names in the electrical business, Mike Holt: http://www.dantespeakheatingcooling.com/upload/Mike_Holt_NEC.pdf See Figure 440-5 and 440-6 in this .pdf for a clear explanation.

  • Not hijacking, but trying to discuss the simplest and cheapest route for the querant to determine whether he has to do anything to his circuit using my system as an example. The sticker on my condensing unit lists the following: RLA 19.9; LRA 107.0; Max overcurrent protection: USA HACR CKT-BKR 40; Canada CKT-BKR 40. What do these mean for breaker size and wire size? Is "HACR" a special type of breaker for a/c condensing units? – Jim Stewart Jan 19 '17 at 20:41
  • From this home inspection site inspectapedia.com/aircond/…: Rated Load Amperage or Running Load Amperage - RLA, also called Rated Load Current or RLC on some equipment. This is the manufacturer's anticipated load during normal usage, that is, the current drawn when the motor is running normally. . .To assist in avoiding nuisance tripping during compressor startup when high current is drawn momentarily, A/C compressor circuit breakers may be permitted to be one size larger than the circuit breaker required by the wire size itself. – Jim Stewart Jan 19 '17 at 21:20
  • That is incorrect. There is no "one size larger" rule. That site is primarily opinion based and not a reliable source for code facts. – Speedy Petey Jan 19 '17 at 21:22
  • To the downvoter, please provide justification. Where was I wrong? – Speedy Petey Jan 19 '17 at 21:22
  • Isn't one size larger what you wrote? Using the spec for my a/c condenser what wire size and breaker is the minimum acceptable? – Jim Stewart Jan 19 '17 at 21:26
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A 30A breaker may be appropriate for certain motor loads on 12 AWG wire.

See ThreePhaseEel's first comment to my answer over here where he calls out chapter and verse. I re-summarized it as follows:

certain motors require a 25A or 30A breaker (NEC 430.52) yet are permitted to use 12AWG wire (430.22 notably 430.22E)

This is deep in the sort of NEC arcana that not everyone will know. So you may need to make them dig out their copy of NEC and do some reading.

  • 440.22 allows the OCPD to be increased 155-225% (my local AHJ requires wire to be sized by the breaker in residential) until we know what the name plate rating is everything is a guess on what was done. – Ed Beal Jan 19 '17 at 20:18
  • @EdBeal, what code does your local AHJ base their requirements on? And what difference does it make of the A/C is residential or commercial??? – Speedy Petey Jan 19 '17 at 21:10
  • They are overly strict on residential branch circuits including 180 va per yoke on outlets and 80% max. This looks like it will change with the 2018 code they are going to the state (not county) adopted rules. 90% of my work is industrial but we do have a construction company also so I do a few homes a year. I have had to add circuits and upgrade wire size 2x in the last 2 years a total waste. – Ed Beal Jan 20 '17 at 0:30
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Most local inspectors only care about what they see at first impression. One can argue semantics all you want, but the inspector is going to want #14 wire matched up to a 15 amp breaker, #12 wire to a 20 amp, #10 to a 30 amp, etc. Arguing will very likely cause no end of future grief for the homeowner. Different insulation material ratings just add more confusion, as this will vary your wires maximum ampacity under code. It is the wire insulation that breaks-down, not the current-carrying capacity. Unless you can show the inspector in the NEC codebook a valid exception, and gently convince said inspector to give you a variance, you will likely be in for a serious uphill battle. As far as oversizing the breaker, good luck getting me to go for that! My six grandchildren were sleeping upstairs when the split hvac unit outside compressor locked-up and completely melted the bus bars in the main breaker panel with the breakers never tripping. I will NEVER over-current a wire, as I have seen way too many breakers fail to do their job, resulting in major repairs. With so much material being manufactured outside of the U.S. and even some materials made right here, I have had issues in the past with sub-standard quality. Remember that the NEC code book states that following the minimum code requirements does not necessarily provide an efficient installation. Overkill will always trump skimping.

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    Absolutely. You can win the argument as I outlined in my answer, but then you have to litigate that. That depends on the inspector's willingness to consider the gory details, and not get indignant about a non-electrician dragging him into the gory details. He can always say "Denied. Have fun fighting city hall. For new work, avoiding that litigation is worth the difference in cost 10 vs 12AWG. For old work that is prohibitive to repair, you might have to fight. – Harper Jan 26 '17 at 21:05
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    Retired, you don't give your inspectors nearly enough credit. If you think they'll automatically want #14 wire matched up to a 15 amp breaker, #12 wire to a 20 amp, #10 to a 30 amp, etc. then you are assuming they don't know the code. From your answer it is clear that you don't know the code very well either, sorry but you wrote it. ........... Wiring to code is ABSOLUTELY safe. That's the reason for the code. May I suggest you read up a bit on Articles 430, 440 & 630. Wiring an A/C unit as the OP describes IS NOT "oversizing" the breaker. It is sizing it appropriately. – Speedy Petey Jan 26 '17 at 21:28

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