Outside AC Unit Wiring / Breaker / Fuse Size Verification


Old 06-03-19, 05:16 PM
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Outside AC Unit Wiring / Breaker / Fuse Size Verification

Hello everyone,

We purchased a home earlier this year, and I am trying to figure out if the AC unit wiring in my house is normal. It does run, but we have not had it on for long periods of time, and I want to make sure it's right before the summer heat comes. I live in Southern California. The data plate on the unit reads:

min. supply circuit amp: 38 amps
min fuse or crk. brk. size: 45 amps
max fuse or crk. brk. size: 60 amps.

data plate image: https://photos.app.goo.gl/x3BBUt7U3Khfs78k8

From the compressor, the wires go to a fuse box which hold 60 amp fuses. From the fuse box, a #8 wire is used to go to the main electric panel, which connect to a 40 amp breaker. The distance from the fuse box to the main panel is about 80-100 feet.

Does this seem correct? What is throwing me off is the number 45 and 60 amps in the data plate of the AC unit.

Thank you for your help in advanced
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Old 06-03-19, 05:41 PM
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Your setup sounds correct. The #8 has an ampacity of 40 amps. The breaker.is under the maximum limit.
Old 06-03-19, 06:43 PM
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Is the "fuse box" on the outside of the house? If so it is more likely being used as a disconnect and the fuse size doesn't come into play as it is higher than the breaker size.

The min. supply determines wire gauge as stated above.

The min fuse or breaker can be higher than wire gauge/capacity as there are special rules for compressors. So your inside breaker really should be 45 amps. This is to prevent nuisance trips due to high startup current.
Old 06-03-19, 06:46 PM
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Depending on the wiring method #8 can have an overcurrent protection of 50 amps (THHN in conduit for example) 45 amp breakers are not very common in retail stores but are available online or electrical suppliers.

The 40 amp breaker in the panel is too small according to the nameplate and might trip when the unit starts up. This breaker needs to be changed to a 45 amp breaker.

If I wired this I would have used #6 wire and put it on a 60 amp breaker/fuse.
Old 06-03-19, 07:48 PM
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Thank you for your responses, I really appreciate it.

Astuff, what you said about the fuse box makes a lot of sense - it is outside, located next to the compressor, so it makes sense that it's just being used as a disconnect.

I think based on what you, Tolyn and pcboss have touched on is that the 40 amp breaker is too small - and that a 45 amp needs to be installed instead. This would match the data plate requirements, and help me avoid any start up issues.

I keep finding that the max size breaker is 40 amps for a #8 wire - but you also mentioned that compressors have special rules? I would like to stay within code.

Thanks again
Old 06-03-19, 07:57 PM
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There is lots of confusion on breaker and wire sizing for air conditioners. A decent discussion: https://www.achrnews.com/articles/13...ut-wire-sizing
Old 06-04-19, 10:54 AM
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Motors and compressors follow different rules for wire and breaker sizing than general-purpose circuits. This unit can be installed with #8 copper and a breaker anywhere from 45 to 60A. They allow this range because different types and brands of breakers have different sensitivity to start-up current. Ideally you would use the smallest allowed breaker that doesn't trip during compressor start-up. In a practical sense there is not a huge difference between 40A and 60A breakers*. If it works I'd leave it alone. If it trips, bump up to a 50A or 60A breaker which should be available at any home center for a common brand.

* In a compressor circuit, the breaker is mostly there to protect against dead shorts and locked rotor; either of which is well in excess of 150A for a compressor that size, making the actual breaker handle not _that_ important. In some situations the code allows a breaker size up to 250% of the motor rating in recognition of this fact.

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