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I am installing a new breaker for my AC unit. My Ac unit says 20A max My emergency shut off near the AC unit says 60A

So what size breaker should i use ?

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    What size breaker is in there now? – mmathis May 23 '17 at 18:24
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    Didn't we just cover this with @whataboutbob. I suppose I shouldn't mention RLA, FLA, & MOCP. – Retired Master Electrician May 23 '17 at 20:22
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The breaker is sized for the protection of the wire and device. The disconnect must be sized larger than the current flowing through so that it doesn't burn up, but is just a switch.

If the AC unit says that it's breaker should be a max of 20A, then use a 20A breaker.

However, just to be clear, you can't change out the breaker to match the unit if the wire sizes are still too small. Since it says to use a 20A, the wires should be 12 gauge or larger. Any smaller and they could burn up before the breaker would trip.

  • Except... – CactusCake May 23 '17 at 18:47
  • @CactusCake Right, the minimum wire size can be obtained by the MCA and the max breaker size from the MOCP... but without more information here it's best to ensure the wires are large enough to be protected by the breaker. – TFK May 23 '17 at 19:00
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    Breaker is sized according to manufacturer, conductors are sized based on rated and full load amps, disconnect is based on locked rotor amps. – Tester101 May 23 '17 at 19:03
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    @TFK agreed, I'm just being a pedant. Here is your +1 :) – CactusCake May 23 '17 at 19:04
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Using a shut off switch at least a couple times higher than the load is a good idea. It's like if you drive 85 mph a lot, you want tires rated for more than 85.

You must use the breaker the labeling or instructons say. NEC 110.3.

It's cheap insurance to use wires generally accepted as large enough for the breaker, even if less might be allowed. I would use 10 AWG wire for in case I get a larger A/C or heat pump in the future.

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Circuit Breaker Size

The size of the circuit breaker is determined by the manufacturer, and should be listed on the nameplate. In your case, the circuit breaker should be a 20 ampere HACR breaker.

Disconnect Size

As for the disconnect. The current rating for the disconnect should be 115% of the rated-load, or branch-circuit selection current. In your situation, the disconnect must be rated at least 23 amperes (20 * 1.15). The horsepower rating of the disconnect, is determined in one of two ways (440.12). In the following example, I'm going to use the nameplate values from this question.

Equivalent Full-Load current

For this calculation, you'll add the compressor-motor rated-load current and the fan motors full-load current. You'll use the resulting value to look up the HP rating using Table 430.248 from the NEC.

6.8 amperes + 0.70 amperes = 7.5 amperes

Looking at Table 430.248, it shows that at 230 volts 8.0 amperes is 1 horsepower. Which means the disconnect must be rated to disconnect 1 horsepower motor loads.

Equivalent Locked-Rotor Current

This calculation uses the locked-rotor current of the motors, to determine the horsepower rating of the disconnect. Since the nameplate does not list the locked-rotor current for the fan motor, we'll use six times the full-load current instead.

40 + (6 x 0.70) = 44.2

Table 430.251(A) from the NEC, shows that at 230 volts 48 amperes is equivalent to 1 horsepower. So again, the disconnect must be rated to disconnect 1 horsepower motor loads.

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