Over-current protection for A/C

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Randall Mearns

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I'm having a little dispute with a local contractor who serves as the inspector for a neighboring town. He states that the circuit breaker for an A/C condensor must be sized per the label on the unit. We have historically sized the breaker to protect the wire size running to the A/C condensor. He cites section 440.41 as his reason. Section 440.41 to me applies to a controller. Questions: what is correct - wire size or label requirements?
is he using a wrong portion of the code?
 
Welcome to the forum.:)

The overcurrent protective device for an A/C unit should be sized at not higher than the maximum overcurrent device size marked on the A/C units nameplate.

Chris
 
To add to what chris stated, you can have a 40 amp breaker protecting a #12 wire. The a/c units have overload protection to protect for overload. The breaker is just for short circuit and ground fault conditions.
 
Condensor overcurrent protection

Condensor overcurrent protection

Always use the "maximum overcurrent protection" number listed on the unit.. for the appropriate breaker size! The wire size should be based on the RLA (Run Load Amps).It might say RLA-17.5 amps... maximum overload protection device 40 amps. So go with #10 wire and a 40 amp breaker. There are several different numbers involved including LRA (Locked Rotor Amps)... and I believe that the max overcurrent protection is a balance between them. See Article 440.12

Jeff
 
Use the MCA for conductor size. The OCPD can be any size between the MCA and the nameplate listed Maximum OCPD.
 
What often happens is you see the running amps at perhaps 17 then you need 125% that puts you at 21.25 amps so you run #10 and likely name plate will say min 30 max 40 breaker. What your method would do is but in a 30. Your wired correctly but not for the reason you think. As said you must use the range of breaker sizes and wire must be ok for the running amps. An AC will likely run 3 or more hours
 
juptonstone said:
The wire size should be based on the RLA (Run Load Amps).It might say RLA-17.5 amps... maximum overload protection device 40 amps. So go with #10 wire and a 40 amp breaker.
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Actually, the label should specify Minimum Circuit Ampacity and Maximum Overcurrent Protection.

If the former is 20a or less, you could use #14 on that 40a breaker, to accomodate starting current.
 
LarryFine said:
Actually, the label should specify Minimum Circuit Ampacity and Maximum Overcurrent Protection.

If the former is 20a or less, you could use #14 on that 40a breaker, to accomodate starting current.
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Agree with Larry. Use MCA and MOP. Only time you need RLA is for multiple units on the same BC.

Mark
 
In this example
Originally Posted by juptonstone
The wire size should be based on the RLA (Run Load Amps).It might say RLA-17.5 amps... maximum overload protection device 40 amps. So go with #10 wire and a 40 amp breaker.
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you could use #12 wire. Article 440.6 has you use the tables 310.16 through 310.19. Table 310.16 has 25 amps listed for 60* wire with a note to see 240.4(D) which generally limits #12 to overcurrent protection of 20 amps except Article 240.4(G) puts it back to article 440. So there is no limitation on the ampacities listed in the tables 310.16 through 310.19.
 
jaylectricity said:
Not around here...three hours to get the house to temperature? No way, half hour tops.
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WorkSafe said:
Unless it's seriously undersized.
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Actually a properly sized A/C unit in accordance with ACCA Manual J will run for an extended period of time.

An A/C unit that comes on and runs for 10 minutes and then turns off and does this every 1/2 hour or hour is seriously over sized and is short cycling.

Short cycling will severely shorten the life of the A/C unit.

Chris
 
I agree. An A/C system reaches its highest efficiency 7-8 minutes after starting so a system that short cycles is not very efficient. Also short cycle times do not remove enough humidity.
 
"We have historically sized the breaker to protect the wire size," .... there's your mistake.

Where do you get that idea? From the tables. Yet, if you look at the notes on the tables, they will direct you to the code section that applies.

That code section lists TEN "exceptions" to the rule. Air conditioners are one of the ten ... so your contractor is spot-on. (Just wait untill you se a welder on a 100 amp breaker, #12 wire - and such a low 'duty cycle' it's leagal!)

Now, nobody asked, but ... IMO, by the time you have ten 'exceptions,' there isn't much of a 'rule' left!
 
If I understand the question, the breaker can be any size* as long as the over current device isn't larger than that listed on the unit. That is if the breaker isn't your only overcurrent device.

*Subject to size of conductors and that it's not smaller than the Min overcurrent allowed.
 
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