AC Equipment Sizing

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growler said:
You can install a 20 amp breaker and it will be safe but may not work.
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david luchini said:
The rule is that the breaker must be capable of carrying the starting current of the motor or the equipment.

It's not really a minimum "size" rule, but a minimum performance rule.
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I was getting carried away when I said you could install a 20 amp breaker. I would bet money that's not going to work.

How would they know the minimum size breaker that would work except for perfect conditions. Ambient temp, voltage drop and age of the unit. Under the right condition it might start with a 35 amp breaker and then not work on a really hot day.
 
Read the following below taken from Mike Holt code resources.

Conductor Sizing and Protection (10-11-2K)
Example. Air-Conditioning

An air-conditioner nameplate indicates a minimum circuit ampacity of 27.8 amperes with a maximum fuse size of 40 amperes. What is the minimum size branch circuit conductor and the maximum size overcurrent protection device for this equipment, Figure 8?

(a) No. 12 with a 40 ampere fuse (b) No. 10 with a 30 ampere fuse

(c) No. 10 with a 40 ampere fuse (d) No. 8 with a 40 ampere fuse

• Answer:(c) No. 10 with a 40 ampere fuse, Section 440-4(b)

Short-Circuit Protection = 40 ampere Fuse. The protection device must not be greater than a 40 ampere fuse, either one-time or dual-element because the nameplate specifies fuses [110-3(b)]. A circuit breaker cannot be used for this application. Conductor Size = No. 10. The conductors must be sized based on the 60ºC column of Table 310-16 [110-14(c)] and they must have an ampacity of not less than 27.8 amperes.
 
So...if the Max breaker size recommended by the manufacture is 50 you would actually be required to use 6awg..

Wire size = 6awg.
Fused Disconnect = 60
Breaker size = 50

I know the wire size seems to be overkill but that's what would be needed to comply with Nec.
 
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LCELECTRIC said:
So...if the Max breaker size recommended by the manufacture is 50 you would actually be required to use 6awg..

Wire size = 6awg.
Fused Disconnect = 60
Breaker size = 50
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Nope. Check out earlier threads here or Mike Holt's free video on the subject.
The breaker provides only short circuit and ground fault protection, and the overload on the motor in the device provides the more sensitive overload protection.
So you can use wire rated to carry only the MCA and a breaker up to 50A will still be considered as protecting it in combination with the overload device built into the unit.

If you want to use 6awg you certainly can.
 
GoldDigger said:
Nope. Check out earlier threads here or Mike Holt's free video on the subject.
The breaker provides only short circuit and ground fault protection, and the overload on the motor in the device provides the more sensitive overload protection.
So you can use wire rated to carry only the MCA and a breaker up to 50A will still be considered as protecting it in combination with the overload device built into the unit.

If you want to use 6awg you certainly can.
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I must be misinterpreting section 440.12 as that's the way I understand it.
 
david luchini said:
I thought you were interpreting 440.12 with regards to the above.

A unit with an MCA of 30.1 would likely be able to use a 30A disconnecting means.
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I agree, since the 25% factor is already in the 30.1 amp MCA. The 15% factor for the disconnecting means is less than 25% in the MCA so you can just use the MCA or in this case something a little smaller than the MCA which would be a 30 amp disconnect.
 
david luchini said:
I thought you were interpreting 440.12 with regards to the above.

A unit with an MCA of 30.1 would likely be able to use a 30A disconnecting means.
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infinity said:
I agree, since the 25% factor is already in the 30.1 amp MCA. The 15% factor for the disconnecting means is less than 25% in the MCA so you can just use the MCA or in this case something a little smaller than the MCA which would be a 30 amp disconnect.
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If this is a fused disconnect are you going to be able to put a 50 Amp fuse in a 30 Amp disconnect ?
 
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