A/C feeder

Status
Not open for further replies.
J

jemsvcs

Senior Member
Confused a bit as to sizing Air Conditioner feeder overcurrent protection -

Let's take a single unit with an RLA of 16 A as an example (assuming no branch circuit selection current is stated on the nameplate). Based on 430.24 the conductors would have to have an ampacity not less than 125 percent of the FLC. Therefore, I would use 12 AWG conductors.

Now, assuming the Max. Fuse Rating is based on 225% as allowed in 440.22(A) the fuses at the disconnect would be rated at 40 A.

Looking at 430.62 it states that the protective device for the feeder shall have a rating not greater than the larges rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder.

So, this would imply the feeder be sized at 12 AWG and have a 40 A circuit breaker. Am I off base on this one?

Thanks,

Jemsvcs
 
Re: A/C feeder

Generally with an A/C it has the minimum circuit ampacity and the maximum overcurrent device rating, both of which already have the math computed as part of the rating. Is this not the case with your's?
 
Re: A/C feeder

I would use 440.32 for sizing the branch circuit conductors to an A/C unit. The 125% requirements still applies.

16A x 125% = 20A, so #12 awg will suffice.

Then using 440.22(A), the code first allows the setting of the protective device to not exceed 175%.

16A x 175% = 28A, so 30A breaker will suffice.

In the event the 30A breaker has nuisance tripping from motor starting current, the code allows up to 225%.

16A x 225% = 36A, so 40A breaker will suffice.
 
Re: A/C feeder

that's for fuse and it is a maximume rating of short cirucit and ground fault protective device.
for breaker is 250%.
but again it's maximum. it can be 125% if you select fuse that can withstand available short circuit rating.
Also you need to know what type of start motor has.
it's confusing. i have the same problem and still learning how to understand that.
 
Re: A/C feeder

Originally posted by snjeza:
that's for fuse and it is a maximume rating of short cirucit and ground fault protective device.
for breaker is 250%.
but again it's maximum. it can be 125% if you select fuse that can withstand available short circuit rating.
Also you need to know what type of start motor has.
it's confusing. i have the same problem and still learning how to understand that.
Click to expand...
Huh? I don't know where you are getting your numbers from. Please cite the code section that allows an OCD to be as low as 125% and as high as 250% for A/C equipment? :confused:
 
Re: A/C feeder

Originally posted by bphgravity:
I would use 440.32 for sizing the branch circuit conductors to an A/C unit. The 125% requirements still applies.

16A x 125% = 20A, so #12 awg will suffice.

Then using 440.22(A), the code first allows the setting of the protective device to not exceed 175%.

16A x 175% = 28A, so 30A breaker will suffice.

In the event the 30A breaker has nuisance tripping from motor starting current, the code allows up to 225%.

16A x 225% = 36A, so 40A breaker will suffice.
Click to expand...
Don't forget to derate the ampacity of the conductor for temperature. Say if ran through an attic. Check tables
 
Re: A/C feeder

Originally posted by tadavidson:
...Don't forget to derate the ampacity of the conductor for temperature. Say if ran through an attic. Check tables
Click to expand...
Yes, temperature derating, terminal ratings, and voltage drop issues always need to be considered.

However in this situation, temperature derating would most certainly not be an issue, even in an attic.

Assume a worse case scenerio where the installation is in a 120 degree F attic and the wiring method is NM cable. Per the code:

#12 is good for 30A x .82 = 24.6A NM can can only be loaded to the 60 degree rating which = 25A. This A/C load will only be 16A, #12 is still sufficent. Voltage drop would be the only possible issue :)
 
Status
Not open for further replies.
Top