Sizing wire and OCPD for HVAC/mini split systems

C

clemver

Member
Location
USA
Occupation
Electrician
I was always under the impression that both are sized per the nameplate specs. If I have a name plate that reads:

"Maximum overcurrent protective device - 25A"
"Minimum circuit ampacity - 18.1A"

That lets me put a 25A breaker in and I can run #12cu to the unit.

https://imgur.com/a/QJLSspP

I'm struggling to decode the code article that allows that to be done. (referencing 2023 code)

I see 440.22(c)
"...........the protective device rating shall not exceed the manufacturers values marked on the equipment"

But for conductor size all I see is 440.32
"Branch circuit conductors supplying a single motor-compressor shall have an ampacity not less than the greater of the following:
-125% of the motor-compressor rated-load current
-125% of the branch-circuit selection current

So can they be sized separately from the OCDP and off the MCA or do the conductors need to be sized at 125% of the RLA?
 
The minimum cir. amp has already 125% figured in. The reason you can install 12 wire on a 25 amp overcurrent protective device is because the unit has overload protection which will protect the #12 wire. The breaker is just for short circuit and ground faults
 
Dennis Alwon said:
The minimum cir. amp has already 125% figured in. The reason you can install 12 wire on a 25 amp overcurrent protective device is because the unit has overload protection which will protect the #12 wire. The breaker is just for short circuit and ground faults
Click to expand...
Got it. But there's nowhere in the code article that simply states breaker is sized to MOCP and wire is sized to MCA?

I ask because I recently installed this as I described, and the home owner had another "electrician" out for something else. He tells the home owner my install is wrong and the wire is too small for the breaker. Now the home owner rightfully doesn't know who to believe. Puts me in a predicament with this other electrician.
 
clemver said:
I was always under the impression that both are sized per the nameplate specs. If I have a name plate that reads:

"Maximum overcurrent protective device - 25A"
"Minimum circuit ampacity - 18.1A"

That lets me put a 25A breaker in and I can run #12cu to the unit.

https://imgur.com/a/QJLSspP

I'm struggling to decode the code article that allows that to be done. (referencing 2023 code)

I see 440.22(c)
"...........the protective device rating shall not exceed the manufacturers values marked on the equipment"

But for conductor size all I see is 440.32
"Branch circuit conductors supplying a single motor-compressor shall have an ampacity not less than the greater of the following:
-125% of the motor-compressor rated-load current
-125% of the branch-circuit selection current

So can they be sized separately from the OCDP and off the MCA or do the conductors need to be sized at 125% of the RLA?
Click to expand...
You are looking at the wrong section. 440.32 is for when the only load being supplied is a single motor compressor. Where you may use this is for commercial refrigeration where you are only supplying a single compressor and nothing else. Most other cases you are supplying the compressor as well as at least a condenser fan motor. 440.35 "Multimotor and combination load equipment" is what applies. And it basically says use the minimum circuit ampacity marked on the equipment.

As Dennis mentioned the branch OCPD protects from short circuits and ground faults, the motor overload protection will inherently protect the conductor from overload current.
 
clemver said:
I was always under the impression that both are sized per the nameplate specs. If I have a name plate that reads:

"Maximum overcurrent protective device - 25A"
"Minimum circuit ampacity - 18.1A"

That lets me put a 25A breaker in and I can run #12cu to the unit.
Click to expand...
clemver said:
I ask because I recently installed this as I described, and the home owner had another "electrician" out for something else. He tells the home owner my install is wrong and the wire is too small for the breaker.
Click to expand...
The other sparky is wrong which when it comes to these circuits happens often. Also if the wiring method were a 75° type like MC cable or conductors in a raceway you could use #14 AWG conductors with the 25 amp OCPD.

https://i.sstatic.net/0g4qp.png
 
Its been this way for eons. MOCP for the breaker or fuses and MCA for the conductors. Done.

Its pretty clear cut in the code. Don't forget you are just not powering the compressor this is multiple load equipment. Compressor, crankcase heater fan motor and controls. The MFG has already sized everything for you.
 
clemver said:
He tells the home owner my install is wrong and the wire is too small for the breaker.
Click to expand...
210.3 and 240.3 both send you to art 440 for conductor ampacity and overcurrent protection for this equipment.

The "small conductor rule" of 240.4(D) that some people kind of get hung up on here doesn't exist in art 440, or for that matter in any the other articles that 240.3 directs you to.
 
clemver said:
Got it. But there's nowhere in the code article that simply states breaker is sized to MOCP and wire is sized to MCA?

I ask because I recently installed this as I described, and the home owner had another "electrician" out for something else. He tells the home owner my install is wrong and the wire is too small for the breaker. Now the home owner rightfully doesn't know who to believe. Puts me in a predicament with this other electrician.
Click to expand...
I agree with the other responses. I have gotten in this bind before with other "electricians" (I put that in quotes because they were not licensed). I convinced the HO that I was correct by posting the situation here and then directing the HO here for the responses. I made sure the HO understood the exclusive nature of this site and the level of knowledge of the members.

Mark
 
kwired said:
You are looking at the wrong section. 440.32 is for when the only load being supplied is a single motor compressor. Where you may use this is for commercial refrigeration where you are only supplying a single compressor and nothing else. Most other cases you are supplying the compressor as well as at least a condenser fan motor. 440.35 "Multimotor and combination load equipment" is what applies. And it basically says use the minimum circuit ampacity marked on the equipment.

As Dennis mentioned the branch OCPD protects from short circuits and ground faults, the motor overload protection will inherently protect the conductor from overload current.
Click to expand...
THANK YOU! This is exactly what I was looking for. 440.32 had me a bit confused but that makes perfect sense.
 
The values on the nameplate seem a bit odd to me: the labeled compressor RLA is 15.5A, and fan FLA is 0.34A. So usually that would mean MCA is 125% * 15.5 + 0.34 = 19.7. But the label MCA is 18.1A. Why the discrepancy?

Doesn't matter as to the install, for either value you could use #12 Cu, protected at 25A as per the label MOCP.

Cheers, Wayne
 
A word of caution here. If you run your 12 gauge conductors to a fused disconnect and use a 25 amp breaker in the panel you will be in violation. If you use a non fused disconnect you are covered just right.
 
busman said:
I agree with the other responses. I have gotten in this bind before with other "electricians" (I put that in quotes because they were not licensed). I convinced the HO that I was correct by posting the situation here and then directing the HO here for the responses. I made sure the HO understood the exclusive nature of this site and the level of knowledge of the members.

Mark
Click to expand...
What a great solution to have homeowner look at this post
 
Crash117 said:
A word of caution here. If you run your 12 gauge conductors to a fused disconnect and use a 25 amp breaker in the panel you will be in violation. If you use a non fused disconnect you are covered just right.
Click to expand...
Not true. Article 430 has the same allowances for a feeder supplying just one branch circuit as for the branch circuit itself.

Cheers, Wayne
 
clemver said:
Got it. But there's nowhere in the code article that simply states breaker is sized to MOCP and wire is sized to MCA?

I ask because I recently installed this as I described, and the home owner had another "electrician" out for something else. He tells the home owner my install is wrong and the wire is too small for the breaker. Now the home owner rightfully doesn't know who to believe. Puts me in a predicament with this other electrician.
Click to expand...

440.35 says that if the branch circuit conductor ampacity is at least that of the MCA listed on the nameplate ,
49857f098c56146cd7d0d4302a78fb3f.jpg

your good . The ocpd does not dictate the size of your conductors in this application and 240.4(d)’s rule for small conductors does not apply to conductors used for the specific applications listed in 240.4(g) and we are not restricted to 60c ampacities under 100 amps , you can use a conductors higher temp rating if the equipment is listed for the use of conductors with higher temps per 110.14(c)(1)(a)(3) .
The other guy is just another one of the electricians out there that doesn’t understand how to size those circuits
 
clemver said:
I was always under the impression that both are sized per the nameplate specs. If I have a name plate that reads:

"Maximum overcurrent protective device - 25A"
"Minimum circuit ampacity - 18.1A"

That lets me put a 25A breaker in and I can run #12cu to the unit.

https://imgur.com/a/QJLSspP

I'm struggling to decode the code article that allows that to be done. (referencing 2023 code)

I see 440.22(c)
"...........the protective device rating shall not exceed the manufacturers values marked on the equipment"

But for conductor size all I see is 440.32
"Branch circuit conductors supplying a single motor-compressor shall have an ampacity not less than the greater of the following:
-125% of the motor-compressor rated-load current
-125% of the branch-circuit selection current

So can they be sized separately from the OCDP and off the MCA or do the conductors need to be sized at 125% of the RLA?
Click to expand...
You couldve used a #14, 75c wiring method with a Mca of 18 amps the max size of the units ground fault/short circuit protection does not dictate the size of the conductors you’re correct here
 
You can use a fused disconnect at the unit or an unfused disconnect if you have the correct breaker in the panel. The fuses could be 25 amp or larger unless the equipment is marked "fuses only"
 
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