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HVAC- Install

Jackalope

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After having this thing running for a day I have to say I like it. Manages the humidity in the house very well and is pretty damn quiet. So much so you can actually hear the refrigerant going through the lines when standing below it instead of a blower running It's a 3-ton unit but since it's a variable speed fan and dual-stage compressor it mostly runs in the low stage and operates as a 1.5-ton unit and at half fan speed. That should be nice on the old energy bill.

I have two more 2-ton units that I'll eventually replace with something similar when they crap out. One does the upstairs and the other does the master bedroom, bath, kitchen side of the house.
 

Jackalope

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Today I learned.

The rule of a 20a circuit requires 12 gauge wire, and 30a circuit requires 10ga wire is really just a rule of thumb. There are variables like solid wire, twisted pair, strand number, copper, aluminum, and length that come in to play when sizing wire for amperage.

When I installed the outdoor condensing unit the data plate said "Max fuse amp circuit breaker 30a (MOP), minimum circuit amp (MCA) 19.5. Well shit. It should be good on the existing 20a circuit as it's above the minimum. Wrong. That sucker has popped that breaker 3 times in the last two weeks. Realizing I needed to run a 30a circuit and upgrade to 10gauge wire I prepared myself to get butt raped as the unit is all the way across the house from the panel.

I pulled up NEC to do some reading. Apparently this exact situation is referenced regarding a single condensing unit on a dedicated circuit. And to my surprise it's perfectly up to code in this situation to put in a 30a breaker and use 12 gauge wire so long as the wire is sized to meet the minimum circuit amperage (MCA). Which kind of makes sense of why the data plate gives a minimum and maximum circuit size instead of just listing an amperage like most things do. The amp spike is temporary when the compressor kicks on. And the overload protection in the condenser is set to trip before the wire would be in any danger of overheating. When it comes to HVAC condensing units the breaker exists for circuit faults and not overload protection.

@Isaacorps Today I learned
 
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finelyshedded

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SW Ohio
Today I learned.

The rule of a 20a circuit requires 14 gauge wire, and 30a circuit requires 10ga wire is really just a rule of thumb. There are variables like solid wire, twisted pair, strand number, copper, aluminum, and length that come in to play when sizing wire for amperage.

When I installed the outdoor condensing unit the data plate said "Max fuse amp circuit breaker 30a (MOP), minimum circuit amp (MCA) 19.5. Well shit. It should be good on the existing 20a circuit as it's above the minimum. Wrong. That sucker has popped that breaker 3 times in the last two weeks. Realizing I needed to run a 30a circuit and upgrade to 10gauge wire I prepared myself to get butt raped as the unit is all the way across the house from the panel.

I pulled up NEC to do some reading. Apparently this exact situation is referenced regarding a single condensing unit on a dedicated circuit. And to my surprise it's perfectly up to code in this situation to put in a 30a breaker and use 14 gauge wire so long as the wire is sized to meet the minimum circuit amperage (MCA). Which kind of makes sense of why the data plate gives a minimum and maximum circuit size instead of just listing an amperage like most things do. The amp spike is temporary when the compressor kicks on. And the overload protection in the condenser is set to trip before the wire would be in any danger of overheating. When it comes to HVAC condensing units the breaker exists for circuit faults and not overload protection.

@Isaacorps Today I learned
You lost me at, Today I learned. 😂🤣
 

Bankfish

Junior Member
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In my 30+ yrs of hvac work I have never seen 14ga wire run for an air conditioner. 12ga sometimes, mostly 10ga. That seems like a fire waiting to happen.
 
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Jackalope

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In my 30+ yrs of hvac work I have never seen 14ga wire run for an air conditioner. 12ga sometimes, mostly 10ga. That seems like a fire waiting to happen.

It's for sure not 10. It could be 12 I assumed 14 as it was on a 20a breaker for the old unit which was installed about 15 years.. I'll go look at the wire now that it's daylight.
 
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Jackalope

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In my 30+ yrs of hvac work I have never seen 14ga wire run for an air conditioner. 12ga sometimes, mostly 10ga. That seems like a fire waiting to happen.
I just went out and checked. You were right it's 12. I don't know why I said 14, I knew the rule of thumb for 20a circuit was 12. I'll correct my original post.

It's my understanding that the MCA is the minimum wire size needed to guarantee that the wiring will not overheat under all operating conditions for the life of the product. The MOP is the maximum allowable circuit breaker size that will properly disconnect power to the equipment under any anticipated fault condition.

The data plate says MCA 19.5 / 30 MOP. So it's a 30a breaker on 12ga wire.

Now from my reading,, it gets more strange if it's a fused disconnect and not just a standard pull. If it was fused it would have to be 10ga between the 30a circuit breaker and the fused disconnect as it's then considered a feeder circuit. But could be 12 from the fused disconnect to the unit as a branch circuit. 😅🙄.
 
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You lost me at, Today I learned. 😂🤣
All I heard was Charlie Brown's teacher........wah..wah...waah...wah..wah..waah.waaaah....

1656680089140.png
 

Jackalope

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There was a couple HVAC guys from a local company at the boys preschool this morning. One was busy trying to remove an indoor fan motor and the other two were watching. After I dropped the boys off I asked them real quick. They confirmed thst it's ok to use a 12ga wire on a dedicated condenser circuit with a 30a breaker if the data plate says 19.5 MCA / 30 MOP and it's not a fused disconnect.

It just seems so strange because I always thought the wire size to breaker size was a requirement, not a rule of thumb with variables.
 
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Isaacorps

Member
4,549
110
Columbus
Today I learned.

The rule of a 20a circuit requires 12 gauge wire, and 30a circuit requires 10ga wire is really just a rule of thumb. There are variables like solid wire, twisted pair, strand number, copper, aluminum, and length that come in to play when sizing wire for amperage.

When I installed the outdoor condensing unit the data plate said "Max fuse amp circuit breaker 30a (MOP), minimum circuit amp (MCA) 19.5. Well shit. It should be good on the existing 20a circuit as it's above the minimum. Wrong. That sucker has popped that breaker 3 times in the last two weeks. Realizing I needed to run a 30a circuit and upgrade to 10gauge wire I prepared myself to get butt raped as the unit is all the way across the house from the panel.

I pulled up NEC to do some reading. Apparently this exact situation is referenced regarding a single condensing unit on a dedicated circuit. And to my surprise it's perfectly up to code in this situation to put in a 30a breaker and use 12 gauge wire so long as the wire is sized to meet the minimum circuit amperage (MCA). Which kind of makes sense of why the data plate gives a minimum and maximum circuit size instead of just listing an amperage like most things do. The amp spike is temporary when the compressor kicks on. And the overload protection in the condenser is set to trip before the wire would be in any danger of overheating. When it comes to HVAC condensing units the breaker exists for circuit faults and not overload protection.

@Isaacorps Today I learned
Absolutely correct. When it comes to condensers, “wire for the minimum, breaker for the maximum” is the rule of thumb and confirmed by the NEC. It comes into play a lot more now with the much more efficient units having greater variations in amperage ratings. Used to be pretty standard for different tonnages. As far as wire ratings, the NEC has already dumbed them down anyway. For example, 14 is actually good for 20 amps but maximum over current protection is 15 amps. 12 is good for 25, max over current 20 and so on. The logic behind being allowed to “overrate” the wire for condensers is that the biggest load is at startup and is only momentary. The running load is well below what the wire can safely handle. Seems counterintuitive until you play it out then it makes perfect sense. Think of it as acceleration vs cruising RPMs in a vehicle.
 

P8riot

Member
I did energy efficiency work for a short while. Make sure you use big zip ties with a zip tie puller and seal all the duct joints for the soft tubing. Then mesh and mastic. Pull back the insulation and only do this to the inner tube, then put the insulation back over it and use a zip tie on that. You would be surprised how much cold air is escaping into the attic!
 

Jackalope

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If you are not comfortable, change it.

I put a meter on the common to compressor and in low stage cooling it was pulling 5.6 amps. The fan was pulling 1.1. It's a two stage compressor so low stage is about 65% capacity.
 

Jackalope

Dignitary Member
Staff member
36,591
235
Absolutely correct. When it comes to condensers, “wire for the minimum, breaker for the maximum” is the rule of thumb and confirmed by the NEC. It comes into play a lot more now with the much more efficient units having greater variations in amperage ratings. Used to be pretty standard for different tonnages. As far as wire ratings, the NEC has already dumbed them down anyway. For example, 14 is actually good for 20 amps but maximum over current protection is 15 amps. 12 is good for 25, max over current 20 and so on. The logic behind being allowed to “overrate” the wire for condensers is that the biggest load is at startup and is only momentary. The running load is well below what the wire can safely handle. Seems counterintuitive until you play it out then it makes perfect sense. Think of it as acceleration vs cruising RPMs in a vehicle.

I was all kinds of confused when I first read it, like I just saw bigfoot or something. This doesn't exist, is this real, that can't be right. 😅
 

Jackalope

Dignitary Member
Staff member
36,591
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I did energy efficiency work for a short while. Make sure you use big zip ties with a zip tie puller and seal all the duct joints for the soft tubing. Then mesh and mastic. Pull back the insulation and only do this to the inner tube, then put the insulation back over it and use a zip tie on that. You would be surprised how much cold air is escaping into the attic!

I'm not sure how ours are done. It's in the attic. I do know the inner ductwork is pretty thick metal. I tried to lift a span to run the lineset under it and couldn't even budge it, I bet you could stand on it. Then it's wrapped in foil backed fiberglass insulation about 3 inches thick. The seams are all foil taped.
 

giles

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I put a meter on the common to compressor and in low stage cooling it was pulling 5.6 amps. The fan was pulling 1.1. It's a two stage compressor so low stage is about 65% capacity.
How about at the panel? Are ypu losing anything in the span?
 

giles

Village idiot and local whore
Supporting Member
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In a bar
I had a weak breaker giving me fits for about a year. Took me a while to figure it out. So you could try switching it with another to see if it's the breaker itself.