Air Conditioning Wire Sizing

[don_resqcapt19]

Branch circuit selection current is higher than RLA, not MCA.

MCA is based on 125% of branch circuit selection current.

Branch circuit selection current only exists where the internal overload protection exceeds 156%. That is not a common condition.

The MCA for equipment that does not have overload protection exceeding 156% is the sum of the rated load current of all motors, plus 25% of the rated load curren of the largest motor.

 

[david luchini]

The MCA for equipment that does not have overload protection exceeding 156% is the sum of the rated load current of all motors, plus 25% of the rated load curren of the largest momotor.

MCA of a single motor compressor with a branch circuit selection current is 125% of the branch circuit selection current.

 

[letgomywago]

So even though often it can be smaller I always do a 10awg minimum for hvac. There's a few reasons for this but the biggest one is that hvac guys suck at giving you proper specs at rough in and I'd rather put in a 2pole 15 on 10 than need to fish in a 12 2 or 10 2 because it turns out they only had a larger unit available.

 

[augie47]

"...........hvac guys suck at giving you proper specs at rough in"

I have certainly found that to be the case. A huge percentage of my rejections as an inspector related to HVAC circuits and a great percentage of the time if ended up in a p******* contest between the m,mechanical and electrical contractors.

 

[Crash117]

I’ve found that hvac contractors will say “it’s a 3 ton unit. That’s all you need to know.” Ask for the MCA & MOCP and they go deaf and dumb

 

[letgomywago]

"...........hvac guys suck at giving you proper specs at rough in"

I have certainly found that to be the case. A huge percentage of my rejections as an inspector related to HVAC circuits and a great percentage of the time if ended up in a p******* contest between the m,mechanical and electrical contractors.

Whenever I've done a furnace heat pump changeand ac change out I've always had to bring a 2pole 15,20,25,30 and 35 for the heat pump and sure enough they're always wrong I make them text me and boom they're on the job looking at the unit and can't read a plate on the side.

Minisplits are also fun I show up with a fuses disconnect and they act like that's a waste and won't hold up an green tag and of course is will and it forces me to be a bit of a jerk to the hvac contractors when I don't want to be.

 

[mtnelect]

I will try to explain it again:
1) California is trying to not build anymore power plants
2) California is trying to make sure the HVAC units are the proper size for the structure.
3) California is trying to make sure the proper size HVAC units ARE based on Engineering standards, not the "Rule of Thumb" estimates.
4) California has instituted a HERS inspection program to ensure the HVAC systems are installed correctly.
5) California has instituted a program that requires that permits are filed so the above items are completed.
6) Eighteen other states have now adopted California's Title 24, part 6 as their requirements for residential HVAC systems.
7) As an Electrician when installing a HVAC system, just follow the name plate information on the condensing unit.

 

[kwired]

Except when an Electrician sees an "Oversized" breaker feeding a HVAC unit and replaces the breaker to meet to match the wire size.

An ignorant electrician does that. One that knows what they are doing maybe looks further into why that breaker exceeds the conductor ampacity. Anything feeding art 430 or 440 items may not even go much further in investigating.

 

[kwired]

Except on a 100 + degree day, when the HVAC unit is called upon to provide cooling. In those conditions with high refrigerant pressure, it will trip with the lower sized breaker every time.

The approximate 2.5 times rated current for the breaker allows starting current.

Assuming you did get it started but it still pulls more than rated ampacity, then motor overload protection should be shutting things down after trip curve time has been met for whatever percent of overload is being drawn.

 

[kwired]

This is 440.4(B) I think it is reasonably clear what is supposed to be marked on the nameplate and indicates what MCA and MOCP are derived from.

If you took RLA and FLA of the items in the unit and did calculations yourself you almost always come up with same numbers that are on the nameplate. This alone makes 440 applications easier than 430 applications - they've done the calculations for you for AC units all you have to do is follow the nameplate. Ampacity adjustments in the wiring method itself as well as voltage drop would be the only things you may need to figure on your own where they may apply.

 

[don_resqcapt19]

MCA of a single motor compressor with a branch circuit selection current is 125% of the branch circuit selection current.

Yes, if that unit has a branch circuit selection current. I have never seen such a unit.

Where a unit has a "branch circuit selection current", UL 1995 requires that current to be marked on the nameplate. Just like the code requirement, the UL requirement for showing the branch circuit selection current only applies when the internal overload protection is set above 156% of the rated load current. If there is no branch circuit selection current shown on the nameplate, the MCA is based on 125% of the rated load current.

 

[Lucia1222]

How do these geniuses want you to wire motors ??? full size conductors ??

Yes I’m told it’s a safety hazard and would get sued because the wire is going to burn the house down this where I started reading I told all this years ago I never had to try to prove it I guess stupid is not fixable

 

[letgomywago]

Yes I’m told it’s a safety hazard and would get sued because the wire is going to burn the house down this where I started reading I told all this years ago I never had to try to prove it I guess stupid is not fixable

Nope. Do you wire a 600a service to a single family house by adding up all the breakers this is the same kind of thinking as running full size to a motor load.

 

[Eddie702]

I always think of it as the breaker is usually sized for the wire connected to it EXCEPT for motors and hvac equipment.

HVAC you follow the name plate for MOCP and wire size.

Straight motors you size off of the NEC tables except for overload protection sizing which is off the name plate

It's not that hard but a lot of electricians can't figure that out.

Same with derating wire. Most can't figure that out either.

 

[Lucia1222]

Nope. Do you wire a 600a service to a single family house by adding up all the breakers this is the same kind of thinking as running full size to a motor load.

I have actually seen somebody count breakers and say the panel wasn’t big enough

 

[letgomywago]

I have actually seen somebody count breakers and say the panel wasn’t big enough

That's like asking for a 2 inch water supply because you have 4 1/2 inch faucets

 

[Lucia1222]

I always think of it as the breaker is usually sized for the wire connected to it EXCEPT for motors and hvac equipment.

HVAC you follow the name plate for MOCP and wire size.

Straight motors you size off of the NEC tables except for overload protection sizing which is off the name plate

It's not that hard but a lot of electricians can't figure that out.

Same with derating wire. Most can't figure that out either.

Makes sense to me and I’m not a electrician I showed everything I got I found the people don’t believe it’s safe but if you watch a air conditioner shut off on overload the breaker doesn’t trip it cools down and comes back on

 

[augie47]

I have actually seen somebody count breakers and say the panel wasn’t big enough

"Oh, my! I had not noticed that. If you get your checkbook out I can fix that"

 

[Lucia1222]

And if you think about it carrier does not want to be known for burnt wires all through their units so they want to protect their small stranded wires too that would mean you would have to protect it at the end off the line with that you make it safe

 

[letgomywago]

And if you think about it carrier does not want to be known for burnt wires all through their units so they want to protect their small stranded wires too that would mean you would have to protect it at the end off the line with that you make it safe

There's a lot of confusion between our industries but every carrier outdoor residential unit I've seen accepts a breaker at the panel not just fussed disconnects. Mitsubishi on the other hand is the one that says fuses only. The internals obviously will have their own requirements but those actually use taps based off of the actual usage of each load. That's how all small controls are done with maybe one fuse for the whole control section.