Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
Air Conditioning Wire Sizing
- Thread starter JesseETG
- Start date
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
I said that wrong I was told the breaker isn’t allowed to handle over 125% of the wire
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The manufacturer of the hvac has 125% built in when they state mca. Somewhere on the label it may say what the actual unit amperage would be.
Think of it.... Code says 125% of the motor load. Since the HVAC says minimum circuit ampacity then they would have to have the 125% built in.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
So someone said that if the conductor was rated for 20 amp you couldn't protect the circuit with a larger OCPD say 30 amps? Did you see Figure 440-5 in post #22?
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
Yea so if I have 3o mca 40 max I can’t use 10 wire and a 40 because 40 amp would be over 125% of the mca
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
That’s where the 37.50 came from long day mixed it up
- Location
- Connecticut
- Occupation
- Engineer
#10 wire and a 40A c/b is perfectly fine for a unit with 30 mca and 40 mocp
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
I’m just go with that and just allow everybody else to say I’m wrong the manufacturer should be telling them this in the book I did the math all the numbers work out right but for some reason people in the field around me do not trust that the people who made the unit know what it’s going to do
It would help if article 440 said that branch circuit selection current was MCA. MCA was always a mystery to me because the code didn't specifically use that term, and 440 makes much more sense when you realize MCA is branch circuit selection current.
What else could “Minimum Circuit Ampacity” mean other than the ampacity used to size the branch circuit?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Stop wasting time doing math. There is no math to do.
Wire must be rated for MCA.
Breaker must be no larger than MOC.
It's that simple. Really.
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
I know it is I was talking about If you do it off the compressor and fan motor separately it works out to the mca pretty dam close anyway I even showed that
- Location
- New Jersey
- Occupation
- Journeyman Electrician
These people that are "showing" this to you need some further education.
Last edited:
Lucia1222
Member
- Location
- Pennsylvania
- Occupation
- HVAC tech
- Location
- Illinois
- Occupation
- retired electrician
Branch circuit selection current and MCA are not the same thing. Branch circuit selection current is higher then the MCA and is a special case. I have never seen a nameplate with a branch circuit selection current on it. The rule in 440.4(B) applies to every unit I have ever seen. The language is clear as to the required markings.
You simply use a conductor with an ampacity that is equal to or greater than the MCA (minimum supply circuit conductor ampacity) and an over current device equal to or smaller than the MOC (maximum rating of the branch-circuit short-circuit and ground-fault protective device}.
The branch circuit selection current it only used for equipment
- Location
- Connecticut
- Occupation
- Engineer
Branch circuit selection current is higher than RLA, not MCA.
MCA is based on 125% of branch circuit selection current.
- Location
- Connecticut
- Occupation
- Engineer
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Don't say that; he might believe you. 