- Thread starter gmayeux
- Start date

- Location
- Cherry Valley NY, Seattle, WA

The simplest way for multiple units is to add up the MCA's, however you will end up over counting since you end up counting that 25% for largest motor on every unit instead of once. Only problem is it's a bit of a hassle to dissect all the loads on all the units.

- Location
- Henrico County, VA

- Occupation
- Electrical Contractor

What about using 80% for all but the largest unit?

- Location
- Cherry Valley NY, Seattle, WA

I think the potential problem with that is if the unit has multiple compressors then you would end up undercounting? Say it has 3 compressors @ 20A each (ignore the small stuff like controls and fan motors). That MCA should be 25+20+20=65. Now say you took 80% of that to try to avoid the over count, you be counting that unit has 52A.What about using 80% for all but the largest unit?

What I have usually done is just add up all the MCA's, then when I have the total service (or a feeder) load I just see where I am at and if it behooves me to be a bit lower due to equipment or conductor sizes, Ill take a closer look.....maybe a few receps servicing dedicated equipment will also get changed to general use receps :angel:

- Location
- Connecticut

- Occupation
- Engineer

And for calculating the load. See Table 220.3.Several replies have suggested using MCA's. but please correct me if I'm wrong based on 440.4(B) the MCA is used for sizing the conductors, minimum supply circuit conductor capacity,

- Location
- West Chester, PA

When you are aggregating several HVAC-type loads (typically condensing units) you need compressor RLA (rated load amps) and fan HP or FLA. You will be over-designed if you consider the MCA to be the load. Over-design is never going to hurt anyone. But it can drive up the cost.

- Location
- Chapel Hill, NC

- Occupation
- Electrical Contractor

Branch Circuit load for an a/c is calculated per Part IV of art 440 but I am not sure that is what you are asking

- Location
- Connecticut

- Occupation
- Engineer

See 430.24I was not clear in my OP, the HVAC calculations are needed to justify the panel load sizing.

- Location
- Cleveland, Ohio

DavidAnd for calculating the load. See Table 220.3.

How does Table 220.3 "Referencing 440" get you to using the MCA for load calculation ?

Is there a specific line in 440 that states this ?

- Location
- Connecticut

- Occupation
- Engineer

220.3 references Art 440, section IV, which tells you the minimum ampacity (mca) required for branch circuit conductors.David

How does Table 220.3 "Referencing 440" get you to using the MCA for load calculation ?

Is there a specific line in 440 that states this ?

In other words, the branch circuit load is calculated by the required branch circuit minimum ampacity.

What others said, MCA (minimum circuit amps) will be used by you in choosing your wire size while MOP(maximum overcurrent protection) will tell you what size of protection device is.

Your problem is not knowing the FLA (or RLA) of your units which will be needed in computing the main circuit protection device! The MCA of the units gives you a clue on what your RLA is. Equate MCA (of your supply conductor) to the unit's maximum continuous current (MCC). UL says you can get the RLA (rated load amps) of the unit by dividing the MCC by 1.56 (units with currents 9.1A up to 20A or by 1.40 if current is above 20A, else use 1.70 ( also see 430.32 (A) (2).

Example: MCA or MCC= 40A. Therefore, your RLA = 40/1.4 = 28.6A. Do this to the rest of the HVAC units you intend to be supplied with. Then you can compute for the group's MCA (125 of greatest unit RLA + sum of the rest) and your MOP (2.25 x biggest unit + the other loads).

- Location
- Connecticut

- Occupation
- Engineer

This is not right. I have a cutsheet of a unit with motors of 11A, 9.7A, 6.2A, 2.1A, 2.1A, 1.6A, 1A and 0.29A.Your problem is not knowing the FLA (or RLA) of your units which will be needed in computing the main circuit protection device! The MCA of the units gives you a clue on what your RLA is. Equate MCA (of your supply conductor) to the unit's maximum continuous current (MCC). UL says you can get the RLA (rated load amps) of the unit by dividing the MCC by 1.56 (units with currents 9.1A up to 20A or by 1.40 if current is above 20A, else use 1.70 ( also see 430.32 (A) (2).

The cutsheet lists the unit FLA as 33.99A (the sum of the currents above) and the unit MCA as 36.7 (the FLA plus 25% of the largest motor.)

Taking the MCA divided by 1.4 is 26.21A which is 7.78A lower than the actual unit FLA. That would lead to the service being undersized.

- Location
- Cherry Valley NY, Seattle, WA

Although we cant use it for calculation purposes (perhaps as a "preview" for using 220.87) Topgone's divide by 1.4 figure is very close to what I find the actual load of HVAC units to be. 1/1.4 is 71%, I usually find 66% of MCA to be almost always right on for the actual load.This is not right. I have a cutsheet of a unit with motors of 11A, 9.7A, 6.2A, 2.1A, 2.1A, 1.6A, 1A and 0.29A.

The cutsheet lists the unit FLA as 33.99A (the sum of the currents above) and the unit MCA as 36.7 (the FLA plus 25% of the largest motor.)

Taking the MCA divided by 1.4 is 26.21A which is 7.78A lower than the actual unit FLA. That would lead to the service being undersized.

- Location
- Connecticut

- Occupation
- Engineer

33.99/36.7 is 92.6%.Although we cant use it for calculation purposes (perhaps as a "preview" for using 220.87) Topgone's divide by 1.4 figure is very close to what I find the actual load of HVAC units to be. 1/1.4 is 71%, I usually find 66% of MCA to be almost always right on for the actual load.

That's nowhere near 66% or 71%.

- Location
- Cherry Valley NY, Seattle, WA

I was referring to actual load, not FLA.33.99/36.7 is 92.6%.

That's nowhere near 66% or 71%.

- Location
- Cherry Valley NY, Seattle, WA

What do you find abnormal?I have attached a cut sheet( I think) from a typical Bard wall mount HVAC, i just don't understand how you use MCA to calculate a circuit load, much less the breaker size,in the example highlighted, you have MCA 38 amps and a MOP 40, just does not add up.