Rooftop Ac Load calculation

[pmoney44]

Looking to add an EV charger to a business. The only panel that I can possibly use has Rooftop AC units. How can I calculate each unit for load to make sure I'm not overloading the panel. Looking through 2023 code. Size largest ac load at 125% and the rest at 100%. other than that, I'm kind of lost.
Thanks in advance.

Only thing that caught my eye on the unit was minimum circuit amps, maximum breaker size, FLA.
units are 3 phase
208/230

 

[Charged]

I’d probably do it a couple different ways to see if I realistically had a problem and then find the NEC method to best reflect it.
I usually hit the mca with a .8/.85 or if the nameplate had fla for the components of the rtu , then calculate the total fla.
If you can add your ev to that and you’re good then I think you’ll be fine. Note per NEC need to consider the ev continuous load.
Also check utility bills if you can , you could try to figure how much of that peak demand is from rtus.
You could do a 30 day load study on the panel in question.
Also don’t take moving circuits around off the table. Sometimes it’s nice because you can use the 40 or 50 amp breaker from the circuit you moved for your ev charger.

 

[pmoney44]

I’d probably do it a couple different ways to see if I realistically had a problem and then find the NEC method to best reflect it.
I usually hit the mca with a .8/.85 or if the nameplate had fla for the components of the rtu , then calculate the total fla.
If you can add your ev to that and you’re good then I think you’ll be fine. Note per NEC need to consider the ev continuous load.
Also check utility bills if you can , you could try to figure how much of that peak demand is from rtus.
You could do a 30 day load study on the panel in question.
Also don’t take moving circuits around off the table. Sometimes it’s nice because you can use the 40 or 50 amp breaker from the circuit you moved for your ev charger.

After more research, I was just thinking of taking the MCA from each load and adding together to get my full load. I would then take my EV charger load at 125% and make my decision on that. I think MCA is above what the unit draws. Just a thought to try to make this a bit easier.

 

[electrofelon]

After more research, I was just thinking of taking the MCA from each load and adding together to get my full load. I would then take my EV charger load at 125% and make my decision on that. I think MCA is above what the unit draws. Just a thought to try to make this a bit easier.

The MCA includes the extra 25%, so you can certainly just add up all the MCA's, but that will over count all those extra 25%. Technically you would find the largest unit and take the MCA of that, and then add up all the individual loads of all the other units at 100%..... I find HVAC very typically and repeatedly draws about 66% of the MCA, so that would be a realistic although technically not code compliant method, and is what I would do if no one was asking for load calcs.

 

[wwhitney]

After more research, I was just thinking of taking the MCA from each load and adding together to get my full load.

MCA is the sum of the internal loads, plus 25% extra for the largest motor. But for a feeder for multiple such pieces of equipment, you only need to add the extra 25% for the largest motor overall, not the largest motor in each unit.

So if you can determine from each nameplate what the largest motor in each unit is, then you can do the sharper calculation for the feeder: take the sum of the MCAs, and subtract from it 25% of the largest motor in each unit, except for the unit with the largest overall motor.

Often the largest motor in a unit is the compressor, whose load is listed as RLA. Pretty sure each motor in a unit will be listed on the nameplate as an RLA or FLA.

Cheers Wayne

 

[electrofelon]

Often the largest motor in a unit is the compressor, whose load is listed as RLA. Pretty sure each motor in a unit will be listed on the nameplate as an RLA or FLA.

Cheers Wayne

I have found that there is frequently some missing loads on the nameplate - when computing the MCA off of the loads shown. It is usually rather small, perhaps there is some allowance for the controls and electronics that they don't bother to list?

 

[wwhitney]

I have found that there is frequently some missing loads on the nameplate - when computing the MCA off of the loads shown. It is usually rather small, perhaps there is some allowance for the controls and electronics that they don't bother to list?

That would be my guess, but as long as the discrepancy is smaller than the largest motor listed, you can have confidence that you've found the largest motor in the unit.

Cheers, Wayne

 

[electrofelon]

That would be my guess, but as long as the discrepancy is smaller than the largest motor listed, you can have confidence that you've found the largest motor in the unit.

Cheers, Wayne

Mostly just a curiosity. Although that means to calculate the load of a given HVAC unit (that is not one of the ones with the largest motor on the feeder) you would want to subtract 25% of the largest motor from the MCA. If you added up all the motors shown, you would miss that mystery load.

 

[augie47]

Be careful to check the actual conductor size on the existing feeder and not the just the overcurrent device as they may taken advantage of the
MCA vs MOCP ratings.

 

[wwhitney]

Although that means to calculate the load of a given HVAC unit (that is not one of the ones with the largest motor on the feeder) you would want to subtract 25% of the largest motor from the MCA.

Which is the procedure I gave in post #5.

Cheers, Wayne

 

[electrofelon]

Which is the procedure I gave in post #5.

Cheers, Wayne

Yes, I just wanted to note the missing load issue in case some were not aware.

 

[Tulsa Electrician]

If all units are three phase and you ev is single phase. Be sure to add load correctly to the the feeder conductors. Do not use total KVA and balance.

 

[kwired]

Seems likely in most cases that they wouldn't size the feeder much larger than minimum needed particularly if said panel is on the roof with the units. Particularly for 150 amp or less feeders.

 

[pmoney44]

Thanks all for the advice!!! Very much appreciated!!!