Load calculations for Rooftop Units

[steve66]

I am trying to size a panel and feeder to run some rooftop units (and a few other loads). I have been adding the "minimum circuit ampacity" (MCA)numbers to determine the feeder size. But doesn't the MCA number include a safety margin? In other words, isn't it really higher than the actual current drawn by the rooftop unit?

Is there any way I can calculate the real current drawn? The only info I am given on these units is volts, phases, MCA and "maximum overcurrent protection". I've tried looking up actual current consumptions in the manufacturers catalogs, but its not easy finding electrical numbers in those HVAC catalogs.

Steve

 

[cpal]

Re: Load calculations for Rooftop Units

It sounds as if you want to size a feeder to the units if this supplies Hermetic Refrigerant Motor-Compressor. loads, you should be consulting article 440. for multiple loads refer to 440.33.

You can determine how your installation is imacted by this and adjacent paragrpha

Charlie

 

[bphgravity]

Re: Load calculations for Rooftop Units

In my opinion, minimum means minimum.

Its the same with any multi-load appliance. Take for example a typical range. The namplate says 8,000 va, but that is really only acheived when all four burners are on high and the oven is on broil. For most uses, a load of much less than 8-kva is used, but that doesn't mean it won't be used at full-rating.

From what I understand, some appliances like dishwashers have mulitple loads where two may be noncoincidental. (heater/motor) I'm not sure A/C units are like that.

 

[steve66]

Re: Load calculations for Rooftop Units

Posted by Bryan:

In my opinion, minimum means minimum

Yes, but I think "minimum circuit ampacity" refers to the branch circuit. In fact, I think article 440.33 that Cpal mentioned (which is also for sizing branch circuits) is what the manufacturer uses to find the "MCA".

But I'm trying to find a feeder size. When I look at 440.33, it says to "add 25% of the hightest rating". If that's how they determine "MCA", and I use those numbers to get a feeder size, I'm adding 25% of the largest motor in each unit . But shouldn't only have to add 25% of one single motor.

I think I have answered my own question (with 440.33). Unless I know the ratings of each motor within a unit, I don't know how much they have added for the "25%", so I can't reduce the MCA when sizing feeders.

Thanks:
Steve

 

[bob]

Re: Load calculations for Rooftop Units

Steve
I think you did answer your own question.
The MCA figure is the FLA of the unit + 25%.
The requirement for the feeder is the FLA of the units + 25% of the largest unit.

[ February 18, 2005, 02:10 PM: Message edited by: bob ]

 

[steve66]

Re: Load calculations for Rooftop Units

Thanks for the replies.

If I wanted to shave a few amps off the load calculation, I could probably look up the RTU specs in detail, and find out how many and what size motors each one actually has. Then find the real current consumption, add the loads for all the RTU's, and then just add 25% of the largest motor.

I don't think it would be worth the effort though.

Steve

 

[arkiesparky]

Re: Load calculations for Rooftop Units

Bob is correct, the MCA that is supplied by the manufacturer already includes 125% of the largest motor plus 100% of the other motors.

Use these numbers for you load calculations.

 

[kiloamp7]

Re: Load calculations for Rooftop Units

steve66 - for ease & to CYA, yes I usually use the totals of the MCA's.

But, if you have plenty of time on your hands, & either the RTU's are on site & you can go through them closely, or you have "real certified" detailed breakdown on each RTU, then there is nothing wrong with shaving a few amps off of the minimum feeder ampacity.

For "fun & practice" you can even compare your Art.440 calculation, for each RTU, to the manufacturer's MCA & MOCP that he puts on the nameplate.