1. Junior Member
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Largest Motor Calculations

I am working on my feeder/service calcs for a commercial building. I am trying to determine which load is the largest motor load. In larger buildings the largest motor is easier to identify, booster pumps, elevators, etc…, but in smaller buildings it is not so apparent. My confusion comes from a single load possibly being calculated multiple times or a load being calculated using multiple sections (specifically in regards to largest motor).

For example, the largest motor is an ac compressor, which would be governed by NEC 220.60 Nonconcident Loads (heating vs. cooling) and NEC 220.50 Motors. What happens when the ac compressor is the largest motor and the cooling is greater than the heating, or when the ac compressor as the largest motor and the heating is greater than cooling? In the first case the ac compressor could be calculated twice (cooling and largest motor). But in the second case the heating load is greater, so would the heating and cooling (cooling as largest motor) be used for the calculations? It would almost seem that if the cooling was the largest motor and the heating was the larger than the cooling, then the next largest motor should be used since that compressor load is not being used in the calculations (as would appear to be the case based on other postings). In this case, as long as the heating is 125% more than the cooling, the compressor as the largest motor would already be accounted for anyway.

As a second example a commercial kitchen appliance is the largest motor. NEC 220.50 says this should be calculated at 125%, but 220.56 says it is permissible to take demands…, which one is it? Do the sections numbers imply priority when taking calculations or does NEC 220.50 imply motor equipment not falling under other sections (i.e. not kitchen appliances, not noncioncident loads, etc…)? I understand the need to adjust connected loads with demands and diversities, I’m just not sure how to accurately calculate these loads.

Am I the only one that finds this confusing or am I missing something? Thanks in advance,

solarEI

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Let me try the first paragraph. You have already calculated the largest of the A/C or HEATING load. If it is the A/C Compressor the VA is calculated at 100% (for example: 40A x 240v = 9600 VA) so that when we get to the largest motor calculation and the compressor is also the largest motor we now add the 25% (40A x 240v x 25% = 2400 VA)

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2nd paragraph: Appliances are calculated in the same manner except that where there are 4 or more the total VA is reduced to 75% due to the fact that they will not be running at the same time. Largest Motor: Again, in this case if one of the appliances is the largest motor multiply its VA x 25% as in the last example.

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Originally Posted by cjnickjr
Let me try the first paragraph. You have already calculated the largest of the A/C or HEATING load. If it is the A/C Compressor the VA is calculated at 100% (for example: 40A x 240v = 9600 VA) so that when we get to the largest motor calculation and the compressor is also the largest motor we now add the 25% (40A x 240v x 25% = 2400 VA)
Thanks! So what if the ac compressor is the largest motor but the heating load is larger than the cooling so the compressor load is omitted from the calcs, do you use the next largest motor?

solarEI

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Originally Posted by cjnickjr
2nd paragraph: Appliances are calculated in the same manner except that where there are 4 or more the total VA is reduced to 75% due to the fact that they will not be running at the same time. Largest Motor: Again, in this case if one of the appliances is the largest motor multiply its VA x 25% as in the last example.
so you would take the equipment VA x 75% x 25%?

Thanks,
solarEI

6. Originally Posted by solarEI
So what if the ac compressor is the largest motor but the heating load is larger than the cooling so the compressor load is omitted from the calcs, do you use the next largest motor?
I'd say that, if the heater load is still greater even after adding the 25% to the compresor load, you should be okay.

Disclaimer: if I'm wrong, kindly disregard this post. :rolleyes:

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Originally Posted by solarEI
so you would take the equipment VA x 75% x 25%?

Thanks,
solarEI
This would be 2 separate calculations. Example for fixed appliance demand: dishwasher 780VA, Disposer 1100 VA, Compactor 700 VA and a Water heater 5000 VA. Total would be 7580 x 75% = 5685 VA. If the disposer was the largest motor load in the building 1100 VA x 25% = 275 VA. Total demand for fixed appliances 5685 VA. Total demand for largest motor 275 VA. Added together 5960 VA.

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Originally Posted by LarryFine
I'd say that, if the heater load is still greater even after adding the 25% to the compresor load, you should be okay.

Disclaimer: if I'm wrong, kindly disregard this post. :rolleyes:
If that were the case I would agree since the demand would be met by the heating unit we would be looking for additional demand created by the heater and a large motor running at the same time. I would chose the next largest motor.

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