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 thecoolingis greater than the heating, or when the ac compressor as the largest motor and theheatingis 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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