NEC calculation for Large number of AC units

L

liqababy

Member
Location
Caribbean
Occupation
Electrical Engineer
Hi,

So I had a question about a huge hospital design i once saw. How does the NEC handle installations like that where there are a large amount of equipment and AC units that if considered to all be taken at full load they would result in a service size that is ridiculously large? Considering that like the MRI machines won't be on all at the same time or 24/7, or VRF AC units may not be on all at the same time at maximum use, or exhaust fans and other medical equipment won't all be on at the same time. Something of that scale would end up with some crazy service size that may never be used.

are there calculations or provisions aside from the receptacle load demand factor than can be applied? or how does someone go about even doing that caluclation without leading them to require a small substation?
 
One thing I do is calculate the real load of the AC units and not just use the Minimum circuit Ampacit , it can be a bit tedious as the manufacturers dont always give you a totaled up Running Load Amps (RLA) they supply a list of numbers like RLA for the comressor and Full Load Amps (FLA) for the fan motors etc.
 
tortuga said:
One thing I do is calculate the real load of the AC units and not just use the Minimum circuit Ampacit , it can be a bit tedious as the manufacturers dont always give you a totaled up Running Load Amps (RLA) they supply a list of numbers like RLA for the comressor and Full Load Amps (FLA) for the fan motors etc.
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That make sense, but a 2023 code change says:
220.50(B) Air-Conditioning Equipment.
The conductor sizing requirements specified in Part IV of Article 440 shall be used to determine air-conditioning loads for hermetic refrigerant motor-compressors.
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The conductor sizing rule in 440.33 is pretty much the same as the calculation required for the minimum circuit ampacity number on the nameplate.
 
don_resqcapt19 said:
The conductor sizing rule in 440.33 is pretty much the same as the calculation required for the minimum circuit ampacity number on the nameplate.
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Sure, for a single piece of equipment, but 440.33 also covers multiple pieces of equipment and only requires the extra 25% on the "highest motor-compressor or motor full load current in the group". So as expected for multiple pieces of equipment.

tortuga said:
One thing I do is calculate the real load of the AC units and not just use the Minimum circuit Ampacit , it can be a bit tedious as the manufacturers dont always give you a totaled up Running Load Amps (RLA) they supply a list of numbers like RLA for the comressor and Full Load Amps (FLA) for the fan motors etc.
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But if you know the MCA is the sum of the loads plus 25% of the largest motor, and you can easily identify the largest motor from the nameplate, you can subtract off 25% of the motor from the MCA, rather than add up all the individual loads. Then in your group of multiple units, add back 25% of the largest motor in the group.

Cheers, Wayne

P.S. Does MCA also include an extra 25% for any resitive heating in the units, and does a feeder load calc require that extra 25% for resistive heating? If the answers are "yes" and "no," respectively, then you'd also need identify all the resistive heating loads incorporated into the MCA and subtract off 25% of those.
 
There is an interesting informational note after 430.26 that says...
Demand factors determined in the design of new facilities can often be validated against actual historical experience from similar installations.
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Of course, Informational Notes are not part of the Code, but why put it in there if it doesn't mean anything?
 
wwhitney said:
Sure, for a single piece of equipment, but 440.33 also covers multiple pieces of equipment and only requires the extra 25% on the "highest motor-compressor or motor full load current in the group". So as expected for multiple pieces of equipment.
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Not sure what group means....multiple separate pieces of equipment, or one piece of equipment with multiple compressors?
 
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