HVAC MCA and Service Calculation

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mkgrady

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Massachusetts
When trying to determine the adequacy of a service for a single family residence and all you know about the HVAC equipment (Mini-split system) is the minimum circuit ampacity, how is this info used to verify the service size? I realize the load in watts is counted at 100% in the calculation but all I have is the MCA. I'm troubled by the outcome of the calculation because when I take an amperage reading of something that has a MCA of say 22 amps it never seems to come close to that much.

For example if I am looking at an existing service that has a calculated load of 80 amps in accordance with 220 and the MCA of the added HVAC is 22 amps (240V) then the added load seems to put it over 100 by 2 amps. Is there another way to calculate it? Obviously I am trying to keep the number under 100.
 
You should never measure what the MCA number says because it includes some 125% factors (compressor and largest motor). Summing the RLAs and FLAs may not be sufficient, as that nameplate may be the RLA for just the compressor and not the control electronics or all the indoor fan units on a mini split.

220.82 says to use 100% of the HVAC nameplate rating. The nameplate has so many values I don't know which one you use, but think it is MCA (which always seems to be there) or BCSC if BCSC is listed (which I've never seen). If the main mini-split is powering the inside units, then I don't believe you need to add the inside units to the calc. In a normal split system, the inside and outside units are separately powered so you need both nameplate values. If the mini-splits have heat strips, that's another complexity.
 
The only electrical ratings I find in six pages of specs are voltage and mca. I guess that’s all I can use. Just seems like more than is needed
 
Look at it this way, if you plug MCA into your load calculation your service/feeder will not be undersized presuming you did everything else correctly.

Most single family dwellings you often install 200 amp supply and are well under on load anyway. If you are that close that the extra 25% of the compressor makes a difference you are probably pretty marginal on your supply and especially for new construction might need to seriously consider going with more than 200 amp supply - usually means step up to 320 meter socket and 2-200 amp disconnecting means.
 
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