Residential Load Calc

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JordanCNelson

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Do I need to include an assumed load for the largest motor in a residence if it is not specified. I am studying for an electrical administrator test and have been told that in residential load calculations I should always include 25% of the largest motor. Should I?
 
IMO, no... but I'm not certain what you mean by an "assumed load". Load calculations are based on what is or will be installed, and does not include uncertain, unspecified future installations. So if it is not specified, 25% of nothing equals nothing!
 
For standard calculation, you take 100% of your fixed appliances (75% if 4 or more), and then the largest motor load of those you would take an additional 25%, (hence 125% for the largest motor). However, you can not assume that b/c a dishwasher has a higher VA then a disposal that it is the larger b/c the VA comes from the heater elements not the actual moter. Usually it may be a disposal or pump or fan of sorts.

If you are trying to do the standard calc, and are not given any of the fixed appl. then you can not calculate for them
 
I think it gets too confusing to answer correctly in residential. Unless there is an obvious motor, like a huge air compressor, you're left scratching your head. Is the bathroom vent fan motor larger than the dishwasher pump? How about the clothes washing machine, or the disposal, or the garage door opener? How many of those items are even on site for you to install?

An airconditioner makes it easier, but there is still a twist. The MCA nameplate value of an airconditioner already inlcudes the 25% factor. So if you've used the MCA of an AC, and it seems like the largest motor, are you done? Or is an Airconditioner compressor not a motor???

In the end, it is only going to make a difference of a few amps. Not enough to be significant in most cases. But enough to get a test question wrong.
 
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