question about largest motor load/ example on pp. 209-210 of House Wiring 3rd. Ed.

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lil-ole-lady

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Raeford, NC, USA
If anyone happens to have this textbook by Greg Fletcher, would you please explain why the trash compactor (8 amps/120 volt) has the largest motor load instead of, for example, the dishwasher (10 amps/120 volt). I'm in a night class (a big mistake) so asking questions is frowned upon (or literally hissed at) even if no one knows the answer. No offense, but it seems like would-be electricians prefer to learn via electric shock verses attending a 4-hour class for more than 2 1/2 hours (neither have happened thus far). Seriously though, my go-to-guy in the class spent three whole snow days on this, so any help would be greatly appreciated.

P.S. The book states "The largest motor load is based on the highest-rated full-load current for the motor listed" and "You choose the largest motor load based on the highest-rated amperage of a motor." So, do these statements mean the same thing or what exactly?
 
Smart $

Smart $

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Location
Ohio
My guess is the 10A of the dishwasher includes heating element(s)... whereas the trash compactor is pretty much all motor.

The largest motor is the one with the greatest line current (the value used to determine OCPD, disconnect, etc. ratings). If all motors are single phase, it's typically the largest HP motor (rule of thumb only)... but if there's a mix of single and three phase motors, you can't rely on HP rating. For example, a 10HP 230V 1? motor draws more line current than a 15HP 230V 3? motor.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
BTW, in the example mentioned, it doesn't make much difference. You are only adding 25% of the largest motor. Even if you used the dishwasher's full rated current, it only amounts to 0.5A difference.

10A * 25% = 2.5A
8A * 25% = 2.0A
 
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