NEC 220.50 Largest Motor for Standard Option Calculation

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Bullwinkle

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I am studying for the Washington Electrical Administrators Exam and have run across a question for which I've found conflicting answers.

When computing the Standard Option for a Dwelling, NEC 220.50 requires that 25 percent of the largest Motor Load be included in the Standard Option calculation. EXAMPLE: A dwelling has: (a) a 1/2 HP, 120-V Garbage Disposal, (b) a 19-A, 240-V Air Conditioner, (c) no other Motors, and (d) the Air Conditioner load is larger than Heating Load.

Which Motor Load is used for computing the 25 percent required under Section 220.50: The Garbage Disposal (FLC of 9.8 A x 120-V x 0.25 = 294 VA) or the Air Conditioner (19-A x 240-V x .25 = 1,140 VA).

Clearly 1,140 VA is larger than 294 VA so that is what I would choose.

I've been told, however, that the correct answer is to use the Garbage Disposal and not the Air Conditioner because "Without knowing how big the Motor is in the Air Conditioner unit there is no way to determine the HP rating. There may be multiple compressors and a couple of fan motors. The garbage disposal is a definite horsepower rating and is therefore used.

What is the correct answer.
 
Use the A/C values. How long will it take to grind your table scraps once or twice a day ?

30, 45, seconds or a whole minute. With a standard A/C vs electric heat, use the largest

of the two loads. Because you cannot operate both at the same time.
 
I agree that is the correct practical answer, but with tests I'm not so sure (especially in Washington). I've wondered about the same issue myself, is an HVAC compressor a motor per strict interpretation of the motor rules? Further confounding this is the MCA value on an HVAC unit already includes the 25% increase. What is the 19A on the AC unit -- MCA, RLA, or what? I ask too many questions of test questions. To me, "nameplate" is too vague on many test questions, as nameplates can have different values (FLA, RLA, LRA, MCA, A) and many compressor devices are combination loads (compressor, condensor fan motor, and controls).

I've also had people tell me that a 20A 120V motor is "larger" than a 15A 240V motor. I don't agree with that either (especially if calculations are done in VA), but when the code sasy "larger" what is meant -- amp rating, HP rating?
 
John120/240 said:
Use the A/C values. How long will it take to grind your table scraps once or twice a day ?

30, 45, seconds or a whole minute. With a standard A/C vs electric heat, use the largest

of the two loads. Because you cannot operate both at the same time.
Click to expand...

Agreed if this was my house that was being wired. How would you answer Exam Question?
 
Bullwinkle said:
How would you answer the Exam Question?
Click to expand...
For the exam, I would assign the 25% factor to the garbage disposal. My reasoning is that the information on the A/C was not given in terms of HP, and therefore you do not know anything about the motor(s) associated with the equipment.

That said, as a real-world project, I would assign the 25% factor to the A/C unit, since that gives the higher (i.e., more conservative) result.
 
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