Determining running watts

[mark32]

I was just on a site that helped one size a generator by a list of typical loads. A 1/2 hp sump pump was rated at, I believe, 2100 running watts. How can that be when one hp = 746 watts? Surely I'm missing something.

 

[iwire]

I was just on a site that helped one size a generator by a list of typical loads. A 1/2 hp sump pump was rated at, I believe, 2100 running watts. How can that be when one hp = 746 watts? Surely I'm missing something.

Yeah you are missing the inefficiency of the motor.

 

[mark32]

Thanks Bob, I figured it would be related to the inefficiency of the motor but to such a large degree?

 

[iwire]

Thanks Bob, I figured it would be related to the inefficiency of the motor but to such a large degree?

A 1/2 hp sump pump was rated at, I believe, 2100 running watts.

It does seem like a mistake, the NEC says a single phase 1/2 HP motor is about 1100 watts.

 

[augie47]

Bob, do you think that since it's for generator sizing they are adding a factor to take care of start-up ?

 

[iwire]

Certainly possible.

 

[mark32]

Ooops

Ooops

My bad, the chart I was referring to listed 1200 watts and not 2100 as I had written, sorry. Nonetheless, even at 1200w, that's more than three times the 746 watts per hp rule/standard (I'm using 373 watts since this is a 1/2 hp motor). The startup for this motor, according to this chart, is 3600w. Here's a link

http://www.gillettegenerators.com/sizing/sizing02.html

 

[iwire]

My bad, the chart I was referring to listed 1200 watts

1200 watts is right in line with the NEC.

NEC Table 430.248 lists a 1/2 HP 115 volt single phase motor at 9.8 amps or 1127 watts.

 

[dmagyar]

1 hp pool pump

1 hp pool pump

I've got a 1 hp 240 volt single phase pool pump, I just measured it @ 7.0 amps, which would put it at 1680 watts.

 

[mark32]

Bob, you are correct as always. I should have checked 430.248 but at the time I didn't have my code book handy and thought I'd get close by using the rule of thumb I had previously mentioned. Thanks!

 

[Bjenks]

1200 watts is right in line with the NEC.

NEC Table 430.248 lists a 1/2 HP 115 volt single phase motor at 9.8 amps or 1127 watts.

If you are doing NEC 220 load calcuations for watt at 115V 1/2HP you would actually do the following:

1) look up amps in table 430.248 at 115V- 1/2 HP or 9.8A.
2) Then to find the watts you would take 9.8A x 120V = 1,176 watts.

I base this on the following Code references:

220.5 (A) says for load calculations you must use nominal system voltages of 120,240,208,480?.

NEC 220.50 Motor loads shall be calculated in accordance with 430.24,430.25, and 430.26 and with 440.6 for hermetic refrigerant motor compressors.

NEC 430.24 says you have to go 125% of NEC 430.6(A) which references the NEC 430.xxx Tables.

NEC 430.6(A)(1) Table Values says Tables 430.248,.249,.250 shall be used to determine the ampacity of conductors or ampere rating of switches, short- circuit and ground fault protection instead of the actual current rating marked on the motor nameplate. If the motor only has ampere on it then you have to use the tables to figure out (interpolated if necessary) the HP and then use the table value for the current.

See note under Table 430.248 which says, ??The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120 and 220 to 240 volts?. Table 430.250 says, ?? The voltages listed are rated motor voltages. The voltages listed are rated motor voltages. The current listed shall b permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts?. The current use is NOT based on the actual voltage, but the motor voltage rating.