calculating amps on 5hp AC motor...

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eric dolphy

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a beginner asks about calculating the service demand load amperage for a 230v 5hp AC unit...

one suggestion was ~745 watts per each hp (3725) divided by 230 equals ~16 amps...however, another procedure had the equation 230v x 28amps - 6440 va... any idea how the '28' figure was derived?

thank you
 
Perhaps from the table FLA for a 5HP motor?
Because of power factor and motor inefficiency one mechanicàl output HP will correspond to more than 745 VA electrical input.
If the motor was sized to the load using a service factor greater than 1 the current can be even higher.
 
IMO, you would reference Art 220 for Service Calculations where motors are covered by 220.14(C) which refers you to 430.22, 430.24 and 440.6 which will direct you to the Art 430 Tables. Table 430.248 lists a 5HP 230v motor as havinga FLA of 28 amps.
 
Yes, use the tables.

But for future reference, in an AC motor, use HP x 746 / V x PF x Eff (x 1.732 if it is 3 phase). So unless you know the power factor and efficiency of the motor, calculating amps is just educated guessing anyway. That's why the tables are a better choice, they are, for the most part, "worst case" scenario.
 
Like already said, just use the tables for code work.




If you REALLY want to know what a specific 5HP motor is pulling, put an ammeter and wattmeter on it.

I have a 5HP 183 frame 5 HP motor that pulls 48A at 240 V rms, so it all depends on the load.

If that makes you wonder; yes, a 5HP motor (25 A nameplate) can pull 48 Amps and drive a 10 HP pressure washer load. You just need to put a big fan on it to keep it from overheating. This example has a ducted 1400 CFM airflow thru it.
 
junkhound said:
Like already said, just use the tables for code work.




If you REALLY want to know what a specific 5HP motor is pulling, put an ammeter and wattmeter on it.

I have a 5HP 183 frame 5 HP motor that pulls 48A at 240 V rms, so it all depends on the load.

If that makes you wonder; yes, a 5HP motor (25 A nameplate) can pull 48 Amps and drive a 10 HP pressure washer load. You just need to put a big fan on it to keep it from overheating. This example has a ducted 1400 CFM airflow thru it.
Click to expand...

You ever measure internal winding temp on that one when it has been running for a while?

Hope you built it out of parts on hand otherwise you were probably better of just getting the right motor in the first place:)
 
junkhound said:
I have a 5HP 183 frame 5 HP motor that pulls 48A at 240 V rms, so it all depends on the load.

If that makes you wonder; yes, a 5HP motor (25 A nameplate) can pull 48 Amps and drive a 10 HP pressure washer load. You just need to put a big fan on it to keep it from overheating. This example has a ducted 1400 CFM airflow thru it.
Click to expand...

Of course that is a code violation and some seriously questionable design.

I can probably run 50 amps through a 20 amp breaker all day long if I blow enough cold air on it.
 
There are a lot of "air over" rated motors that are built to similar specs of a smaller rated motor.

Crop drying fan motors are ones I run into the most - you see motors that look more like a 5 hp general purpose motor with a nameplate rating of 7-9 HP, and the other most popular one is what looks like about a 10 HP motor with nameplate rating of 12-15. But they must be located in the airstream to get that rating, and have a service factor or 1.0.
 
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