im stumped on this question

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Amps = W / V x pf x eff. x 1.732, and W = HP x 746
So A = 50 x 746 / 208 x .8 x .9, = 143.8
Pretty damed close if you ask me...

If you don't know the EXACT numbers on the PF and eff, you are never going to get the exact answer with a calculator, and even then, it's not going to be the same as the nameplate, never is. The nameplate data is an actual TESTED value by the manufacturer.
 
john gary

john gary

Not the full load current is the input current and is used to select the size of the disconnect and the wire size to feed the motor. If you are using the 2011 code book you find this information on page 338 - the table show that for a 50HP 208V 3 phase motor the FULL LOAD CURRENT - 143Amps. -- not the full load Amps. NOTE at the bottom of Table 430.250 that the Full Load Current for 90% and 80% power factor, the figures must be multiplied [increased] by 1.1 and 1.25 respectively

The full load Amps are selected from the Motors name plate and are used to select the proper HEATER COILS for the Starter.

If you are wanting to calculate the FULL LOAD AMPS the formula used by the previous responder is accurate - However note that if you do not know the motors efficiency rating and the Power Factor rating, which is required to be listed on the motors name plate - you can only guess at these ratings - the 80% efficiency rating and the 90% Power factor use in the previous listing are representative of a better than average motor
JG
 
lukeolson said:
a 208-volt, 50hp, 3-phase squirrel cage motor has a full load current rating of is 143 amps

How did they figure this out? I am completely stumped, thank you
Click to expand...

Maybe like this?

HP kW Volts pf effy Current
50 37 208 0.800 90.0% 144 A
 
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