250HP Motor Amp Ratings 1

[clanda]

I'm having trouble finding the ampere ratings for a 250 HP 480V, 3-Phase motor.

 

[dpc]

302 A per 2005 NEC. But at this size, little things like rpm, motor design type, enclosure type, etc can have meaningful impact on the FLA. YMMV.

 

[oftenlost]

What is the RPM of this unit? You can generalize on current draws based on old rules of thumb and assumptions about efficiencies but the low power factor of many slow speed machines can increase line current by 30 percent or more.

 

[jraef]

I take it you don't yet have a motor? If you do, you should only use the motor nameplate information. Everything else is a guesstimate, i.e. for determining conductor sizes during the pre-construction phase of a project.

Cheap-and-dirty rule of thumb for 460V motors is 1.2 Amps/HP by the way. 250 x 1.2 = 300A, so pretty close to what dpc said.

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[thewellguy]

It depends on the motor type. I was working on a submersible well pump motor yesterday and the FLA is 344

 

[resqcapt19]

jraef,

If you do, you should only use the motor nameplate information.

In general the NEC says that the motor currents shown in Tables 430.247, 430.248, 430.249, and 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. 430.6(A)(1)
Don

 

[jraef]

resqcapt19,
Yes you're right of course. I was stuck thinking only of overload protection devices (I'm in the middle of a large project doing that right now).

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