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250HP Motor Amp Ratings

250HP Motor Amp Ratings

250HP Motor Amp Ratings

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

RE: 250HP Motor Amp Ratings

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.

RE: 250HP Motor Amp Ratings

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.

RE: 250HP Motor Amp Ratings

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.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

RE: 250HP Motor Amp Ratings

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

RE: 250HP Motor Amp Ratings

jraef,

Quote:

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

RE: 250HP Motor Amp Ratings

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).

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

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