FLA for 1000 HP motor 1

[McMichael]

The NEC gives the full load currents for motors up to 500 HP. Do we extrapolate for larger machines?

 

[CJCPE]

See: thread237-142080

 

[aolalde]

McMichel.

Yes, extrapolation is a way to get it when you do not have nameplate data. Another way is assuming the motor Efficiency (EFF) and Power factor (PF).

For your specific motor, at full load; EFF~0.93 and PF~0.87, and for 4000 VL.

IFL ~= 1000*746/(1.732*4000*0.93*0.87)= 133 Amperes.

 

[jraef]

Where did he say it was a 4000V motor?

 

[McMichael]

I didn't, but I should've, specified a voltage. aolalde's answer satisfied my needs.

Thanks for the reminder

 

[aolalde]

Jraef:
Pardon me, I assumed.

 

[jraef]

LOL, pretty good assumption as it turned out!

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Billgas]

I got the formula clearly understood. Any of you guys know why the NEC 430 tables are not linear? ie: A 10HP flc is 14A, the formula would suggest the 20HP would be double, but it is only 27, similiarly the the 100HO might be 10x, but is only 124A. I have not graphed HP .vs. flc but it would make an interesting tool.

 

[electricpete]

As the horsepower goes up, there tends to be slightly better efficiency and power factor which makes FLA go up less than linear with hp (for the same speed).

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[Billgas]

What is best to use when the motor HP goes well beyond the tables? 1000-1500-2000 horsepower. In comparing my answers using extrapolation with an Excel spreadsheet and my calculations using the "amps when HP is known" formula are way different. The calculated value is nearly twice the NEC value.

 

[dpc]

It's best to call a motor manufacturer and get some real numbers if you're really trying to get an accurate answer with these large motors. If you're just trying to size electrical distribution equipment the old 1 kVA per horsepower will keep you safe.

The full load amps depends on many factors beside horsepower, including motor speed, motor voltage, motor design type, specified efficiency, etc. Low speed motors can have significantly higher amp requirements due to lower power factor.

And none of this applies to submersible motors - I don't even trust the NEC tables for those.

 

[waross]

I don't have my NEC with me, but isn't there direction in the NEC to extrapolate motor current for larger size motors. Also, extrapolating for a 1000 hp. motor from the rating of a 500 hp. motor will be on the safe side.
I had assumed that the NEC anticipated a possible future motor replacement with a less efficient motor and made allowances in the tables. That is, if a high efficiency motor was to be replaced with a normal efficiency motor, the ampacity of the circuit will still be adequate.
respectfully