# how do you calculate VA for a motor?



## sparkyboys (May 3, 2009)

20hp x 746/208v x 99.46%eff x ?pf x 1.73

i need the power factor

how many kw?


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## TOU (Jan 30, 2009)

i am not sure, the spec says nothing about kw....

it seems the formula you're using is totally new to me...or maybe i forgot my formulas...but i thought

va = 208*1.732*(the amp...one of those amp given in the spec)...am i attacking this thing wrong?


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## sparkyboys (May 3, 2009)

oh my bad 

208 x 65.1 x 1.73/ 1000=23.425584 va 

had to change answer, i divided by 100 instead of 1000, sorry.

this is correct tho


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## TOU (Jan 30, 2009)

thanks...

by the way, is there any time when you use 95 instead of 65?


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## MDShunk (Jan 7, 2007)

I think it's awesome that they give you the across-the-line starting amps and wye-delta starting amps. That's great information to have when you're sizing a panel, feeder, or a service. Pretty rare information to come across. Nothing like winking everyone's lights every time some calls the elevator.


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## MDShunk (Jan 7, 2007)

By the way, I'd use the "running at full load going up" amp listing in the watt calc. That's going to be your worst case scenario, which is sorta the whole reason you're probably calculating KVA.


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## sparkyboys (May 3, 2009)

TOU said:


> thanks...
> 
> by the way, is there any time when you use 95 instead of 65?


i always use 75. use 95 when derating, especially when ambient temps are above 30 deg C, like for ridgid conduit on a roof that unprotected by sunlight.
mostly will use 75 90% of the time. most lugs are rated for 75. rarly use 65


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## captkirk (Nov 21, 2007)

tony Likes this thread....good refresher info


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## sparkyboys (May 3, 2009)

MDShunk said:


> By the way, I'd use the "running at full load going up" amp listing in the watt calc. That's going to be your worst case scenario, which is sorta the whole reason you're probably calculating KVA.


so would that be the 91a or the 91a + 4a?

never done an elevator before, GC always has elevator crew do that.


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## TOU (Jan 30, 2009)

in summary from all of your comments....

use 65 for quick and dirty calcs..

use 75 (where did this number come from?)

and use 95 for safest calcs..

am i correct so far?

another thing...why is it wrong to just use 20hp and convert that into kva?


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## sparkyboys (May 3, 2009)

TOU said:


> in summary from all of your comments....
> 
> use 65 for quick and dirty calcs..
> 
> ...


75 is lug rating. i do not understand what you are asking in the rest of your post


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## micromind (Aug 11, 2007)

Calculating the KVA here is really simple. Use the stuff under 'full load up speed'. Everything else is useless. 

You'll need to do two calculations. The first is the motor. Multiply the amps by the voltage, and since it's 3 phase, multiply the result by 1.73. This is VA. Divide by 1000 to get KVA. 

Next, assuming the control transformer is single phase, multiply the volts by the amps. The result is VA. Divide by 1000 to get KVA. 

On one leg, you'll have the motor only. On the other two, you'll have the motor plus the controls. 

Rob


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## Magnettica (Jan 23, 2007)

TOU said:


> thanks in advance guys, again , be nice...fresh out of college here..
> 
> i'm trying to calculate the KVA of an elevator motor, 20 horse power, 120/208, 3phase. i'm looking at the spec now, but confused on the info.
> 
> ...



A 20 HP motor at 208 volts 3Ø is 59.4 amps.


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## steelersman (Mar 15, 2009)

sparkyboys said:


> i always use 75. use 95 when derating, especially when ambient temps are above 30 deg C, like for ridgid conduit on a roof that unprotected by sunlight.
> mostly will use 75 90% of the time. most lugs are rated for 75. rarly use 65


He wasn't asking about degrees celcius columns in table 310.16. He was asking about the 2 different as amps values given on the nameplate. :thumbsup:

I wasn't aware that there was a 95 degree column in the code book. I thought it was 90 degrees. Not sure what codebook you're looking at. :no:

Also there's no 65 degree column either. It's 60.


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## steelersman (Mar 15, 2009)

captkirk said:


> tony Likes this thread....good refresher info


you'd better take sparkyboys posts with a grain of salt then cause that boy sounds like he's back on the weed again.


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## sparkyboys (May 3, 2009)

steelersman said:


> He wasn't asking about degrees celcius columns in table 310.16. He was asking about the 2 different as amps values given on the nameplate. :thumbsup:
> 
> I wasn't aware that there was a 95 degree column in the code book. I thought it was 90 degrees. Not sure what codebook you're looking at. :no:
> 
> Also there's no 65 degree column either. It's 60.


who cares. i got better answers from mike holt site anyways


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## user4818 (Jan 15, 2009)

steelersman said:


> you'd better take sparkyboys posts with a grain of salt then cause that boy sounds like he's back on the weed again.


You didn't capitalize this sentence and you truncated "because" into "cause."


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## steelersman (Mar 15, 2009)

sparkyboys said:


> who cares. i got better answers from mike holt site anyways


you weren't the one looking for an answer now were you. So crack open that NEC, put the bong down and start reading!


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## steelersman (Mar 15, 2009)

Peter D said:


> You didn't capitalize this sentence and you truncated "because" into "cause."


Sometimes when I'm speaking to a *******, I like to speak the same as them.


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