# Brain Tease:  Power (Wattage or VA) for a 3 phase motor



## jar546 (Feb 12, 2011)

Lets assume 3phase squirrel cage induction type wound motor.

15HP 3ph 460vac

What is the VoltAmps of this motor?

What size Cu conductors will you need to run to this motor?

Hint, you will have to look up FLC.


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## chris kennedy (Feb 13, 2011)

jar546 said:
			
		

> Lets assume 3phase squirrel cage induction type wound motor.15HP 3ph 460vac
> 
> What is the VoltAmps of this motor?


Just using T430.250 I get 17,430. If you gave PF and %EFF I would get a more realistic number.



> What size Cu conductors will you need to run to this motor?Hint, you will have to look up FLC.


Assuming continuous duty, 21A×1.25=26.25A or #10cu.


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## jar546 (Feb 13, 2011)

We need to hash this one out then.

From the FLC table I got 21

460x21x1.732=16,731

Using the label of 460, I have 36.37A

How did we differ?


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## chris kennedy (Feb 13, 2011)

jar546 said:
			
		

> We need to hash this one out then.From the FLC table I got 21
> 
> 460x21x1.732=16,731
> 
> ...


I used the real world voltage the 460 motor will run at. 21×830=17,430.


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## chris kennedy (Feb 13, 2011)

jar546 said:
			
		

> Using the label of 460, I have 36.37A


How did you come up with this???


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## jar546 (Feb 13, 2011)

Holy crap, I realized what I did.

1st, I took the wattage and divided it by the voltage of 460 which gave me 36.37

Verifying, I took the 21 FLC and multiplied it by 1.732 for 3ph which gave me 36.37


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## jar546 (Feb 13, 2011)

I keep forgetting that chart is already taking into consideration it is a 3 phase motor for the FLC

I stand by my VA calculation though.


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## jar546 (Feb 13, 2011)

I see where you added the 1.25 for the motor and I did not.  Now I am coming up with the exact 26.25A

Now, for branch circuit protection, I would go with 60A  (52 real answer) but I know a 50 would work


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## chris kennedy (Feb 13, 2011)

jar546 said:
			
		

> I stand by my VA calculation though.


Yeah, I'm going to have to give you that one Jeff. Looking at some example calcs in Ugly's now and they also use the name-plate voltage.

I stand corrected.


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## jar546 (Feb 13, 2011)

We are taught to use the nameplate voltage but I personally use the actual voltage when accuracy counts


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## chris kennedy (Feb 13, 2011)

jar546 said:
			
		

> We are taught to use the nameplate voltage but I personally use the actual voltage when accuracy counts


Force of habit for me, although if this exercise had been a real test question, I as you say "read to much into it". FAIL!


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## Daddy-0- (Mar 2, 2011)

Is the contractor section where yall come to ask the REALLY hard questions?? My contractor's can't even read their own plans!


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