Which formula to use?

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Re: Which formula to use?

iwire,

Yes Bob, you can point out a lot to me. I am long on theory but short on practice and even shorter on the Code.

Rattus
 
Re: Which formula to use?

It basically boils down to, how do you know when you need to use the sq. root term
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I think I answered this one.

and when to use the low voltage (e.g. 277V) or the high voltage (e.g. 480V)?
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I was out of time and didn't get this one yesterday.

You use the lower voltage or neutral to phase voltage when you are actually or physically using those conductors that originate from there.

If you have a load that requires three phase wye (one that needs three phases and a neutral) then some of that load will be one (or more) phase to neutral. There will be a three phase load(s) (not using neutral) and a single phase load(s) to neutral.

They are calculated separately.

Charlie and Rattus. You even confused me. :D
 
Re: Which formula to use?

Sam,

I am saying that you can treat the load as a black box where we do not know the load configuration. I choose wye, therefore wye it is!

Then, for a balanced load,

Papp = 3 x Iline x Vy
= 3 x 7.6A x 277V = 6.32KVA

This is the most straightforward thing one can do to figure Papp.

I am also saying that if we are interested only in sizing the wires and breakers, all we need is current. And, if the three motors are identical, the calculated load current is,

Imax = 3.25 x Iline = 3.25 x 7.6A = 24.7A

My motto is Keep It Simple Sam.

Furthermore, I disagree with Charlie on the value of putting extra steps in the calculation--more chances for error.
 
Re: Which formula to use?

Originally posted by Kuzu:
... The unit has two parts, the compressor and the fan motor. I have the HP of the fan motor as well as the FLA of it. For the compressor I only have the RLA and LRA values. Total unit FLA is also given. ...
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Kuzu -
Whoops, I missed a key word in your post, "compressor". The terms "RLA" and "LRA" suggest this may well be a hermetic refrigerant motor compressor. If it is, you need to look at Article 440.

The "Branch Circuit Selection Current" is also discussed and is used for selecting OCP and couductor sizes - not the horsepower rating.

carl
 
Re: Which formula to use?

My motto is Keep It Simple Sam.
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Exactly my point.

I don't mean to attack you or Charlie in any way. I just thought it got a little complicated with regard to the OP.

You know I respect you two guys.
 
Re: Which formula to use?

Originally posted by rattus:
... But, if you assume the applied voltage to be 480, the magnetizing current, Im, might go up a bit, but we must remember we are still supplying 5hp. In that case the load current would go down and this change would be much more than the increase in Im. ...
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Rattus -
Yes, you are right. At a 4% increase over nameplate voltage, one could expect a 1% increase in efficiency and a 3% reduction in full load curent. Reference GE Co. AC Motor Selection and Application Guide GET-6812B

carl
 
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