3 phase curreent calculation 1

[mally]

This will be a bit basic to some I know, but here goes.
Using ohms law in calculating single phase, I would do the following:
I=P/V
eg = I=2000/240 = 8.3 amps,

However in three phase do you do the same ie.

I=P/V
eg I = 5000/415 = 12 amps

When sizing a circuit breaker for the last calc would you use the 12 amps in total or 12 amps per phase, totalling to a 36 amp figure to size with?

Thanks in advance
Mal

 

[mpparent]

Actually, you would want to go w/ this equation for a 3 phase system:

I=P/(sq. rt(3)*V) giving you 6.95A. Now, if you're using the NEC, you would put a 1.25 factor on top of that giving you 8.7A. You could use a 15A, 3 pole breaker.

 

[nitro]

mpparent, you equation only holds true if you have a unity (1.0)power factor. To accurately determine 3 phase ac you must factor in you pf.
I=P/(sq.rt(3)*V*pf)

 

[mally]

Thanks mm
I am now very confused, let me explain
A Eurotherm engineer came into our plant this moprning to size up a AC drive/motor replacement for a 22kw Kopp variator and motor (mechanical speed control). The supply to the existing motor is sized at 100amp/3 phase. I asked what was the largest motor we could have with the existing supply and he said 50Kw. If I use your calcs then he is well out ie:
I = 50,000/(sq.rt(3)*V)
I = 50,000/1.73*415
I = 50,000/718
I = 70 amp

Have I interpreted something wrongly?? One would think he would know his job. Perhaps not?

 

[peterb]

Start with your 100A/3-ph/415 suply circuit.
The rated kVA of this circuit is -
100A*0.415 kV*sqrt(3)= 71.88 kVA

Assume a power factor of 0.8 -
Rated power of circuit = 71.88*0.8 = 57.5 kW

This is the input power to the motor. Assuming an efficiency of 0.95, then output power = 0.95*57.5 = 54.6 kW
The closest standard size would be a 50 kW motor.

Note that this only allows for a 9.2% margin in sizing the circuit, which may be adequate for your location but, as mpparent points out, the NEC would require a 25% margin. This would limit the motor size to (57.5/1.25)= 46 kW or below.

 

[mpparent]

Nitro,

true...i had assumed mally was referring to apparent power, which is the P/pf you're referring to. My "P" is your "P/pf".

 

[jbartos]

Suggestion: Although PeterB posting last paragraph complies with NEC, it does not guarantee that the 46kW motor will work in all applications. I recommend that you also check the following:
1. Motor Locked-Rotor Amperes. If they are high, the motor kW rating shall be even smaller to have motor start quaranteed without any worry about the motor stalling while starting because of a high voltage drop.
2. Somewhat related to the comment in 1. Check the length of the motor branch circuit and control circuit. If they are excessively long the motor rating will also have to be lower than 46kW to have guaranteed the motor start.