HP Vs Brake HP

[tm7128]

Should we use HP or BHP for the calculation of loads? For example a 20HP load has a BHP=18. We have to size transformer for eight 20HP motors.

If we take HP of motors then: 8*20*0.746 = 119.36 kVA (150kVA transformer required)


But if we take BHP of motors, then: 8*18*0.746 = 107.42 kVA (112.5kVA transformer required)

Please let me know commenst.

 

[MikeHalloran]

What happens while the motors are starting?


Mike Halloran
Pembroke Pines, FL, USA

 

[dpc]

You have to size the transformer based on the electrical load, not the mechanical load. More specifically, you have to size it based on the motor full load amps as listed in the NEC, if you are in NEC-land.

The mechanical load does not matter. You could have a motor with nothing connected to it and you'd size the motor feeders and electrical supply the same way.

And your formula is wrong. You've left our motor power factor and motor efficiency.

 

[tm7128]

The problem is that motor FLA in NEC are calculated on 460V, while the transformer secondary is 480V. So current at 480V will be little less than the current at 460V.

As of motor starting, if the full load current of the transformer is at least six times larger than the FLA of the largest motor, then motor starting should not be a problem and motor starting study should not be required. Is it correct?

 

[davidbeach]

460V motors are used on 480V systems. That's what the code is written around. Code FLA is the only load you need to know unless you have some reason to suspect that your motor will draw more than the value in the code tables. No rhyme, no reason, just use the code values and move on; it doesn't matter how much it might not make sense. If you really think there is a better way, there's a form at the back of every code book for proposals for the next cycle and you should submit your proposal. But until it's accepted you'll need to continue using the code FLA values; 460V motors on 480V systems, 230V motors on 240V systems, 200V motors on 208V systems.

 

[dpc]

The voltage at the transformer might be 480 V but what is the voltage at the motor? The motor nameplate rating is 460 V for a reason.

But as David said, you can't use logic or the laws of physics - you have to comply with the NEC. That's why we have codes - to prevent people lacking basic understanding from cutting so many corners that they burn down the building.

 

[Kiribanda]

tm7128,
You cannot use the BHP to size the transformer.The BHP is given by the pump performance curve.Then you should assume a reasonable coupling efficiency and then get the motor HP.Then you have to go the next available motor HP rating to get the actual motor HP.Then you have to see how many are running contineously,or as stand-by or as intermittent to size the transformer.Finally you have to see whether the transformer is enough to start the motors depending on your operation requirements.As dpc pointed out the motor rated voltage is 460 on a 480 V network and your transformer is having 480V no-load sec voltage.