circuit specs for 1ph 20hp VFD to run 10HP motor

Hi All,
I would like some advice on the circuit (wire size and breaker) required to run a 3ph 10hp inverter duty motor motor with a 20hp VFD on 1ph 220 power. This combo is probably a little overkill for my hobby shop lathe (monarch) but it's what I have. I'm wondering if the fact that I will rarely if ever fully load the motor will allow me to go to a slightly smaller breaker than would be required if I were really using the full potential of the motor. In other words I am trying to minimize the service I direct from my 150amp breaker box.

Thanks for the advice

A 10 hp motor will need 27 amps at 230v 3ph, or 47 amps 1ph (3ph amps X 1.73 = 1ph amps). Wire size for a run 25' or shorter is #8 AWG, 25'-50' requires #6, 50'-100' uses #4.

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Barry Milton

ZTRUK said:

Hi All,
I'm wondering if the fact that I will rarely if ever fully load the motor will allow me to go to a slightly smaller breaker than would be required if I were really using the full potential of the motor.

Click to expand...

You "fully load" the motor every time you turn it on. If you undersize the circuit, the feed breaker when you power it up.

Chris

Soft-start.

You 'fully load' the motor every time you start it... if you're using Across-The-Line starting.

When using a VFD, the amount of starting current is limited by the VFD's programmed accelleration ramp AND any current and slip parameters you've programmed.

The limitation you WILL see, ultimately, is that you cannot command more power to the motor than your circuit breaker can allow... if you ramp it slow and easy, it'll be fine.

The one thing that MAY throw a wrench in the works, is when you apply initial power to VFD... the capacitor banks charging up may cause enough inrush to trip the breaker... particularly if you're using a very quick-acting breaker.

The primary side is easy. The NEC requires you to size the conductors feeding a drive at a minimum of 125% of the VFDs maximum amp rating. There is no exception regarding the size of the load connected to it. So even if you had the 20HP VFD connected to a 1/4HP motor, you STILL have to size the wires FEEDING the VFD the same. Chances are your VFD is going to be rated for around 64A maximum, so you have to size the conductors to no less than 80A. So then you have to size the OCPD (Over Current Protective Device, i.e. fuses or CB) to protect that wire, so again, 80A. You could size the OCPD smaller, but the cost would be the same.

What you do on the OUTPUT side of the VFD is based on how low the VFD's output overload protection can be set. Most VFDs stop at 20% of the VFD's output current rating; below that and you would have to use separate OL and OCPD protection. But in your case, you should be fine to size the wires for the motor size just as you normally would.

Don't forget this will be a three phase VFD running derated for single phase. This
really doesn't fit in the code, but one might be tempted to downsize the wiring, and
adjust the input overcurrent protection based on this fact.

Don't think they make 10 hp single phase VFDs.

one might be tempted to downsize the wiring

Click to expand...

+1

The reason I listed AWG #8, #6 or #4 is that full load amps should not exceed 47a. I understand what Jraef said, and he is correct, but that does result in sizing the wiring for a load that it will never carry when single phase input is used.

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Barry Milton