# Thread: Power Draw for 7.5hp VFD

1. ## Power Draw for 7.5hp VFD

I am trying to figure out if I can power a 7.5hp 3 phase CNC lathe in my home shop. I would like to use a VFD and have determined that the output maximum amperage for a 7.5 VFD is about 25 amps 3 phase. What I need to know is how many amps will a VFD of that size draw from a 220 volt single phase circuit?

2. Originally Posted by Southbend Sam
I am trying to figure out if I can power a 7.5hp 3 phase CNC lathe in my home shop. I would like to use a VFD and have determined that the output maximum amperage for a 7.5 VFD is about 25 amps 3 phase. What I need to know is how many amps will a VFD of that size draw from a 220 volt single phase circuit?
With 150+ pages of data in a VFD handbook, you'd think the answer to that would be in there.

But, it's not.

However, my 3 hp VFD calls for a circuit breaker of 20 amps or 30 amps, or a fuse of 30 amps or 40 amps.

That may give you some idea. I'd guess that you'd at least double the breaker size for 7.5 amps.

You might want to call one of the VFD suppliers and ask them. Of call GE. Their tech support number is listed in my VFD manual.

3. yep, 7.5hp 230v drive is usually rated around 25amps 3ph output. means input is a tad higher from efficiency, but you can ignore that it is small extra. but from 1ph input you will draw about 25/.6=41 amps. this is to get full load out of your 7.5hp motor at rated speed. Keep in mind that if you only load it partially, you will pull less current. the motor will pull about 10 amps to make it run (to make a magnetic field inside) so you always have that. the rest is for load. also the slower you go the less input amps pulled. but for 220v 1ph current draw I'd say wire for 40-45amps and in reality you will pull 20 amps most of the time....

4. If it is a case of running a fresh circuit, I would go with 50amp breaker and #8 conductor.
Although with a VFD you won't have such a large inrush, you are probably going to have other loads on a CNC machine, and there is usually local protection in the way of fusing in the enclosure anyway.
The cost difference for a 50amp circuit will not be that great.
Al.

5. Thanks for all the input. I've got a 50 amp circuit in my garage so it looks like I am good to go if I can just find the right machine.

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