Conductor/Breaker sizing question

Tomzo

Tomzo

H-M Supporter - Silver Member
H-M Supporter - Silver Member
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Greetings

I am bringing a subpanel into my shop so I can install a 240V circuit for a VFD conversion on my RF31 clone mill. My main panel is right on the other side of the exterior wall from where I am planning to install the subpanel. My main is full, but I currently have a 50A GFI breaker for a hot tub. I intend to use that space in the main and install the GFI hot tub breaker in the subpanel - about 5 feet away (but probably 15 feet of wire as it needs to go up through the top plate of the wall across the top of a window, and back down). Ultimately I want to ditch the hot tub, but my wife likes it in theory even though it only gets used a few times a year. Here are some questions:

- An electrician friend of mine indicated that putting a 60A breaker in the main to feed the subpanel would work and #6 wire should be fine with that sort of a run. From what I can tell that checks out.
- I have a 2HP three phase motor that will run off the VFD and from what I understand that size motor will only draw about 6 amps. From looking at my local home improvement store, it seems that 20A is about as small a breaker that I can find for 240V circuits. Is that oversized?
- In my mind I had thought that this circuit would require huge conductors, but from what I can tell 12ga wire will handle this load easily. Does that sound right?

I think I have this worked out but I figured some of you have probably done this before so any tips or advice is appreciated.

Thanks

Tom
 
You need to size your panel breaker, and wiring, based on the rated current draw of the VFD times 1.25. For a 2 hp VFD, that could either be a 20 amp breaker, or 30 amp depending on what the rating of the VFD is. The current draw of the motor does not matter since the breaker is connected to the VFD.
 
Ah - excellent point. The one I am looking at indicates that it is rated at 10A


I should note that this VFD is rated up to 3 HP but I only have a 2 HP motor - I figured being a bit oversized shouldn't hurt - especially with these cheaper offshore VFDs.

Clearly much smaller conductors can do 10A but a 20A breaker requires 12 ga wire from what I understand. I guess in this situation the difference between 6 A and 10A is handled by the 12ga wire and 20 amp breaker pretty handily.
 
Breakers protect the wire they feed not the end use device. A 20amp breaker in a panel will protect 12 gage wire regardless of what is plugged into that wire.

In the case of your setup, you should have a fused (or breakered) disconnect before your VFD to protect it and the motor.

I would let your electrician friend wire everything up to whatever disconnect you choose for the VFD.
 
I have one of these for my ELS on my lathe

uxcell RT18-32X AC 380V 32A 1 Pole 10x38mm DIN Rail Mount Fuse Holder Base: Electrical Fuse Holders: Amazon.com: Tools & Home Improvement

uxcell RT18-32X AC 380V 32A 1 Pole 10x38mm DIN Rail Mount Fuse Holder Base: Electrical Fuse Holders: Amazon.com: Tools & Home Improvement
www.amazon.com

I can certainly add this into the circuit with a 10A fuse. That is a good idea as if the breaker won't trip until 20A the whole thing might be toast before that happens.

Thanks
 
Thinking about it, I think I will just go ahead and build a control panel box. That way I can add in the disconnects and have space for potential future solenoids for air misting at the work location for chip removal. I mean, why not?
 
^^^^^^ That’s an excellent idea and exactly where I hoped you would end up.
 
A breaker protects the wiring and needs to be sized accordingly. On the VFD the input current is not 10A, this is the output current. Breaker sizing/fusing is 125% of the VFD input current (or as outlined in the manual) regardless of the motor current it is driving and motor wiring is 125% of the rated output current to the motor. Three phase to single phase plus a fudge factor for efficiency, would be ~2X output current x 125%, so technically you would need a 30A breaker for a 3 Hp VFD. Wiring to the VFD would be sized according to the breaker that is feeding it.
 
My big issue with these sorts of panels is the insane cost of the boxes themselves. For my ELS project I used an old PC case I had lying around. For something with 220V in it I may consider finding something a bit more heavy duty....Craigslist here I come!
 
The only opinion I can offer is to realize that any machine should protect itself. A circuit breaker protects the iine it feeds. It is distressing to me how many people use specificly rated fuses or breakers. I concur that you should set up a 'sub-main' panel so you can feed other future devices. Whatever the input rating is for the VFD, give it a little wiggle room, say plus 20%, for the feed. Then size the wire to that and use the appropriate size breaker. Just remember that 3 phase from a single phase source is calculated as +1.73. So calculate your machine load as a base line. Then that load times 1.73 to calculate the single phase source. Then add in that wiggle room and wire for that.

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