Wire size Question

B

Beef

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Recently posted a question about a 2.2 KW VFD and received great input. So now the question is what wire size to run for the outlet for said VFD.
Length from breaker box to outlet will be around 30 feet. Circuit will be 220 volt single phase. Planning on using a 20 amp breaker in breaker box.
Thanks for the input !!
 
(2.2*1000)/220=10A
A 20A circuit should use 12 gage wire.
The breaker is protecting the wire not the VFD!
The VFD will/should have it's own internal protection.
 
The VFD manual should specify the breaker/fusing as well as recommended gauge. NEC specifies that the wire/breaker size minimum must be sized at 125% of the VFD input rated current, and not what it is driving. A 2.2kW (3 Hp) VFD typically is on a 30A breaker which would require 10AWG wire for the circuit to the socket. Beyond the socket, my understanding is the code falls under UL.

I recently ran into this issue with another person who is installing a 3 Hp VFD and had a similar discussion. The VFD can output 12A 3 phase, 240VAC, the manual recommends 30A delay fusing, and up to #8 input wiring. He has a 20A circuit or a 50A circuit. The issue boils down to will it run on a 20A circuit, maybe if you dial down some of the motor overload parameters, but code says it should be on a 30A breaker (12 x 1.73 x 1.25 = 26A at 100% efficiency). The breaker size typically dictates the motor wire gauge of what is hardwired from the breaker panel to the socket, the breaker protects the wiring. If you are running say a 2 Hp motor, yes it will run off of a 20A breaker, but my understanding is it is not to code. Also be aware that a 20A breaker/socket is not rated at 100% continuous duty, it is only rated at 90%. If he uses a 50A circuit, then all the wiring to the VFD needs to be rated at 50A, and the VFD should have 30A fusing.

Other issue I have had with some of the generic Chinese VFD's is the terminals do not support the amperage rating/wire gauge/crimp terminals. It was almost impossible to attach the specified gauge because the terminals were to small. I have also seen the input circuit boards traces blow due to being undersized. VFD's protect their output, but not their input circuitry. On smaller VFD's they are pretty much through away if something fails, bigger more expensive VFD's I often use input fusing which can react faster than breakers.
 
Hum, I'm not quite following, what does the 1.73 multiplier represent?
 
conversion from current in one leg of 3 phase to total current in single phase input.
 
This to me is a judgement call. First they tell you that you must oversize the VFD because OMG Chinese. Then they turn around and tell you to oversize the wiring because of theoretical capacity.

If the VFD was used as a hard-wired power station to be used with many unknown devices, then I would agree that the wiring must support the maximum potential load. However, in this case the VFD is driving a single device, your 1HP lathe. It will be hardwired to the lathe and plugged into the wall outlet. Therefore, I consider it part of the lathe. Sizing the wiring on the lathe VFD in this configuration would be the same logic as sizing the wiring to 50A because the target 20A piece of equipment has a contactor rated at 40A.

That lathe will draw less than 10A. Wiring it on a 20A circuit will be fine. 20A breaker, 12ga wire, and a 20A outlet on the wall for the lathe (w/VFD) is what I run.
 
I certainly run my 3HP VFD converted lathe on a 20A 220V circuit. Several tools that I have came factory with VFD + 3 phase motor come with a 6-20p plug, and call for a 20A circuit. On a scientific basis, 3HP = 2.2KW. At 220V single phase, that requires about 10.25 amps (theoretical 100% efficient motor). 65% efficiency for VFD+motor would put you at 16A, right at the 80% for a 20A circuit. That would be a pretty poor efficiency.

A motor may draw more that rated current temporarily (startup for example). The VFD will normally be spec'd to supply that current even though the motor nameplate doesn't shows that. To me this is a bit of a rating/load game, what number constitutes the "load", in one case (a motor) we'd use the nameplate number which is NOT the peak, in another we're using something that is basically the VFD peak load. I will agree with @mksj, that straightforward reading of the NEC calls for wiring installation to meet 125% of that spec. I would also argue that a VFD is not an appliance, or tool. It is a component. When you replace a manufacturers controls with a VFD, you are becoming a designer and voiding the UL approval of that tool. A component's rated load is not the overall system's rated load.

As a personal judgement call in my own shop, I feel using 125% of the VFD's maximum is over doing the safety margins. I'd have to submit that to an approval body such as UL to really get it approved. As home hobbyist, not happening. Also, In part the NEC is mandating what a new wiring design is expected to accommodate. We all know in our homes we don't quit plugging things into a wall circuit until the breaker pops, 100% load not 80%. I interpret this as a design criteria to give people a bit of growth room, not strictly a safety issue. As long as the wall wiring gauge meets or exceed the breaker rating (typically 12ga for a 20A circuit), I would use this logic and feel quite comfortable and will continue to use 3HP VFD's on a 20A circuit.

Four questions: 1) Does it work? 2) Is it safe? 3) Is it per code? 4) Will it pass inspection?
Ideally the last two are the same, but there is some room for argument. Code is trying not only for safe, but does it meet reasonable expected use, so 2 & 3 are not always the same.
 
mksj said:
and not what it is driving
Click to expand...
This based on the idea that you "could/will" add more loads to this VFD device. Valid point, sort of. But even though the VFD "could" consume more power than what a 20A circuit is designed for, the breaker should still protect everything down stream from a dangerous overload.

This strikes me as similar to the idea that if you have a 2hp motor on a machine you need to provide protection and wiring for a 10hp motor because you might change motors on the machine or add another machine to the circuit.

Is my understanding wrong?
 
RandyWilson said:
This to me is a judgement call. First they tell you that you must oversize the VFD because OMG Chinese. Then they turn around and tell you to oversize the wiring because of theoretical capacity.

If the VFD was used as a hard-wired power station to be used with many unknown devices, then I would agree that the wiring must support the maximum potential load. However, in this case the VFD is driving a single device, your 1HP lathe. It will be hardwired to the lathe and plugged into the wall outlet. Therefore, I consider it part of the lathe. Sizing the wiring on the lathe VFD in this configuration would be the same logic as sizing the wiring to 50A because the target 20A piece of equipment has a contactor rated at 40A.

That lathe will draw less than 10A. Wiring it on a 20A circuit will be fine. 20A breaker, 12ga wire, and a 20A outlet on the wall for the lathe (w/VFD) is what I run.
Click to expand...

Randy, I agree with everything in your response. There is no point in going over the top when completely unnecessary.
 
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