Wire Sizing / Branch Protection for 120VAC - 230VAC VFD

J

JovialBulge

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New England
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Programmer
The unit as a whole is going to be portable so it will be plugged into a wall outlet with a NEMA 5-20 plug, the wire size coming off of that receptacle and feeding the VFD branch and the PSU branch I would use 12 AWG since the plug is 20A.
VFD maximum input current is 19.2A @ 120VAC, so the wire size for the line should be 12AWG. Typically I would have a 25A circuit breaker or Class CC fuse protecting the input side of that size VFD. Should I bring that down to a 20A breaker of fuse at that point since the NEMA 5-20 plug will only support that amperage anyway?
 
Is this a permanent install or some temporary install for testing or demonstration?

Normally with a VFD max input is 19.2 x 125% for a motor load or 24 amps. You might need #10s or #12s depending on the wiring method and a 25 or 30a breaker
 
This will be a permanent installation however it will be portable in nature. So the plug on the unit has been specced to be a NEMA 5-20 style, therefore it will only have 20A max on the unit based on that limiting factor. While I agree with your above statement, I’m not sure how to get around the 5-20 receptacle specification and have more amps available to me to then utilize a larger BCPD as you listed above.
 
I'm not even sure that a cord and plug for a VFD is code compliant but the conductors feeding the VFD are as Eddie stated required to be a minimum size based on 125% of the input current. Drive OCPD sizes are usually determined by the drive manufacturer not the conductor size.
 
So the OCPD for the drive if you were to follow the UL 61800-5-1 installation table, lists the maximum allowable fuse rating which is 40 ( Class CC, J or T) and the recommended breaker which is 20A.

19.2A drive rated input current, so 24A. According to UL Table 28.1 I’d be using 10AWG wire for that amperage rating, I’m following all the way up to there. What I’m not getting is why it’s sized that way versus saying “it’s got a 20A CB on the line side, 20A requires 12AWG wire, done.”
I assume that the code takes into account someone removing that OCPD that’s rated at 20A and replacing it with one that’s rated for 25A or even more, and at that point the 12AWG wire becomes the OCPD, which is less than ideal. Is that the correct train of thought here?

Even further down that path, the NEMA 5-20 receptacle feeding this entire portable piece of equipment “should” be protected by a 20A breaker in the facility regardless, so putting say a 25A or higher breaker on the line side of the VFD seems useless to me no?
 
Any motor circuit requires motor amps (from the NEC chart In 430 not the motor nameplate.) x 1.25 for the starting current and to size the wire.

With a drive there is no VFD chart in the code so you use the VFD input x 1.25 to size the wire

If you use a wiring method rated 75C you can use #12

To size a CB or fuses for a motor with no VFD you use the code chart. 1 time fuses are usually 300% of code motor amps. Time delay fuses are usually 175% of code motor amps. Breakers may be up to 225% of motor amps all with 75C #12 or 60C #10 wire.

For a VFD to size the breaker refer to the VFD instructions

Motor overloads in a motor starter for thermal protection are usually sized at 115% of the motor name plate amps (not code amps

VFD overload setting are the same as above.

As with anything in the code there can be exceptions but the above is the general rule
 
This panel will be built to UL508A specs, not stamped but built as if it would be, so I believe a few of the wire amperage capacities per a specific AWG differs from the NEC that you referenced. I’ll continue to compare the two and take into account your information you posted above. Thank you.
 
As to the breaker, the VFD listing will give you a MAXIMUM breaker size it is listed with, you can install a smaller one if you like, it's just more likely to nuisance trip. VFDs create line side harmonics, harmonics cause additional heating in components, ESPECIALLY thermal sensing elements in circuit breakers. In other words you CAN use a 20A breaker, because the MAXIMUM is 25A, and 20A<25A. Good luck with that though...

Really, the issue here is the initial basis of design was incorrect. You CANNOT use 12 ga wire on a VFD that has an input current of 19.2A, it would violate 430.122, which REQUIRES that line side conductors feeding an inverter be sized at 125% of the inverter input amps (due to the same harmonic heating effects as stated above). So the initial idea of running a VFD of this size from a 20A outlet was flawed from the very beginning. It would require 10ga wire, all the way back to the panelboard. Given the numbers, I'm guessing this is a 1HP VFD set up for 120V input feeding a 1HP 230V 3 phase motor. The max. VFD you can install that can plug into a 20A outlet is 3/4HP. Someone should have investigated that up front.

Now that all said, WILL it end up functioning? Probably. I always like to apply a test of considering what would happen if there were NO VFD: in this case a 1HP 115V motor. That motor would be 16A FLC, the NEC requires that the conductors be sized at 125% there too, so 20A would have been fine and given that, this will PROBLABLY work OK. It's just not going to be Code compliant. But then again, is a plug-in device required to be Code compliant? That's not a clear cut situation.

CAN you apply a UL508A label to it though? Nope, UL508A will have the SAME conductor sizing requirements as the NEC. So that's going to require that the wires be10ga minimum. CAN you put 10ga portable cord into a NEMA 5-20P cord cap? Yes, in most cases. TECHNICALLY, the fact that the wiring in the WALLS to the NEMA 5-20R outlet will likely have 12ga, is the Code compliance conundrum, along with the possibility of a 20A breaker nuisance tripping. so that's something to consider as you see fit.
 
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