Disconnect switches for your equipment

Ischgl99 said:
For wiring a VFD, you need to size the wire at least 125% of the VFD input current rating ...
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I've seen posts on electrical/wiring forums that indicate that 125% only applies to continuous loads (expected to be on for three or more hours), and that you can use 100% for lesser loads? This seems consistent with google search results.
 
rabler said:
I've seen posts on electrical/wiring forums that indicate that 125% only applies to continuous loads (expected to be on for three or more hours), and that you can use 100% for lesser loads? This seems consistent with google search results.
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I don't recall seeing that anywhere in the code, the section I found when wiring my VFDs did not say anything about a difference between continuous or non-continuous loads. I didn't save the links I used, so going from memory on this, and there could be a newer version of the code that changed that.
My interpretation of the code is that the 125% wire size is there to protect a failure of the drive, not the motor, and that protection is needed any time there is power to the VFD since the motor does not need to be running for a drive failure. So, I would think the 125% is needed if the drive itself could be powered up for longer than the 3 hours, and there are plenty of times my drives are powered up all day even though I might be using the machines for short periods of time. I think the people stating the 100% for non-continuous loads are taking a different section that applies to motors wired conventually and incorrectly applying that to the VFD.
 
@jareese Don't overthink it. It seems like when someone asks a question about electrical, folks like to dive into industrial applications. You have basically 14-ish amps. A 12AWG wire will handle 20. If you want to be super conservative, you could run 10AWG. I have an appropriate breaker on my circuit breaker box, and the wire ran to where the lathe is sitting via the walls when I built the shop. My 1440 sits in a slightly different spot than my old lathe, so I had to run some conduit to my wall-mounted switch. I somewhat wish I had used a different wall switch because the one I linked snaps on and off quite loudly, which I am not a fan of. Personal preference.
I can get some pictures tonight.
 
Ischgl99 said:
y interpretation of the code is that the 125% wire size is there to protect a failure of the drive, not the motor, and that protection is needed any time there is power to
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Code more or less quits at the outlet. Load at outlet/disconnect is really all that matters. It is to protect the wiring and outlets. Any difference between motor and VFD and protecting them, is not really a code issue. That’s a UL approval issue, reliability, etc.

The “continuous” load is well below 50% if the motor isn’t running. Again I’ll defer to a qualified electrician but perusing electrical forums seems pretty clear this isn’t a continuous load. Otherwise you could argue my thermostat is always on too, so my furnace is always running. Most electricians in turn basically making a case to an electrical inspector….

Yes, you need to go by VFD current not motor current. Most good VFDs will list an input current. 1.7xmotor amps is a good estimate (that's basically a single phase to 3 phase conversion factor), but go by the VFD spec.
 
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Ischgl99 said:
For wiring a VFD, you need to size the wire at least 125% of the VFD input current rating to be compliant with the NEC, the motor size does not matter with wiring calculations into the VFD. Your VFD is rated at 7.5 kw for normal duty, so that is the input parameters you need to use for your calculations. The 5.5kw rating is for heavy duty operation, but that is not enough power for your 5hp motor run on single phase input to the VFD anyways. So, take 37 amps x 1.25 to get a minimum wire ampacity of 46 amps. That’s some pretty big wire, so a disconnect might be better than a plug at that size.
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rabler said:
Code more or less quits at the outlet. Load at outlet/disconnect is really all that matters. It is to protect the wiring and outlets. Any difference between motor and VFD and protecting them, is not really a code issue. That’s a UL approval issue, reliability, etc.

The “continuous” load is well below 50% if the motor isn’t running. Again I’ll defer to a qualified electrician but perusing electrical forums seems pretty clear this isn’t a continuous load. Otherwise you could argue my thermostat is always on too, so my furnace is always running. Most electricians in turn basically making a case to an electrical inspector….

Yes, you need to go by VFD current not motor current. Most good VFDs will list an input current. 1.7xmotor amps is a good estimate , but go by the VFD spec.
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So, if he uses an outlet it is a UL issue, but if he hard wires it with a disconnect it becomes a code issue? I don’t see the difference, both are the only things connected to that branch circuit.

Your thermostat comparison is ridiculous, it is not powering the furnace, it is only telling it when to come on, but yes your thermostat is actually running continuously, but your furnace is only running when it is told to come on, so it is not running continuously.

I’ve worked with enough licensed electricians to know they are not always right, and when they disagree with the code, they try and get away with whatever they can. In the end, it comes down to what the building inspector, and your insurance company, require, as well as your tolerance for risk. I’ve seen first hand the damage VFD failures do, so I am not willing to risk my house to save some money by cutting corners.
 
Ischgl99 said:
Call an electrician.
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OR - I can just use 10 gauge wire from the 30 amp breaker in my panel running the 15 feet to where I'm going to put my disconnect, then run some heavy stranded from there to my lathe. I think that's what I'm going to do.

I am looking at a disconnect because I don't have an ON/OFF switch for my lathe. As soon as power is applied (breaker flipped on) the fans in my cabinet start running and I have power to the transformer.
 
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7milesup said:
@jareese Don't overthink it. It seems like when someone asks a question about electrical, folks like to dive into industrial applications. You have basically 14-ish amps. A 12AWG wire will handle 20. If you want to be super conservative, you could run 10AWG. I have an appropriate breaker on my circuit breaker box, and the wire ran to where the lathe is sitting via the walls when I built the shop. My 1440 sits in a slightly different spot than my old lathe, so I had to run some conduit to my wall-mounted switch. I somewhat wish I had used a different wall switch because the one I linked snaps on and off quite loudly, which I am not a fan of. Personal preference.
I can get some pictures tonight.
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The box I chose doesn't seem to have any springs according to the pictures anyway. I'll let you know once I get it, if it's quiet
 
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