Sizing Fuse for Motor

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Thread Starter

AnonymousMan

This is kind of a "back to basics" question that I thought I could ask the pros who do this regularly. I've recently run into an application regarding selecting a fuse for a motor. In my case it is a single phase 230V supply going to an inverter for a 2HP application. In looking in the NEC book all it says is that you should not select a fuse less than 125% of full load current, but does not mention anything about max fuse size (that I can see). I realize that a time delay fuse is needed for motors.

Should I be upsizing my wiring to handle up to 125% -> 200% of the full load current? How will the motor be protected by such an upsize in fuse? If there is anywhere in the NEC or other handbooks I can reference please let me know where to find this.

If using a motor starter (i.e. no inverter in the mix), is it typical to use the fuse to protect the wiring for 150-200% of full load current, and then set the motor starter dial to the actual full load current in order to protect the motor itself?

Thanks
AnonymousMan
 
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Bob Peterson

The code requires that you abide by the instruction manual for VFDs. It will tell you what size over current protection is required. Usually it is a max of 250% of the rated input current of the VFD rounded up to the next standard size circuit breaker or fuse.

There is some question about the size of the incoming wiring. Most times I size it to the fuse or CB, but some claim the code could be read to allow the incoming wiring to be sized at 125% of the rated input current of the drive. It seems to me that there is merit in this approach as it is actually a motor circuit and wiring it in accordance with other motor circuits makes sense.

The wiring from the drive to the motor is just regular motor wiring. The short circuit protection is supplied by the fuse or Cb and the overload protection by the (usually) built in overload protection of the drive.
 
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AnonymousMan


> The code requires that you abide by the instruction manual for VFDs.

OK, I'll RTFM. 250% seems like some honking oversized fusing/wiring, but maybe it is for the inrush of the VFD capacitors.

What about if there is no VFD (fuse direct to motor)? Is it common to use a 125% fuse with an optional motor starter (set at 100% on the dial)?

Thank you,
AnonymousMan
 
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Bob Peterson

The code specifically allows for TM circuit breaker or fuses to be set at 250-300% on motors for short circuit protection ONLY.

You are still required to have overload protection, in most cases provided via either the overloads on a motor starter or electronically as part of a drive.

On "normal" circuits, a thermal magnetic breaker or fuse is used that provides both short circuit and overload protection. This often does not work especially well on motor circuits due to the inrush on start-up.

I would suggest this is not an especially good place to discuss the US NEC. It has a lot of subtleties that are not real obvious. If the moderators will allow me to suggest it, there is a code forum I am on that is quite good and is a much better place to explore this kind of thing in depth.

http://forums.mikeholt.com/
 
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