# Single phasing and VFD



## nrp3 (Jan 24, 2009)

If I use single phase to feed a three phase motor through a vfd, how do I size the fuses? I have a pet food manufacturer that buys used equipment and forgets he needs single phase equipment. I know now that I have to double the horsepower of the motor to properly size the vfd, but what to do with the fuse sizes at the disconnect?


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## varmit (Apr 19, 2009)

The line side fuses would be based on 125% of the rated drive input current.


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## John Valdes (May 17, 2007)

nrp3 said:


> If I use single phase to feed a three phase motor through a vfd, how do I size the fuses? I have a pet food manufacturer that buys used equipment and forgets he needs single phase equipment. I know now that I have to double the horsepower of the motor to properly size the vfd, but what to do with the fuse sizes at the disconnect?


Usually if the VFD is under 5 Hp, it requires no derating. Look at the drive specs/user manual or ask the manufacturer. After 5 hp it is usually one drive size up, not double. Meaning, if you have a 7.5 HP motor, you would use a 10 HP drive, not a 15 HP. As the motors get bigger, you must consult the manufacturer for the proper derate.
If you look at a motor and drive catalog, you can see the cutoff for derating and how to apply the derating.
Also, the fuse size is determined by the manufacturer. I personally would not use the 125% unless the manufacturer said so. The fuses are to protect the drive, not the motor.
All drive user manuals will tell you what size fuses to use and what type.


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