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Recommended electric motor feeds

Out of idle curiosity:

At what power level (horsepower rating or wattage) is it recommended for a 3 phase motor be used over a single phase if 3 phase is available? At what power level is 3 phase service considered required? (that is, don't install single phase motors over X hp, order a 3 phase service)

For bigger motors, at what HP would you wire to a 277/480 panel over

120/208 (or 120/240 high leg) if such a panel was available? At what HP would a 277/480 feed be required? (you'd order new service to run such a beast)?

I'm assuming standalone motors, not equipment with motors where the "choice" was made by its designer, and US service.

Reply to
Michael Moroney
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For sure below one hp. If the motor is loaded and running constantly then 3 phase for sure. If less than 50% loaded or running then below 5 hp use single phase. My thoughts only.

Reply to
SQLit

If given a choice I always go for 3 phase and the highest voltage normally available (480 in the states). You won't have the starting caps to deal with or replace and the motor will run more efficiently and cooler at the higher voltage........have a great holiday, Ross

Reply to
Ross Mac

In a "perfect world" I would agree. BUT good protection for a single phase motor is a LOT cheaper than good protection for a single phase motor.

Moreover, a 2 or 3 wire switch is a LOT more expensive than a 1 wire switch.

Given all this, I would GUESS that the breakeven point is at around the 2hp level.

Just my opinion.

Reply to
John Gilmer

Sounds like good advice John....I wrote equipment specs at one time and that was just my rule of thumb...I know there are a bunch of folks out there much more educated on this subject than I.....have a great holiday season!...Ross

Reply to
Ross Mac

Another consideration might be distance. How far will the motor be from the utility's transformer? Voltage drop is a lot less for three phase than single phase. I don't know if there is a rule of thumb for this.

Dean

Reply to
Dean Hoffman

It is hard to find a single phase motor over 5hp. Residential single phase air conditioning stops at 5 hp....if you need more than that they usually install multiple units.

120/208 will keep a future janitor from creating a 180+ volt receptacle circuit vs a 120/240 4wire system. 3 phase motors are just as efficient at 208/240 as they are at 480 given proper wire sizing and overload protection. 480/277 is great, too, but requires step down equipment for receptacle and possibly some lighting.

If you have many motors over 100hp you are probably going to want a 480 volt service.

Reply to
User 1.nospam

You can get an idea looking at the amperage tables for motors in the NEC.

Sincerely,

Donald L. Phillips, Jr., P.E. Worthington Engineering, Inc.

145 Greenglade Avenue Worthington, OH 43085-2264

snipped-for-privacy@worthingtonNSengineering.com (remove NS to use the address)

614.937.0463 voice 208.975.1011 fax

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Reply to
Don Phillips

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