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Recommended electric motor feeds

  • Thread starter Michael Moroney
  • Start date
M

Michael Moroney

Jan 1, 1970
0
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.
 
S

SQLit

Jan 1, 1970
0
Michael Moroney said:
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.

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.
 
R

Ross Mac

Jan 1, 1970
0
Michael Moroney said:
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.
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
 
J

John Gilmer

Jan 1, 1970
0
Ross Mac said:
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

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.
 
R

Ross Mac

Jan 1, 1970
0
John Gilmer said:
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.
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
 
U

User 1.nospam

Jan 1, 1970
0
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)

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.
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)?

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.
 
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