Electric motors on one circuit--Circuit breaker sizing
Electric motors on one circuit--Circuit breaker sizing
(OP)
Hello!
I am trying to put several, up to 5 single phase 750Watt on a single circuit. I know the full load amps total is 4.3A (at 240Volts) per motor so, with 5 on the phase its 21.5A.
It is the start up current i am concerned about. All 5 motors can be started at once and i want to avoid neusance triping of the protecting circuit breaker.
How do i Size this circuit breaker?
Thankyou.
Marko
I am trying to put several, up to 5 single phase 750Watt on a single circuit. I know the full load amps total is 4.3A (at 240Volts) per motor so, with 5 on the phase its 21.5A.
It is the start up current i am concerned about. All 5 motors can be started at once and i want to avoid neusance triping of the protecting circuit breaker.
How do i Size this circuit breaker?
Thankyou.
Marko





RE: Electric motors on one circuit--Circuit breaker sizing
But NEC also requires that the overcurrent protection for motors be limited to 250% of FLA, or 10.75 amps, so your installation would not be legal without additional downstream overcurrent protective devices sized at about 10 or 15 amps.
You could use two 15-amp breakers, feeding up to three motors each. Those two 15A's could be fed by the 30 amp breaker. 15 amps is the smallest standard breaker size, but fuses go down to smaller standard sizes per NEC 240.
Or you could use three 10-amp fuses, feeding up to two motors each. Those three fuses could be fed by your 30 amp breaker.
RE: Electric motors on one circuit--Circuit breaker sizing
Most C/Bs in the 30-40A range do not allow for adjustments in the Inst setting, but they will typically trip on Inst for approximately 10 times the overload rating.
RE: Electric motors on one circuit--Circuit breaker sizing
Bear in mind the purpose of the breaker is to protect the cables. Therefore you may find a single breaker on the supply side of these starters may be to large to protect the cables to the individual motors. Individual breakers may be required.
Regards,
GGOSS
RE: Electric motors on one circuit--Circuit breaker sizing
The breakers are there to provide high-level short-circuit (overcurrent) protection for the cables, not overload protection.
The overload devices provide low-level overload protection for the cables, even though they are downstream of the cables.
This is why NEC permits the breakers to be rated at 250% FLA, but the cables at 125% (breaker at 2x cable rating). This is generally not permitted for most other loads, where all overcurrent/overload protection is upstream of the cable, and the cable ampacity must be equal or greater than the breaker trip rating. This seemingly "unprotected" cable (really, downstream protection) is permitted only for motors.
RE: Electric motors on one circuit--Circuit breaker sizing
how to select MCCB and the bus bar rating and the main breaker as well?
RE: Electric motors on one circuit--Circuit breaker sizing