Electric motors on one circuit--Circuit breaker sizing 3

[marko333]

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

 

[peebee]

NEC would requre you to size the breaker at 125% of largest motor + 100% of rest of motors. In your case, that's 5.25xFLA = 22.6A, which would imply a 25 or 30A breaker.

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.

 

[DanDel]

If each motor has adequate overload protection, use a C/B sized at least 125% of the FLA(verify that your feeder cable is properly sized and refer to the appropriate codes for your location) and set the Instantaneous setting to allow simultaneous starting. This is typically 6-7 times the FLA, or 150A or so for your application. You can check what the Locked Rotor Current is for your motors from the nameplate or the manufacturer to get the actual value.
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.

 

[GGOSS]

Hello marko333,

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

 

[peebee]

A clarification to GGOSS's post:

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.

 

[somang]

i'm designing switchgear for the first time please help me and give me enough information to size MCCB for feeders and for starters DOL.
how to select MCCB and the bus bar rating and the main breaker as well?

 

[peebee]

Somang: If you're designing to NEC then check out NEC 430. I've already summarized the 430 requirements for MCCB's in my 8/6/03 post above.