Bussed motor starters

[sii]

I inherited a machine which has five motor starters all bussed togethere across the top of the OL unit. Originally the five motors were .75, 1.5, 2, 2, and 3 horsepower and the fuse feeding all five starters was specified to be 30 amps. I have replaced the 3 hp motor with a 10 hp including the proper OL unit and contactor. I am unsure of how to properly size the fuse now. Do I size the fuses by figuring the total horsepower? If that's the case, 30 amps seems appropriate given the total hp before I made the changes.

Voltage is 480 FWIW.

Thanks in advance.

 

[Jraef]

Assuming you are using those IEC starters with motor protection switches, total FLA x 1.25, minimum but not to exceed the rating of the bus bar. Most of those feeder bus bar systems are rated 63A minimum so it shouldn't be an issue, just double check. Some of the cheap ones are less, maybe 32A.

So using NEC charts, 1.4A + 2.6A + 3.4A + 3.4A + 14A = 24.8A, x 125% = 31A. I would fuse it at 35A because you don't want the inrush of the 10HP motor to pop your fuses. Those Motor Protection Switches are the motor SCPD, so all the fuses are for is "group fusing" and branch protection for the bus bars. The MPS devices themselves will tell you the maximum fuse size ahead of them right on the label, but it's probably a lot higher than you need.

 

[mrmike]

Do Away with those fuses- They can create problems, like Single Phasing which can burn up your motors. You have already got the O/L for each one so I would also install a sized breaker for each motor to be on the safe side..............

 

[sii]

That's the plan in the future....no time now, gotta make it run.

 

[Smart $]

Assuming you are using those IEC starters with motor protection switches, total FLA x 1.25, minimum but not to exceed the rating of the bus bar. Most of those feeder bus bar systems are rated 63A minimum so it shouldn't be an issue, just double check. Some of the cheap ones are less, maybe 32A.

So using NEC charts, 1.4A + 2.6A + 3.4A + 3.4A + 14A = 24.8A, x 125% = 31A. I would fuse it at 35A because you don't want the inrush of the 10HP motor to pop your fuses. Those Motor Protection Switches are the motor SCPD, so all the fuses are for is "group fusing" and branch protection for the bus bars. The MPS devices themselves will tell you the maximum fuse size ahead of them right on the label, but it's probably a lot higher than you need.

For a single multi-motor machine, I thought it was 125% of largest motor's FLC plus 100% of the others'.

1.4A + 2.6A + 3.4A + 3.4A = 10.8
14A ? 125% = 17.5A
10.8A + 17.5A = 28.3A

 

[iwire]

Do Away with those fuses- They can create problems, like Single Phasing which can burn up your motors. You have already got the O/L for each one so I would also install a sized breaker for each motor to be on the safe side..............

Even with a blown fuse if the OLs are sized properly they should open the circuit before the motor is damaged.

You can also add phase monitors if you wish, fuses are still very popular choice for motor protection.

 

[millelec]

Even with a blown fuse if the OLs are sized properly they should open the circuit before the motor is damaged.

You can also add phase monitors if you wish, fuses are still very popular choice for motor protection.

I like the phase monitors for critical applications. Have seen motors w/a common power supply run "single" phase for an extended period of time due to the regenerative voltage created by the motors when lost a phase on incoming line power. They don't like it.

 

[weressl]

I inherited a machine which has five motor starters all bussed togethere across the top of the OL unit. Originally the five motors were .75, 1.5, 2, 2, and 3 horsepower and the fuse feeding all five starters was specified to be 30 amps. I have replaced the 3 hp motor with a 10 hp including the proper OL unit and contactor. I am unsure of how to properly size the fuse now. Do I size the fuses by figuring the total horsepower? If that's the case, 30 amps seems appropriate given the total hp before I made the changes.

Voltage is 480 FWIW.

Thanks in advance.

I am not sure you're aware that any of this modification will likely void the listing of the equipment!?