Feeding two motors from same breaker

[mark sceeles]

Recently I ran across a installation that was two blower motors, each independent systems--that were fed from the same circuit breaker--the voltage is 460 3 phase with each motor having its own mag starter--motors are 1 HP each. Is this installation per NEC?

 

[augie47]

Possibly.
The starters take care of the overload so the breaker is for ground-fault/short circuit protection.
The smallest standard size breaker is a 15 amp, so if you have one 15 amp breaker it is allowable for each individual motor.
Once you exceed 15 amp, the story changes.
You should, however, check the specs for the magnetic starter. Those may call for a smaller short-circuit device and if not installed it could be viewed as a violation of 110.3(B)>

 

[kwired]

To see where Augie is coming from with some of his information see 430.53.

 

[spark master]

How is this illegal? Please explain. I am installing 2 machines. --> 2 disconnects from 1 breaker. I am mounting a gutter above the disconnect, and splicing in the gutter, to go from disconnect to disconnect. #6 wire, on a 60 amp breaker. Each machine has it's own fancy control panel. Each machine has a 45 amp name plate, but the running amps is much less. and they will never run at the same time. Even if they did run at the same time, it's not continuous.

 

[kwired]

How is this illegal? Please explain. I am installing 2 machines. --> 2 disconnects from 1 breaker. I am mounting a gutter above the disconnect, and splicing in the gutter, to go from disconnect to disconnect. #6 wire, on a 60 amp breaker. Each machine has it's own fancy control panel. Each machine has a 45 amp name plate, but the running amps is much less. and they will never run at the same time. Even if they did run at the same time, it's not continuous.

What kind of assurance is there that they will not run simultaneously, any interlocking controls, otherwise an overcurrent device sized at 250% to allow for starting could still allow starting the second unit if starting current isn't all that high. Then you have a possibility of two motors drawing full load current on a conductor only sized for one motor.

 

[spark master]

And thus the breaker would trip. What's to stop someone from using 2 1800watt hair dryers in a bathroom. Is there anything illegal about it ? My experience with these imported machines, is the name plates are way over rated. The motors have a fancy VFD control panel, with a PLC, and they never run at full speed. The current draw is about 1/2 of name plate, with a few spikes here and there. Another example of my thinking: If you add up all the breakers in any panel, you are certainly going over the main breaker size. My house for example has 42 breakers in 1 panel. another 12 in a sub panel. If you add up all the breakers, it's 900+ amps running off a 200 amp service. And I know I never drew more than 100 amps at any given time.

 

[kwired]

And thus the breaker would trip. What's to stop someone from using 2 1800watt hair dryers in a bathroom. Is there anything illegal about it ? My experience with these imported machines, is the name plates are way over rated. The motors have a fancy VFD control panel, with a PLC, and they never run at full speed. The current draw is about 1/2 of name plate, with a few spikes here and there. Another example of my thinking: If you add up all the breakers in any panel, you are certainly going over the main breaker size. My house for example has 42 breakers in 1 panel. another 12 in a sub panel. If you add up all the breakers, it's 900+ amps running off a 200 amp service. And I know I never drew more than 100 amps at any given time.

Your examples are all examples of conductors protected at their ampacity. Motor circuits have conductors sized for motor ampacity but the branch circuit fuse or breaker is sized to allow starting current, and motor overload protective device keeps the motor as well as conductor from being overloaded.

In your example, maybe there isn't much of a problem as you said the main feed is a 6AWG protected by a 60 amp breaker. But if you had 10 AWG protected by a 60 amp breaker - which would be possible with a motor that has a FLA of say 28 amps and you then placed two of those motors on one circuit and they happened to run at same time without tripping anything while starting, then that 10 AWG could be loaded to 56 amps and never trip any overcurrent devices.

 

[petersonra]

Your examples are all examples of conductors protected at their ampacity. Motor circuits have conductors sized for motor ampacity but the branch circuit fuse or breaker is sized to allow starting current, and motor overload protective device keeps the motor as well as conductor from being overloaded.

In your example, maybe there isn't much of a problem as you said the main feed is a 6AWG protected by a 60 amp breaker. But if you had 10 AWG protected by a 60 amp breaker - which would be possible with a motor that has a FLA of say 28 amps and you then placed two of those motors on one circuit and they happened to run at same time without tripping anything while starting, then that 10 AWG could be loaded to 56 amps and never trip any overcurrent devices.

but if you had two motors on the same breaker that would either be a motor feeder or a group motor installation and the conductor sizing rules would not allow that small of a conductor.

 

[kwired]

but if you had two motors on the same breaker that would either be a motor feeder or a group motor installation and the conductor sizing rules would not allow that small of a conductor.

I think that is what I said, but in less words:happyyes: