Trying to plan a project where I will take a Hoffman enclosure that will house two motor starters that control 1 50hp motor, and 1 20 horsepower motor that goto a mixer. The feeder will come from a 600a 480V MDP. Without getting to deep into distances, what is the best way to start figuring my cb size? I would like to also have a cb in the hoffman enlosure with a rotary handle on the outside. So do I size enclosure CB the way the code says for multiple motor loads? Should I oversize my MDP CB? Just trying to figure out the best coordination.
Multiple Motor Branch circuit and circuit breaker selection....
- Thread starter gk351
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kwired
Electron manager
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- NE Nebraska
Take a look at 430.53. You need separate branch circuit short circuit and ground fault protective device for each motor sized to 430.52. An overcurrent device that will allow both motors to start without tripping will likely not comply with 430.52.
You may run a feeder to supply both motors and put two overcurrent devices in your hoffman enclosure. They may be allowed to have a single disconnect but must have separate overcurrent protection.
Distance has nothing to do with circuit breaker size, but may affect voltage drop and require larger conductors - the breaker size should remain the same no matter what size the conductor needs to be.
So I couldnt run a feeder from the MDP to the Hoffman enclosure which would have an inverse time breaker that was calculated for both motors. On the load side oif that breaker run to a isolated lug assembly and run appropriately sized taps to each of my starters?
( Trying to watch a National Championship game and read the NEC is hard!!)
( Trying to watch a National Championship game and read the NEC is hard!!)
kwired
Electron manager
- Location
- NE Nebraska
Your circuit breaker will be sized too large if you want the motors to start without tripping the breaker, especially for the smaller motor, which is only 40% of the size of the larger motor.
Read 430.52 and 430.53
Read 430.52 and 430.53
kwired
Electron manager
- Location
- NE Nebraska
After doing some reading, here is what I have. If the feeder CB is rated for the LRC of both motors, which I have figured it would need to be 200 amp. The total LRC is 508A. I use 430.251(B) and find that 75hp equivalency is needed. I used square d digest motor table to find that 200A cb I mentioned earlier. Now would it be permissible For me to use the same breaker at the Hoffman enclosure coupled to a rotary disconnect handle so I have disconnecting means at the enclosure. I know I have double protection with there being 2 CBs. Then I could run appropriate sized taps to my motor starters that are in the same encloseure(taps would be less than 10 ft).....if this doesn't work, I will just use separate combo starters I suppose. I just want to make one control panel where they could control everything from one enclosure with one disconnecting means.
kwired
Electron manager
- Location
- NE Nebraska
You appear to be set on putting both these motors on one branch circuit, I have mentioned at least three times that you can not do this because of 430.52 and 430.53. If you find a way around these sections please point it out to all of us.
Most of what you are trying to justify this with is acceptable for the feeder supplying this. The two motors still need individual branch circuits tapped somehow from the feeder.
Your best installation is the separate combination starters supplied by common feeder.
Most of what you are trying to justify this with is acceptable for the feeder supplying this. The two motors still need individual branch circuits tapped somehow from the feeder.
A legal tap has an overcurrent device at the load end of it not a motor controller.
Your best installation is the separate combination starters supplied by common feeder.
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