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Old 01-14-2013, 12:20 PM   #1
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7.5 hp motor


I am looking at wiring up a 7.5 hp 230 volt 31 amp motor in my garage for a steamer. I beleive I will need #8 copper to wire it up with a 75 amp breaker. I would like to be able to plug it into the wall. What should the amperage of the receptacle be? My thinking is if I sell the place and someone decides to plug in a heater or something that it could be a problem.

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Old 01-14-2013, 12:55 PM   #2
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According to the NEC the motor need will conductors rated for 40A. #8 copper will comply. The breaker can be as high as 100A. You really need to consult the manufacturers installation instructions for what they require. If you decide to cord and plug connect it, the plug and receptacle have to be horsepower rated.

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Old 01-14-2013, 01:12 PM   #3
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Thanks I appreciate your quick response. Just so I am clear the plugin 15-50R horsepower rated would comply with the code?
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Old 01-14-2013, 01:16 PM   #4
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7.5 hp motor


The conductors need to be 1.25 x the listed amperage in the NEC motor tables for single phase motors. So a single motor branch circuit would need 1.25 x 40 amps = 50 amps. I you use thhn individual conductors in a listed conduit like emt #8 awg copper will work. NM-b cable will not comply with 50 amps.

I doubt you will need the maximum amperage breaker which is 2.5 times the table value of 40 amps or 100 amp inverse time breaker. The idea is to hold the start-up current long enough for the motor to return to running amps under load. Anyway I would be surprised if you needed anything over 50 amps to start the motor though 75 will work if that is what you already have.

The branch circuit is rated by the breaker installed on the circuit so if you stick to a 50 amp breaker you could find a 6-50 receptacle and plug at the store and not have to find a special plug at an electrical supply.

What is the method for starting and stopping the motor?
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Old 01-14-2013, 01:19 PM   #5
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Thanks I appreciate your quick response. Just so I am clear the plugin 15-50R horsepower rated would comply with the code?
Are you talking about a 3 phase motor ? If you are then we need to rethink the previous replies?

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Old 01-14-2013, 01:26 PM   #6
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Thanks I appreciate your quick response. Just so I am clear the plugin 15-50R horsepower rated would comply with the code?
You have to check with the manufacturer and find out the horse power rating. A 15-50R is HP rated for 3 HP. A 6-50R is HP rated for 3 HP. I can't find anything rated for 7.5 HP, 1 ph. You may have to hard wire it.
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Old 01-14-2013, 01:37 PM   #7
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7.5 hp motor


It is single phase 240 volt 7.5hp 31 amp
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Old 01-14-2013, 01:40 PM   #8
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I checked the Hubbel website and the largest 1ph plug/receptacle HP rating is for 3HP.
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Old 01-14-2013, 01:42 PM   #9
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You have to check with the manufacturer and find out the horse power rating. A 15-50R is HP rated for 3 HP. A 6-50R is HP rated for 3 HP. I can't find anything rated for 7.5 HP, 1 ph. You may have to hard wire it.
Thanks electures, I was looking at the 3 phase receptacles not the 1 phase.
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Old 01-14-2013, 01:43 PM   #10
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7.5 hp motor


I think you should use an inexpensive manual starter. That way you'll have motor overload, short circuit protection, and a disconnect all in one.
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Old 01-14-2013, 06:31 PM   #11
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7.5 hp motor


Quote:
Originally Posted by electures View Post
You have to check with the manufacturer and find out the horse power rating. A 15-50R is HP rated for 3 HP. A 6-50R is HP rated for 3 HP. I can't find anything rated for 7.5 HP, 1 ph. You may have to hard wire it.

This is why I'm asking what he is going to use to start and stop the motor. The receptacle and plug only needs to be horsepower rated to equal the motor ratings if it is the disconnecting means.

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