Hi,
What size wire and breaker are needed for a 5Hp, 20Amp 230 V motor on an air compressor? Also, what is the best way to wire between the pressure switch and the panel box when the compressor will be 10-15 feet away? I am thinking a plug and a receptacle...

Thanks

Frank

Please email answer to:
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The electrical code Motor Full Load Currents Table 430.248 give the full load of a 5 hp, 230V, single phase motor as 28 amperes...
If the motor is a single phase motor then 25 amperes is too small for a circuit breaker.
There is information on the plate affixed to the motor that we need before we can offer the most accurate information...
Can you provide all of this data?

I wouldn't even bother replying until the OP comes back (not likely) with the actual amperage of this motor.
"A 5Hp, 20Amp 230 V motor on an air compressor" could mean so many things in today's world.

I get the impression that the OP dropped this post on multiple boards and wants everyone to hand deliver the correct answer to his e-mail inbox.

As noted by Hrcarson, Sizing motor branch circuit conductors, disconnecting means, overcurrent protection, short circuit protection, controllers, switches, shall use the Tables in Article 430 .

Not the nameplate full load current.

The FLC listed on a nameplate is only used to size overload protection.

Thing is, many of these compressors do not have a HP rating on the plate.

So, it occurred to me, now that I have more time, that it did not appear anyone answered the question.

Referring to the 2008 edition of the NEC, Using Table 430.248 a 5 HP 230 volt motor amp rating is 28 amps. Wire is to be 125% of 28= 35 amps

#10 copper THHN is rated 35 amps, but cannot exceed 30 amp CB rating, per 240.4 (D).

So, 2- #8 THHN and 1- #10 green in conduit, or #8 2 wire with ground Romex cable on a min of 35 amp breaker, a 2 pole 40 will be allowed, per Table 430.52.

The pressure switch should be wired to the coil of a magnetic motor starter, with overload protection, unless OLP is built into the motor, to start the motor.

Now Stan picked up on that this may be one of those air compressors that give false ratings, fake HP based on the Locked Rotor Amps of a smaller motor, to impress the buyer. It could have a 20 amp 240 volt male plug on the cord, but those usually have the pressure switch pre wired.

So I have to believe the unit is a true 5 HP motor. But if that is not true, then some input from Frank would be helpful, no, required. I keep trying to highly suggest, that if the question does not make sense, or can have multiple scenarios, then ask for more information.

Technically, the rules at this website we all agreed to is if an answer is incorrect, the use the Report Inappropriate Post button, and report it. Perhaps this should be utilized more often. Keep this bickering and sniping back and forth going will bring Admin of the site down on this forum.

Not sure if Frank has been turned off by the conversation going on here, or has not checked back and waiting for an email.

So, I have deleted any posts I deemed unproductive to the question. Everyone go back to their corners, and cool it. We all have different backgrounds and experience due to opportunities, chance, culture, location, etc. I, personally, am one smart electrician, and have a vast. Wide spanning career, but I learn something new, frequently. Does not make me any better than anyone else.

I am opening this thread for further, gentlemanly, discussion.

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Now Stan picked up on that this may be one of those air compressors that give false ratings, fake HP based on the Locked Rotor Amps of a smaller motor, to impress the buyer. It could have a 20 amp 240 volt male plug on the cord, but those usually have the pressure switch pre wired.

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I said this because of this:

Quote:

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5Hp, 20Amp 230 V motor on an air compressor

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A 5HP 20A motor just does not jive, right?

Quote:

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#10 copper THHN is rated 35 amps, but cannot exceed 30 amp CB rating, per 240.4 (D)

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Keep in mind, 240.4(D) basically goes out the window for motors, A/C's and welders.

I double checked, but can miss something,can you show where?

240.4(G) & Table 240.4(G) Specific Conductor Applications

I knew that dammit, did not find it. In real life I would have used #10, but trying to be accurate with Codes here answering. Got a little distracted. So went conservative.

Thanks Stan.

OK, #10 wire with a 35 or 40 Amp CB is allowed.

However, my point is using the Tables of Article 430 is the correct method of sizing these motor circuit conductors.

Any other comments or criticism from anyone?

For resi work I don't like to apply 240.4(G). You never know what someone will put there later.

Commercial/industrial you had almost better use it because the next guy will and his bid will be a lot lower.

With bid work, absolutely, I agree. I do some bids, but often still get hired due to reputation of company, workers, quality of work, etc. I do lose bids to low bidder, even with repeat customers. I tell them, pay me now or pay me later. So often I get hired later to fix the low bidders work.

The Code can be your friend when you know how to use it.

I tend to not use 240.4 G when it comes to #14, #12, and #10, as I am only in the commercial and industrial climate, and not much bid work. Sure I give prices for various projects, but not competitive bids. When I design and connect a datacenter server, medical equipment in a hospital, a paper machine, a brick making plant, security lighting, etc, it needs to work reliably without any problems. Last I want to worry about is exceeding the amp rating of conductor.

And this application Frank asks about is a very short distance. Seldom do I encounter short distances, You should see where they put some electrical panels and rooms. I got a Walmart that has 100 feet of wire just to get from electric room to the edge of the sales floor, then a hike to the load.

So, Frank, if your still listening, wire can be 2- #10 for the hots, and 1 #12 green for the ground.