air compressor hook up ?

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J

jes25

Senior Member
Location
Midwest
Occupation
Electrician
Ok guys I need you to see if you can find any problems with this install. I do 99% resedential so I am a little unsure. I have a compressor to hook up
230 volt
7.5 hp
single phase
32 A

It is used and has a motor starter already on it it is a furnas (cat. no. 16cf15 alu). I plan to use no. 8 thhn (voltage drop is not a factor) and hook it to a 40A brkr.(32A * 1.25 = 40A) I will have a non fused disco. within sight.
sound good?

Does the motor starter have thermal protection in it? It does have a reset button on it. How do you know when you need a motor starter? Like I said it has a reset button on it does that take the place of the overloads ( heaters) Thanks for the help. Sorry for my ignorance. Any general htips for hooking up motors would be appreciated as far as the motor starter/ overloads go.
 
Re: air compressor hook up ?

Table 430.148 gives the Full Load current value for a 7.5HP single phase 230V motor as 40A. No. 8AWG conductors are OK if terminations are rated 75C. The breaker used for short circuit and ground fault protection can be 100A per 430.52. The reset button may be for the thermal overload relay in the starter if the motor does not have internal thermal protection. Make sure they are sized for the motor FL amps. Use the nameplate value to calculate per 430.52(A)(1)
 
Re: air compressor hook up ?

Take a look at Table 430.148 which shows the full load current at 40 amps for a 7 ? HP single phase motor @ 230 volts. You will need to use branch circuit conductors rated for at least 50 amps. As John said if all your terminations are rated for 75C the #8?s would be fine but you will need to use a larger circuit breaker (see Table 430.52). The starter should have thermal elements to protect the motor from overload but you should verify that they are the correct size.
 
Re: air compressor hook up ?

A little embarassed I ask how do I check if they are the correct size?
 
Re: air compressor hook up ?

If you have interchangeable overload elements (typical of NEMA starters), you will have to check their numbers against the manufacturers ampacity chart. This chart is sometimes found inside the cover.I am not familiar with the Furnas equipment. If it is an IEC type it may have a dial that covers a certian amperage range.
 
Re: air compressor hook up ?

I checked the Seimens-Furnace catalog and Yes you do have a starter. According to the catalog this is a magnetic "Definite Purpose Control" device rated at 40 amps at 240 volt. Yes these do require Bimetal type of overloads in them. I would think that if the compressor came with the starter unit attached then the overloads have already been sized for the compressor load. It wouldn't hurt to check for the proper sizes, but it should be ready to hook-up.
 
Re: air compressor hook up ?

It's got thermals protection if there is a reset button on it. Therefore, you do not need overloads. You can run #8 wire. I would probably settle for a 50A breaker, but see if it holds. As far as the chart in the code for a 7.5 hp motor, use that if there is no nameplate. There is a nameplate on this compressor with a smaller amperage, because we know the load. And with that load they can determine that the motor will only draw 32 amps. I'd go for a 50A breaker. As mentioned, you can go 2.5 times 32 amps = 80 amps(430.52), which is legal, but most likely too high. The closer you get without nuisance tripping, the better.

Goodluck
 
Re: air compressor hook up ?

Thanks Guys with your help and a little research on my own I am confident I got a good job.

I used to do a commercial when I was an apprentice but never "ran" any jobs. Now as a contractor I would like to do light commercial but I am a little unsure of myself at times this website really helps me. I try to be there with the inspector when I am a little unsure of myself to make sure he looks it over good, so I can learn that way as well. Its funny, I am a licsensed master but I have a lot to learn. Thanks again. :)
 
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