7.5hp 220v motor, need help with breaker and wire size


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Old 11-15-07, 11:36 AM
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7.5hp 220v motor, need help with breaker and wire size

Hey all. Found this wonderful forum and I need a second opinion. Here is the scoop. I just bought a large air compressor that has a 7.5hp single phase 220v motor on it. The FLA shown on the motor tag is 32A and the Serv Factor is 1.15. I plan on running a dedicated line from the main breaker box to the garage, where i will have a nonfusible disconnect switch for this compressor. The run length will only be 20' and will probably be a 5 minute job to run the wire (got lucky!). What would be your suggestion on a breaker size and wire size/type? I was told by someone that a 60A dual pole breaker should be used and that i need to run #4 AWG copper wire from the breaker box to the disconnect switch, and from the disconnect switch to the magnetic starter box on the compressor. I kinda like the idea of using an outlet and plug to eliminate the clutter of the disconnect switch, but all i could find were 50A plugs made for ranges. Would these not work? Also, is there a code requirement for the height of an outlet or disconnect box in a garage? I would like to stay within code. I'm located in central Illinois.

While i'm on this topic, I'll definitely need the two hot wires for the 220V. I assume i'll need to run a ground wire between the breaker box and the disconnect box, as well as between the disconnect and the magnetic starter box? Should this be strand or solid wire and what size? I won't need the typical white wire, will I? Thanks for the tips and help!
 
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Old 11-15-07, 12:21 PM
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For 7.5 HP 1 240 v motors i useally used the 50 amp breaker with #6 wires

you did mention maganatic starter so you will have overload protection there if have it.

20 feet is pretty short distance and is this location is in sight to see the breaker box from air compressor location ??


If so you dont need to put a disconnect switch at all. But if have divied wall or any partaion wall or something that cant see it direct then you have to put in the disconnect switch.

this is speficed in the code.

you say #4 wire IMO it is allready overkill on that size dont need that big unless you have very long run [ more than 80 feet away one way ]

for the repectale for the air compressor btw the 50 amp repctale / plug combo IMO it dont need it because you are very close to the breaker box , second thing that most 50 amp 240v plugs are limted on HP rating [ if that dont show the HP rating you cant used that ]

is this loaction is attched or detached garage and also is the subfeed box there or not ??

this will help us more to do the proper way.

Merci, Marc
 
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Old 11-15-07, 12:28 PM
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First of all, thanks for the response! The compressor will be in an attached garage. Basically, i have the wire ran for my previous compressor but the wire and breaker is too small. If i remove the outlet and box for my old compressor and look in the hole, i am looking along the basement wall that the breaker box is. I'll just feed the wire along the sill area till i reach above the breaker box and then down into the box. Can't see the breaker box from the garage, so some kind of disconnect is needed to comply with the code.

I initially thought the #4 wire was overkill, which was why i decided to ask for a second opinion. Not trying to be a an el cheapo, but the price of #4 wire isn't cheap if you consider the number of wires i'll be running. But then again i don't want any startup problems with the compressor, nor do i want the wires to overheat and cause a fire. The compressor is an 80 gallon unit, so it is quite large. I'll be doing some sandblasting, so it may cycle more than i think it will.

And yeah this compressor has the magnetic starter box. It uses two heaters from what i'm told. It is similar to the picture below, but not exact.

 
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Old 11-15-07, 02:19 PM
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I agree that #6/2 cable and a 50A breaker should be okay given the distance. A disconnect box is required because the breaker is not in line-of-sight, but a receptacle is not required. Just use a flexible metal cable whip or #6 THHN installed in flexible conduit for the last few feet between the disconnect and the motor. If you do want a receptacle, you'll need to use one of the very expensive industrial 50A receptacles that are rated for motors.

The 50A breaker will protect against any wire overheating. Many local codes require at least 18" off the garage floor for any receptacles or switches to avoid sparks if there are low hanging gas vapors.
 
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Old 11-15-07, 02:57 PM
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Check the coil voltage on the starter, you may need a neutral.
 
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Old 11-15-07, 04:37 PM
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Originally Posted by classicsat View Post
Check the coil voltage on the starter, you may need a neutral.
You have a point there but normally most air commpressor i ran into have few diffrent voltage in the coil.

most common is 24, 120 , 240 volt and few others as well

but normally for 1 phase useage i will never used the netrual for coil useage because one main reason that if one of the leg go out for some reason and the contractor will not open.
that one reason why i always hook up the coil line to line unless this is a strictally a 120 v device.

The other thing with magatic switch make sure you have proper heater[s] [ for the overload protection ]

it will be listed in the starter cover it will tell you the correct heater size and you have single phase set up some will required 2 some will use 1 heater so check it out carefully with this part.

sized the heater just a hair over the full load amp on the motor like you say motor rated at 32 amp find a heater setting for 32-35 amp range normally this will work fine with this set up.

Merci, Marc
 
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Old 11-15-07, 05:04 PM
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Originally Posted by jbuening View Post
While i'm on this topic, I'll definitely need the two hot wires for the 220V. I assume i'll need to run a ground wire between the breaker box and the disconnect box, as well as between the disconnect and the magnetic starter box? Should this be strand or solid wire and what size? I won't need the typical white wire, will I? Thanks for the tips and help!
Most of the time stranded wire is easier to run, but more annoying to terminate. Most of the time it's personal choice. A white wire is a neutral though, the ground you will need should be either green or bare. It should be 10AWG, which probably means green.
Given the size wire and length of run, I'd use 6AWG range cable. Chances are you could get that for about what it'd cost to run separate conductors, and you'd get a neutral too. Just in case it might come in handy at some point.
 
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Old 11-16-07, 05:40 AM
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Excellent advice so far! Let me summarize what i've interpreted from all of the replies:

- Use either #6 AWG or Romex (does Romex have lower amperage rating? Would it be cheaper?)
- Use either a 50A or 60A dual pole breaker (should it match the disconnect amperage?)
- Run wiring through conduit at all times, hard conduit up to disconnect and flexible from disconnect to magnetic starter.
- Use either 50 or 60A nonfused disconnect switch (think i can only find 60A at Lowes)
- Ground wire is definitely needed between breaker box and disconnect, and from disconnect to magnetic starter. Stranded or solid is acceptible
- The white neutral wire is not needed? Therefore i will need to purchase two power wires and a ground, or get Romex which will have the extra neutral wire just in case it's needed in the future? Should i check my coil??
- I'll need to check the heaters that are in the starter. One is original and the other was replaced by the previous owner. Need to buy a few backups in case one goes out on me. This starter uses two heaters, whereas the three phase units use three (from what i'm told, one for each hot wire)


I think that is it. Now i just have to figure out a way to attach the conduit to my concrete basement wall and get all of the proper fittings
 
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Old 11-16-07, 03:05 PM
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#6 Romex has the same amperage as individual #6 conductors, with the additional advantage that individual conductors have to be run inside some kind of conduit protection in any case. Whereas Romex can just be stapled up wherever handy. Putting Romex cable inside conduit makes it more protected, but as long as it's indoors and not vulnerable to physical damage, it can usually be run on its own.
You want to use a 50A breaker on #6 wire, #6 isn't rated for 60 amps other than industrial motor applications and the like. However the disconnect can be 60 amps since it's being protected by the breaker... that 60 is really just a maximum for the box anyways.
The neutral you'd need to check the unit to be certain on. It's likely that buying the cable with a neutral won't cost more than buying three separate individual conductors. Retailers put a higher margin on single wire than on cable, since it costs them more labor to cut three lengths of wire than one length of cable. And at the retail level, cable is more competitive.
 
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Old 11-17-07, 07:48 AM
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I just noticed in my compressor's manual that it states to use a minimum of 60A breaker. I've already purchased the #6 wire and the 60A disconnect. Do you think the 50A breaker will be fine given the manufacturer's recommendation? Will i have wire overheating problems if i use the #6 romex-type wire and a 60A breaker? The wire i bought says "Type NM-B 6/3 with Ground"
 
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Old 11-17-07, 08:38 AM
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I think that either the 50A or 60A breaker is acceptable. If the motor will not start with the 50A, bump up to 60A. When you're dealing with a motor circuit, the traditional wire and breaker sizing rules do not apply because the motor has thermal overload protection which will prevent overheating. The breaker provides only short circuit protection, so it may be sized much larger than the wire would otherwise allow.

In either case, #6/3 cable is rated for 55A and the code allows you to round up to the next common breaker size which is 60A. The motor FLA is 32A which is well within the ampacity of #6.
 
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Old 11-17-07, 09:35 AM
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Excellent, because i had already bought the 60A breaker and was about to go back to exchange it. What is typically done when you have conduit going through a wall (in my case from the basement into the garage) and then up to a disconnect box? This garage has wood plywood behind the drywall, so it would be a huge pain to put the conduit in the wall. Would a simple 90 pull elbow work at the point where the conduit exits the wall? Wasn't sure what the code requirement is on this and i don't see how any kind of box would help in this case. I'm almost there
 
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Old 11-17-07, 10:59 AM
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When installing Romex in conduit, a simple "LB" fitting will make a clean 90 through the wall to vertical.
 
 

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