New 7.5hp Compressor!! Breaker/wiring size help needed - Vintage Mustang Forums

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post #1 of 53 (permalink) Old 11-12-2007, 11:53 AM Thread Starter
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Just bought a gently used IMC Bel-Aire 80gal two stage air compressor with a Baldor 7.5hp motor on it. It's a pretty stout unit that puts out 25.25 ACFM @ 100psi and 22.28 ACFM @ 175psi. Pump is spec'd at 35cfm displacement. Mine is the 318VLE model, which has the automatic drain, low oil level switch, and magnetic starter. I'll get some pictures soon, still have to figure out how to get it out of the back of my pickup

Link to the company's website for those interested: IMC Belair

Haven't heard much about them, but they are pretty darn expensive when new. Example



I need help in choosing the size of wire and breaker for this monster. The plate from the Baldor reads as follows (only pertinent info):

Amps: 32 - 29
7.5 HP
3450 RPM
PH 1
V: 220
Serv Factor: 1.15
NEMA Nom Eff: 85.5%
PF 98%

Not sure if the last two were needed, but figured what the heck. The power box on the compressor has two heaters, not sure if that is relevant either. Based on my research online, people tend to use 40-50 amp breakers with a 7.5hp compressor motor. Haven't found much with wire size though. The length of wire run is about 15' max, probably less so the conversion will be really painless. I have a little 20A and 12/3 wire running to my current little compressor, obviously not large enough. Thanks


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post #2 of 53 (permalink) Old 11-12-2007, 01:34 PM
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You want the good or bad news? On my table, NEC 430.248, a 7.5 HP single phase motor has a full load amp rating of 46 amps with a locked rotor amps rating of 480 amps ( the amount of current it draws when staring up) on table 430.251A. The circuit condutor must be 1.25% of the FLA or 57.5 amps which leaves you using #4. The circuit breaker must be 250% of the FLA which comes to 115 amps and since they don't make a 115 amp breaker, use a 125 amp breaker. That is if it's inverse time delay, if instantanous trip breaker, you'll have to go 800% of FLA or 368 amps!

Copper is getting expensive and you'll need 1" conduit. The main purpose of the circuit breaker is to provide short circuit protection, that's why you'll see high amp rating, this covers the inrush of amperage when the motor starts. The motor has it's own over load protection, again that's why you'll have such a high rating on the circuit breaker. The circuit breaker will work very fast on a short circuit!

Tom

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post #3 of 53 (permalink) Old 11-12-2007, 01:53 PM Thread Starter
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Holy hell! That is nuts. So what about a 7.5hp with 32 amps? I was trying to follow this post but there was a lot of jumping around on which to use. His motor is a 10hp unit at 46amp, whereas mine is 7.5hp with 32. From what i gather on that post, the 90A dual pole breaker was recommended with #6 THHN wire.

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post #4 of 53 (permalink) Old 11-12-2007, 02:40 PM
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I believe the locked rotor amps are actually 265

And table 430.52 is the MAXIMUM allowable - not the requried - plus the 800% is for instantaneous trip breakers, not inverse-time (thermal-mag). Inverse-Time is max 250%.

As a rule of thumb here, we generally size motor over-current at 175% to 200% here in our office.


If it were me Jeremy (ha!), I would size it at 175% of 32-Amps, which would equate to a 60-Amp. And run the #4-AWG conductors. I think you will be fine.

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post #5 of 53 (permalink) Old 11-12-2007, 02:53 PM
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Quote:
Originally Posted by 70_Fastback
I believe the locked rotor amps are actually 265

And table 430.52 is the MAXIMUM allowable - not the requried - plus the 800% is for instantaneous trip breakers, not inverse-time (thermal-mag). Inverse-Time is max 250%.

As a rule of thumb here, we generally size motor over-current at 175% to 200% here in our office.


If it were me Jeremy (ha!), I would size it at 175% of 32-Amps, which would equate to a 60-Amp. And run the #4-AWG conductors. I think you will be fine.
Thanks for checking. Man I blew this one! 480 amps...@ 115 volts. At 230 volts it's 240 amps and 208 volts it's 265. And you're right, up to 250 or 800% percent, not required. I guess I was in a little too much of a rush. I was also going to say and forgot to was to mention was to go by the 32 amp name plate rating as well.

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post #6 of 53 (permalink) Old 11-12-2007, 02:59 PM Thread Starter
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Cool. So run with a 60 amp dual pole breaker and #4-AWG wire? Isn't #4 a bit large for only 60 amp? I'm no EE, just making sure. That other post mentioned #6 THHN (which i have no idea what that means) wire.

Might have to do some digging to find that size wire. Not sure if the local Menards or Lowes has anything that big.

One other question, do you guys wire in a 220V extension from the compressor switch box to an outlet or do you hard wire straight to the electrical box? This compressor didn't come with any kind of plug-in, so i assumed the PO had it hard wired. I have the hole in the wall for the outlet, but wanted to make sure there wouldn't be any large losses with using a outlet. Thanks!!

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post #7 of 53 (permalink) Old 11-12-2007, 03:02 PM
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LOL - that's what you get for me being slow at work today! J/K!!

Plus with a service factor of 1.15 - doesn't that classify his motor as "continuous-Duty" [430.32 (A)(1)] which could allow for an overload device trip rating of 125%, with modifications per 430.32(C) ?

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post #8 of 53 (permalink) Old 11-12-2007, 03:03 PM
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I've seen both. I'd place air lines throughout the garage so you only move the tank once. Hardwire it in there and go play...
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post #9 of 53 (permalink) Old 11-12-2007, 03:09 PM
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I would terminate in a 60-Amp, non-fusible disconnect switch. I think that may be cheaper than buying a 60-Amp Plug & a 60-Amp receptacle.

But might want to do a cost comparison still just to double check. And I know Home Depot carries #4-AWG.

(AWG is American Wire Guage - the size. And THHN is the type of insulation on the conductor.)

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post #10 of 53 (permalink) Old 11-12-2007, 03:12 PM Thread Starter
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Quote:
Originally Posted by 70_Fastback
Plus with a service factor of 1.15 - doesn't that classify his motor as "continuous-Duty" [430.32 (A)(1)] which could allow for an overload device trip rating of 125%, with modifications per 430.32(C) ?
:flies over my head: Sure Jeremy, whatever you say i guess I'm a structural who could barely pass circuits class :p

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post #11 of 53 (permalink) Old 11-12-2007, 03:19 PM Thread Starter
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Quote:
Originally Posted by 70_Fastback
I would terminate in a 60-Amp, non-fusible disconnect switch. I think that may be cheaper than buying a 60-Amp Plug & a 60-Amp receptacle.

But might want to do a cost comparison still just to double check. And I know Home Depot carries #4-AWG.

(AWG is American Wire Guage - the size. And THHN is the type of insulation on the conductor.)

OK. I'll have to check these non-fusible disconnect switches. Is this what you speak of? http://www.lowes.com/lowes/lkn?actio...0TR&lpage=none

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post #12 of 53 (permalink) Old 11-12-2007, 03:20 PM
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And I could possibly be interpreting it incorrectly though too...

Jeremy



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post #13 of 53 (permalink) Old 11-12-2007, 03:28 PM
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No, I personally don't like those plug style. I think that is a light duty switch - I would opt for the general duty, or heavy duty, levered disconnect switch.

I like these: Square-D

Jeremy



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post #14 of 53 (permalink) Old 11-12-2007, 03:35 PM Thread Starter
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Ok, so is this a box that mounts on the wall? That link didn't bring any certain one up.

If you look at http://www.abacamerican.com/318VLE%2...high%20res.pdf you will notice there is a box mounted to the tank. This has the two heater fuses, a starter coil (i think that's what it's called), and a breaker/reset switch that is accessible from the outside of the box cover. Just making sure you were aware of this, as it appears the "disconnect switches" are similar to this box, no??

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post #15 of 53 (permalink) Old 11-12-2007, 03:52 PM
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Crap... sorry

http://www.squared.com/us/products/s...switchpage.htm

and I think the box on yours is the pressure switch and contactor, etc. Not a disconnecting means. You want the disconnect locally so you can service the unit with it de-energized. But with your panel with-in line of site.... you could just hardwire it... and be ok.

Jeremy



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