relocating a compressor receptacle

[laser3kw]

here is a variation of "how do I wire a air compressor." This is located on the upper level of our fire department. The compressor is an "Industrial Air" brand and the motor data plate shows 3.7hp and 17.2 fla at 240v (zoom in on motor pic).
The previous electrician ran a 50amp (breaker) three phase (6ga) run from the main panel to the upper level to it's own disconnect. The air compressor receptacle is is wired (6ga) to disconnect.
We relocated the compressor down stairs. and need to relocate the receptacle downstairs, literally, three feet down and 15 feet across the ceiling.
Here is what I purpose: re-fuse the upper disconnect to 20 amp fuses. Run a conduit run to new location with 12 ga wire to a second smaller disconnect with 20 amp fuses and the receptacle.
Would this be an acceptable course? This compressor will cycle several times a day 24/7/365. I drawn on the many cases I have read here and it seems "logical"
pictures of equipment - forgive me if I have misstated anything

 

[Don1357]

50amp is one hell of an overkill for a 15 amp motor.

My understanding is that as long as the breaker protects the weakest link you should be fine, as in if you are putting 12ga wire on that circuit your breaker must be 20amps or less.

 

[Bert_]

#12 thhn should be fine. Should have 30A fuses though, 25A minimum. No need for a second set of fuses.

My understanding is that as long as the breaker protects the weakest link you should be fine

This shows that you do not understand motor circuits. Breaker or fuse is commonly larger than the wire.

 

[laser3kw]

#12 thhn should be fine. Should have 30A fuses though, 25A minimum. No need for a second set of fuses.

the second disconnect and fuses are just for convenience at the compressor site. If I could find a knife throw switch I'd use that, but I have the fused disconnect.
thanks for the response

 

[Don1357]

This shows that you do not understand motor circuits. Breaker or fuse is commonly larger than the wire.

My understanding is that you want the breaker to trip before the wire burns. Please explain how that's wrong.

 

[Bert_]

My understanding is that you want the breaker to trip before the wire burns. Please explain how that's wrong.

In a motor circuit the breaker does not protect from overload. It protects from short circuit only. The motor will have some form of overload protection. That will also protect the wire.

In this application with #12 thhn he is allowed a maximum 30A time delay fuse or 40A breaker.

 

[wyliesdiesels]

the second disconnect and fuses are just for convenience at the compressor site. If I could find a knife throw switch I'd use that, but I have the fused disconnect.
thanks for the response

Get an AC pullout disconnect.

My understanding is that you want the breaker to trip before the wire burns. Please explain how that's wrong.

Breakers for motor circuits need to be oversized due to in-rush current

So the breaker is NOT sized for overload protection. Overload protection for the motor AND WIRE is accomplished with integral thermal overloads in the motor (red button) or overloads in the motor starter...

 

[pattenp]

Sizing the breaker based on the conductor size is normally correct. Oversizing the breaker is allowed for motors when hardwired and the motor has its own overload protection. An outlet circuit used for a motor is not to have a breaker larger than specified for the wire and or outlet size.

 

[nadogail]

Pull out HVAC disconnects very inexpensive and effective.

 

[Don1357]

In a motor circuit the breaker does not protect from overload. It protects from short circuit only. The motor will have some form of overload protection. That will also protect the wire.

In this application with #12 thhn he is allowed a maximum 30A time delay fuse or 40A breaker.

That will work perfectly fine and dandy, until somebody else connects a device to that plug that doesn't have an overload protection. heck a failure of the overload protection of the motor (which has been seen to happen if a jolt fuses it) the undersized cable can burn before the breaker trips.

I'll take my chances and wire according to the breaker. Oh, wait... that's the opposite of what I would be doing, taking any chances.

 

[Bert_]

That will work perfectly fine and dandy, until somebody else connects a device to that plug that doesn't have an overload protection. heck a failure of the overload protection of the motor (which has been seen to happen if a jolt fuses it) the undersized cable can burn before the breaker trips.

I'll take my chances and wire according to the breaker. Oh, wait... that's the opposite of what I would be doing, taking any chances.

Putting a motor this size on a 20A breaker is asking for it to trip. Nobody says you should use a plug.

 

[Don1357]

Putting a motor this size on a 20A breaker is asking for it to trip. Nobody says you should use a plug.

The plate provides the F.L.A. at 17.5amps. You don't need to use the 125% factor when the F.L.A. is provided.

 

[wyliesdiesels]

The plate provides the F.L.A. at 17.5amps. You don't need to use the 125% factor when the F.L.A. is provided.

When sizing wire for motor circuits you’re not even supposed to use the FLA on the nameplate. Code requires the use of the listed horsepower and corresponding FLC current in table 430.248

Technically the plug and receptacle he is using is not rated for the horsepower listed on the motor.

 

[laser3kw]

Get an AC pullout disconnect.
Breakers for motor circuits need to be oversized due to in-rush current

So the breaker is NOT sized for overload protection. Overload protection for the motor AND WIRE is accomplished with integral thermal overloads in the motor (red button) or overloads in the motor starter...

When sizing wire for motor circuits you’re not even supposed to use the FLA on the nameplate. Code requires the use of the listed horsepower and corresponding FLC current in table 430.248

Technically the plug and receptacle he is using is not rated for the horsepower listed on the motor.

Thanks for all the reply's guys. It's exactly why I posted this question.
I considered the AC disconnect but didn't know if it is allowed in other uses.
From what I read (and between the lines):
Re-fuse the upper (3phase) panel to 25amp fuse (30 amp option)
12 ga THHN wire - OK - from upper panel to lower disconnect (AC disconnect or fuse disconnect) The smaller disconnect I have is fused panel (not breaker). If I were to use it, what would be the correct fuse size? 25amp? 30 amp?
Plug / receptacles ok - what would be the correct one for an "3.7hp" (name plat) 17.5 amp FLA ?

thanks again in advance

 

[pattenp]

Code wise there isn't a plug and outlet you'd be willing to pay for to handle more than 3hp. The nema 6-50 is 3hp so that's the best you can get without spending and arm and a leg for a pin and sleeve type plug. Anything over 3hp should be hardwired. The issue is the pulling of the plug as a disconnect when the motor is under load.

 

[sberry]

Using the receipt wouldn't bother me as much as having it underwired. You do gotta wonder sometimes, my steamer is 5 and comes 30 cord and plug. Got it's own switch though.
This thread does demonstrate the need for some more work on the fundamentals of circuit design,,, say,,, the difference between short circuit and thermal,, and maybe even a bit about cord and plug,,,, ha ha

 

[sberry]

30 is probably right, Might be legal to 50 on a 12 wire hard wired.

 

[theoldwizard1]

No one commented on the fact that 3 phase "Y" only gets you 208V ?

 

[sberry]

We got caught up in other stuff.

 

[laser3kw]

Code wise there isn't a plug and outlet you'd be willing to pay for to handle more than 3hp. The nema 6-50 is 3hp so that's the best you can get without spending and arm and a leg for a pin and sleeve type plug. Anything over 3hp should be hardwired. The issue is the pulling of the plug as a disconnect when the motor is under load.

I won't have a problem hard wiring it. The original install used the plug that came with it(?). That is another reason I want to put a disconnect near the compressor. I also don't believe it is a true "3.7hp" (advertised). Data plate on motor states 17.5 FLA. where would that put us as far as a plug would go?

No one commented on the fact that 3 phase "Y" only gets you 208V ?

good point. Again, it has been running this way for several years (right or wrong)

 

[wyliesdiesels]

No one commented on the fact that 3 phase "Y" only gets you 208V ?

Good eye

I won't have a problem hard wiring it. The original install used the plug that came with it(?). That is another reason I want to put a disconnect near the compressor. I also don't believe it is a true "3.7hp" (advertised). Data plate on motor states 17.5 FLA. where would that put us as far as a plug would go?

good point. Again, it has been running this way for several years (right or wrong)

Actually, 17.2a is about right. The NEC lists 3HP FLC as max 17a and the avg 3HP motor has a 15a FLA.

One problem youre gonna run into is that motor is not rated to run on 200v (208v) and thus will pull more amps and run hotter. Does this motor have integral overloads (red reset button)? I would make sure its not running too hot as that will shorten its life...

 

[sberry]

It might have lasted due to short duty cycles?

 

[laser3kw]

Good eye
Actually, 17.2a is about right. The NEC lists 3HP FLC as max 17a and the avg 3HP motor has a 15a FLA.

One problem youre gonna run into is that motor is not rated to run on 200v (208v) and thus will pull more amps and run hotter. Does this motor have integral overloads (red reset button)? I would make sure its not running too hot as that will shorten its life...

yes, the motor has the red reset button.
Moving it downstairs was prompted by two factors: noise - it is near sleeping quarters and heat: in the summer it get hot on the second level

 

[Norcal]

Might be simplest to buy some 60A to 30A fuse reducers rather then do anything with the existing disco, and with 208V, adding a buck/boost transformer to boost the voltage.

 

[laser3kw]

Might be simplest to buy some 60A to 30A fuse reducers rather then do anything with the existing disco, and with 208V, adding a buck/boost transformer to boost the voltage.

That was one of my questions: can I swap he 60amp fuses for 20~ 30 amp fuses in the upper level box? Your comment leads me to believe that the 20 amp fuse won't fit where the 60 amp fuse is.
This gets messy fast.

 

[Norcal]

That was one of my questions: can I swap he 60amp fuses for 20~ 30 amp fuses in the upper level box? Your comment leads me to believe that the 20 amp fuse won't fit where the 60 amp fuse is.
This gets messy fast.

A 60A fuse clip will only accept 35-60A fuses, fuse reducers are a acceptable way to install a smaller fuse then what it will normally accept.

 

[wyliesdiesels]

I would get rid of the upstairs fused disconnect and just install a non-fused AC pullout disconnect downstairs next to the compressor along with a buck boost transformer since the unit is already overheating. running it at rated voltage will help reduce the operating temps...

Too many points of failure and added cost. Remember, KISS

 

[theoldwizard1]

I would get rid of the upstairs fused disconnect and just install a non-fused AC pullout disconnect downstairs next to the compressor along with a buck boost transformer since the unit is already overheating. running it at rated voltage will help reduce the operating temps...

Too many points of failure and added cost. Remember, KISS

Oh yeah !

Shop around on eBay or Craigslist for a used 5KVA 208/240 transformer. The outer case may look like ****, but it is the wiring inside that matters. Used <$200.

 

[Bert_]

Oh yeah !

Shop around on eBay or Craigslist for a used 5KVA 208/240 transformer. The outer case may look like ****, but it is the wiring inside that matters. Used <$200.

No. A buck/boost transformer would be much smaller and cheaper. A 240 x 32v 0.75kva transformer would accomplish the same thing and costs less than $200 NEW.

 

[Bert_]

We know he has 3 phase but I don't think we know that it's 208.

 

[laser3kw]

I would get rid of the upstairs fused disconnect and just install a non-fused AC pullout disconnect downstairs next to the compressor along with a buck boost transformer since the unit is already overheating. running it at rated voltage will help reduce the operating temps...
Too many points of failure and added cost. Remember, KISS

this is the route I want to take.
The plan is to use the existing upstairs three phase, 60amp disconnect, re fuse it to 25 / 30 amp, run new conduit / 12ga from that disconnect to the new downstairs location, install a AC pull out disconnect and possible eliminate the plug / receptacle and hard wire. I will have to look at the buck boost transformer as a near future add on. We need to get this up this weekend. The compressor keeps the air system on the equipment up so they can respond instantly (instead of waiting for on board systems to pump up)

PS - are all FRN-R-** fuses the same length? The current fuses are FRN-R-60, I can get FRN-R-20 (or 25) for the upper three phase panel for the new run.

 

[brewchief]

If there is a possibility that a future replacement compressor might be a 5hp then I would use #10 thhn instead of #12. I try to avoid overdoing things too much in the name of future proofing but this might be a case that slightly larger wire might be a good investment.

Sent from my SM-T510 using Tapatalk

 

[sberry]

Among other things that was something worth considering.

 

[theoldwizard1]

We know he has 3 phase but I don't think we know that it's 208.

Based on this picture

and the fact that there are 2 wires exiting this fused switch, it is highly likely that is 208V coming out of the 3 phases

Altough I guess it could be a "wild leg" delta (less common).

There is no neutral, but none is required.

 

[Bert_]

Based on this picture

and the fact that there are 2 wires exiting this fused switch, it is highly likely that is 208V coming out of the 3 phases

Altough I guess it could be a "wild leg" delta (less common).

There is no neutral, but none is required.

240v delta is common in many areas. It very well might be 208v. But we can't assume

 

[Norcal]

We are assuming that it’s 208V based on the color coding of the conductors but that can be dangerous to assume anything with electrical the OP could verify by checking the voltage, if one of the legs is a nominal 208V then it ain’t 208 3 phase.

 

[sberry]

Could stick a volt meter in to the socket and see if its 240?
I see my local outfit has a couple reels and they served with 3/8 brake line. Just looks dinky. 1/2 would have been acceptable. They should have bought a couple 50 ft air hoses and hooked it up.
They got a nice shack now, it took them a while but the improvements are there. Most of the utilities have been upgraded when they did a new addition and they did a good job of integration with the old. Both locals have pretty good fire halls. They might each have a truck or 2 on air, most of the stuff is probably on juice brakes.

 

[sberry]

I would have metered it out of curiosity. They had a big fukkup at a county garage, had a new steamer (hot pressure washer) installed, it was on a new build and dont recall what happened other than it was the wrong voltage and they were trying to find out who gonna pay.
The installer was general hired help, all that looks ok to a lot of people. I had one ask,,, how long you figure I been doing this. I told him I couldnt guess but would have blurted out 2 weeks but it was 6 months. They got it done, they had a heavy drill. Their rigging was a bit shabby, was worth watching someone else do it. They had an old guy helper tag along that I am sure might have been keeping this on track.
There were a couple fukkit mistakes they made I had to fix. Seems they had a 3 ph plug they sent with all the installs and thats "just what they use" .
I am about 15 ft from the panel and in the permanant install wire nutted in a box, there was so wired to the motor with that plug on it. I was going to put a dp switch in it, I even scored one but its about as good without, people wont mess with it.

 

[sberry]

For that comp and this install would have ran 2 10s to the 30 recept thru one of those 2 space 70A jobs and put a 30 breaker in it. Wouldnt mess with the fuses, would leave it all alone, no adapters, none of that ****, simply take the old recept box off and extend the pipe to the new disconnect/breaker box, only 2 spaces so no one can get real creative with it.
I think I would let the motor rating slide a little on this one and match the wire to it all like one of the posters suggested,,, yes, the 12 is legal if direct and how this was done would certainly be impacted by available materials to some extent.
I forget what the existing fuses are now??? You could change them but wouldnt need to 10 I believe? Wanna put a 5 on it in the future, remove outlet, wire it on, change a breaker if needed.

 

[sberry]

The original install totally not correct due to 60 fuses on a 30 outlet. I bet that machine is designed to be limited to 50 also.