Seeking advice on wiring an air compressor
- Thread starter carpoor
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crkleve
Well-known member
Personally I might move the compressor (if possible) to shorten the length of the electrical run. Airline is MUCH less expensive than wiring a 100' run with 6AWG wire.
matt_i
Well-known member
I think I would compromise on #8 fed by a 30A breaker. Not that #6 won't work but it is expensive as stated. Just so you are thinking like me, this is copper wire.
Cyberbear
Well-known member
#10 wire is the smallest I'd use for a 100' run. Anything over that you should bump up to next size larger wire, I think for your projected run, #6 would be over kill and a waste of money.
Tim The Tool Man
Well-known member
Normally #10 is fine but for a run that far, jump up to #8. You don't want to burn up motors. #6 will work great but it, as stated, is over kill.
Also post this question in the electrical section and see what the Sparkys have to say.
Also post this question in the electrical section and see what the Sparkys have to say.
I have a similar sized compressor (7.5HP, 220v, 16A, 60gal) and when I originally had it hooked up, the electrician ran a 20A outlet and put a plug in cord on it. So that's how I'm going to do it again in my new garage. That way I can unplug it and move it if needed.
sberry
Banned
To make is simple it needs an 8
Dagny
Well-known member
Whats the amp draw? standard motor is 28 if it is single phase
tlmartin84
Well-known member
Is it a "true" true 5hp motor? Aren't a lot of these new electric motors capable of a lot less amperage since they are using capacitors?
My 60 gallon 5HP manual reccommends a 15 amp, 14 ga, 120V or 240V Circuit...
It also depends on whether it is 240V or 120V.....
My 60 gallon 5HP manual reccommends a 15 amp, 14 ga, 120V or 240V Circuit...
It also depends on whether it is 240V or 120V.....
matt_i
Well-known member
If its a 5hp multiply by 745 Watts per hp. I use 750 for rough calculations. 3750W. Volts * Amps is a power flow calculation, so 3750W / 240vac = 15A in steady state full load. I always use extra due to the full load inrush (since a compressor motor starts typically against a load), double this to a 30A circuit on #10awg in normal conditions, normal length wire run.
Your "developed horsepower" can draw less amperage via physics wierdness, but it won't be more.
Your "developed horsepower" can draw less amperage via physics wierdness, but it won't be more.
tlmartin84
Well-known member
What type of plug/outlet did he use?
pattenp
Well-known member
If the motor is a true 5HP there should be a data label on the motor that states that is is 5HP. To comply with the NEC provided the OP is in the US, if the data label list the HP as 5HP then the wiring needs to be hardwired using #8 NM-b or #10 THHN in pipe and can be on a 40A breaker. The NEC list 5HP as 28A, so 28 X 1.25 = 35A circuit size.
Honestly I'm not sure. I will take a pic of it when I get home. As far as I know it's a 20A 3 prong (IIRC) 220V. The work was done by a local electrician.
tlmartin84
Well-known member
About this.......
I checked my manual for my air compressor.
3.7 HP rating ....... 3.7*745/240 = 11.5 amps * 1.25 = 14.4 amps so a 15 Amp Circuit.
The manual list minimum 12AWG wire (up to 75'), but then in the needed to install lists a 15 Amp breaker. It also says a plug may be used......
Doesn't the code refer you to the manufacturers recommendations? And then there is a nice little note in the manual that says wire to meet all local codes..........
So what trumps what? It's like the manufacturer has a get of jail free card....
I had intentions of running, 12-2 with a 15 amp double pole breaker with NEMA 6-15R Plug and receptacle based off of the manufacturers specs.
Heck the mill and lathe Im purchasing both come with 6-15R Plugs on them with 2hp motors.........What gives?
According to NEC I need a better plug on my AC than what comes standard on my large welder do to the HP rating............. how is that?
I bought my Campbell Hausfeld a few years ago which had a 7 hp motor. I knew it wasn't 7 HP, but it didn't stop me from buying it. A couple years later, I got a letter in the mail that Cambell Hausfeld lost a law suit for misrepresenting their motor HP rating and I could claim my "win" by selecting from a list of accessories (about $100 value).
pattenp
Well-known member
Is that 3.7HP on the data label that's on the motor? Or is it on a label on the compressor tank? See if the motor HP is listed as SPL with an amp amount that's on the motor.
If the compressor comes with an installed cord and plug from the manufacturer then provide a circuit sized as stated by the manufacturer. Compressors without a cord and plug with motors greater than 3HP should be hardwired and the circuit sized per the NEC based on HP of motor from motor data plate.
Edit: My suggestion is if the manufacturer gives precise instructions on circuit size then follow the manufacturers instructions. Be it corded or not corded. Just beaware typical larger amp outlets and plugs have a max HP rating of 3HP.
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myredracer
Well-known member
Sorry, but that is not how it's done. You never do a HP to KW conversion like that. You need the current rating off the motor and then go to the NEC to determine the wire and breaker size.
tlmartin84
Well-known member
I understand this, I was trying to give validity to the manuals 15 amp claim.
The motor itself is an AO smith........
View media item 55456
It list 15.0 Amps, so based off of the manuals information 15 amp breaker, you tell me is it 15 amps the maximum current, or is that with the 125% already figured in?
Notice the HP.........SPL, what does that mean?
tlmartin84
Well-known member
FWIW mine is wired on a 30 amp breaker with 10-2 NM in my current shop. So I am not arguing with anyone here. For the most part I agree. I was going to use 12-2 and the 15 amp breaker in the new shop though, I am within 10' of the panel.
It is just extremely frustrating trying to plan to wire a shop using manufacturer specs that don't concur with NEC. You dang near have to buy everything, prior to sizing the ****. This is a good case for using conduit and pulling wire after the fact.
And as mentioned above, you can't use any of the listed specs because they are overated.
It is just extremely frustrating trying to plan to wire a shop using manufacturer specs that don't concur with NEC. You dang near have to buy everything, prior to sizing the ****. This is a good case for using conduit and pulling wire after the fact.
And as mentioned above, you can't use any of the listed specs because they are overated.
myredracer
Well-known member
That 15.0 amp figure *should* be the max. running current. Min. wire would be #12. Am guessing the required breaker size would be 20 amps but you should refer to the NEC for sizing. You never want the wire undersized. But a smaller than NEC size breaker can often work and the worst that will happen is a tripped breaker.
However... That current rating at 15.0 amps is suspect tho. being exactly "15.0". Usually for a motor that has a 208/240 rating, current at both 208 and 240 volts is on the nameplate. At 208 volts, current is higher. Maybe the 15.0 is "worst case" at 208 volts? If it is, current at 240 would be 208/240 x 15.0 = 13.0 amps - still indicating #12 wire. But I would expect this is a DIYer/homeowner compressor and the voltage would normally be 240 volts.
The nameplate lists the LRA current (locked rotor amps) which is the inrush current at startup. Typically 6 times running current. Working back, the max. running current would be 81.0/6 = 13.5 amps. That would still indicate min. #12 wire size. But is the LRA at 208 or 240 volts?
SPL is a contrived inflated HP rating and the actual HP rating is less than what is indicated on the compressor. Here is a good discussion on the Mike Holt electrical forum about someone want to size wire & breaker for an SPL 3.5 HP compressor. http://forums.mikeholt.com/showthread.php?t=97782 One poster mentions AO Smith as one manufacturer who does this. Have not gone back and reread this thread and not sure what the consensus was. Very confusing even to all the electrical experts there!!
Based on the motor's nameplate, I would say that the wire is okay at #14, but I would really suggest #12 Will reduce voltage drop if you have a long run to the panel and/or if the voltage at the main panel drops much below 120V sometimes. You could simply try a 15 amp breaker and see if it holds okay on startup, if not, move up to a 20 amp. Does the compressor have a 15 amp rated cord and plug on it?
If and when you find that the big 3.7 HP compressor isn't enough, #12 wire would allow a somewhat larger compressor. Perhaps installing #10 now might be wise if there's a chance of upgrading down the road.
It would be interesting to take a clamp-on ammeter reading (cheap at HF).
Great fun...
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wyliesdiesels
Well-known member
Wow a lot of back and forth on this one.
Wiring for motor circuits is sized at 125% of NEC table FLC.
So a TRUE 5HP motor would need to have #10 THHN or #8 NM-b as pattenp stated
@ 100' with #10 wire, the voltage drop will be acceptable.
Breakers for motor circuits are sized larger than the wire ampacity to compensate for in-rush currents. This is okay because the wire and motor is protected by either overloads integral to the motor or in the motor starter.
The breaker can be sized max 250% of FLC. The minimum size would obviously be one where the breaker doesnt nuisance trip.
As far as outlets go- unless u can find an outlet and plug with a HP rating equal to or greater than the motor HP rating, then it needs to be hardwired.
A disconnect for hardwired motors is required if the breaker panel is more than 50' from the motor and isn't visiable at the motor. If the equipment is cord and plug connected, then that is the disconnect.
Wiring for motor circuits is sized at 125% of NEC table FLC.
So a TRUE 5HP motor would need to have #10 THHN or #8 NM-b as pattenp stated
@ 100' with #10 wire, the voltage drop will be acceptable.
Breakers for motor circuits are sized larger than the wire ampacity to compensate for in-rush currents. This is okay because the wire and motor is protected by either overloads integral to the motor or in the motor starter.
The breaker can be sized max 250% of FLC. The minimum size would obviously be one where the breaker doesnt nuisance trip.
As far as outlets go- unless u can find an outlet and plug with a HP rating equal to or greater than the motor HP rating, then it needs to be hardwired.
A disconnect for hardwired motors is required if the breaker panel is more than 50' from the motor and isn't visiable at the motor. If the equipment is cord and plug connected, then that is the disconnect.
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wyliesdiesels
Well-known member
When SPL is listed u look at the FLA on the motor then go to the NEC FLC tables and see how the FLA fits in the table.
For a 240v motor with 15a FLA, it would be close to a 3HP motor.
So then u would choose your wire based on a 17a FLC- 17*125% = 21.25a. So technically u would need either #12 THHN or #10 NM-b because NM-b wiring is limited to 60* ampacity ratings.
myredracer
Well-known member
Curious - does the NEC require that for even smaller plug-in compressors (or even other shop equipment) like at 15 amps or 20 amps? Some compressors have a pressure switch with an off - auto position. Does that suffice?
wyliesdiesels
Well-known member
Sorry I forgot to put that in there. I will edit my post.
Yes, if the compressor is cord and plug connected then that is a suitable disconnect.
The disconnect requirement only applies to hardwired equipment.
tlmartin84
Well-known member
View media item 55457
Here is that table.
How/Where does the NEC address plugs?
If you terminate in a JB near the compressor, are standard wire nuts ok to make the connection with?
Here is that table.
How/Where does the NEC address plugs?
If you terminate in a JB near the compressor, are standard wire nuts ok to make the connection with?
pattenp
Well-known member
NEC 430.42(c) & 210.21(B)
penright
ALLIANCE MEMBER
I know this is an old post, but it does touch on a question I been researching. Not sure what the trade name is, but the kind of disconnect that you pull out a blade. It that kind ok for the compress (assuming proper amps)? I am assuming the disconnect is for servicing. Is the 50' and visible has to do with, if I turn the breaker off, I want to see if someone could possibly flip it back on?
Another question. While researching, I have looked at a lot of posts and I see different HP ratings for when you have to hardwire. Some say 3 and above and others say 5 and above. Which one is right? I will need to double-check, but I believe I saw FLA at 17.2. The compress is advertised as a 3.7 HP.
I have some THHN #10 copper (red, white, black, green). The breaker panel is halfway down of a 30' wall. So that puts the breaker about 15' from the compressor. Say 10' up and down, I should not be over 35'. I was going to pull the neutral and cap it off in the Jbox so the hardwire could be converted to an outlet. Would #10 SO cord be ok for the hardwire?
mike93lx
ALLIANCE MEMBER
That 7.5hp rating is ********, no point in even quoting it. That's like saying my shop vac is 6.5hp. Only when struck by lightning.
A real 7.5hp motor needs a 50a circuit. Your cord and plug would be en fuego.
Slednut
Well-known member
mike93lx
ALLIANCE MEMBER
Slednut, are you asking a question?