Wire Guage

[stppinz]

I am wiring up a new compressor. It has a 6HP (Speed Air). I am hardwiring it in, replacing the plug it had on it when I got it. The wire it has on it is 10 guage. Can I get away with using 12 guage? I already have a bunch of 12/3 that would work vs over $1.50 a foot for 10/3 at the Depot. If I need to I will go and get the 10/3, but it would just be nice to save the $60+.

Just woundering.

Thanks for your time.

Also, I noticed that the ground wasn't hooked up to the switch on the compressor side. Where should that be connected? Looks like it was just on a screw that attaches the cover to the switch. Thanks again.

 

[bmwpower]

Is this a 240v compressor motor? Why do you need 10/3? What does the nameplate say on the motor? Should tell you the amps. How far of a run is it? You may have to upsize anyway due to voltage drop.

 

[VvvJRvvV]

Look on your compressor, you should see FLA. This is Full Load Amps, this is what you need to gauge the wire size on your compressor for. Let us know what that says...

 

[stppinz]

I don't see a FLA but it has a FA on the label of 93 amps.

Sorry for the lack of info. Yes, it is 240V and the line to the pannel is going to be about 30-40 feet.

Thanks

 

[May Pop]

If the cord is 10 gauge it must be there for a reason. The motor will always have a FLA [ full load amperage] on the tag. Use that to size the feed for your new comp. Dont get cheap with the install or you may do it a second time. The size of the feed should be 1.25 times the FLA according to NEC.
Good luck with your new compressor.

 

[ron in sc]

I bought a new compressor, still waiting on delivery.

It has a 5 hp Baldor motor turning about 1700 rpm, 230 volt, single phase. Draws 32 amps while running and much, much more on startup. Manufactor says use 60 amp circuit breaker.

Wire gauge required is 8 gauge.

I don't see how you can use less than 8 gauge with your motor having more hp than mine.

 

[Charles (in GA)]

Big question is why you are doing away with the plug. It sounds like a portable compressor if it has a plug, however, a FLA of 93 is a HUGE motor from looking at table 430.248 in the NEC (which NEC 430.6(A)(1) specifically requires you use instead of the actual FLA on the motor, for determining conductor sizes.)

Table 430.248 does not have a 6 hp. it has 5 hp and 7.5 hp. The FLA for a 7.5 hp motor at 230 volt (highest given) is 40 amps. Multiply that by 125% for conductor sizing (50 amps) but use the motor nameplate FLA for breaker or fuse sizing. Mutiply that number by 250% for breaker sizing (yes I know that seems excessive, and probably is, but I didn't make up the rules)

Grainger catalog does not list any 6 hp compressors in the Speedair line.

how about a Grainger number for the unit?

Charles

 

[stppinz]

I guess I should of looked closer at the tank and compressor. It answered my question about wire guage, but I still can't find what horse power the motor is. I thought it was 6 from this picture - But is doesn't say HP after the 6 but it does say 60 gallon.

The tank states that 26' to 50 feet at 240V 12 guage wire would work

Here is the motor

 

[bmwpower]

15 amps is it. Looks like a 12 gauge will work. You will only need 12/2 (2 hots and a gound).

Not 100% sure, but I think that 93 amps has to do with the amps drawn when the motor locks up. Too late to search for it. I'm sure Charles will know, though...

 

[Charles (in GA)]

The 4B221 number and 4B245 for the replacement pump are both discontinued numbers, but the specs compared to current models put it at 3 hp most likely.

The 93 is locked rotor amps.

Charles

 

[stppinz]

Thanks guys.

 

[cj7jeep81]

Thanks guys. Think a 60amp breaker would be sufficent?

If its pulling 15 amps, I'd put it on a 20 amp circuit. 60 amps wouldn't give you any protection.

 

[Charles (in GA)]

Actually, a twenty amp (two pole for 240V) breaker probably won't be enough. You should never exceed 80% of a devices capacity, which for a 20 amp braker is 16 amps. At 15 amps you are pushing the limit. Also, The example in one of my books for calculating wiring and breaker sizing uses a 3 hp/15 amp motor as the example. It uses Full Load Current of 17 amps as determined from NEC code table 430.248 for a 3 hp motor (do not use data plate info for this, per code) and mutiplies this by 125% to arrive at 21.25 amps of conductor ampacity. Even using the bottom temp range of 60 degree C a 12 gauge wire is still capable of 25 amps per the charts, so 12 is OK.

For a breaker you take the 17 amps derived from the table and mutiply by 250% to arrive at 42.5 amps. The nearest larger breaker is a 45 amp. Sound large but this is how the code calculates breaker sizes for motors on dedicated circuits.

Charles

 

[stppinz]

For a breaker you take the 17 amps derived from the table and mutiply by 250% to arrive at 42.5 amps. The nearest larger breaker is a 45 amp. Sound large but this is how the code calculates breaker sizes for motors on dedicated circuits.

Charles

Thanks guys - You rock. I am going to get that compressor going today.

 

[Charles (in GA)]

Sorry to get so technical with it, but the only true answer is a correct one. I don't claim to know or understand the NEC, just that I can read and sometimes interpet it, usually after reading one of the other books I have that gives me the references, then I can go read it for myself. The NEC is over 700 pages of fine print and not a picture in sight. I use it in conjunction with the Illustrated Guide to the National Electric Code by Charles Miller. The two books will cost you $122 from Amazon. I also use a little flip book called Code Check Electrical you can find at the big box home suppliers. Ugly's is also a good reference.

Charles

 

[ModSquad]

so what your saying is you need min 12 gauge wire on a 45 Amp breaker to run that compressor safley.

Thanks for the calculations this will definatly help for future references..

 

[Gummi Bear]

so what your saying is you need min 12 gauge wire on a 45 Amp breaker to run that compressor safley.

Thanks for the calculations this will definatly help for future references..

For that particular motor and compressor, yes. If you get another machine from another manufacturer, or even an updated model of the same one from the same manufacturer, it may be different.


Like Charles explained, all the rules that most folks know about code, ampacities, and protection go right out the window with 2 things: Motors and Welders. They have their own special rules. There's a reason that apprenticeship schools spend a lot of time focusing on these two things, IIRC, I spent nearly a whole semester of my 3rd year doing nothing but motor calcs.

 

[ModSquad]

yeah, k.. I was mostly refering to the calculation he used. so I could use similar calculation on my situation. What are the main differences for welders and compressors? I'm assuming distance becomes a large factor as well (voltage drop) (right term?) basically how far the tool is from the panel.. Example, I'm running a 60 AMp breaker and 8 gauge wire from my panel about 15 ft to my welder (miller 250X) as well as running same size line and breaker for my compressor which is about 25ft away. (240V compressor, light industrial).. 2 different circuits.

 

[ModSquad]

Gummi, your on POR4x4 board too aren't you? I recognize your sig.. hehe LOVE it, can't miss it!..

 

[JCByrd24]

so what your saying is you need min 12 gauge wire on a 45 Amp breaker to run that compressor safley.

Thanks for the calculations this will definatly help for future references..

Charles, this doesn't make any sense to me. I don't have a copy of NEC, but I am an engineer, do have a book from the depot, and have done a fair amount of wiring...including my 240V, 15A compressor in my basement. A 45A breaker on a 12 awg wire sounds just wrong to me, especially in the name of safety. The whole idea of a breakr is to protect the wiring it serves. Under certain conditions, that breaker will not protect that wire and is a fire hazard. Breakers are a time sensitive device, meaning that you will not necessarily trip the breaker just because the motor draws 25 A at startup on a 15A breaker. It may take 5 seconds at 25A. In my house I wired 12 awg (could have done 14 probably) protected by a 20A breaker and its never tripped, just as the label on the OPs compressor suggests. Maybe NEC is allowing larger breakers in the instance of motors, but I'd start at 20A and go up to the "allowable" only if I had tripping problems, and even then I'd be weary....Can anyone educate me out of this opinion.

 

[Charles (in GA)]

As I noted, this is how the code calculates dedicated motor circuits. Not to say I care for it, or that a smaller breaker won't work, but if you are trying to go by code, that is how they do it. In this case, the sizing of the breaker is more for the benefit of the motor, so it won't trip. If there is a good short, it will trip and still protect the circuit. Indeed, in this case I would probably use a 30 amp breaker to start, 20 is just too small, and would be running too close to the 80% limit of the components.

I have found over time that there is almost always a "method to their madness" when it comes to the NEC.

Charles

 

[Gummi Bear]

Charles, this doesn't make any sense to me. I don't have a copy of NEC, but I am an engineer, do have a book from the depot, and have done a fair amount of wiring...including my 240V, 15A compressor in my basement. A 45A breaker on a 12 awg wire sounds just wrong to me, especially in the name of safety. The whole idea of a breakr is to protect the wiring it serves. Under certain conditions, that breaker will not protect that wire and is a fire hazard. Breakers are a time sensitive device, meaning that you will not necessarily trip the breaker just because the motor draws 25 A at startup on a 15A breaker. It may take 5 seconds at 25A. In my house I wired 12 awg (could have done 14 probably) protected by a 20A breaker and its never tripped, just as the label on the OPs compressor suggests. Maybe NEC is allowing larger breakers in the instance of motors, but I'd start at 20A and go up to the "allowable" only if I had tripping problems, and even then I'd be weary....Can anyone educate me out of this opinion.

Go through and re-read Article 430.

If it still doesn't make sense, check out, borrow, or buy a copy of Mike Holt or Tom Henry's DVD and book about motor calculations. If you have questions, let me know, I'll be happy to answer them.


Like I mentioned before, everything you thought you knew about the code, goes flapping out the window when it comes to motors and welders.


Depending on the motor and application, there are instances where your protection of a given load can be as much as 1,100% of the actual load.

 

[Gummi Bear]

Gummi, your on POR4x4 board too aren't you? I recognize your sig.. hehe LOVE it, can't miss it!..

Yeah, I'm everywhere

 

[Charles (in GA)]

If you think the 250% mutiplier for the amperage (to determine the breaker size) is excessive you ought to look at Table 430.52 of the NEC. This says that for inverse time breakers you use the 250% (inverse time breakers are common garden variety residential type breakers, among others) and for time delay fuses you use 175% for the mutiplier, but for intantaneous trip breakers you use 800% and for standard non-time delay fuses you use 300%.

As GummiBear noted, the code is quite complex when it comes to motors, and everything about them. If it were not for the instructional books sending me to the right places in the NEC, I'd be lost.

Charles

 

[ron in sc]

Does 8 gauge wire with 60 amp breaker sound right for a Saylor-Beall 5 hp compressor, single phase, 230 volts, draws 32 amps while running? That's the size wire and breaker the Saylor-Beall guy told me to use. Also the lenght of the run is less than 10 ft.

 

[Charles (in GA)]

Wow! thats a lot of running amps for a 5 horse! I think my 7.5 hp emerson motor on my compressor only runs at about 33 amps. I used 6 gauge but I also ran about 70 ft (as the "wire" flies, up and over a 14 ft high garage door) and also I am powering a subpanel where I have the chance to draw more current, so I needed a little extra.

By the charts, you would use 28 amps for the FLC for a 5 hp motor and mutiplying by 125% you get 35 amps. Dependiing on the exact type of wire (THHN, etc) it is probably good for 40 amps... depending on temperature... so you are OK there, and the same 28 amps mutiplied by 250% equals 70 amps... the proper breaker size (by the code calculations). If it works and it were me, I certainly would not waste any time worrying about it. For the FLC amps you use the chart in the code, NOT data plate numbers.

Again, I am not a professional in the field,, I'm just an aircraft mechanic who understands electricity, sorta.

Charles

 

[ModSquad]

I haven't read any of the manuals, but Im gonna look into getting some for personal reference.. but what's the 80% rule? when it comes to wires and breakers, everything is rated at 80% of it's actual capacity? so a 70 Amp breaker can actually handle 125 Amps absolute Max?

 

[Stuart in MN]

I haven't read any of the manuals, but Im gonna look into getting some for personal reference.. but what's the 80% rule? when it comes to wires and breakers, everything is rated at 80% of it's actual capacity? so a 70 Amp breaker can actually handle 125 Amps absolute Max?

A 70 amp circuit breaker can handle 70 amps. The point of the 80% rule is to give you a margin of safety - for instance, on that 70 amp circuit you don't want to load it any more than 56 amps. For a 20 amp circuit, no more than 16 amps; for a 15 amp circuit no more than 12 amps. If the load was greater than that you'd probably start getting nuisance trips, plus it would be hard on the circuit breaker.

Think of it like this - if you owned a car that can go 100mph, theoretically you could drive it at 100mph all the time but it probably wouldn't last very long.

 

[ModSquad]

yeah, I was just gonna say.. it can handle the 70 amps maybe a spike here or there but not constant 70 amps as it would cause tripping and premature failure of the circuit.

good analogy

I just found this link for a reference, seems to have good info and is based off of the NEC.
http://ourworld.compuserve.com/homepages/boyce_smith/basic.htm

 

[ron in sc]

Wow! thats a lot of running amps for a 5 horse!

Thanks Charles

Maybe there will be a plate on the motor that states the running amps. The motor I specified is an industrial Baldor running at about 1700 rpm.

Hopefully I'll get the unit this week coming.

 

[z28toz06]

You have to keep in mind the triple amp draw requirements. In order to get a motor, or ac compressor going, especially when it is under compression, the initial start up is roughly 3 times the running requirements.

 

[VvvJRvvV]

Ron,

Look for FLA (full load amps), it should be on your nameplate. This is what you should be sizing your wire and breaker off of. If you don't want to do the math or don't have the books, post back and Charles or Gummi or I or someone will let you know what you need.

 

[Gummi Bear]

You'll want to size your feeder and over current device from the motor nameplate, it trumps the NEC tables. The tables in the NEC are for calculation purposes, and will suit if a nameplate is not available.

Motors are one of the most misunderstood electrical applications out there. WIth as much information as there is available, and all the resources you can find, it's easy to get turned around and heading in the wrong direction, even if your intentions are pure. (that kind of sounds like something I heard in a kung fu movie )

 

[trackwelder]

I just recently was given a curtis 5 hp 120 gal air compressor. The motorplate shows for amps 19.8 at 230 volt. My question would be what wire and size breaker should I use on this. Thanks for any help.

 

[ModSquad]

testing myself here, but I'm thinking you'll need a 20 Amp circuit and #12 wire. lets wait and see what the others say.. is that full load or running amps?

 

[Charles (in GA)]

You'll want to size your feeder and over current device from the motor nameplate, it trumps the NEC tables. The tables in the NEC are for calculation purposes, and will suit if a nameplate is not available.

I realize you probably have quite a bit of training/education at this, I have not, and don't wish to argue, just to state what I read in the code. This appears to be a change in the 2005 code from previous versions (looking at the page markings), and this may be the cause of the misunderstanding. I too would think you would want to use actual nameplate numbers for calculating values but the code state specifically otherwise (as I read it). I have omitted text in exceptions 1 and 2 that is irrelevant to the discussion. Exception No. 3 could carry the day, but I suspect that most large compressors are not "listed appliances", being built of off the shelf component pieces. Note that none of this discussion pertains to portable, plug in compressors, but rather to hardwired compressors on dedicated branch circuits.

Note that 90.5(A) defines shall as mandatory.

2005 NEC
Article 430 - Motors, Motor Circuits, and Controllers
430.6(A)(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not in horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Exception No. 1: Multispeed motors …………………..

Exception No. 2: For equipment that employs a shaded pole or permanent-split capacitor-type fan or blower motor that is marked with the motor type…………

Exception No. 3: For a listed motor-operated appliance tha is marked with both motor horsepower and full-load current the motor full-load current marked on the nameplate of the appliance shall be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection and any separate overload protection.

 

[Charles (in GA)]

I just recently was given a curtis 5 hp 120 gal air compressor. The motorplate shows for amps 19.8 at 230 volt. My question would be what wire and size breaker should I use on this. Thanks for any help.

Wow, thats a lot of compressor to be given. The 120 gal tank will make for long run time on the motor per cycle, but fewer cycles, and that is what kills motors and pumps, is startup.

Look at posts #25 and #26 above for 5 hp discussion.

If you use the amperage you give, then 20 amps (rounded) x 125% = 25 amps for the conductor, that would be 10 gauge copper.

20 amps x 250% = 50 amps for the breaker sizing. or 20 amps x 175% = 35 amps for a time delay fuse.

Charles

 

[ModSquad]

what's the diff between time delay and regular breaker? I was reading that but there were no descriptions..

 

[Vermaraj]

Exception #3 is applicable to most compressors. In this case "listed appliance" refers to any appliance (powered load) that has been tested and certified by a listing agency (such as UL). Therefore the tag accurately reflects the operating parameters and there is no need to use standardized tables.

Where you are connecting a motor that is sourced independently from the load. The manufacturer cannot know what the operating characteristics of the load will be. Then the table would be used, regardless of the tag numbers.

For instance a 1hp motor running a compressor may get bogged down and draw more current if an unloader valve is not used. Code is designed to capture this uncertainty. A listing agency testing a piece of equipment would not have this uncertainty. Either the appliance has an unloader valve or it doesn't.

 

[trackwelder]

My neighbor and I hooked the compressor up and it pumped up with no problems. I had a spare 30amp breaker and 10 gauge wire to power it up. So to be wired correctly I should install a 50 amp breaker correct? One other question is what should the compressor pump up to before kicking off. It shuts off at 110# and cycles back on when bleed down. I broke the original gauge off which went to 300#. It was replaced with one that reaches 200#. Thanks for all the answers it is much appreciated.