I ended up with a NAPA air compressor, I believe it is made by BelAire. There is no mention in the manual about what size breaker to use.
It is 5hp, 2 stage 230v. The motor is a Baldor and says 21amps on the label, compressor label says 22amps. So I’m not sure if that’s when it’s running or if that is peak when it starts. So what size is typical for this type compressor? Also there is a generic sticker on the motor that says to make sure it is properly grounded but as far as I know by the diagrams for wiring it just requires 2 110’s.
I would suggest wiring that for at least a 30 amp breaker, so 10 gauge wire. If you use standard Romex make sure that you label the white wire as hot with black or red tape at the box.
If you get standard 10/2 wire (10 gauge, 2 load carrying + 1 ground) the colors are typically white, black, and bare copper (sometimes green). You’ll wire the white and black to separate legs on your mains or subpanel, but since the white wire isn’t being used for neutral it has to be labeled on both ends for safety.
You may want to get a local electrician to wire this up for you if you want it done safety and to code.
Ok, the labeling remark confused me. I’m good to wire it and I have some bulk 6ga I plan to run in EMT from the panel to a box by the compressor because it’s about a 40ft run up and over to the far side of the garage.
I did the entire panel and have a 220v plug and a 220 heater all wired to code.
base breaker on wire size not use from appliance, so 6ga is good for 55 or so amps based on heat and such plus the enclosed issues (I am not electrician)
I would throw a 40 amp breaker or even a 50 and say f-it…I used 50 amp breakers because I couldn’t find 40 amp ones local, my plasma,welders only need less than 40 so I feel good about my selection…licensed electricians would probably disagree…oh…here, hold my beer … …so says the…
Oversizing the wire gauges is a good idea, but the breaker should be sized to trip in the event of a fault for what you actually have loaded. I’m not going to say it’ll burn your house down or anything, but oversizing it too much will allow energy to go somewhere before it actually trips. I’ve heard of goat cheese, but not goat beer .
If you are are running 6ga, make sure to use at least 3/4 emt, and stick a 50 amp dual pole (240v) breaker on it. You are most likely going to put a 50a plug on the end for the compressor anyway. I’ve never really liked putting 30amp breakers where 50amp plugs are used, but you could do this circuit with 10ga / 30amp just fine. The motor will only pull what it needs (around 20 amps) unless something shorts. It will use more at startup, but it shouldn’t trip a 30a breaker. Breaker is there to stop the wires from getting too hot and burning up / causing a fire - most can make it over a motor start unless the load is too close to the rated trip.
Note: 6 ga is good for 75a with THHN (common) insulation. 8 ga for 55a THHN. I did my compressor on 6ga as well because I had some left over. I pulled 3/1 (with neutral) so I could wire in a 4 prong 50amp if I wanted to in the future, but I left the neutral capped in the box.
Some local codes require 3/1 with a 4 prong outlet now so check. Where I live we don’t have codes for residential (it’s your crap if you burn it down it’s your fault).
According to code and chart I found I can run 2 6ga in 1/2" EMT and use the conduit for the ground. I was also planning on doing the drop to a box and hard wire the compressor, no plug. All references I found say this is GTG, but maybe I’m wrong.
2 6ga in 1/2" is going to be tight, but may be ok code wise. I always pull a ground regardless. Can you rely on emt? Probably, and it may be ok with code, but I prefer to have a ground wire just in case.
Hardwire - I still like having the plug…can be used as an disconnect, and also used for other stuff if needed. Just my opine
Update / Edit to my post to mainly to clarify - I was trying to say 2 different things, but it reads like I was crossing up the ground / neutral. They are different and should not be confused. Let me try again because it’s electricity
I don’t rely on EMT for ground - I use EMT for physical protection only - so I always pull a bare (or green insulated) ground wire in EMT. Do you have to? - no - if code allows you to use EMT for a ground, you’re ok. I still pull a ground wire regardless - that’s just me.
For neutral - you don’t need one for this 240v circuit, but I’d still upsize the EMT to create space for one in case you want it in the future. Even if you don’t want / need a neutral in future, upsizing the EMT will make the wire pull easier - pulling 6ga is not terrible, but it can be. You could also decide in future to run some 12ga / 20a 120v circuits to that end of the shop or run a small subpanel without having to add new conduit.
Just some opines both of which are optional - I don’t know your shop. If 1/2" works for you and it’s up to code, you’re GTG!
I don’t think anyone was planning to run a neutral over the conduit, but it’s a good point.
The best way to think about a safety ground is you should be able to eliminate it and have the device still work. The safety ground only carries current when there is an issue, which is why you want to have it in place. If you needed operational all the time it’s not actually a ground, it’s a return path, and you either have an architectural issue, or don’t understand the system fully.
Just finished wiring up my Shop and Dewalt 5HP 60 gallon compressor. Some good info on this thread but I did see a couple conflicting points that probably should be clarified.
If using EMT for the run, you will need two line conductors (wires) between the compressor motor and the two pole breaker since the compressor is a 240v single phase load. You need to run wire that is rated for running in conduit. This would typically be THHN wire you can purchase from HD or Lowe’s. I would use one Red and one Black. By the way, dont use wire cut out of NM cable you may have lying around. It’s not rated to go into conduit. As far as a neutral, you don’t need one for the compressor if is the only load you are feeding through the EMT run. I also agree with others running a separate green or bare copper ground in the EMT even though code does not require it. Just ensures you have a low impedance path back to the breaker for ground fault protection.
To size the conductors/wires/ breaker properly meaning code compliant, can you provide the FLA, Service Factor, HP, Temp Rise from the motor nameplate? Better yet a picture of the nameplate. Another key piece of information is if your motor on the compressor has built in overload protection? Would typically look like a reset button on the motor.
Protecting motor loads like compressors correctly need to consider two different fault conditions: 1) Overload and 2) Short Circuit/Ground Fault. There is a entire complicated article 430 in the National Electric Code that covers it. It can be found here for reference: Motor Calculations Part 1: Motors and Branch-Circuit Conductors | EC&M
Assuming your compressor motor has built in overload protection, I suspect your 5HP compressor will work out with a 10awg conductor size and a breaker size of 40 to 50 amps (150 to 300% of the Full Load Current of the motor). The two pole circuit breaker in this case is sized that way to avoid nuisance tripping due to the motor inrush current when starting but still protect for short circuit/ground faults. The breaker is not protecting from overloading the wires because that Fault condition is taken care of by the built in motor overload protection assuming it has it.
Lastly, I would suggest thinking about using a magnetic starter if you plan to use compressor a lot. The contactors built into the pressure switch on the compressor are typically not that heavy duty and tend to fail over many compressor starts. A magnetic starter has much heavier duty contactors that will last much longer. A good mag starter is around $100 to $150 dollars. Model number I used, got it from Amazon:
EATON B27CGF30B040 MAGNETIC MOTOR STARTER 5HP 1 PHASE 208-230 VOLT 30 AMP
I find it interesting that no one considers emt with a surface area probably at least 50 times greater than a small ground wire inadequate.
The deal is done. 2 6ga in 40ft of 1/2" EMT to the box by the compressor, from box to compressor is 8ga with a ground because that is in flex. For now I used my 30amp dual breaker for my heater because, well it’s hot right now and I haven’t turned it on in months. I think the 30amp breaker is ok but we’ll see how it works when it’s cycling a few times.
It’s overkill all the way and I’m good with that. The 6ga was free, a leftover from a friends spa install.
EMT as the ground path is perfectly fine if it is installed correctly. The key it to make sure all the various EMT connectors are securely tightened and the fitting connection into the electrical panel has a good metal to metal contact. Electrical panels are painted so it’s good practice to use some emery cloth to remove some paint to ensure good contact. The recommendation to use a separate ground wire avoids any concerns about how well the EMT path is bonded together. You want to have a low impedance path back to the panel to ensure any short circuit/ground faults are cleared immediately by the breaker. If there was a break or bad connection in the ground path through the EMT run, you would become the path to ground resulting in possible electrical shock risk and the breaker not clearing the fault. Hope this helps😊
Codes are developed assuming that an apprentice is doing the bulk of the labor and they either don’t understand what they are doing or are to lazy to do it 100% every time. So you develop them in such a way that it take multiple unlikely events to happen in concert before there’s a safety issue. The code is basically Idiot proof (trust me I’ve met the Idiot 2.0 model…They will find a way). IT’s also overkill. As others have said if it’s done right it’s not an issue. You’re doing the work you double check everything because it’s your butt on the line. The kid that’s throwing outlets in Track homes that’s been on the job 2 weeks has no skin in the game…He’s not going to double check every join, fitting and screw.
I don’t go beyond the three points in this article whenever I’m breaker sizing for motors. Sure, you’ll have variations when it comes to motor and circuit breaker types, but these three steps never fail to help me get the exact sizing information I need.
…so says the… 
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that no one considers emt with a surface area probably at least 50 times greater than a small ground wire inadequate.