"It's a 10 horse, so a 30 amp breaker is fine, right?"
We get some version of that question every week, usually from somebody standing next to a new compressor with a roll of wire and a free Saturday. The answer is almost always no, and the reason is that motor circuits do not follow the same rules as the rest of the wiring in your building.
Getting air compressor wire and breaker size wrong goes one of two ways. Undersize it and you get nuisance trips every time the motor starts, or worse, warm conductors and a fire hazard. Oversize the breaker without oversizing the wire and you have removed the protection that keeps the wire from cooking. Neither is a good Saturday.
Here is how motor circuits actually get sized, and a chart you can sanity check your installer against. To be clear up front: this is background so you know what you are looking at. The actual installation should be done by a licensed electrician who is working to your local code.
Why a motor circuit is different
A normal circuit is sized to the load. A motor circuit has to handle two very different things at once.
Running current is modest and steady. Starting current is not. When a motor starts it can pull six to eight times its running current for a second or two while the rotor comes up to speed. If you sized the breaker to the running current, that inrush would trip it every single start.
So the code splits the job in two. The conductors get sized for the continuous running load, with margin. The breaker gets sized much larger, big enough to ride through inrush, because a separate device handles overload protection. That separate device is the motor starter's overload relay, and it is the reason the oversized breaker is safe.
That last point trips people up, so it is worth saying plainly. On a properly built motor circuit, the breaker is not protecting the motor from overload. The overload relay in the motor starter does that. The breaker is there for short circuits and ground faults.
Use the table value, not the nameplate
This is the part that surprises almost everybody. For sizing conductors and overcurrent protection, the code has you use the full load current from its own standard tables, based on horsepower and voltage, rather than the FLA number stamped on your motor's nameplate.
The nameplate FLA does get used, but for the overload relay, not for the wire and breaker. Using nameplate amps to size conductors will often leave you short, because manufacturers rate motors a little differently from one another and the table values are deliberately conservative.
So step one is always: look up the horsepower and voltage in the table.
The two rules
Once you have the table full load current, the math is not complicated.
Conductors: at least 125 percent of the full load current. A continuous duty motor needs conductor ampacity of no less than 1.25 times the table FLC.
Inverse time breaker: up to 250 percent of the full load current, then rounded up to the next standard breaker size. That headroom is what lets the circuit ride through starting inrush without nuisance tripping.
Worked example on a 10 HP, 230 volt, three phase compressor:
- Table full load current: 28 amps
- Conductors: 28 x 1.25 = 35 amps minimum ampacity
- Breaker: 28 x 2.5 = 70 amps, which is already a standard size
So a 70 amp breaker feeding conductors rated for at least 35 amps. Notice how far that is from the "30 amp breaker for a 10 horse" guess. Notice also that the breaker is double the conductor rating, which looks alarming until you remember the overload relay is doing the overload job.
Full load current by horsepower
These are the standard three phase table values that installers work from. Use them to check the math, not to replace an electrician.
| Motor HP | FLC at 230V (amps) | FLC at 460V (amps) | Min conductor ampacity at 230V | Breaker at 250% (230V) |
|---|---|---|---|---|
| 5 | 15.2 | 7.6 | 19 | 40 |
| 7.5 | 22 | 11 | 27.5 | 60 |
| 10 | 28 | 14 | 35 | 70 |
| 15 | 42 | 21 | 52.5 | 110 |
| 20 | 54 | 27 | 67.5 | 150 |
| 25 | 68 | 34 | 85 | 175 |
| 30 | 80 | 40 | 100 | 200 |
Breaker column is 250 percent rounded up to the next standard size. Your electrician may land on a smaller breaker if the motor starts fine on it, since the code treats 250 percent as a ceiling for inverse time breakers rather than a required value.
The things that change the answer
The table above assumes a fairly ideal install. Several real world factors push conductor size up:
Distance. Voltage drop is the most common reason a shop compressor runs hot and trips overloads. On a long run you size up regardless of what the ampacity table says. A rule many electricians use is keeping voltage drop under 3 percent on the branch circuit. On a 150 foot run to the back of a building, that alone can bump you a wire size or two.
Temperature and conduit fill. Conductors in a hot attic, or bundled with several other circuits in the same conduit, have to be derated. Your usable ampacity goes down.
Wire material. Aluminum carries less current than copper for the same size, so an aluminum feeder has to be larger.
Single phase versus three phase. Single phase motors draw considerably more current for the same horsepower. A 5 HP single phase 230 volt motor is up around 28 amps where the three phase version is 15.2. If you are on single phase, do not borrow numbers from a three phase chart.
Variable speed drives. A VSD compressor has its own input current rating and its own protection requirements. Size to the drive's documentation, not to the motor horsepower.
Common mistakes we see
- Sizing off the nameplate FLA instead of the table value.
- Putting a big breaker on small wire with no overload relay in the circuit. This is the dangerous one.
- Running an extension cord to a big compressor. Do not. Voltage drop on a cord will burn up a motor faster than almost anything else.
- Ignoring the disconnect. A motor needs a disconnecting means within sight of the equipment, and inspectors will look for it.
- Reusing the old circuit for a bigger machine. A 5 HP circuit does not feed a 10 HP compressor, even briefly.
If it trips on startup
A breaker that trips only during starting, and only sometimes, is usually one of three things: a breaker sized too tight for inrush, a bad start capacitor on a single phase motor, or voltage drop from a long run or a tired supply. A breaker that trips after the compressor has been running a while is a different animal and points at overload, a failing motor, or excessive discharge pressure.
If the motor itself is the problem rather than the circuit, our air compressor motors section is organized by frame, voltage, and RPM so you can match a replacement properly.
Frequently Asked Questions
Can I use the FLA on my motor nameplate to size the wire?
No. For conductors and overcurrent protection the code directs you to the standard full load current tables by horsepower and voltage. The nameplate FLA is what you use to set the overload relay. Using nameplate amps for the wire will often leave the circuit undersized.
Why is my breaker so much bigger than my wire rating?
Because on a motor circuit the breaker only handles short circuits and ground faults. Overload protection comes from the overload relay in the starter. That is what makes a 70 amp breaker on 35 amp conductors a legitimate design instead of a hazard.
Do I need a disconnect next to the compressor?
Yes. A disconnecting means is required within sight of the motor and the equipment so it can be locked out for service. Many compressors do not ship with one, so it usually gets installed as part of the hookup.
Can I run my 5 HP compressor on a regular 240 volt outlet?
Usually not on a standard dryer or range circuit. A 5 HP single phase motor draws around 28 amps running, which needs a dedicated circuit sized well above that. Check the compressor documentation and have an electrician run a proper circuit.
Does a longer wire run really matter that much?
Yes, more than most people expect. Voltage drop makes the motor draw more current to do the same work, which makes it run hotter and shortens its life. On runs over about 100 feet it is normal to go up a wire size or two purely for voltage drop.
The short version
Look up the full load current for your horsepower and voltage in the standard table. Size conductors at 125 percent of it. Size the inverse time breaker at up to 250 percent and round up. Add margin for distance, heat, and conduit fill. Make sure there is an overload relay and a disconnect.
That is the shape of how air compressor wire and breaker size gets determined, and it is enough to have an informed conversation with your electrician. It is not a substitute for one. Local codes vary, inspectors vary, and the cost of getting a motor circuit wrong is a lot higher than the cost of an hour of an electrician's time.
