Powering up a compressor

[berone]

I resolved my compressor dilemma with a surprise find on Craig's List. A guy was looking to trade an 80 gallon, 7.5 hp BelAire 318VL for a mig welder. I've used my mig once in 10 years. Long story short- this is now in the van:
The motor plate reads 29 amps. Trying to sort out if the 40 amp breaker I have in an existing disconnect is sufficient or if I have to step up to a 50 amp. Any ideas?
Thanks,

[kurt18947]

Disclaimer - I am not a professional electrician, electrical engineer or possess other credentials. Having said that, I don't see why a 40 amp circuit would not be sufficient. Do not be taken in by the "but the starting amp draw is 60 or 80 amps!!!!" Circuit breakers are designed to tolerate a few seconds load in excess of the rated amperage to allow for starting surges. I doubt a motor with a 15 amp dataplate rating would be happy on a 15 amp circuit, especially an air compressor or other motor starting with under load but there's no need to go overboard. My gut feeling is that a 40 amp circuit for a 29 amp load should be sufficient. You could look for a manual for your compressor online and see what the recommended electrical service is.

[TDKPE]

I'm assuming that's a single-phase motor, with start and run capacitors in the big box on the right, since the current you stated is much too high for for 3-phase at that hp and (presumably) 230V. What is the value in the box labelled "Code" or "kVA Code"?

Doing it by the numbers, which means following NEC Article 430 (Motors, Motor Circuits, and Controllers), you would size wiring and disconnects for a 40A load (Table 430-138), upsized to 125% (single motor on this circuit), so wiring for 50A. The circuit breaker can be upsized over that to up to 250% of the 40A design load (that's not a misprint). That's to prevent the CB from tripping on startup. The overcurrent and overload protection for both the motor and the branch circuit conductors and disconnect is via the motor control, which also has to conform to Art. 430. Overload settings are based on the actual motor nameplate value, even though conductors are sized to the table values.

The reason for the big current value from the table is because in industry a motor is likely to be replaced, and even though it would have the same hp and frame, the FLA may be higher so the NEC states current values to design for that are high enough to cover just about any motor of the same hp and voltage. Overload heaters may have to be changed based on the actual nameplate value.

In your case, since you're not a factory, I would size conductors for the FLA of the actual motor. 29*1.25=36.25A, so size conductors for 40A. That would mean 8 gauge copper, either at 60C rating (40A) or 75C (50A). Assuming it has a real motor control, you can upsize the breaker (dedicated circuit, of course) to protect against startup tripping. A mid-range inrush (kVA Code = K) translates to 232-261A, which may require an 'oversized' breaker. Larger compressors tend to take longer to get up to speed, and even though motor current is controlled by rotor speed (not load, or not directly at least), longer startups may cause tripping. But you can legally (and safely) use a larger breaker later if needed, as long as the it has a real motor control with overload. If it doesn't (and it may not, with that little overload reset button on the motor), then size conductors and breaker for 50A and hope it's enough.

[berone]

Yes- single phase. Code reads "F"
The current version of this compressor comes with a magnetic starter. This one does not, and I was wondering if I should add one. Not sure what it neans, but the motor says "thermally protected."
Thanks!

[TDKPE]

"F" translates to a starting current of 5.00-5.59 times rated FLA at rated voltage, so expect 145-162A while starting.

The 'thermally protected' is the little klixon-type button on the end bell. Since the motor was probably part of the compressor originally, and was supplied with overload protection, it conforms to NEC 430-32(a)(2) (Continuous Duty Motors, More than 1 horsepower, thermal protector integral with the motor), and if the pressure switch is rated for the hp of the motor, you're fine. If it's not rated for 7-1/2 hp, it should have at least a relay that is. One danger (if it isn't) is the contacts welding shut and not turning the motor off at rated pressure - make sure your ASME valve is in good condition and has the proper pressure rating.

Nice score, by the way. That's a real industrial unit, and should last as long as you want it to.

[berone]

So I'm good without the magnetic starter, 8 gauge copper, and breaker... Still not clear on that. I want to run a disconnect next to it. I currently have one (from the welder that's now gone) with a 40 amp breaker. Should I replace that with a 50 amp? Or use that with a 50 amp in the panel? Or can I use the 40 with a 40 amp in the panel?
Thanks,

[TDKPE]

I would try it with what you have. If it trips, upsize the breaker and/or go to a 'high-magnetic' type, which has a higher instant-trip threshold than the standard inverse-time with magnetic trip type. If you're uncomfortable with 8-gauge copper and a larger breaker, go bigger on the wire, but remember that conductors with 75C or higher insulation (like THHN/THWN) are already rated 50A (55A at 90C, but the terminations aren't going to be 90C).

As to the disconnect, yes you need a disconnect within some distance of the machine (don't recall off-hand), unless the motor is within sight of the breaker, but I would use the disconnect so you can shut it down at night and know that it's off. And because it's a clean by-the-numbers installation.

That's what I would do, at least.

[Mandrake]

Here is a semi-dumb question: If the motor's overload protection protects both the motor and the branch circuit conductors (assuming dedicated circuit), then what purpose does the circuit breaker serve?

[TDKPE]

Short-circuits and ground-faults, which can happen anywhere in the circuit, which the motor control can't protect against. The control also (unlikely, at least) does not have the short-circuit current interrupt rating of a breaker or fuse, like 10kA for typical home grade stuff, and higher from there, like 22kA and 65kA. It's amazing how much current can pass through even a long 14 gauge wire during a direct short event, at least for a cycle or so until the breaker trips. That's why the breaker can be 'oversized', as it's only a short-time event, and not a long-time heating issue, which the motor control protects against. But if following Art. 430, you have to follow all of the bits that apply, as the slack it seems to cut at one end is compensated for at the other. And if in doubt, you can never go wrong with the 15/20/30/40/50A for 14/12/10/8/6-gauge copper 'rule'. It's just that in industry, things start getting pretty expensive when talking about 100 hp motors. Or 500 hp. Or 5000 hp (NEC table stops at 500). Unnecessarily oversized conductors would cost a fortune, and get someone fired.

[BloomingtonMike]

Circuit breakers are there to protect the wiring from short circuits and overload. Not there to protect the devices connected to the wiring or the people around them.