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Machine 5HP compressor 20A breaker?

Machine
slow-poke

slow-poke

Ultra Member
I'm actively searching for a 5HP, 60 gallon vertical air compressor to replace my existing 20 gallon.

The Sanborn I'm considering states 15A @ 240V. I have to assume the 5HP rating is exaggerated because even if one assumes 100% efficiency, then current would be 15.5A.

Cable run is very short, about 10', and I have plenty of #12 and spare 20A breakers. Using the 80% rule in theory the 20A breaker should be marginally okay. If I had some #10 and a 25 or 30A breaker I would use it, I'm inclined to give what I have a try unless someone tells me I tried that and the breaker was constantly tripping on startup.

Anyone running their 5HP on a 20A breaker?
 
slow-poke said:
I'm actively searching for a 5HP, 60 gallon vertical air compressor to replace my existing 20 gallon.

The Sanborn I'm considering states 15A @ 240V. I have to assume the 5HP rating is exaggerated because even if one assumes 100% efficiency, then current would be 15.5A.

Cable run is very short, about 10', and I have plenty of #12 and spare 20A breakers. Using the 80% rule in theory the 20A breaker should be marginally okay. If I had some #10 and a 25 or 30A breaker I would use it, I'm inclined to give what I have a try unless someone tells me I tried that and the breaker was constantly tripping on startup.

Anyone running their 5HP on a 20A breaker?
Click to expand...
My 5HP Sanborn runs fine on 20A 240. Never had a problem in the last 30 years.
 
I don't think that's a 5Hp motor. If it draws 15a @ 240v, then its probably 2-3hp. My motor FLA is 22amps. And I use a 60amp breaker and 60amp disconnect safety switch. I cant remember the wire size I used, but I can check.

Also for motors, I believe you have to use a wire gauge at 125% of FLA.
 
You should be fine on 20A/240V -- Some of the newer breakers trip faster, so you might run into the occasional trip on startup - it is not a high probability, but depending on your brand of box/breakers, it might. The older breakers take longer to trip, especially Federal Stab-lok (the original good ones).

You can't go wrong with a 30A breaker and a 10 gauge lead in, either...
 
Thanks for the feedback. I found a short piece of 10-3, so I used that for future proofing. I will try the 20A breakers.
 
Okay so now I'm confused ( I know, surprise).

I have a 5hp/26 gal CH that I bought in 96 or 97. The data plate on the Emerson motor lists 120/240v, 15/7.5 amp and 2.25kw, none of the math I know makes 5hp anywhere in those numbers using 750watts for one hp. The HP rating of the motor is "SPL", I'm guessing that means "Special" lol.

5hp takes around 3.75kw so how does the magic math work for the hp on this motor? 5hp on 120v would take around 31amps and on 240 would be around 15.5amps.

I've been running this machine on 15amps for years so either the motor is over rated or all the electrical engineers calculations are goofed.

@slow-poke - not trying to hijack, I find the numbers interesting and am trying to figure out how the manufacturers come up with their calculations.
 
I run a compressor labeled as 5 HP on a 115 volt 15 amp dedicated circuit. Since the circuit supplies 1,800 watts and 5 HP is 3,730 watts the label is probably a lie.
 
Even the new stuff has questionable numbers, a new CH at HD is rated 230 Volt, 15.7 Amps, 1PH, 60 Hz Electrical and advertised as 3.7hp.
230v @15.7amps is 3611watts which is 5hp.
 
Not sure this will help but I'll try.

Motor HP ratings are always maximums and even at that only potential maximums. Just cuz your motor is rated at 500 HP doesn't mean you can or will use it. The HP rating for a motor is also rated at an RPM.

Its easy to imagine a 5HP motor running at just 1/4HP when there is no load.

Now you guys are talking compressors, which are usually high hp devices. However, if the compressor on your 5HP machine is only capable of absorbing 2HP at the motors rated RPM, then that's all the motor will deliver.

To determine the power supply requirements, one must know the load requirements, not just the motor hp.
 
Compressor motors usually have 'special' ("SPL") ratings, due to high torque requirements to start moving the piston when power is first applied. It can be in a compression portion of the stroke, so there is a lot of force to overcome. Startup current in these motors is somewhere around 2X the running current under load. That current can be for the greater part of a second, so the windings and the rest of the construction has to handle the greater heating caused by these startup conditions.

The motor plate shows the running current under design load.

You need a slow tripping breaker at 20A or a 30A breaker to be sure it won't trip during startup.
 
I would hope there is an unloader on most compressors, which makes it much easier for a motor to start a compressor. Some compressors will be set up to unload anytime compressor stops, others may need to be set and will only unload at stop set point. Some units will not start pumping air unless compressor is turning a certain rpm, a few of the ways to allow a motor to start working, with out pulling a much higher amperage then normal operation. Many motors will need 2X or more amperage to start compared to "normal" running amperage, as per Dabbler. This maybe marked as "FLA" full load amps on motor plate.
Yes, the listed wattage does not work out with hp. and amps X volts. Look at the newer vacuum cleaners, these "suckers" should be able start an engine, a very Big LOL. I have not got an answer or even a good idea of one about that.
 
I know my true 5hp IR compressor has tripped the 30a 240v breaker a couple of times, not usually, but in the years I've had it it has managed to trip on a start. I wanted to wire for 40a, but the wiring box on the compressor wouldn't fit 8/3 wire, so had to downsize to 10/3 and 30a.
 
IIRC there was a class action lawsuit regarding all this "peak" horsepower nonsense.

Reminds me of the peak amplifier ratings so common for car stereo In the 70's. Assuming the amp did not use a switcher, the peak voltage was 12V and if you do the math for an 8 Ohm speaker that works out to less than 9W RMS. The funky arithmetic used for advertising claimed up to 400peak Watts!

Funky math:
Car is running so battery voltage is 14V
2 Ohm speakers
14*14/2 = 98 peak Watts, but we have 4 speaker, so 400 Watts

When in doubt, just multiple the Voltage by the current and divide by 750 for a ballpark number.

My old CH is advertised as 4.5 peak HP, but will actually run on 120V, 15A circuit.
120*15/750 = 2.4HP, not exactly sure on the losses assume 20% and it is closer to a 2 actual horsepower motor.
 
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Actually, if you read the literature carefully you find that this is Peak HP, as in start up when the compressor draws most (momentarily) comoared running load, once its running the data plate gives the draw to be expected, which is why the lower fused systems have no issue.

If you have a motor rated at 15amps and a 15amp fuse you will wear out the fuse as start up will be around 17-25amps causing it to trip (and trip more easily as it ages). However a motor rated for 8-10 will only draw 12-18 on start up which is within the limits as it is not stressed at the 15amp running full load.

My guess you have 2-2.5hp motor with a starting momentary performance of 5hp.

Want to ease start up draw go 3phase VFD. Draw remains constant as compressor spools up.
 
I installed a 80G/5HP Devair on a 30amp stablok but also put a small knife switch fuse box between the two for good measure.
 
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