Compressor issues and breakers

There was a posting about a compressor with a 1.8HP motor tripping breakers.

According to my calculations, that would draw 20 amps, but the owner stated the manual said use a 20 amp breaker. It would not be the first time a manufacturer pushed the limit past the reasonable.

I learned that a circuit should never be run at more than 80% continuous. That means a 1.5HP motor would draw 16 amps, and would fill up a 20A breaker.

These numbers would also need derated if using to small or long of cord.

Please correct my math if wrong.

It depends if you are didviding the wattage by 110 v or 220 v. If I use 746 watts per HP I get like 5 amps @ 22oV.

Well in his reply to his post he said that set up has worked for years and is now over loading the motor, so I agree with him that the motor is probably the cause, but yes he needs a bigger breaker!

Your MATH may be OK, but air compressor makers used OUTLANDISH HP claims for years, especially for the consumer-grade units.

HOPEFULLY, when he looks at the motor nameplate the FLA posted there will be accurate.

That's how I did it when I designed, if I had a 20 amp breaker Id use 20 amp rated wire but the maximum continuous full load amps Id limit the branch circuit to would be 80% x 20 or 16 amps.

There were exceptions in motor branch circuits where if the breaker wouldn't allow the motor to start it could be oversized subject to thermal overload motor protection being present.

John T

David G,
For less than $20 HF sells a good digital amprobe that will measure peak amps. I have one. Recommend everyone who has an electricial question, do the math and buy a cheap digital amprobe. I also have an old analog amprobe, but the digital is better at giving you the starting amps.

My experience with air compressors is there is a very good chance either the capacitor is bad or the end switch is bad. I would first take motor apart, clean end switch before I replaced the cap. There is a smaller chance you have a bad start winding.

Stop guessing, stop doing the math, get an amprobe and find exactly what happening.
George

A 1.8HP single phase 120V motor without power factor correction caps would draw 22amps.
The proper sized supply breaker to supply short circuit protection would be 22.0 X 250% = 55amp which rounds up to 60 amp.
Thermal overload protection located in the motor or in the motor starter would be likely 115% or 25.3amp.
There is a long line of people who don't seem to know the difference between short circuit protection and thermal protection.

Nice feedback,

I think you are on target with the HP label meaning NOTHING, would need to get FLA off motor. The average consumer would never know about this.

I assumed 750W per HP and 120VAC nominal voltage.

Most compressor "HP" rating are total fiction. The motor nameplate "amps" ratings are usually fairly accurate.

If you have trouble tripping the recommended circuit breaker, you should check for voltage drop. If the voltage at the compressor is much less than 120 volts under load, you need to beef up your wiring and/or look for bad connections.

The thing to understand is a motor will draw however much current it needs to handle its load. If the voltage drops, it needs to draw more current to make up for it.

Compressors are like vacuums and power saws. Horsepower ratings are vastly overstated. Many times horsepower is based on starting amperes. Check the FLA amps on the nameplate. Breaker should not be loaded at more than 80% of rating on a continuous basis. By the NEC, continuous load is defined at more than three hours duration. If it had worked fine before, look at a bad start switch, bad capacitor, or faulty unloader valve on the pressure switch.

My understanding of the outlandish HP ratings we see and have seen is that since electrically developed HP is measured in watts (746 of em per HP, aka 0.746 kW) which are volts x amps (and a little power factor thrown in with ac circuits). So they monitor the volts and lock the rotor monitoring the amps. Throw the switch. As the amps ramp up they watch the volts and as soon as they fall through a pre determined minimum where the product of volts and amps is a maximum that is the number used for the rating.

My opinion it's nothing but a marketing ploy to help in justifying the cost of things making you think you are getting something for your money when you aren't.

So when you are running your vacuum cleaner with an advertised 1 hp motor on a 25' 18AWG cord plugged into 120 volts and it doesn't get hot from current overloading, you know why.

Finally if you are buying a motor and want to know what it really is, multiply the line running amps (stamped on the ID plate) by the applied voltage, multiply that by 0.85 to account for internal osses, and another 0.85 for worst case power factor and you are at about what you have for a motor.

So running (120v and 16a x .85 x .85)/746 = 1.9 or about 2 hp.

Buttttttttt what's new?

Mark