wildbill46
New member
I recently purchased an air compressor with an induction motor marked "6.5 hp peak". I need the rated hp to size the breaker at 240 volts.
Gentlemen
- Thread starter wildbill46
- Start date
- Status
stickboy1375
Senior Member
- Location
- Litchfield, CT
mdshunk
Senior Member
- Location
- Right here.
Vacuum sweepers use the same game too.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
My Craftsman is rated for 6.5 HP. When I removed the cover and looked at the motor is said HP__, Volts 120, Amps 15. Since the HP rating was conveniently left blank I looked in 430.248 and came up with about 1 HP.
cadpoint
Senior Member
- Location
- Durham, NC
- Occupation
- Electrician - but not by NC Law.
Come on Teach, there's no free answer here, Welcome to the Forum !wildbill46 said:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080427-2125 EST
wildbill46:
What is the purpose of your breaker and where is the breaker located?
If the breaker is on the pump to protect the motor, then size it so that you do not get startup trips. Obviously this breaker would be smaller than the main panel breaker.
If the breaker is at your main breaker box then it is sized based on the load wires, and these at a minimum are sized on load current. For example #12 wire and a 20 A breaker may be what you need.
Your motor probably has internal thermal overtemperature protection and a breaker sized to do a better protection job may not economically exist. Obviously for a locked rotor fault condition the breaker will respond sooner than the internal temperature rise protector. Locked rotor fault on this pump is very unlikely. But if the motor is overheated because of continuous operation and high ambient temperature a typical breaker sized to tolerate the inrush current will not trip.
I looked at my 20 yr old or more Sears compressor and the motor reads --- 2 HP continuous duty at 40 deg C (104 deg F), 3450 RPM, 230 V, at 12 A. This calculates to an input of 2760 VA. The output power calculates to 1492 W. At 70% efficiency that is 2131 W. These all seem reasonable. My judgement is you would want a main panel breaker of 20 A (Sq-D QO20) and #12 wire. A 50 ft run from the main breaker box to the pump with #12 would be reasonable. I have not measured inrush current on this pump, but my guess is greater than 80 A.
Long wire runs and motors present a problem on inrush current that may require increasing the wire size to reduce voltage drop.
My guess is that you are not a licensed electrican. If this is the case, then when you figure out what you need hire one to do the work, and he should check your conclusions.
The basic philosophy of circuit protection is that the protector protects the wiring or device directly connected to the protector.
.
wildbill46:
What is the purpose of your breaker and where is the breaker located?
If the breaker is on the pump to protect the motor, then size it so that you do not get startup trips. Obviously this breaker would be smaller than the main panel breaker.
If the breaker is at your main breaker box then it is sized based on the load wires, and these at a minimum are sized on load current. For example #12 wire and a 20 A breaker may be what you need.
Your motor probably has internal thermal overtemperature protection and a breaker sized to do a better protection job may not economically exist. Obviously for a locked rotor fault condition the breaker will respond sooner than the internal temperature rise protector. Locked rotor fault on this pump is very unlikely. But if the motor is overheated because of continuous operation and high ambient temperature a typical breaker sized to tolerate the inrush current will not trip.
I looked at my 20 yr old or more Sears compressor and the motor reads --- 2 HP continuous duty at 40 deg C (104 deg F), 3450 RPM, 230 V, at 12 A. This calculates to an input of 2760 VA. The output power calculates to 1492 W. At 70% efficiency that is 2131 W. These all seem reasonable. My judgement is you would want a main panel breaker of 20 A (Sq-D QO20) and #12 wire. A 50 ft run from the main breaker box to the pump with #12 would be reasonable. I have not measured inrush current on this pump, but my guess is greater than 80 A.
Long wire runs and motors present a problem on inrush current that may require increasing the wire size to reduce voltage drop.
My guess is that you are not a licensed electrican. If this is the case, then when you figure out what you need hire one to do the work, and he should check your conclusions.
The basic philosophy of circuit protection is that the protector protects the wiring or device directly connected to the protector.
.
cadpoint
Senior Member
- Location
- Durham, NC
- Occupation
- Electrician - but not by NC Law.
One might take the time to read one's Bio / Profile, it might cause some reflection of a before / or after thought.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080428-2112 EST
cadpoint:
Good point. I should have done that.
.
cadpoint:
Good point. I should have done that.
.
- Status