I have a 5-hp air compressor, 230-volt, single phase. The compressor has a Baldor motor with overload protection, a FLA of 28 amps and a magnetic starter. Wire run distance from the circuit breaker panel to the compressor is less than 40 feet. I plan on having THHN wire (2 conductors plus ground) installed in the appropriate sized EMT conduit dedicated circuit. What gauge wire and circuit breaker is required for this service. THANKS!
I should also mention I live in Scottsdale AZ where summer temperatures can get to 110 degrees F in my shop.
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1) The branch circuit breaker for an electric motor is calculated at 1.25 times the FLA rating due to the "in-rush" of current upon the motor start-up. Therefore, the breaker is sized at 1.25 x 28 amps = 35 amps. Round the breaker up to the next standard size of a 40 amp double pole breaker.
2) Using 240 volts AC, A 40 foot run and a 35 amp load results in a .9 % voltage drop (VD) or a 2.1 voltage drop. Code recommends a maximum of 3 % voltage drop. This calculation is based on using 8 AWG Stranded Copper, Type THHN for the (2) two hot conductors. Therefore the VD is OK.
3) The equipment grounding conductor will be 10 AWG Stranded Copper, Type THHN.
4) The conduit fill ratio should be no more than 40% per code. Using (2) two 8 AWG's and (1) one 10 AWG results in a 31% conduit fill ratio using 1/2" EMT. Therefore, 1/2" EMT is acceptable as the raceway size.
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