What Wire Size for a 10 HP, 208 Volt, Three Phase Water Pump?
A user asked the app to size conductors for a 10 horsepower, 208 volt, three phase pump motor. Motor circuits follow their own logic in Article 430, and the first rule is the one most often missed: you size conductors from the Code table current, not from the nameplate. The answer here is 8 AWG copper at 75 degrees C.
The Article 430 walkthrough
Start at Table 430.250, which lists full load current for three phase AC motors. A 10 HP motor at 208 volts draws 30.8 amps per the table. Section 430.22 requires branch circuit conductors for a single continuous duty motor to have an ampacity of at least 125 percent of that table value: 30.8 times 1.25 equals 38.5 amps. In the 75 degree C column of Table 310.16, 8 AWG copper is rated 50 amps, comfortably above 38.5, while 10 AWG at 35 amps falls short. So 8 AWG THHN or THWN copper is the minimum, with 8 AWG aluminum at 40 amps also technically passing if terminations are rated for aluminum.
Short circuit and ground fault protection comes from Table 430.52. For an inverse time breaker the maximum is 250 percent of full load current: 30.8 times 2.5 equals 77 amps, and Section 430.52(C)(1) Exception 1 allows the next standard size up, which is 80 amps. That large breaker startles people used to matching breakers to wire, but it is correct: the breaker only handles faults and locked rotor inrush. Running overload protection is separate under Section 430.32, set from the motor nameplate current, typically 115 or 125 percent depending on service factor and temperature rise, and it usually lives in the starter or the pump controller.
Pump specific details
Long runs to well heads and irrigation pumps make voltage drop the real conductor driver; at 208 volts there is little margin, so many pump installers upsize to 6 AWG past 100 feet and let the numbers, not habit, decide. A submersible deep well pump also brings Section 430.31 overload requirements into the controller, and if the pump serves a residence from a separate structure, disconnect rules in 430.102 apply at the controller location. If the motor is inverter driven, size conductors per the drive instructions under 430.122, which uses 125 percent of the drive rated input current instead.