How Do You Size Overload Protection for a 1.15 Service Factor Motor?
A student working an exam question on a 40 horsepower, 460 volt, three phase, 1.15 service factor, Class B, totally enclosed wound rotor motor asked the app how the service factor changes the overload setting. It changes it by exactly one step: a motor marked 1.15 service factor or higher gets 125 percent instead of 115 percent.
The 430.32 decision tree
Section 430.32(A)(1) covers continuous duty motors rated more than 1 horsepower. Three cases, and only three: a motor with a marked service factor of 1.15 or greater is set at not more than 125 percent of the motor nameplate full load current, a motor with a marked temperature rise of 40 degrees C or less is also 125 percent, and all other motors are 115 percent.
The critical detail people miss is which current you use. Overload protection is sized from the nameplate full load amperes, not from the Table 430.250 value. Table 430.250 is for conductors, short circuit protection, and disconnect sizing. So for the 40 horsepower motor, Table 430.250 lists 52 amps at 460 volts and that number sizes the conductors at 65 amps per 430.22, but the overload relay is set from whatever the nameplate says, which might be 51 amps or 54 amps depending on the manufacturer. If the nameplate reads 52 amps, the maximum overload setting is 52 times 1.25, or 65 amps.
When 125 percent will not let the motor start
Section 430.32(C) is the relief valve. If the overload device selected under 430.32(A)(1) is not sufficient to start the motor or carry the load, the next higher size may be used, but not beyond 140 percent of nameplate full load current for a 1.15 service factor or 40 degree C rise motor, and not beyond 130 percent for everything else. For the 52 amp example that ceiling is 72.8 amps. You do not jump straight there. You go up one size at a time and stop as soon as the motor starts reliably.
Wound rotor motors bring one extra requirement. Section 430.23 sizes the secondary conductors between the slip rings and the controller at 125 percent of the secondary full load current for continuous duty, and the resistor conductors get their own multipliers from Table 430.23(C) based on duty class. The secondary circuit does not get overload protection of its own. The primary overload relay protects the whole machine.
Do not confuse overload with short circuit and ground fault protection. Table 430.52 sets the breaker or fuse, and for a design B energy efficient motor an inverse time breaker may be up to 250 percent of the Table 430.250 current, with 430.52(C)(1) Exception 1 allowing the next standard size. That breaker will look enormous next to the conductors, which is correct. The overload relay is what actually protects the conductors and the windings from sustained overcurrent.