Recently, a question came up about applying the National Electrical Code (NEC) to achieve minimum compliant overcurrent protection for a general motor circuit to protect both the motor and conductors. This column reviews the steps to use and Code sections that apply.
The motor circuit to protect is a continuous- duty, 75-horsepower (hp), 208-volt (V), three-phase motor (without internal thermal protection), and the branch circuit conductors (XHHW copper) are routed through an ambient temperature of 86°F. The service factor marked on this motor is 1.15, and the branch-circuit short-circuit ground-fault protective device selected for this application is a dual-element, time-delay (DETD) fuse. The temperature ratings of the terminals at the motor and within the equipment to which the circuit conductors are connected are rated 75°C.
This scenario contains key information. First, the motor is continuous duty, and the circuit is installed through an 86-degree temperature that relates directly to conductor sizing. To start, it must be understood that the total overcurrent protection for the motor and circuit is the combination of the branch-circuit short-circuit ground-fault protective device and the required overload protective device. Let’s break this all down.
Section 430.6(A)(1) indicates, for general motor applications, the ampacity values in the tables (430.247, 430.248, 430.249 and 430.250) as applicable shall be used instead of the actual motor nameplate current. 430.6(A)(2) requires the use of the motor nameplate current for sizing separate overload protection. Referring to Table 430.250, a value of 211 amperes (A) must be used for a 208V, 75-hp motor.
Recalling that for continuous-duty motor applications, motor circuit conductor sizing is achieved by referring to Section 430.22(A), which indicates to size the conductors at 125 percent of the applicable table value current. The value 211A at 125 percent is 263.75A. Section 430.6 instructs users to refer to Section 310.15(B) for minimum conductor ampacities.
Remember the ambient temperature in this scenario (86°F). Table 310.15(B)(16) indicates a 300 kcmil XHHW conductor (75°F insulation) carries 285A. Because the ambient temperature is 86°F, no additional correction for ambient temperature is required. The minimum size motor circuit conductors are established at 300 kcmil XHHW copper.
Now, on to sizing the branch-circuit short-circuit ground-fault protective device. Section 430.52(B) indicates the motor branch-circuit short-circuit ground-fault protective device shall be capable of carrying the starting current of the motor, which is typically the motor’s locked-rotor current (roughly six times the motor’s full-load amperes). According to 430.52(C)(1) and Table 430.52, the maximum size DETD fuse that can be used is 175 percent of the 430.250 table ampacity of 211. Doing the math, 369.25A cannot be exceeded using a DETD fuse.
Exception No. 1 to 430.52(C)(1) indicates the next standard size DETD fuse can be used. Referring to Section 240.6 and Table 240.6(A), it appears that a 400A fuse is the next standard size above 369.25A. The maximum size branch-circuit short-circuit ground-fault protective device is established at 400A.
On to sizing the overload protection for this motor circuit. Remember that, if the motor is internally thermally protected, separate overload protection is not required. That is not the case in this example.
Section 430.32(A)(1) provides sizing requirements for overload protection that is supplied separately. In this case, the overload protection is included in a magnetic motor starter. Use of the manufacturer’s overload relay table is required, according to 430.52(C)(2). A close look at 430.32(A)(1) generally indicates that, for a motor having a service factor of 1.15 or greater, the maximum rating of the overload protective device is not to exceed 125 percent of the motor’s nameplate current. If the overload device does not carry the motor’s starting current, it may be increased, but it shall not exceed 140 percent of the nameplate current rating. The motor’s nameplate current indicates 203A. Doing the math, the overload protective device generally must not be greater than 253.75A. Use the manufacturer’s relay table on the motor controller to find the corresponding maximum overload protective unit for this application.
When attaining minimum NEC compliance for motor and motor-circuit overcurrent protection, multiple factors are in play. First, the motor has to start, so sizing the motor branch-circuit short-circuit ground-fault protective device may require larger sizes than what would normally be required for conductor protection in a nonmotor circuit. In this example, a 400A DETD fuse could be installed on the motor circuit conductors that are sized at 300 kcmil XHHW copper. Remember that the DETD fuse protects against ground-fault and short-circuit events only, not overload. The separate overload protection is sized at not to exceed 253.75A, which protects the motor and the circuit conductors from overload. The sizes determined for this motor example are the minimum and maximum sizes required. The NEC is the minimum, meaning one must do at least that much.