As stated in 240.4 in the National Electrical Code (NEC), conductors, other than flexible cords, flexible cables and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). The main rule in this section states the ampere rating of the overcurrent device must not be higher than the ampere rating of the conductor being protected, but there are other provisions in the subsections below the main rule.

One of those other provisions is often referred to as the round-up rule. As stated in 240.4(B), it is permissible to use the next higher standard overcurrent device rating (above the ampacity of the conductors being protected), provided all of three conditions in 240.4(B) are met. The first condition states the conductors being protected cannot be part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. The second condition states it would be permissible to round up to the next size fuse or breaker as long as the ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

As stated in the third condition in 240.4(B), the next higher standard rating of the overcurrent device selected cannot exceed 800 amperes (A). Another subsection in 240.4 pertains to overcurrent protection for specific conductor applications. Overcurrent protection for specific conductors can be provided as referenced in Table 240.4(G). Motors and motor-control circuit conductors are listed in this table for specific conductor applications.

Some of the conductor types in Table 240.4(G) reference an article and one or more section numbers. With some of the conductor types, there are too many sections to reference. A few of the conductor types listed in this table reference an article and one or more part numbers. With most loads, fuses or circuit breakers are installed to provide both overcurrent and overload protection. Quite often with motors, fuses or circuit breakers are not the devices used for overload protection.

Requirements pertaining to motor and branch-circuit overload protection are in Part III of Article 430. As stated in 430.31, Part III specifies overload devices intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. Overload protective devices are usually installed in the motor controller. For the purpose of Article 430, a controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current (430.2).

Out in the field, motor controllers are frequently called “starters” or “motor starters.” Each motor used in a continuous duty application and rated more than 1 horsepower (hp) shall be protected against overload by one of the means in 430.32(A)(1) through (A)(4). The first means listed to protect against overload is a separate overload device. These devices are usually called “heaters” or “overload” out in the field. A separate overload device that is responsive to motor current shall be selected to trip or shall be rated at no more than the percent of the motor nameplate full-load current (FLC) rating shown in 430.32(A)(1). The percentage for motors with a marked service factor of 1.15 or greater is 125 percent. The percentage for motors with a temperature rise 40°C or less is also 125 percent. For all other motors, the percentage is 115 percent.

When sizing branch-circuit conductors for motors, it is required to use the FLC values given in Table 430.247, Table 430.248, Table 430.249 and Table 430.250. When determining the rating of the motor overload device, it is permissible to use the amperes on the motor’s nameplate.

For example, what size motor overload protective device is required for a 10-hp, 208-volt (V), three-phase motor? This motor will be used in a continuous-duty application. The motor’s nameplate shows this motor will draw 26.6A when the voltage is 208V, three-phase. This motor has a marked service factor of 1.00 and a temperature rise of 50°C. First, look in 430.32(A)(1) for the percent that will be used to find the rating of the overload device. This motor has a marked service factor, but it is not 1.15 or greater. This motor also has a marked temperature rise, but it is not 40°C or less. Therefore, the percent needed for this motor is 115 percent. Because it is permissible to use the motor nameplate FLC rating when calculating motor overload protection, use 26.6A. The rating of the separate overload device for this 10-hp, 208V, three-phase motor shall not exceed 30.59A (26.6 × 115 percent = 30.59) (see Figure 1).

If a motor has either a marked service factor or a temperature rise that meets the parameters in 430.32(A)(1), the percentage is 125 percent. For example, what size motor overload protective device is required for a 2-hp, 208V, three-phase motor? This motor will be used in a continuous-duty application. The motor’s nameplate shows this motor will draw 5.7A when the voltage is 208V, three-phase. It has a marked service factor of 1.15 and a temperature rise of 50°C. This motor has a marked service factor that meets the parameter shown. This motor also has a marked temperature rise, but it is not 40°C or less. Because it meets one of the first two types of motors with the percentage of 125 percent, use 125 percent when calculating motor overload protection. The nameplate FLC rating for this motor when the source voltage is 208V, three-phase, is 5.7A. The rating of the separate overload device for this 2-hp, 208V, three-phase motor shall not exceed 7.13A (5.7 × 125 percent = 7.125 = 7.13) (see Figure 2).

If a motor has both a marked service factor and a temperature rise that meets the parameters in 430.32(A)(1), the percentage is 125. For example, what size motor overload protective device is required for a 5-hp, 208V, three-phase motor? This motor will be used in a continuous-duty application. The motor’s nameplate shows it will draw 15A when the voltage is 208V, three-phase. This motor has a marked service factor of 1.15 and a temperature rise of 40°C. This motor has a marked service factor that meets the parameter shown and has a marked temperature rise that meets the parameter shown. Use 125 percent if the motor meets the parameter for service factor or temperature rise or if a motor meets the parameters for both. The nameplate FLC rating for this motor when the source voltage is 208V, three-phase, is 15A. The rating of the separate overload device for this 5-hp, 208V, three-phase motor shall not exceed 18.75A (15 × 125 percent = 18.75) (see Figure 3).

Section 430.32(C) also shows motors with a marked service factor and a temperature rise, but the percentages in this subsection are 140 and 130 percent instead of 125 and 115 percent. It is permissible to use these percentages but not without selecting motor overloads in accordance with 430.32(A)(1) first. Where the sensing element or setting or sizing of the overload device selected in accordance with 430.32(A)(1) and 430.32(B)(1) is not sufficient to start the motor or to carry the load, higher size sensing elements or incremental settings or sizing shall be permitted to be used, provided the trip current of the overload device does not exceed the percentage of motor nameplate FLC rating shown in 430.32(C).

For example, motor overload protective devices were selected in accordance with 430.32(A)(1) for a 5-hp, 208V, three-phase motor, but the size selected was not sufficient to allow the motor to start. As long as incremental ratings above the rating specified in 430.32(A)(1) are used, what is the maximum overload rating that can be used for this 5-hp, 208V, three-phase motor? This motor will be used in a continuous duty application. The motor’s nameplate shows this motor will draw 15A when the voltage is 208V, three-phase. This motor has a marked service factor of 1.15 and a temperature rise of 40°C. Because the service factor and the temperature rise meets the parameters, use 140 percent when determining the rating for the overloads. The nameplate full-load current rating for this motor when the source voltage is 208V, three-phase, is 15A. The maximum rating of the separate overload device shall not exceed 21A (15 × 140 percent = 21) (see Figure 4).

Next month’s column continues the discussion of requirements for motors, motor circuits and controllers.

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