Most of Article 240 in the National Electrical Code (NEC) provides general requirements for overcurrent protection and overcurrent protective devices not more than 1,000 volts (V), nominal. As a general rule, the overcurrent device rating shall not exceed the ampacity of a conductor. As stated in 240.4, 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 first part of this rule states that the conductors shall be protected against overcurrent in accordance with their ampacities. This means that the overcurrent device rating shall be equal to or less than the conductor ampacity.

For example, in the 75°C column of Table 310.15(B)(16), 2 AWG THWN copper conductors have an ampacity of 115 amperes (A). Of course, these conductors are only good for 115A, assuming the ampacity has not been lowered because of ambient temperature or more than three current-carrying conductors. In accordance with the first part of 240.4, the maximum rating for an overcurrent device protecting 2 AWG THWN copper conductors would be 115A, but 115 is not a standard ampere rating. See 240.6(A) for a list of standard ampere ratings for fuses and inverse-time circuit breakers. Since 115 is not a standard rating and the conductors must be protected against overcurrent in accordance with their ampacity, the maximum rating for an overcurrent device protecting these conductors would be 100A. But, there is more to 240.4. This section continues by stating conductors 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 other provisions in 240.4(A) through (G) do not necessarily require the rating of the overcurrent device to be equal to or less than the ampacity of the conductor. These other provisions can be thought of as alternative rules.

A well-known alternative rule—sometimes referred to as the round-up rule—pertains to overcurrent devices rated 800A or less. Where all three conditions in 240.4(B) are met, the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used. The first condition states that the conductors being protected shall not be part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. The second condition pertains to the rating of the overcurrent device. If the ampacity of the conductors is the same as a standard rating, rounding up is not permissible. The second condition will be met if the ampacity of the conductors does not correspond with a 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). The last condition will be met as long as the next higher standard rating selected does not exceed 800A when rounding up. If the 2 AWG THWN copper conductors previously mentioned are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, and correction and/or adjustment factors have not lowered the allowable ampacity to 100A (or less), an overcurrent device with a rating of 125A can be installed to protect the conductor (see Figure 1).

Besides the round-up rule, there are other alternative rules in 240.4. Some pertain to small conductors, tap conductors and transformer secondary conductors. Section 240.4(G) pertains to overcurrent protection for specific conductor applications. Overcurrent protection for the specific conductor applications shall be permitted to be provided as referenced in Table 240.4(G). With items listed in Table 240.4(G), it is permissible to size the rating of the overcurrent protective device by the article or section shown in the table. For example, Table 240.4(G) shows Article 440, Parts III and VI as the references for air conditioning and refrigeration equipment circuit conductors. Therefore, when sizing branch-circuit short-circuit and ground-fault protection for air conditioning and refrigeration equipment, use the provisions in Article 440, Part III.

Motors and motor-control circuit conductors also are in Table 240.4(G). Instead of using Article 240 to determine the overcurrent device rating for motors and motor-control circuit conductors, it shall be permitted to use Article 430, parts II, III, IV, V, VI and VII. Article 430 covers motors, motor branch-circuit and feeder conductors, motor branch-circuit and feeder protection, motor overload protection, motor-control circuits, motor controllers and motor-control centers.

When looking for requirements in Article 430, Figure 430.1—which serves as a type of table of contents to the article—can be very helpful. This figure is divided into two halves. The top half is a sequential listing of all 14 parts as they appear in the article. For example, provisions pertaining to motor-circuit conductors are in Part II, sections 430.21 through 430.29. The bottom half shows the components or elements in relationship to the motor. For example, the rectangle or box above the drawing of the motor represents motor-overload protection and requirements pertaining to motor-overload protection are in Part III (see Figure 2).

While conductors for many loads are sized by the actual load or the Article 220 calculated load, conductors for motors are not sized by the motor’s actual load, which is shown on the nameplate. The amperes shown on a motor’s nameplate are usually referred to as full-load amperes (FLA). Section 430.6 covers ampacity and motor-rating determinations. The required ampacity and motor ratings shall be determined as specified in 430.6(A), (B), (C) and (D). The section that is referenced most often is 430.6(A), which pertains to general motor applications. Current ratings for general motor applications shall be determined based on 430.6(A)(1) and (A)(2). Other than for motors built for low speeds (less than 1,200 rpm) or high torques and for multispeed motors, the values given in tables 430.247 through .250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate [430.6(A)(1)]. The amperes shown in tables 430.247 through .250 are full-load currents (FLC). Motors built for low speeds (less than 1,200 rpm) or high torques may have higher FLCs, and multispeed motors will have FLC, varying with speed, in which case the nameplate current ratings shall be used.

Motor-overload protection was not in the list of items that must be sized in accordance with Tables 430.247 through .250. That’s because, in accordance with 430.6(A)(2), separate motor-overload protection shall be based on the motor’s nameplate current rating. Motor-­overload protection usually is referred to as overloads or heaters.

In short, the FLCs in Tables 430.247 through .250 shall be used for most motor calculations. For example, branch-circuit conductors, a branch-circuit short-circuit and ground-fault protective device, a disconnect (safety switch), and a motor controller (starter) will be needed for a 15-horsepower (hp), three-phase motor. The nameplate on the 15-hp motor shows the motor will draw 34A when supplied by 230V. What ampacity or current rating shall be used when determining the branch-circuit conductor size, the branch-circuit short-circuit and ground-fault protective device rating, the ampere rating for the disconnect, and the separate motor overload device rating?

In accordance with Table 430.250, the FLC of a 15-hp, three-phase, 230V motor is 42A. Although the nameplate shows the motor will draw only 34A, the FLC of 42A shall be used for most of these motor calculations. With this motor, use the table value of 42A when determining the branch-circuit conductor size, the branch-circuit short-circuit and ground-fault protective device rating, and the ampere rating for the disconnect. Use the nameplate value of 34A when determining the separate motor-overload device rating (see Figure 3).

Next month’s article will continue the discussion of sizing conductors.