10.6 Motors Part I: Full-Load Current, Conductors and Overload Protection

The Rule That Organizes Article 430

Motor circuits use two different current values, and knowing which one applies where is most of the battle:

The reason is design intent. The tables give conservative, standardized values so that conductors and short-circuit protection are sized generously and consistently. Overload protection is meant to track the actual motor, so it uses the actual nameplate.

430.6(A)(1) Table Values. Other than for motors built for low speeds or high torque, and for multispeed motors, the values given in Tables 430.247, 430.248, 430.249, and 430.250 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. Exception No. 1 applies to multispeed motors; Exception No. 2 to listed motor-operated appliances; Exception No. 3 to torque motors.

430.6(A)(2) Nameplate Values. Separate motor overload protection shall be based on the motor nameplate current rating.

430.6(B) Torque Motors. For torque motors, the rated current shall be locked-rotor current, and this nameplate current shall be used to determine the ampacity of the branch-circuit conductors, the ampere rating of motor overload protection, and the ampere rating of motor branch-circuit short-circuit and ground-fault protection.

430.6(D) Ampacity and Motor Rating Determination — Wye-Start, Delta-Run. For a wye-start delta-run connected motor, the selection of branch-circuit conductors on the line side of the controller shall be based on the motor full-load current, and the conductors between the controller and the motor shall be permitted to be sized at 72 percent of the motor full-load current rating.

The Tables

Selected values from Table 430.250, three-phase, full-load current in amperes:

Selected values from Table 430.248, single-phase:

Conductor Sizing — Part II

430.22 Single Motor. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current rating, as determined by 430.6(A)(1), or not less than specified in 430.22(A) through (G).

• (A) Direct-Current Motor-Rectifier Supplied. For DC motors operating from a rectified power supply, specific multipliers apply.
• (B) Multispeed Motor. For a multispeed motor, the selection of branch-circuit conductors on the line side of the controller shall be based on the highest of the full-load current ratings shown on the motor nameplate.
• (E) Duty-Cycle Service. Conductors for a motor used in a short-time, intermittent, periodic, or varying duty application shall have an ampacity of not less than the percentage of the motor nameplate current rating shown in Table 430.22(E).
• (G) Conductors for Small Motors. Conductors for small motors shall not be smaller than 14 AWG, with exceptions permitting 18 AWG and 16 AWG under stated conditions.

430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor and other loads, shall have an ampacity not less than the sum of each of the following:

1. 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A);
2. Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A);
3. 100 percent of the noncontinuous non-motor load;
4. 125 percent of the continuous non-motor load.

The 125 percent applies to only one motor — the highest rated. Where two or more motors of the same rating are the largest, only one is taken at 125 percent.

430.25 Multimotor and Combination-Load Equipment. The ampacity of the conductors supplying multimotor and combination-load equipment shall not be less than the minimum circuit ampacity marked on the equipment.

430.26 Feeder Demand Factor. Where reduced heating of the conductors results from motors operating on duty-cycle, intermittently, or from all motors not operating at one time, the authority having jurisdiction may grant permission for feeder conductors to have an ampacity less than specified in 430.24.

430.28 Feeder Taps. Feeder tap conductors shall have an ampacity not less than that required by Part II, shall terminate in a branch-circuit protective device, and shall meet one of three length and ampacity conditions: not over 10 feet long and having an ampacity at least one tenth of the feeder rating; not over 25 feet long with an ampacity at least one third of the feeder conductors; or the same ampacity as the feeder conductors.

Worked Example — Single Motor Conductors

A 460-volt, three-phase, 30 horsepower continuous-duty motor with a nameplate FLA of 38 amperes and a service factor of 1.15.

Table 430.250, 30 hp at 460 V ......... FLC = 40 A
Conductor ampacity, 430.22 ............ 40 x 1.25 = 50 A
Table 310.16, 75 C copper ............. 8 AWG = 50 A   -> acceptable

Notice that the nameplate 38 amperes is not used for the conductor. The table FLC of 40 amperes is.

Worked Example — Several Motors

A feeder supplies three 460-volt three-phase motors: 30 hp (FLC 40 A), 10 hp (FLC 14 A), and 5 hp (FLC 7.6 A), plus 12 amperes of continuous lighting load.

125% of largest motor ......... 40 x 1.25 = 50.0 A
Other motors at 100% .......... 14 + 7.6  = 21.6 A
125% of continuous other load . 12 x 1.25 = 15.0 A
                                            -------
Minimum feeder ampacity ....................  86.6 A

Overload Protection — Part III

Overload protection guards against sustained operating current above rating — a jammed conveyor, a failed bearing, a motor running in a hot enclosure. It is not short-circuit protection.

430.31 General. 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 is not intended to protect against short-circuit or ground-fault currents.

430.32(A) Continuous-Duty Motors — More Than 1 Horsepower. Each continuous-duty motor rated more than 1 horsepower shall be protected against overload by one of the following means:

• (A)(1) Separate Overload Device. A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

• (A)(2) Thermal Protector. A thermal protector integral with the motor, approved for use with the motor it protects, that will prevent dangerous overheating due to overload and failure to start.
• (A)(3) Integral with Motor. A protective device integral with a motor that will protect the motor against damage due to failure to start.
• (A)(4) Larger Than 1500 Horsepower. A protective device integral with the motor that is responsive to motor temperature.

430.32(B) Continuous-Duty Motors — 1 Horsepower or Less, Not Permanently Installed. Shall be permitted to be protected by the branch-circuit short-circuit and ground-fault protective device where the motor is manually started and within sight from the controller.

430.32(C) Selection of Overload Relay. Where the sensing element or setting or sizing of the overload device selected in accordance with 430.32(A)(1) and (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, provided the trip current of the overload device does not exceed the following percentage of motor nameplate full-load current rating:

430.36 Fuses — In Which Conductor. Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded conductor and also in the grounded conductor if the supply system is 3-wire, 3-phase AC with one conductor grounded.

430.37 Devices Other Than Fuses — In Which Conductor and Table 430.37 specify the number and location of overload units for each system type — for example, a three-phase AC motor requires three overload units, one in each phase.

Worked Example — Overload Sizing

A continuous-duty motor has a nameplate FLA of 28 amperes and a marked service factor of 1.15. What is the maximum initial overload device rating, and what is the maximum after adjustment under 430.32(C)?

Initial, 430.32(A)(1) ........ 28 x 1.25 = 35.0 A
Maximum adjusted, 430.32(C) .. 28 x 1.40 = 39.2 A

Note that both use the nameplate 28 amperes, not a table value. That is the one place in Article 430 where nameplate governs, and questions are written specifically to test whether you know it.