Conductors that supply several motors must be sized to at least the sum of the four quantities enumerated in Sec. 430.24 (1) through (4). These are:
(1) 125% of the highest rated motor’s FLC as listed in Tables 430.247 through 250.
(2) The sum of the FLC of the other motors as listed in Tables 430.247 through 250.
(3) 100% of the noncontinuous non-motor load.
(4) 125% of the continuous non-motor load.
The last sentence above each table allows us to use the ampacity columns for a range of system voltages without any adjustment. The conductor size must be selected from Table 310.16 per the terminal temperature rating (60°C or 75°C) of the equipment [Sec. 110.14(C)(1)]. Don’t use the motor nameplate full-load amperes (FLA) [Sec. 430.6(A)(2)] to determine the motor conductor size.
Motor applications are considered continuous duty unless the nature of the control or apparatus the motor drives is designed so the motor will not operate continuously under load [Table 430.22(E) Note]. When a motor is not continuous duty because of this type of application, size the conductors using the percentages of Table 430.22(E). If a motor must stop when performing its function (such as in the case of an elevator motor), it is a good sign the motor is intermittent duty.
Conductors for a motor used in a short-time, intermittent, periodic, or varying-duty application must have an ampacity of at least the percentage of the motor nameplate FLA rating shown in Table 430.22(E).
Overload
equipment operates above its current rating,or where the current exceeds the conductor ampacity. When an overload condition persists, equipment failure or fire from damaging or dangerous overheating can result. A fault, such as a short circuit or ground fault, is not an overload [Art. 100].
Overload devices protect motors, motor control equipment, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start — but not against short circuits or ground faults. Overload protection is not required where it might introduce additional or increased hazards, as in the case of fire pumps [Sec. 430.31(A)].
Overload devices can be:
- Thermal overloads (heaters) in an overload relay of a motor contactor (starter). These heater units are selected using a chart or size given by the manufacturer.
- Solid-state (electronic) overloads have an adjustment dial for setting the trip level. They are installed in an overload relay of a motor contactor (starter).
- Inverse time circuit breakers and dual element fuses can serve as both motor overload protection and the motor short-circuit ground-fault protection if the requirements of Sec. 430.32 are met [Sec. 430.55].
- Fuses, when sized per Sec. 430.32(A) [Sec. 430.36].
Overload sizing for continuous-duty motors
Motors rated more than 1 hp (used in a continuous-duty application without integral thermal protection) must have the overload device(s) sized per one of the four methods required in Sec. 430.32(A)(1) through (4).
For example, you can use a separate overload device. This device must be selected to open at no more than 125% of the motor nameplate FLA rating depending on service factor or temperature rise:
- Service Factor (SF). Motors with a marked service factor of 1.15 or more on the nameplate must have the overload device sized at not more than 125% of the motor nameplate current rating. Motor service factors are safety factors; they indicate how much the motor capacity can be exceeded for short periods without overheating. For example, a motor with a service factor of 1.15 can operate at 15% more than its rated output without overheating. This is important for motors where loads vary and may peak slightly above the rated torque.
- Temperature Rise. Motors with a nameplate temperature rise of 40°C or less must have the overload device sized no more than 125% of the motor nameplate current rating. A motor with a nameplate temperature rise of 40°C means the motor is designed to operate so it will not heat up more than 40°C above its rated ambient temperature when operated at its rated load and voltage. Studies have shown that when the operating temperature of a motor is increased 10°C above its rating, the motor winding insulating material’s anticipated life is reduced by 50%.
Example
Question: A motor has a nameplate that specifies a service factor of 1.12 with a temperature rise of 41°C and a nameplate full load current rating of 25A. What size dual-element time-delay fuse is required for the overload protection of this motor?
Solution:Since the service factor of 1.12 is less than 1.15, and 41°C is over 40°C, the overload protection is sized based on 115 percent of the motor nameplate ampere rating [Sec. 430.6(A)(2)].
Overload Protection = 25A × 115%
Overload Protection = 28.75A
Answer:Therefore, use a 25A dual-element time-delay fuse [Sec. 240.6(A)]
Other Motors. No more than 115% of the motor “nameplate current rating.”
Branch-circuit short-circuit and ground-fault protection
A branch-circuit short-circuit and ground-fault protective device (OCPD) protects the motor, the motor control equipment, and the conductors against short circuits or ground faults, but not against overload [Sec. 430.51] (Fig. 3).
