10.7 Motors Part II: Short-Circuit Protection, Controllers, Disconnects and HVAC

Branch-Circuit Short-Circuit and Ground-Fault Protection — Part IV

This is the device that protects the motor branch circuit against short circuits and ground faults. It is deliberately sized far above the motor's running current so it does not open on starting inrush, which for an across-the-line start is typically six to eight times full-load current for several seconds.

430.52(B) All Motors. The motor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor.

430.52(C)(1) Rating or Setting. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used.

Table 430.52 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices — percentage of full-load current:

The four numbers to hold in memory for the common squirrel-cage case are 300 / 175 / 800 / 250.

430.52(C)(1) Exception No. 1. Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.

430.52(C)(1) Exception No. 2. Where the rating specified in Table 430.52, as modified by Exception No. 1, is not sufficient for the starting current of the motor, the ratings shall be permitted to be increased but shall in no case exceed:

• 400 percent for full-load currents of 100 amperes or less, or 300 percent for full-load currents greater than 100 amperes, for non-time-delay fuses not exceeding 600 amperes;
• 225 percent for time-delay (dual-element) fuses;
• 400 percent for full-load currents of 100 amperes or less, or 300 percent for full-load currents greater than 100 amperes, for inverse time circuit breakers;
• 11 amperes for fuses rated 601 through 6000 amperes.

Worked Example — Inverse Time Breaker

A 230-volt, three-phase, 10 horsepower squirrel-cage motor has a Table 430.250 FLC of 28 amperes. What is the maximum inverse time circuit breaker?

Table 430.52, inverse time breaker, squirrel cage ..... 250%
28 x 2.50 = 70 A
70 A is a standard size from 240.6(A) -> 70 A

If the calculation had produced 72.5 amperes, Exception No. 1 would permit rounding up to the next standard size, 80 amperes.

Worked Example — Dual-Element Fuse

The same motor with dual-element time-delay fuses.

Table 430.52, dual-element time-delay fuse ........... 175%
28 x 1.75 = 49 A
49 A is not a standard size; Exception No. 1 permits
the next higher standard rating ...................... 50 A

If 50 amperes proves insufficient to start the motor, Exception No. 2 permits increasing to 225 percent — 28 x 2.25 = 63 amperes, so a 60-ampere fuse.

430.52(C)(2) Overload Relay Table. Where maximum branch-circuit short-circuit and ground-fault protective device ratings are shown in the manufacturer's overload relay table for use with a motor controller, or are otherwise marked on the equipment, they shall not be exceeded even if higher values are allowed as shown above.

430.53 Several Motors or Loads on One Branch Circuit. Two or more motors or one or more motors and other loads shall be permitted on the same branch circuit under the conditions in (A) through (D), including the well-known allowance for motors of 1 horsepower or less taking not more than 6 amperes each on a branch circuit protected at not more than 20 amperes at 120 volts or 15 amperes at 1000 volts or less.

Feeder Protection — Part V

430.62(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder, based on the maximum permitted value for the specific type of protective device in accordance with 430.52, plus the sum of the full-load currents of the other motors supplied by the feeder.

Worked Example — Feeder Protection

A feeder supplies three motors with table FLCs of 40, 14, and 7.6 amperes, all squirrel-cage, protected by inverse time breakers.

Largest branch-circuit device: 40 x 2.50 = 100 A
Sum of other motors' FLC: 14 + 7.6 = 21.6 A
Maximum feeder device: 100 + 21.6 = 121.6 A
Next STANDARD size BELOW: 110 A  (no round-up permitted here)

Note the direction: 430.62 states "not greater than," so you round down to 110 amperes, unlike 240.4(B) where you round up. This asymmetry is a favorite exam trap.

430.63 Rating or Setting — Motor Load and Other Load(s). Where a feeder supplies a motor load and other loads, the protective device shall have a rating not less than that required for the sum of the other loads plus the value from 430.62 for the motor load.

Motor Control Circuits, Controllers and Disconnects

430.72 Overcurrent Protection (Control Circuits). A motor control circuit tapped from the load side of a motor branch-circuit short-circuit and ground-fault protective device and functioning to control the motor shall be protected against overcurrent in accordance with 430.72.

430.75 Disconnection (Control Circuits). Motor control circuits shall be arranged so that they will be disconnected from all sources of supply when the disconnecting means is in the open position.

430.83(A) General (Controllers). Controllers other than inverse time circuit breakers and molded case switches shall have a horsepower rating at the application voltage not lower than the horsepower rating of the motor.

430.87 Number of Motors Served by Each Controller. Each motor shall be provided with an individual controller, with an exception for a group of motors under specified conditions in industrial and similar installations.

430.102(A) Controller. An individual disconnecting means shall be provided for each controller and shall be located in sight from the controller location.

430.102(B) Motor. A disconnecting means shall be provided for a motor in accordance with (B)(1) or (B)(2):

• (B)(1) Separate Motor Disconnect. A disconnecting means for the motor shall be located in sight from the motor location and the driven machinery location.
• (B)(2) Controller Disconnect. The controller disconnecting means shall be permitted to serve as the disconnecting means for the motor if it is in sight from the motor location and the driven machinery location.

Exception to (B): The disconnecting means shall not be required to be in sight from the motor and the driven machinery location under either condition (a) or (b), provided the controller disconnecting means is individually capable of being locked in the open position, with the provision for locking remaining in place with or without the lock installed:

(a) Where such a location of the disconnecting means is impracticable or introduces additional or increased hazards to persons or property; (b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

"In sight from" is defined in Article 100: visible and not more than 50 feet (15 m) distant from the other.

430.107 Readily Accessible. At least one of the disconnecting means shall be readily accessible.

430.109 Type. The disconnecting means shall be a type specified in 430.109(A) unless otherwise permitted — a listed motor-circuit switch rated in horsepower, a molded case circuit breaker, a molded case switch, an instantaneous trip circuit breaker that is part of a listed combination motor controller, a self-protected combination controller, a manual motor controller marked "Suitable as Motor Disconnect", or a system isolation equipment device.

430.110(A) General. The disconnecting means for motor circuits rated 1000 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.

Article 440 — Air-Conditioning and Refrigerating Equipment

Hermetic refrigerant motor-compressors break the Article 430 pattern because they do not have a conventional horsepower-based full-load current. Article 440 substitutes two nameplate values:

• Rated-Load Current (RLA) — the current resulting when the motor-compressor is operated at the rated load, voltage, and frequency of the equipment it serves.
• Branch-Circuit Selection Current (BCSC) — the value in amperes used instead of the rated-load current in determining ratings of motor branch-circuit conductors, disconnecting means, controllers, and short-circuit and ground-fault protection whenever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. Where BCSC is marked, it is always equal to or greater than the RLA and must be used.

440.6(A) Hermetic Refrigerant Motor-Compressor. For a hermetic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. Where no rated-load current is shown, the rated-load current shown on the compressor nameplate shall be used.

440.22(A) Rating or Setting for Individual Motor-Compressor. The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted. Where the protection specified is not sufficient for the starting current, the rating or setting shall be permitted to be increased but shall not exceed 225 percent.

440.32 Single Motor-Compressor. Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.

440.52 Application and Selection (Overload Protection). The motor-compressor shall be protected against overload and failure to start by one of: a separate overload relay responsive to motor-compressor current, selected to trip at not more than 140 percent of the rated-load current; a thermal protector integral with the motor-compressor; a fuse or inverse time circuit breaker responsive to motor current, rated at not more than 125 percent of the rated-load current; or a protective system.

440.14 Location (Disconnecting Means). Disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnecting means shall be permitted to be installed on or within the air-conditioning or refrigerating equipment. The disconnecting means shall not be located on panels that are designed to allow access to the equipment or that obscure the equipment nameplate.

That last sentence is a common field violation: mounting the disconnect on the removable service panel of a condensing unit.

440.4(B) Multimotor and Combination-Load Equipment. Multimotor and combination-load equipment shall be provided with a visible nameplate marked with the maker's name, the rating in volts, frequency, and number of phases, the minimum supply circuit conductor ampacity, and the maximum rating of the branch-circuit short-circuit and ground-fault protective device.

Where those two nameplate values are present — MCA and MOCP — they govern. You do not recalculate from Article 440 percentages; you size the conductor to the minimum circuit ampacity and select an overcurrent device not exceeding the maximum overcurrent protective device rating. That is a 110.3(B) and 440.4(B) requirement, and it is how nearly every packaged heat pump, mini-split, and rooftop unit is actually installed.