NEC 430-450 — Motors, HVAC, Generators & Transformers (Washington)

NEC 430-450 — Motors, HVAC, Generators & Transformers (Washington)

Learning Objectives

1.1 Which Article Governs

Article 430 covers motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers [430.1]. Article 440 applies to electric motor-driven air-conditioning and refrigerating equipment and its branch circuits and controllers [440.1]. Article 445 covers installation and other requirements for generators [445.1]. Article 450 covers all transformers except those excluded by 450.1, which include current transformers, dry-type transformers that are a component part of other apparatus, transformers integral to X-ray, high-frequency, or electrostatic-coating apparatus, Class 2 and Class 3 circuit transformers, sign and outline, electric-discharge lighting, and power-limited fire alarm transformers, and transformers used for research, development, or testing. Transformer overcurrent protection is in 450.5, not 450.3, which the 2026 edition titles Reconditioned Equipment.

1.2 Table Current versus Nameplate Current (430.6)

Under 430.6(A)(1), the values in Table 430.247 (dc motors), Table 430.248 (single-phase ac motors), Table 430.249 (two-phase ac motors), and Table 430.250 (three-phase ac motors) are used instead of the current marked on the nameplate to determine the ampacity of conductors, the current ratings of switches, and the current ratings of branch-circuit short-circuit and ground-fault protection. Under 430.6(A)(2), nameplate current ratings are used for separate motor overload protection and, for motors built for low speeds (less than 1200 RPM), high torques, canned pumps, or multispeed motors, also for conductor ampacity, switch ratings, and branch-circuit protection. For torque motors, 430.6(B) makes the rated current equal to the locked-rotor current.

1.3 Conductors for Motors (430.22, 430.24)

Section 430.22 governs a single motor in continuous duty: conductors need an ampacity of not less than 125 percent of the motor full-load current rating as determined by 430.6(A)(1). From the table above, a 7.5 hp, 230-volt, three-phase motor needs 22 × 1.25 = 27.5 A; a 10 hp, 575-volt, three-phase motor needs 11 × 1.25 = _13.75_ A; a 30 hp, 230-volt, three-phase motor needs 80 × 1.25 = 100 A; a 50 hp, 460-volt, three-phase motor needs 65 × 1.25 = _81.25_ A; a 15 hp, 208-volt, three-phase motor has a table full-load current of 46.2 amperes, and a 3 hp, 115-volt, single-phase motor needs 34 × 1.25 = 42.5 A.

Section 430.24 covers conductors supplying several motors or motors and other loads: ampacity must be at least 125 percent of the full-load current of the highest rated motor as determined by 430.6(A), plus the full-load currents of the other motors, plus 100 percent of the noncontinuous non-motor load, plus 125 percent of the continuous non-motor load.

1.4 Motor Overload Protection (430.32, 430.37, 430.4)

Overload protection protects the motor against overload and failure to start; it is not short-circuit protection. Under 430.32(A), each motor in continuous duty rated more than 1 hp must be protected by one of the means in 430.32(A)(1) through (A)(4), including a separate overload device responsive to motor current or a thermal protector or electronically protected motor approved with the motor.

The percentages of 430.32(A)(1) apply to the nameplate full-load current in two rows: 125 percent for motors with a marked service factor 1.15 or greater and for motors with a marked temperature rise 40 degrees C or less, and 115 percent for all other motors. The rows are fixed, never interpolated. A 34-ampere nameplate with a marked service factor of 1.15 is a 125 percent motor, so the device may be set at no more than 42.5 A. A 45-ampere nameplate with a marked 50 degrees C rise is an all other motors case: 45 × 1.15 = _51.75_ A, not the 125 percent result of _56.25_, not 45 × 1.50 = 67.5 A, and not a figure such as _38.3_ that matches neither row. A 28-ampere nameplate with a marked rise of 40 degrees C permits 28 × 1.25 = 35 A, while 32.2 is the 115 percent result for that motor.

Where the device chosen under 430.32(A)(1) cannot start the motor or carry the load, 430.32(C) permits a higher setting, but the trip current may not exceed 140 percent of nameplate full-load current for motors with a marked service factor 1.15 or greater and for motors with a marked temperature rise of 40 degrees C or less, and 130 percent for all other motors. Those are modification allowances, not routine multipliers: on a 30-ampere base, 115 percent gives 34.5 A. Section 430.37 and Table 430.37 give the number and location of overload units such as trip coils or relays where devices other than fuses are used — for a three-phase ac motor, three units, one in each phase. Section 430.4 covers part-winding motors, where each half of the winding is protected with a trip current one-half that specified by 430.32 and 430.37, and each winding connection has branch-circuit short-circuit and ground-fault protection rated at not more than one-half that specified by 430.52.

1.5 Short-Circuit and Ground-Fault Protection (430.52, 240.6)

Article 430 Part IV protects the branch-circuit conductors, control apparatus, and the motor against short circuits and ground faults. Section 430.52(B) requires the device to carry the starting current of the motor, and 430.52(C)(1) requires a rating or setting not exceeding the value calculated from Table 430.52(C)(1):

The current in the calculation is the table value required by 430.6(A)(1), never the nameplate value. A three-phase, 230-volt, 15-hp motor has a Table 430.250 full-load current of 42 amperes, so an inverse time circuit breaker may be rated at 42 × 2.50 = 105 A; because 105 A is not a standard rating, 430.52(C)(1)(a) permits the next higher standard rating from 240.6, which is 110 A. A dual-element (time-delay) fuse on a 25-hp, 208-volt motor calculates at 74.8 × 1.75 = _130.9_ A.

Section 240.6 standard ampere ratings for fuses and inverse time circuit breakers are 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000. Where even the increased value is insufficient for starting current, 430.52(C)(1)(b) sets hard ceilings: 400 percent for a nontime-delay or Class CC fuse, 225 percent for a dual-element fuse, and 400 or 300 percent for an inverse time circuit breaker depending on full-load current.

1.6 Controllers, Control Circuits, Disconnecting Means

Section 430.14 requires motors to be located so that adequate ventilation is provided and maintenance such as lubrication of bearings and replacement of brushes can be readily accomplished, and requires open motors having commutators or collector rings to be protected so that sparks cannot reach adjacent combustible material; generators must also meet that rule [445.10].

Section 430.72 protects motor control circuits: a control circuit tapped from the load side of the motor branch-circuit protective device is not a branch circuit, its conductors are protected under 430.72(B)(1) or 430.72(B)(2), and a control circuit transformer is protected by one of the methods in 430.72(C)(1) through 430.72(C)(5), which include protection in accordance with 450.5 — the cross-reference that matters because 450.5(B) exempts a transformer installed as a motor control circuit transformer in accordance with those provisions.

Section 430.102(A) requires an individual disconnecting means for each motor controller, located in sight from the controller location, with exceptions for motor circuits over 1000 volts ac or 1500 volts dc and for coordinated controllers driving several parts of one machine. Section 430.102(B) requires a disconnecting means for the motor, either in sight from the motor location and the driven machinery location [430.102(B)(1)] or the controller disconnecting means of 430.102(A) where it is in sight from the motor [430.102(B)(2)]; the exception to both permits the separate motor disconnect to be omitted where its location is impracticable or introduces additional or increased hazards, or in industrial installations with written safety procedures, but the disconnecting means of 430.102(A) must then be lockable open in accordance with 110.25.

Section 430.109 lists the acceptable types. Under 430.109(A) the disconnecting means must be a listed motor-circuit switch rated in horsepower, a listed molded case circuit breaker or molded case switch, an instantaneous-trip circuit breaker or self-protected combination motor controller in a listed combination motor controller, a listed manual motor controller marked "Suitable as Motor Disconnect", or listed system isolation equipment. Section 430.109(B) permits the branch-circuit overcurrent protective device to serve for stationary motors of 1/8 hp or less, and 430.109(C) covers stationary motors rated 2 hp or less and 300 volts or less, where a general-use switch needs an ampere rating of at least twice the motor full-load current and an ac-only snap switch only where the motor full-load current is not more than 80 percent of that rating. Section 430.110(A) requires a current rating of at least 115 percent of the motor full-load current.

1.7 Article 440 — Air-Conditioning and Refrigerating Equipment

Section 440.4 requires a hermetic refrigerant motor-compressor nameplate to carry the manufacturer's name, trademark, or symbol; an identifying designation; the rated number of phases; rated voltage; and rated frequency [440.4(A)(1)], with the rated-load current marked on the motor-compressor or equipment nameplate [440.4(A)(2)] and the locked-rotor current marked for single-phase units above 9 amperes at 115 volts or above 4.5 amperes at 230 volts and for polyphase units [440.4(A)(3)]. Multimotor and combination-load equipment carries marking that includes the minimum conductor ampacity and maximum branch-circuit protective device rating [440.4(B)], and equipment permitting current above the 440.52(B) percentages is marked with a branch-circuit selection current [440.4(C)].

Section 440.6(A) states which current governs: the rated-load current marked on the equipment nameplate determines the rating or ampacity of the disconnecting means, branch-circuit conductors, controller, short-circuit and ground-fault protection, and separate motor overload protection, with the branch-circuit selection current used instead where so marked [440.6(A)].

Air-conditioning and refrigeration equipment may not be installed within a zone measured 900 mm (3 ft) horizontally and 2.5 m (8 ft) vertically from the top of a bathtub rim or shower stall threshold, including the space directly over the tub or shower stall, although the motors of a system may be located remotely from each other [440.8]. Where equipment is installed outdoors on a roof, an equipment grounding conductor of the wire type must be installed in outdoor portions of metallic raceway systems that use compression-type fittings [440.9], and controllers may not be installed where the available fault current exceeds their marked short-circuit current rating [440.10].

Section 440.11 requires a disconnecting means for air-conditioning and refrigerating equipment, including motor-compressors and controllers, and where it is readily accessible to unqualified persons, any enclosure door or hinged cover that exposes energized parts when open must be locked or require a tool to be opened. Section 440.12(A) selects the disconnecting means for a hermetic refrigerant motor-compressor from the nameplate rated-load current or branch-circuit selection current, whichever is greater: the ampere rating must be at least 115 percent of the greater current [440.12(A)(1)], except that a listed unfused motor circuit switch without fuseholders whose horsepower rating is not less than the equivalent horsepower of 440.12(A)(2) may have a lower rating. Equivalent horsepower comes from Table 430.248 through Table 430.250 for the greater current and from Table 430.251(A) through Table 430.251(C) for the locked-rotor current [440.12(A)(2)]. Worked example: rated-load current 24 A and branch-circuit selection current 27 A, so 27 A governs and 27 × 1.15 = _31.05_ A (about 31.1 A), while using the smaller 24.0 A gives 27.6 A.

Under 440.12(B), where the combined load of two or more motor-compressors may be simultaneous on one disconnecting means, 440.12(B)(1) derives the horsepower rating from the sum of all currents at the rated-load and locked-rotor conditions and treats them as a single motor, while 440.12(B)(2) requires an ampere rating of at least 115 percent of the sum of all currents at the rated-load condition. Conductors follow Article 440 Part IV, introduced by 440.31.

Section 440.62(A) permits a room air conditioner to be treated as a single motor unit where four conditions are met: it is cord- and attachment-plug-connected; rated not more than 40 amperes and 250 volts, single phase; total rated-load current is shown on the nameplate rather than individual motor currents; and the branch-circuit protective device rating does not exceed the ampacity of the conductors or the receptacle rating, whichever is less. Where no other loads are supplied, 440.62(B) limits the total marked current rating to 80 percent of the branch-circuit current rating; where lighting outlets, other appliances, or general-use receptacles are also supplied, 440.62(C) limits it to 50 percent, with an 80 percent allowance where the circuitry is interlocked against simultaneous operation. Section 440.63 permits an attachment plug and receptacle or cord connector to serve as the disconnecting means for a single-phase room air conditioner rated 250 volts or less if the manual controls are readily accessible and located within 1.8 m (6 ft) of the floor, or if an approved manually operable disconnecting means is installed within sight from the unit.

1.8 Article 445 — Generators

Section 445.1 states that Article 445 contains the installation and other requirements for generators. Section 445.2 requires stationary generators to be listed, with an exception permitting one-of-a-kind or custom manufactured generators to be field labeled.

Section 445.11(A)(1) requires each generator to have an accessible nameplate giving the manufacturer's name; rated frequency; number of phases if ac; rating in kilowatts or kilovolt-amperes; power factor; normal volts and amperes corresponding to the rating; and rated ambient temperature. Available fault current, overcurrent device interrupting rating, and conductor insulation temperature rating are not on that list. Section 445.11(A)(2) covers neutral status: each generator must be marked by the manufacturer to indicate whether or not the neutral is bonded to its frame [445.11(A)(2)(a)], and where the neutral bonding is modified in the field, additional marking is required to indicate whether or not the neutral is bonded to the frame [445.11(A)(2)(b)]. Generators rated more than 15 kW require additional data such as reactances, power rating category, temperature rise and insulation class, and the means of overload protection [445.11(B)].

1.9 Article 450 — Transformers

Section 450.1 covers all transformers other than the eight categories listed in 1.1, and Article 450 also reaches transformer vaults and secondary ties. Section 450.4 requires interconnected transformers to comply individually with Article 450 unless specific provisions allow interconnection and operation as a single unit. Section 450.5 carries the overcurrent protection rules and defines the word transformer as a transformer or a polyphase bank of two or more single-phase transformers operating as a unit; protection must comply with 450.5(A), 450.5(B), or 450.5(C).

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Common Exam Traps

Practical Field Points

Summary

The four articles form one system, and the central skill is knowing which current the question wants. Table full-load current from Table 430.247 through Table 430.250 governs conductor ampacity, switches, and branch-circuit short-circuit and ground-fault protection [430.6(A)(1)]; nameplate current governs separate motor overload protection [430.6(A)(2), 430.32(A)(1)], where 125 percent applies to motors with a marked service factor of 1.15 or greater or a marked temperature rise of 40 degrees C or less and 115 percent applies to all other motors; and the greater of rated-load current or branch-circuit selection current governs hermetic refrigerant motor-compressors [440.6(A), 440.12]. Single-motor conductors are 125 percent of the table value [430.22], while groups of motors add 125 percent of the largest plus the full-load currents of the rest [430.24]. Branch-circuit protection starts with Table 430.52(C)(1) and finishes with the standard ratings of 240.6, where 430.52(C)(1)(a) allows the next higher standard size and 430.52(C)(1)(b) sets hard ceilings. Disconnecting means come from the types in 430.109, are located by 430.102(A) for the controller and 430.102(B) for the motor, and where a disconnect cannot be placed in sight, lockability in the open position is the price of the exception. Article 440 restates that logic through 440.4 marking, the bathtub zone of 440.8, the 115 percent rule of 440.12, and the 80 and 50 percent limits of 440.62, while Article 445 requires stationary generators to be listed, located as 445.10 and 430.14 require, and marked with the 445.11 nameplate data, and Article 450 sends protection over 1000 volts to Table 450.5(A) and protection at 1000 volts or less to Table 450.5(B) unless the transformer is a motor control circuit transformer.