Motors change how a circuit is sized. A motor pulls several times its running current for a moment at start, so its branch circuit is built from separate parts, and each part has its own rule: conductors, short-circuit and ground-fault protection, overload protection, controller, and disconnect. Air-conditioning equipment adds a nameplate that does most of the arithmetic for you. This lesson covers Article 430 for general motors and Article 440 for air-conditioning and refrigeration equipment, with one worked motor example and one rooftop unit. Local adoption governs which edition applies (see Lesson 1).
Left, what to check at a rooftop unit: nameplate MCA and MOCP, conductors, breaker, disconnect within sight, and the service receptacle. Right, the parts of a motor branch circuit and the number each one is sized from.The two values answer different questions in the existing rooftop unit example. Conductors meet or exceed MCA; the breaker or fuse does not exceed MOCP. See the lesson for conductor selection and installation checks.
Which current to use
A motor has two currents. One is marked on its nameplate. The other comes from the NEC tables for its horsepower, voltage, and phase: Table 430.248 for single-phase motors and Table 430.250 for 3-phase induction motors. For general motor applications, the table value sets the conductor ampacity, the switch rating, and the short-circuit and ground-fault protection. The nameplate current sets the separate overload protection (2023 NEC 430.6(A)(1) and (A)(2)). Using the wrong one is the most common arithmetic error in motor plan review. Motors built for low speeds, high torques, or multispeed operation are exceptions that use the nameplate for more items, and that is why the nameplate is always read too.
Horsepower (3-phase induction)
208 V
230 V
460 V
5
16.7 A
15.2 A
7.6 A
10
30.8 A
28 A
14 A
15
46.2 A
42 A
21 A
20
59.4 A
54 A
27 A
Motor conductors
Conductors that supply a single continuous-duty motor need an ampacity of at least 125 percent of the table full-load current (430.22). A 20 hp, 460 V motor has a table current of 27 A, so its conductors need 33.75 A of ampacity. Then Table 310.16 and the termination rules of Lesson 8 and Lesson 2 pick the conductor. For circuits of 100 A or less, equipment terminals are treated as 60 C unless the equipment is marked otherwise, so the 60 C column is the safe place to look (110.14(C)(1)(a)).
Short-circuit and ground-fault protection
The branch-circuit device protects against short circuits and ground faults, not overloads, so it is allowed to be much larger than the motor current. It has to carry the starting surge. The maximum comes from Table 430.52(C)(1), as a percentage of the table full-load current. If the result is not a standard rating in 240.6, the next higher standard rating is permitted (430.52(C)(1)(a)).
Device (squirrel-cage motor)
Maximum, percent of full-load current
Nontime-delay fuse
300
Dual-element (time-delay) fuse
175
Inverse-time circuit breaker
250
Instantaneous-trip breaker (part of a listed combination controller)
800
If the device still trips when the motor starts, 430.52(C)(1)(b) allows more, but never beyond a hard ceiling: 400 percent for a nontime-delay fuse up to 600 A, 225 percent for a dual-element fuse, and for an inverse-time breaker 400 percent when the motor current is 100 A or less and 300 percent when it is over 100 A. The ceiling is a maximum, so the next-higher-size rule does not apply after it.
Overload protection
An overload device protects the motor from running too long above its rating. Using the nameplate current, a separate overload device may be set at no more than 125 percent for motors with a marked service factor of 1.15 or more, or a marked temperature rise of 40 C or less, and 115 percent for all others (430.32(A)(1)). If that setting will not let the motor start or carry its load, the device may be raised to 140 percent or 130 percent, respectively (430.32(C)).
Disconnecting means
Each motor controller needs a disconnect in sight from the controller, and the motor itself needs a disconnect in sight from both the motor and the driven machinery, unless the controller disconnect is in sight of the motor and serves both (430.102). Article 100 defines in sight as visible and not more than 50 ft away. The motor disconnect may be left out when the controller disconnect is lockable and in-sight placement is impracticable or adds hazard, such as motors over 100 hp, multimotor equipment, submersible motors, and motors on adjustable-speed drives, or in industrial installations with written safety procedures (430.102(B)(2)). The disconnect must have a current rating at least 115 percent of the motor full-load current, unless it is a listed unfused motor-circuit switch with a horsepower rating at least equal to the motor's (430.110(A)). Disconnect types include hp-rated motor-circuit switches, molded case breakers and switches, and manual motor controllers marked as suitable for use as a motor disconnect (430.109(A)).
Air-conditioning nameplates: MCA and MOCP
Article 440 covers equipment with a hermetic refrigerant motor-compressor. Multimotor and combination-load equipment, such as a packaged rooftop unit, must be marked with its minimum supply circuit conductor ampacity (MCA) and the maximum rating of its short-circuit and ground-fault protective device (MOCP) (440.4(B)). Conductors supplying the unit must have an ampacity not less than the MCA (440.35), and the breaker or fuse must not exceed the MOCP (440.4(B), 440.22(B)). The nameplate does the arithmetic once, so the field check is a comparison: conductor at or above MCA, protective device at or below MOCP.
Single compressors and the disconnect
For a single hermetic compressor, use the rated-load current on the equipment nameplate, or the branch-circuit selection current where one is marked, in place of the motor tables (440.6(A)). Conductors need at least 125 percent of that current (440.32). The short-circuit device may be up to 175 percent, and up to 225 percent if that will not start the compressor, with a floor of 15 A that the device need not go below (440.22(A)). The disconnect rating must be at least 115 percent of the same current (440.12(A)(1)).
Location, receptacles, and rooftops
The disconnect must be within sight of and readily accessible from the equipment, and may be mounted on or inside it. It needs the working space of 110.26(A), and it may not sit on a panel that must be removed to service the equipment or cover the nameplate (440.14). On the roof, a 125 V, 15 or 20 A receptacle for servicing must be within 25 ft of the unit, on the same level, and not on the load side of the unit disconnect (210.63(A)), with GFCI protection (210.8(E)). Conductors in sunlight must be sunlight resistant (310.10(D)), and a raceway or cable less than 3/4 in. above the roof takes a 33 C temperature adder for ampacity correction, except type XHHW-2 (310.15(B)(2)). Raising the conduit off the roof avoids the adder.
Worked example: a 15 hp motor and a rooftop unit
A GCHS community center adds a 15 hp, 460 V, 3-phase exhaust fan motor with a nameplate current of 19.6 A and a marked service factor of 1.15. Table 430.250 gives 21 A. Conductors: 21 x 1.25 = 26.25 A, so 10 AWG copper, rated 30 A in the 60 C column. Short-circuit protection with an inverse-time breaker: 21 x 2.5 = 52.5 A, so the next standard size, 60 A, is the largest allowed. Overload: 19.6 x 1.25 = 24.5 A maximum. Disconnect: 21 x 1.15 = 24.15 A, so a 30 A switch, in sight of the motor.
The rooftop unit on the same building is marked MCA 24 A and MOCP 40 A. The conductors need at least 24 A, so 12 AWG copper (20 A) is too small and 10 AWG (30 A at 60 C) is the smallest that works. The breaker may not exceed 40 A. The disconnect goes within sight of the unit, and the service receptacle within 25 ft on the roof.
Key numbers
Motor conductors: 125 percent of table full-load current (430.22)
Inverse-time breaker: up to 250 percent; dual-element fuse 175; nontime-delay fuse 300 (Table 430.52(C)(1))
Overload: 125 percent of nameplate with service factor 1.15 or more, otherwise 115 (430.32(A)(1))
Motor disconnect: at least 115 percent of full-load current (430.110(A)); in sight means visible and within 50 ft
Rooftop unit: conductors at or above MCA, protective device at or below MOCP (440.35, 440.4(B))
Service receptacle: within 25 ft, same level, GFCI protected (210.63(A), 210.8(E))
In the field
A GCHS rehab replaces the failed rooftop unit on a small clinic. The new unit is one size larger, and the old 30 A breaker is still there. The inspector reads the new nameplate first: MCA 24 A, MOCP 40 A. The old conductors are 10 AWG, which is enough. The breaker is under the maximum, so it can stay. Then the walk: the disconnect is within sight but is screwed to the service panel that the technician removes to reach the compressor, and the service receptacle is 60 ft across the roof. Both get corrected before the unit is approved.
Practice questions
1. A Group B building has a 10 hp, 208 V, 3-phase squirrel-cage motor that runs continuously. Using Table 430.250 and 430.22, what is the minimum ampacity of its branch-circuit conductors?
30.8 A, because the table current is the conductor ampacity
38.5 A, which is 125 percent of the 30.8 A table current
35.4 A, which is 115 percent of the 30.8 A table current
44.0 A, which is 125 percent of a 35.2 A nameplate current
Answer: b. The table gives 30.8 A for 10 hp at 208 V. Section 430.22 requires at least 125 percent of that, so 30.8 x 1.25 = 38.5 A.
2. A 15 hp, 460 V, 3-phase squirrel-cage motor in a community center mechanical room has a nameplate current of 19.4 A. Which current is used to size its branch-circuit conductors?
The 19.4 A nameplate value, because it is the actual current the motor draws
The 19.4 A nameplate value, but only when the motor is over 10 hp
The average of the nameplate and table values, which is 20.2 A
The 21 A table value from Table 430.250, because 430.6(A)(1) uses the table for conductors
Answer: d. For general motor applications, the tables in 430.6(A)(1) set the conductor ampacity, switch rating, and short-circuit protection. The nameplate is used for the separate overload device.
3. A 10 hp, 460 V motor in a Group F shop has a nameplate current of 13.2 A and a marked service factor of 1.15. What is the largest setting for a separate overload relay under 430.32(A)(1)?
16.5 A, which is 125 percent of the 13.2 A nameplate current
15.2 A, which is 115 percent of the 13.2 A nameplate current
17.5 A, which is 125 percent of the 14 A table current
18.5 A, which is 140 percent of the 13.2 A nameplate current
Answer: a. A motor with a marked service factor of 1.15 or more allows 125 percent of nameplate current, so 13.2 x 1.25 = 16.5 A. The 140 percent value applies only if the first setting will not start the motor, and the table current is not used for overloads.
4. An inverse-time circuit breaker will protect a 20 hp, 460 V squirrel-cage motor, which has a table current of 27 A. What is the largest standard breaker allowed by Table 430.52(C)(1) and 430.52(C)(1)(a)?
60 A, because a device may never be larger than the calculated value
90 A, because 300 percent of 27 A is 81 A and the next standard size above that is 90 A
70 A, because 250 percent of 27 A is 67.5 A and the next standard size is 70 A
110 A, because 400 percent of 27 A is 108 A and the next size is 110 A
Answer: c. The inverse-time breaker column is 250 percent, or 67.5 A, and 430.52(C)(1)(a) allows the next higher standard rating, 70 A. The 300 and 400 percent values are for other devices or for the starting exception.
5. A dual-element fuse protects a 5 hp, 208 V, 3-phase squirrel-cage motor with a table current of 16.7 A. What is the largest standard fuse under Table 430.52(C)(1) and the next-higher-size rule?
25 A, because 150 percent of 16.7 A is 25 A
40 A, because 250 percent of 16.7 A is 41.8 A, which rounds down to a standard 40 A rating
50 A, because 300 percent of 16.7 A is 50.1 A
30 A, because 175 percent of 16.7 A is 29.2 A and the next standard size is 30 A
Answer: d. The dual-element column is 175 percent. 16.7 x 1.75 = 29.2 A, and the next higher standard rating is 30 A.
6. A 10 hp, 460 V motor has a table current of 14 A. The 35 A inverse-time breaker trips every time the motor starts. What is the largest standard breaker that 430.52(C)(1)(b) permits?
60 A, because the next higher standard size above 56 A is allowed under the next-size rule
50 A, because the breaker may not exceed 400 percent of 14 A, which is 56 A
45 A, because the breaker may be raised only to 300 percent of 14 A
70 A, because 500 percent is allowed when the motor will not start
Answer: b. For full-load currents of 100 A or less, an inverse-time breaker may be increased but shall in no case exceed 400 percent. 14 x 4 = 56 A, and the largest standard size that does not exceed that is 50 A.
7. A 15 hp, 460 V motor in a Group B building has a table current of 21 A, and the disconnect is a fusible switch that is not an hp-rated motor-circuit switch. What is the minimum ampere rating of the switch under 430.110(A)?
21 A, because the switch must equal the motor full-load current
26.3 A, because the switch is sized like the conductors at 125 percent
30 A, because 115 percent of 21 A is 24.2 A and the next standard switch rating is 30 A
60 A, because the switch must match the 60 A branch-circuit breaker that protects the motor circuit
Answer: c. A motor disconnect must be rated not less than 115 percent of the motor full-load current, and 21 x 1.15 = 24.15 A. A 30 A switch is the smallest standard rating that meets it.
8. A 5 hp exhaust fan motor on the roof of a Group B building has a fused disconnect in the second-floor mechanical room, 80 ft away and out of sight of the motor. No exception applies. What is the finding under 430.102?
The motor disconnect must be in sight of the motor and driven machinery, meaning visible and within 50 ft
The disconnect is acceptable, because in sight allows up to 100 ft
The disconnect is acceptable, because any lockable disconnect may be out of sight
The disconnect is acceptable, because it is on the same circuit as the motor
Answer: a. Section 430.102(B) requires the motor disconnect in sight from the motor and the driven machinery, and Article 100 defines in sight as visible and not more than 50 ft. The lockable out-of-sight option applies only where in-sight placement is impracticable or adds hazard, or at industrial sites with written safety procedures.
9. A rooftop unit on a Group A-3 community building is marked MCA 24 A and MOCP 40 A. What is the smallest copper conductor that meets the MCA using the 60 C column of Table 310.16?
12 AWG, which is 20 A in the 60 C column
14 AWG, which is 15 A in the 60 C column
8 AWG, which is 40 A in the 60 C column
10 AWG, which is 30 A in the 60 C column
Answer: d. Conductors must have ampacity not less than the 24 A MCA. In the 60 C column, 12 AWG is 20 A, which is too small, and 10 AWG is 30 A, the smallest size that works.
10. The nameplate of a rooftop unit on a Group B office reads MOCP 40 A. The installer wants to use a 50 A breaker because one is on the truck. What do 440.4(B) and 440.22(B) require?
Allowed, because 175 percent of the compressor current replaces the nameplate
Not allowed, because the branch-circuit device may not exceed the 40 A maximum on the nameplate
Allowed, if the conductors are upsized to 6 AWG copper so that they match the 50 A breaker rating
Allowed, if the disconnect is rated 60 A or more
Answer: b. For multimotor and combination-load equipment, the short-circuit and ground-fault device may not exceed the maximum marked under 440.4(B). Conductor size and disconnect rating do not change that limit.
11. During a final inspection of a Group B clinic, the disconnect for a rooftop unit is mounted on the panel that the technician removes to reach the compressor. What is the finding under 440.14?
No finding, because a disconnect mounted on the unit is always acceptable wherever it is placed
The disconnect must be at least 6 ft away from the unit in every case
The disconnect may not sit on an access panel or where it hides the nameplate
The disconnect is acceptable if the unit is also GFCI protected
Answer: c. Section 440.14 allows the disconnect on or within the equipment but not on access panels and not where it obscures the nameplate.
12. A rooftop unit on a Group B building has its only service receptacle 40 ft away at the roof hatch, connected to the load side of the unit disconnect. What does 210.63(A) require?
A 125 V, 15 or 20 A receptacle within 25 ft of the unit on the same level, not on the load side of the disconnect
A receptacle within 50 ft, which is the in-sight distance for disconnects
A receptacle within 25 ft, and it may be on the load side of the disconnect
No receptacle, because a rooftop unit is served by a portable generator
Answer: a. The servicing receptacle must be accessible, within 25 ft of the equipment, on the same level, and must not be connected to the load side of the equipment disconnect. It also needs GFCI protection under 210.8(E).
13. A single hermetic compressor in a Group M walk-in cooler has a nameplate rated-load current of 17.9 A and no branch-circuit selection current marked. What is the minimum conductor ampacity under 440.32?
17.9 A, which is the rated-load current
20.6 A, which is 115 percent of the rated-load current
31.3 A, which is 175 percent of the rated-load current of the compressor
22.4 A, which is 125 percent of the 17.9 A rated-load current
Answer: d. Conductors for a single motor-compressor need at least 125 percent of the rated-load current, so 17.9 x 1.25 = 22.4 A. The 115 percent figure is for the disconnect and 175 percent is for the short-circuit device.
14. A single hermetic compressor has a rated-load current of 17.9 A and is the only load on its circuit. What is the largest standard short-circuit device under 440.22(A), using the 175 percent value and the next-higher-size rule?
30 A, because the device may never exceed 31.3 A
35 A, because 175 percent of 17.9 A is 31.3 A and the next standard size is 35 A
40 A, because 225 percent of 17.9 A is 40.3 A, and a larger device is always allowed
45 A, because 250 percent of 17.9 A is 44.8 A
Answer: b. A device up to 175 percent of rated-load current is permitted, and 17.9 x 1.75 = 31.3 A. The next higher standard rating, 35 A, is allowed. The 225 percent value applies only if 175 percent will not start the compressor.
Glossary
Full-load current (table)
The current in Tables 430.248 and 430.250 for a given horsepower and voltage, used for motor conductors, switches, and short-circuit protection.
Overload protection
A device responsive to motor current that trips or is rated at no more than 115 or 125 percent of nameplate current (430.32).
Short-circuit and ground-fault protection
The branch-circuit breaker or fuse that clears faults and may be much larger than motor current so the motor can start (Table 430.52(C)(1)).
Within sight
Equipment that is visible and not more than 50 ft distant from other equipment (Article 100).
Minimum circuit ampacity (MCA)
The nameplate value that the supply conductors to multimotor or combination-load equipment must not be smaller than (440.4(B), 440.35).
Maximum overcurrent protective device (MOCP)
The nameplate value that the branch-circuit breaker or fuse must not exceed (440.4(B), 440.22(B)).
Rated-load current
The current marked on a hermetic compressor or its equipment nameplate, used with Article 440 instead of the motor tables (440.6(A)).
Service factor
A number marked on a motor showing how much overload it can carry; 1.15 or more allows an overload setting of 125 percent (430.32(A)(1)).