12.4 Motor Feeders, Disconnecting Means & Controllers

12.4 Motor Feeders, Disconnecting Means & Controllers

Exam Fast Fact: If you remember only one rule for sizing motor feeder overcurrent protection under NEC 430.62(A), remember this: YOU CANNOT ROUND UP! While branch-circuit protective devices under 430.52(C)(1) permit rounding UP to the next standard rating, feeder overcurrent devices must NEVER exceed the calculated value. If your feeder calculation equals 87 amperes, the maximum permitted standard breaker under 240.6(A) is 80 amperes, not 90 amperes.

In commercial and industrial facilities, motors are rarely wired individually back to the main service switchboard. Instead, subpanels and distribution feeders supply groups of motors, motor control centers (MCCs), and combinations of motor and lighting loads. Correctly engineering these systems requires mastering feeder conductor ampacity (NEC 430.24), feeder overcurrent coordination (NEC 430.62), safe disconnect placement (NEC 430 Part IX), and controller operational ratings (NEC 430 Part VII).

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Motor Feeder Conductor Sizing: NEC 430.24

Feeder conductors supplying two or more motors must have an ampacity of not less than 125 percent of the highest-rated motor's full-load current, plus the sum of the full-load currents of all other motors in the group:

Feeder Ampacity = (FLC_largest * 1.25) + Sum(FLC_other_motors)

If the feeder also supplies non-motor loads, NEC 430.24 incorporates standard load calculation factors:

• Add 100% of noncontinuous non-motor loads.
• Add 125% of continuous non-motor loads.

Determining the "Highest-Rated Motor"

The "highest-rated motor" is strictly determined by highest full-load current (amperes) from the NEC tables, not by horsepower! For example, a 10 HP, 208V single-phase motor draws 55 A (Table 430.248), whereas a 15 HP, 460V three-phase motor draws only 21 A (Table 430.250). If both were on the same system, the 55 A motor is the "highest-rated motor." If two motors in a group have the exact same highest FLC, apply the 125% multiplier to only one of them.

Step-by-Step Feeder Conductor Sizing Example

Scenario: Calculate the minimum feeder conductor ampacity to supply three 460-volt, 3-phase induction motors:

• Motor 1: 40 HP (FLC = 52.0 A from Table 430.250)
• Motor 2: 20 HP (FLC = 27.0 A from Table 430.250)
• Motor 3: 10 HP (FLC = 14.0 A from Table 430.250)

1. Step 1 — Identify the largest motor FLC: Motor 1 has the largest FLC at 52.0 A.
2. Step 2 — Apply 125% to the largest motor: 52.0 A * 1.25 = 65.0 A
3. Step 3 — Add the FLC of all remaining motors: Sum of Other Motors = 27.0 A + 14.0 A = 41.0 A
4. Step 4 — Calculate total minimum feeder ampacity: Minimum Feeder Ampacity = 65.0 A + 41.0 A = 106.0 A
   • Using Table 310.16 (75°C Copper), a 1 AWG Copper conductor (rated 130 A) is required, since 2 AWG is rated only 115 A.

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Motor Feeder Overcurrent Protection: NEC 430.62(A)

A motor feeder overcurrent protective device is designed to allow the largest motor in the group to start while all other connected motors are running at full load.

The Feeder OCPD Sizing Formula

Under NEC 430.62(A), the maximum rating of the feeder overcurrent protective device equals:

1. The largest branch-circuit short-circuit and ground-fault protective device permitted for any motor in the group (calculated or standard size permitted under 430.52);
2. PLUS the sum of the full-load currents (FLCs) of all other motors in the group:

Max Feeder OCPD = Branch OCPD_largest_motor + Sum(FLC_other_motors)

The Mandatory Round-Down Rule

The Most Critical Feeder Trap on the Exam: NEC 430.62(A) states that the feeder protective device "shall have a rating or setting not greater than..." the calculated value. Unlike branch circuits under 430.52(C)(1) Exception No. 1, NEC 430.62(A) CONTAINS NO PROVISION TO ROUND UP TO THE NEXT STANDARD SIZE!

If your calculation does not land exactly on a standard rating in NEC 240.6(A), you MUST SELECT THE NEXT LOWER STANDARD SIZE.

Step-by-Step Feeder OCPD Sizing Example

Scenario: Using the same three 460-volt motors (40 HP / 52 A, 20 HP / 27 A, 10 HP / 14 A), calculate the maximum standard rating of an inverse-time circuit breaker permitted to protect the feeder. All branch circuits use inverse-time breakers.

1. Step 1 — Calculate maximum permitted branch-circuit OCPD for largest motor (40 HP / 52 A):
   • Under Table 430.52, inverse-time breaker = 250%.
   • 52.0 A * 2.50 = 130.0 A.
   • Check standard sizes in 240.6(A): 130 A is not standard. Round UP for the branch circuit (430.52(C)(1) Ex. 1) to 150 A.
2. Step 2 — Add the FLCs of all other motors in the group: Sum of Other Motors = 27.0 A + 14.0 A = 41.0 A
3. Step 3 — Calculate the maximum permitted feeder OCPD value: Calculated Feeder OCPD = 150 A + 41.0 A = 191.0 A
4. Step 4 — Select the standard rating (NEC 240.6(A)):
   • Standard ratings around 191 A: 175 A, 200 A.
   • Apply the Round-Down Rule: Because 430.62(A) mandates not greater than, you cannot round up to 200 A.
   • Answer: The maximum permitted feeder circuit breaker is 175 amperes.

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Motor Disconnecting Means: NEC Article 430 Part IX

Motor disconnects isolate equipment to safeguard electricians and maintenance technicians from electrical shock and mechanical entanglement.

[Feeder Panelboard] 
       │
       ▼
[Controller Disconnect] ─── (Must be "in sight from" Controller, NEC 430.102(A))
       │
       ▼
[Motor Controller] 
       │
       ▼
[Motor Disconnect]      ─── (Must be "in sight from" Motor & Machinery, NEC 430.102(B))
       │
       ▼
[Electric Motor]

Location Requirements: The "In Sight From" Rule

Under NEC 430.102(A) and (B):

1. A disconnecting means must be located in sight from the controller location.
2. A disconnecting means must be located in sight from the motor location and driven machinery.

What "In Sight From" Means (NEC Article 100 Definition)

Under NEC Article 100, where equipment is required to be "in sight from," "within sight from," or "within sight of" other equipment, two non-negotiable criteria must be met:

1. The equipment must be visible (not hidden behind walls, partitions, ductwork, or around corners).
2. The equipment must be located not more than 50 feet (15 meters) distant from the other equipment.

If a disconnect is 40 feet away but around a partition where you cannot see it, it violates the Code. If a disconnect is visible down a clear hallway but is 55 feet away, it violates the Code! Both visibility AND <= 50 ft are mandatory.

Exception: Lockable Controller Disconnect (NEC 430.102(B)(2) Ex.)

A separate disconnect at the motor location is not required if the disconnect for the controller is capable of being locked in the open position in accordance with NEC 110.25 (lock remains in place with or without the padlock installed), under any of the following conditions:

• Locating a disconnect adjacent to the motor is impracticable or introduces increased hazards to persons or property (e.g., submerged pumps, explosive atmospheres).
• In industrial installations with written safety procedures and where conditions of maintenance and supervision ensure only qualified persons service the equipment.

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Sizing the Disconnecting Means: Ampere & Horsepower Ratings

Every motor disconnect switch must satisfy two independent ratings:

1. Ampere Rating: NEC 430.110(A)

The disconnecting means for motor circuits rated 1000 volts or less must have an ampere rating of not less than 115 percent of the full-load current rating determined from Tables 430.247–430.250:

Minimum Disconnect Ampere Rating = Table FLC * 1.15

2. Horsepower Rating: NEC 430.109

The disconnecting means must have a horsepower rating not less than the horsepower rating of the motor (as marked on the motor nameplate). The switch contacts must be physically rated to interrupt the locked-rotor current of that motor horsepower.

Exceptions to HP Rating (NEC 430.109):

• Molded-Case Circuit Breakers: A molded-case circuit breaker or molded-case switch is permitted as a disconnect if rated in amperes.
• Stationary Motors <= 1/8 HP: Branch-circuit OCPD permitted as disconnect.
• Stationary Motors <= 2 HP (300V or less): General-use AC snap switch permitted if rated at least 2 * motor FLC.

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Motor Controllers: NEC Article 430 Part VII

A motor controller is defined in Article 100 as any switch or device normally used to start and stop a motor by making and breaking the motor circuit current (e.g., manual motor starters, magnetic contactors, variable frequency drives [VFDs]).

• Controller Rating (NEC 430.83): Must have a horsepower rating not less than the motor horsepower, except general-use switches, inverse-time circuit breakers, or listed inverse-time breakers are permitted under specific conditions.
• Opening All Conductors (NEC 430.84): The controller does not need to open all conductors to stop the motor unless it also serves as the disconnecting means.
• Control Circuit Protection (NEC 430.72): Motor control circuit conductors tapped from the load side of the branch-circuit OCPD must be protected against overcurrent in accordance with Table 430.72(B).

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Common Exam Traps & Practical Scenarios