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Part 12 – Sizing Motor Starter & Contactor – NEMA – NEC

Electrical Technology
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How to Size a Motor Starter, Contactor and Controller? NEMA – NEC

Motor starters and controllers are a combination of devices in a single unit to safely start, run and stop a motor. It makes possible to protect the motor from overcurrent caused by ground and short circuit faults or overload conditions.

A controller unit or stater includes the following components and devices in an enclosure:

  • Contactor(s) & Breakers
  • Control Circuit (Auto transfer, solenoid and auxiliary contacts)
  • Thermal Overload Relay
  • Push buttons for Control and Controller Disconnect (Start, Stop, Reverse, Forward)
  • Pilot Lamps (Indicating the motor current state)

Contactors are sized based on the NEMA ICS 2-2000 standard, which covers Industrial Control and Systems Controllers, Contactors, and Overload Relays rated up to 600 volts. This standard includes guidelines for the selection of contactors, overcurrent protective devices (OCPDs), and thermal overload relays for motor circuits.

In contrast, the IEC 60947-4-1 standard provides guidelines for the selection of electromechanical contactors, motor starters, and contactors used in motor control applications.

Sizing Motor Starter, Contactor & Controller

Why We Need a Motor Controller?

When voltage is first applied to the field winding of an induction motor, only the conductor’s resistance opposes the flow of current in the motor windings. This resistance is very low (almost equivalent to a dead short) which  results in a large inrush current flowing through the windings.

At this initial stage, the motor armature is not yet rotating, so no back EMF (or counter electromotive force) is generated. The absence of back EMF leads to very low impedance, effectively creating a short-circuit condition.

As a result, the motor attempts to draw the locked-rotor current (LRA), which is typically 6 to 8 times greater than the full-load current. If this high inrush current is not quickly reduced, the motor windings will overheat and may eventually burn out.

To prevent this, it is required to install a motor starter, contactor, or controller (such as a direct-on-line (DOL) starter, star-delta starter, etc.) in the motor circuit to reduce the high starting current to the normal rated current.

Rating of the Motor Controller (Starter & Contactor)

Motor starters and controllers can include a contactor with an overload relay, molded case circuit switches, or simply a circuit breaker, as specified in NEC 430.83(A). A general-use snap switch may also be used as a motor controller, provided that the motor’s full-load current (FLC) does not exceed 80% of the switch’s ampere rating, in accordance with NEC 430.83(C)(2).

Any other device used as a controller (whether a contactor, starter, or circuit breaker) must have a power rating at least equal to the horsepower (HP) rating of the motor.

Sizing a Starter for NEMA Motors

Sizing a motor starter involves determining the appropriate full-load current (FLC), overload protection, ground fault and short-circuit protection. Here’s a step-by-step guide with solved example to correctly size a motor starter, contactor or combined controller.

Example: Sizing Motor Starter Components

What is the Correct Size of Controller including Starter and Contactor Components required for a 10 HP, 460V, 3-Phase Motor having the nameplate FLA = 13.5A and service factor 1.15?

Solution:

Step 1: Determine Motor Full-Load Current (FLC)

The Full-Load Current (FLC) of the motor is found in NEC Table 430.250 for three-phase AC motors. If the exact motor is not listed, use the motor’s nameplate FLA (Full Load Amps).

FLC : 14A

Step 2: Select the Motor Starter Size

Motor starters are rated in horsepower (HP) and amperes. It is recommended that the selected controller or starter matches the motor’s FLC and voltage rating. Using the NEMA Size Chart, a 10 HP, 460V motor would typically use a NEMA Size 1 starter having 27A max. continuous amperes and Max. 10 HP at 480/575 V, AC .

Starter: NEMA Size 1 Starter

Step 3: Size the Conductor (Wires)

Per 430.22(A) in accordance with 430.6(A)(1), motor branch-circuit conductors must be sized at 125% of the motor’s FLC.

Conductor Ampacity = 14A × 1.25 = 17.5A

From Table 310.16, using 75°C rated copper conductors, we select #12 AWG THHN (20A rated).

Step 4: Size the Short-Circuit and Ground-Fault Protection

According to NEC 430.52 and Table 430.52(C)(1), inverse-time circuit breaker are rated at 250% while instantaneous circuit breakers are rated at 800% of the FLC of the motor. On the other hand, dual-element time delay fuses are rated at 175% of the full load current of the motor.

  • Inverse Time Breaker Size = 14A × 2.5 = 35A
  • Instantaneous circuit Breaker Size = 14A × 800% = 112A
  • Fuse Size= 14A × 1.75 = 24.5A ≈ 25A

Thus, a 35A inverse time or 112A instantaneous circuit breaker or a 25A time-delay fuse is acceptable to protect the branch circuit and control circuit conductors from short circuit and ground faults.

Good to Know:

  • In an inverse time circuit breaker, the tripping time is inversely related to the magnitude of the overcurrent. This means the breaker trips faster for higher overcurrents and slower for lower overcurrents. Essentially, it provides a delay that shortens as the current level increases.
  • Inverse time circuit breakers are designed to respond to overcurrents with a delay, but the delay decreases as the overcurrent increases. This is different from instantaneous circuit breakers, which trip immediately when the current exceeds a certain threshold.

Step 5: Size the Overload Protection (Thermal Overload Relay)

Per NEC 430.32(A)(1), the overload relay should be 125% of the motor’s FLA with marked service factor 1.15 or greater or temperature rise 40°C or less. For other motors, the rating of overload protective device shall be 115% of the motor full load amperes.

Overload Setting = 12.5A × 1.25 =15.5A

Thus, choose an overload relay setting below 15.5A which is 15A. Keep in mind that the overload protective device will only protect the motor against the overload conditions, not the short circuit and ground faults.

Step 6: Size the Motor Disconnect

Per 430.109(A)(1), 430.110 and Table 430.350 for fuses, the disconnecting means should be rated for at least 115% of the FLC.

Disconnect Rating = 14A × 1.15 =16.1A

Choose the next available standard 20A-rated motor disconnect switch.

Step 7: Size the Contactor/Controller

Motor contactors and controller should be rated for at least the motor’s FLC (other than fuse and circuit breakers). For a 10 HP, 460V motor, a NEMA Size 1 contactor is appropriate. For more details, refer to the NEMA ICS 2: Table 2-4-1 for horsepower (HP) and Locked-Rotor Current (LRA) ratings for three-phase magnetic controller for motors.

  • NEMA Contactor: Size 1 – Min. 10 HP
  • IEC Contactor: AC4: 13.5A  (Rapid Start/Stop)

Example Circuit for a 10 HP, 460V Motor

Here’s a complete motor starter / controller circuit setup based on the above calculations:

Component Sizing
Motor Voltage 460V, 3-Phase
FLC (NEC Table 430.250) 14A
Overload Relay Setting 15A
Breaker Size (430.52) 35A (inverse time)
Fuse Size (430.52) 25A (time delay)
Conductor Size (430.22(A)) #12 AWG THHN
Motor Starter NEMA Size 1
Disconnect Switch (430.110) 20A
Contactor NEMA Size 1 – IEC Size – AC 3

Series Overview: Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

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Electrical Technology
0 6 minutes read

Electrical Technology

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