NEC Article 430 Requirements for Motor Control Centers

NEC Article 430 is the primary code reference for motor installations in the United States. It governs everything from conductor sizing to short-circuit protection to disconnect requirements. For anyone designing, installing, or maintaining MCCs, understanding Article 430 is essential.

Article 430 Structure

Article 430 is organized into parts that follow the power path from supply to motor:

Part I: General provisions and definitions
Part II: Motor circuit conductors
Part III: Motor and branch circuit overload protection
Part IV: Motor branch circuit short-circuit and ground-fault protection
Part V: Motor feeder short-circuit and ground-fault protection
Part VI: Motor control circuits
Part VII: Motor controllers
Part VIII: Motor control centers (specific requirements)
Part IX: Disconnecting means
Part X: Adjustable-speed drive systems

Part II: Conductor Sizing (430.22)

Motor branch circuit conductors must be rated at not less than 125% of the motor full-load current (FLC). This is one of the most commonly applied sections:

Key Rule: Use NEC Tables 430.247 through 430.250 for full-load current values, NOT the motor nameplate current. The nameplate shows the actual motor FLA, which may differ from the table values.

Example: A 25 HP, 480V, 3-phase motor has a table FLC of 34 amps (Table 430.250). Minimum conductor ampacity = 34A x 1.25 = 42.5A. Select a conductor rated for at least 42.5A.

Inside an MCC bucket, the internal wiring must also be rated for 125% of the motor FLC. This is handled by the bucket manufacturer during design and assembly.

Part III: Overload Protection (430.31 - 430.44)

Motor overload protection prevents the motor from drawing excessive current during normal operation. This is the overload relay in an MCC starter bucket.

430.32 - Continuous-Duty Motors

Overload devices must trip at no more than:

115% of motor nameplate FLA for motors with a service factor of 1.15 or greater
115% of motor nameplate FLA for motors with a temperature rise not exceeding 40 degrees C
125% of motor nameplate FLA for all other motors

430.33 - Intermittent and Other Duty

Motors rated for intermittent, periodic, or varying duty have different overload requirements based on the duty cycle classification.

430.35 - Shunting During Starting

It is permissible to shunt (bypass) the overload device during motor starting if:

The device is automatically reinserted after starting
Fuses or a time-delay circuit provides protection during the starting period

430.36 - Fuses for Overload Protection

When fuses are used for overload protection (rare in MCC applications), each ungrounded conductor must have a fuse.

430.44 - Automatic Restarting

Overload devices must not automatically restart the motor unless the restart cannot create a hazard. This is particularly important for conveyors, saws, and other equipment where unexpected restart could injure personnel.

Part IV: Short-Circuit Protection (430.51 - 430.58)

Motor branch circuit short-circuit and ground-fault protective devices (SCPD) protect the circuit wiring and motor from damage during fault conditions. This is the breaker or fuse in the MCC bucket disconnect.

430.52 - Rating or Setting

Maximum SCPD ratings are specified as percentages of motor FLC:

Device Type	Maximum % of Motor FLC
Dual-element time-delay fuse	175%
Non-time-delay fuse	300%
Inverse-time circuit breaker	250%
Instantaneous-trip breaker (MCP)	800% (adjusted) or 1300% (non-adjustable)

If the calculated value doesn't correspond to a standard device size, the next higher standard size is permitted.

Example: 25 HP motor, 480V, FLC = 34A

Dual-element fuse: 34A x 1.75 = 59.5A, use 60A fuses
Inverse-time breaker: 34A x 2.50 = 85A, use 90A breaker
MCP: 34A x 8.0 = 272A, set at 272A or next lower setting

430.53 - Several Motors on One Branch Circuit

Multiple motors can share a single branch circuit under specific conditions. This is common in MCCs where small motors on the same process share a single feeder.

Part VII: Controllers (430.81 - 430.91)

430.83 - Controller Rating

Motor controllers (contactors in MCC buckets) must have a horsepower rating not less than the motor HP rating. For motors over 600V, the controller must be rated for the motor voltage and locked-rotor current.

430.87 - Number of Motors per Controller

Each motor generally requires its own controller. Exceptions exist for groups of motors driving a single machine.

Part VIII: Motor Control Centers (430.92 - 430.98)

This part applies specifically to MCCs:

430.94 - Overcurrent Protection

The MCC must be provided with overcurrent protection not exceeding the rating of the common power bus. This is the main breaker or main fuses.

430.96 - Grounding

The MCC enclosure and all metal parts must be grounded per Article 250. An equipment grounding terminal bar must be provided.

430.97 - Busbars and Conductors

Busbars must be protected from physical damage and arranged to avoid induced heating effects. Bus connections must be accessible for maintenance.

430.98 - Marking

Each MCC must be marked with:

Manufacturer's name
Voltage
Common bus current rating
Short-circuit current rating
Modification record (if field-modified)

This marking is usually on the MCC main section nameplate.

Part IX: Disconnecting Means (430.101 - 430.113)

430.102 - Location

A disconnect must be in sight from the controller (within 50 feet and visible). In an MCC, the bucket disconnect satisfies this requirement because it is integral to the unit.

430.109 - Type

Motor circuit disconnects must be one of:

Listed motor circuit switch (rated in HP)
Listed molded case circuit breaker
Listed molded case switch
Instantaneous-trip circuit breaker (as part of a combination starter)
Listed self-protected combination controller

430.110 - Ampere Rating

The disconnect ampere rating must be at least 115% of the motor FLC for individual motors.

Practical Application in MCC Design

When specifying a new MCC or adding a bucket to an existing MCC, use this checklist:

Determine motor FLC from NEC Table 430.250
Size conductors at 125% of FLC (430.22)
Select short-circuit protection device (430.52)
Select contactor with proper HP rating (430.83)
Select overload relay with proper trip class and FLA range (430.32)
Verify disconnect rating at 115% of FLC (430.110)
Ensure proper grounding (430.96)
Label the bucket with required information (430.98)

Get Expert Help

MCC Depot builds code-compliant replacement buckets and custom MCC solutions for all major brands. Our engineering team ensures every bucket meets NEC Article 430 requirements.

Call 307-442-0382 or email sales@mccdepot.com for assistance with your motor control needs.