8536DB0901R03/21

Structure of a Motor Starter in Accordance with UL 508A

Feeder Circuit and Branch Circuits

The branch circuit consists of the last short-circuit protective device before a load and all power conductors and components between the last short-circuit protective device (SCPD) and the load.

The feeder circuit consists of all power conductors and components on the supply side of a branch circuit.

Feeder Circuit and Branch Circuit

Table 2 contains a description of the numbered circuit components called out in the figure Feeder Circuit and Branch Circuit.

Circuit Components

1

Load—provided in the field

14

Control transformer ground (for 1000 VA max. control transformer)

2

Field wiring terminals

15

Control circuit devices and wiring—Class 1 circuit—isolated secondary circuit

3

Alternate field terminals

16

Solenoid or other control device—provided in the field

4

Field wiring

17

Supplementary protection

5

Internal wiring of the power circuit

18

Class 2 transformer

6

Overload relay and heater elements

19

Class 2 circuit

7

Contactor or controller

20

Power transformer fuse—branch circuit protection

8

Starter

21

Power transformer—for motor load and control protector

9

Combination motor controller

22

Control circuit—Class 1 circuit—common control circuit

10

Branch circuit protection

23

Equipment ground and equipment ground terminal

11

Fused disconnect switch or circuit breaker

24

Bonding conductor—bonding jumper

12

Control transformer

25

Grounding electrode conductor terminal

13

Control transformer fuse or supplementary protection

26

Grounding electrode conductor—provided in the field

Disconnecting Means

A disconnect means is required for each incoming supply, and must open each ungrounded conductor of the supply circuit. A disconnect is also required for each controller and motor, and must be within line of sight of the controller and motor. However, it is permissible to use a single disconnect for a group of controllers. A single disconnect can also serve as the disconnect for both the motor and controller.

Disconnect Types

The following components are considered an acceptable means of disconnect for the feeder circuit:

  • UL 489 inverse-time circuit breaker or molded case switch

  • UL 98 switch unit, either open type or enclosed.

The following components are an acceptable means of disconnect in the branch circuit:

  • UL 489 inverse-time circuit breaker, instantaneous trip circuit breaker (motor circuit protector (MCP)), or molded case switch

  • UL 98 switch unit, either open type or enclosed

  • UL 60947-4-1 disconnect or manual motor controller marked "Suitable as Motor Disconnect"

  • Type E self-protected combination motor controller

Application of Disconnect Switches

There are two UL standards that govern disconnect switches: UL 98 and UL 60947-4-1 (formerly UL 508).

  • UL 98 listed disconnects are suitable for both branch and feeder circuit applications.

  • Disconnects listed to UL 60947-4-1 are intended for motor switching, and should only be used below the branch short-circuit protective device (SCPD).

See NEC 430 and UL 508A for additional requirements and exceptions.

Group Motor Installations

In a group motor installation, a single circuit breaker or fuse may be used for a group of motors. The specific rules are described in NEC Sections 430-112 and 430-53.

Group motor installations are used in the following applications:

  • Fractional HP motors—NEC Section 430.53 (A):

  • For several motors, each rated 1 HP maximum on a branch circuit of 1000 V maximum, a single SCPD not exceeding 15 A is permitted under the following circumstances:

    • No single motor is rated greater than 6 A Full Load Amperes (FLA).

    • The branch-circuit short-circuit device rating marked on any controller is not exceeded.

    • Appropriate motor overload protection is used. Refer to NEC Section 430.32.

  • Smallest rated motor protected—NEC Section 430.53 (B):

    The SCPD is sized for the motor in the group with the lowest rating.

    • If the SCPD is a circuit breaker, it is sized to a maximum of 250% of the smallest motor FLA.

    • If the SCPD is a time-delay fuse, it is sized to a maximum of 175% of the smallest motor FLA.

    • If the SCPD is a non-time delay fuse, it is sized to a maximum of 300% of the smallest motor FLA.

  • Other group installations—NEC Section 430.53 (C):

    Two or more motors of any rating, or one or more motors and other loads, where all the following conditions are met:

    • Each motor controller and overload protection device carries a Motor Group Installation listing.

    • Each circuit breaker (if used) is listed and is of the inverse time type.

    • The SCPD is sized as follows:

      — If the SCPD is a circuit breaker, it is sized to a maximum of 250% of the largest motor FLA plus the sum of the full load current rating of all other motors and loads.

      — If the SCPD is a time-delay fuse, it is sized to a maximum of 175% of the largest motor FLA plus the sum of the full load current rating of all other motors and loads.

      — If the SCPD is a non-time delay fuse, it is sized to a maximum of 300% of the largest motor FLA plus the sum of the full load current rating of all other motors and loads.

    • Appropriate overcurrent protection is provided for non-motor loads.

Tap Conductor Sizing Rules

The tap conductor sizing rules for group motor installations are governed by NEC Section 430.53 (D). Conductors from the load side of the SCPD for the motor group must comply with one of the following:

  • The ampacity is not less than that of the branch circuit conductors.

  • The ampacity is not less than 1/3 of the branch circuit conductors, and the length from the point of the tap to the motor overload protection device is not more than 7.5 m (25 ft.).

  • The ampacity is not less than 1/10 of the SCPD for the motor group, the length from the point of the tap to the controller(s) is not more than 3 m (10 ft.), and the manual motor controller is marked “Suitable for Tap Conductor Protection in Group Installations.”

NOTE: Products certified for motor grouping must carry the markings on the product label or instruction sheet required by NEC and UL (such as “Suitable for Tap Conductor Protection in Group Installations”).

For more information, see data bulletin 8502DB0701 entitled “Group Motor Installations: Understanding NEC 430.53 Requirements.”

Required Functions in Branch Circuits for a Motor Load

NEC 430 requires four essential functions in the branch circuit for a motor load. Each function plays an important role in the control and protection of the motor load. See the figure Required Functions of Combination Starters for the four functions that compose a complete motor branch circuit.

Required Functions of Combination Starters

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