Eaton HMCP400R5 Circuit Breaker

Application of multi-wire terminals

for molded case circuit breakers

Abstract

Multi-wire terminals for molded case circuit

breakers are a means to provide more than one

load side termination while maintaining the circuit

breaker’s ULT listing. This paper examines the

application of multi-wire terminals in conjunction

with UL Standard 508A as well as with the National

Electrical CodeT, NFPAT 70-2014 (NECT). The paper

addresses multi-wire terminals used with circuit

breakers in industrial control panels as defined in

NEC Section 409 and UL Standard 508A.

Codes and standards requirements

NEC requirements

NEC Section 430.28 addresses the topic of feeder taps.

430.28 Feeder Taps—Feeder tap conductors shall have an ampacity

not less than that required by Part II (Part II addresses the sizing

of conductors for specific loads; consult the NEC for details), shall

terminate in a branch-circuit protective device, and, in addition, shall

meet one of the following requirements:

Be enclosed either by an enclosed controller or by a raceway,

be not more than 10 feet (3 m) in length, and, for field

installation, be protected by an overcurrent device on the line

side of the tap conductor, the rating or setting of which shall not

exceed 1000 percent of the tap conductor ampacity.

2. Have an ampacity of at least 1/3 that of the feeder conductors,

be suitably protected from physical damage or enclosed in a

raceway, and be not more than 25 feet (7.5 m) in length.

3. Have an ampacity not less than the feeder conductors

[Reference 1].

UL 508A requirements

UL 508A Section 31.4 addresses the issue using similar

language.

Section 31.4.3—The ampacity of the tap conductors, the internal

conductors to the individual loads, shall be:

A. Not less than 1/3 the ampacity of the branch circuit conductor,

calculated as in 28.3.3 (28.3.3 addresses the sizing of field-wiring

conductors for specific loads. Consult UL 508A for details.); or

B. Not less than 1/10 the ampere rating of the branch circuit

protection for the group for each motor circuit provided with

a manual motor controller marked “Suitable as tap conductor

protection in group installations” and complies with the Standard

for Industrial Control Equipment, UL 508. The conductors on the

load side of the manual motor controller shall have an ampacity

not less than calculated in 28.3.2 (28.3.2 addresses the sizing

of field-wiring conductors for motor loads; consult UL 508A for

details) [Reference 2].

NEC 430.28 (1) and UL 508A 31.4.3 (b) state the same requirement

in different terms. Both agree, however, that a tap conductor no more

than 10 feet (3 m) long, emanating from a multi-wire terminal (tap),

must have an ampacity equal to at least 10 percent of the ampacity

of the feeder breaker’s trip current. Some term this the 10-foot

tap rule.

Likewise, NEC 430.28 (2) and UL 508A 31.4.3 (a) state the same

requirement for tap conductors up to 25 feet (7.5 m) long, sometimes

termed the 25-foot tap rule. A tap conductor up to 25 feet (7.5 m)

long must have an ampacity equal to at least 1/3 the ampacity of

the feeder breaker’s trip current.

Sizing conductors

Application complying with UL 508A

10-foot tap rule

The ampacity for a tap conductor applied to a 200 A circuit breaker

must be equal to 20 A or greater. Referring to Table 28.1 in UL 508A,

the conductor would have to be at least 12 AWG. The conductor must

terminate in an appropriately sized overcurrent protective device,

for example, a circuit breaker or fuse.

25-foot tap rule

The ampacity for a conductor not exceeding 25 feet (7.5 m) in length

must be at least 1/3 the ampacity of the circuit breaker’s trip rating.

The ampacity for a tap conductor applied to a 300 A circuit breaker

must be equal to 100 A or greater. Referring to Table 28.1

in UL 508A, the conductor would have to be at least 3 AWG.

The conductor must terminate in an appropriately sized overcurrent

protective device, for example, a circuit breaker or fuse.

Size range of conductors

While a wide range of conductors is listed for a particular multi-wire

terminal, the entire range may not apply to every breaker frame to

which a terminal is affixed. A K-frame breaker, for example, can have

trip ranges from 70 A to 40 A. While a 14 AWG conductor may be

applied to a 70 A trip unit, it may not be applied to a 400 A trip unit.

Proper selection must be made as noted in the rules elucidated in

the application sections above.

Conductor temperature ratings

Circuit breaker terminals are rated for a maximum temperature of

75 °C. Attention must be paid to conductor temperature ratings and

the corresponding ampacities tabulated in the appropriate standard.

Table 28.1. UL 508A, contains columns for conductor temperatures

of 60 °C and 75 °C. Either is within the range of the multi-wire

terminal temperature rating.

While NEC Table 310.15 (b) (16) contains like ratings, it also contains

columns for 90 °C and higher. Conductors having higher temperature

ratings, such as 90 °C, may be used; however, the ampacity must be

de-rated to that in the 75 °C column.