Rules for tapping molded case circuit breakers
An in-depth focus on tap rules for NEC and UL feeder circuits, the need for branch and feeder circuit taps, short circuit interrupting ratings, and creating branch or feeder circuit taps from circuit breakers.
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In many cases, a molded case circuit breaker is employed as the main overcurrent protective device (OCPD), as well as the main disconnecting means, for an industrial control panel or as a main or feeder application. If branch circuits must emanate from the main OCPD, the means must be available to fan out from its terminals. While the examples described in this article are accurate, they are not the only configurations available. Circuit breaker terminal tables should be consulted for other terminals, wire sizes, and quantities of wire.
Tap rules for NEC and UL 508A feeder circuits
In brief, the rules for tapping feeder circuits indicate that a conductor rated 10% of the ampacity of a feeder conductor may be tapped from the feeder, provided it is not greater than 10 ft in length and it terminates in an appropriately sized OCPD. Also, a conductor rated one-third the ampacity of a feeder conductor may be tapped from the feeder provided its length is not greater than 25 ft and it terminates in an appropriately sized OCPD.
Addressing the need for branch and feeder circuit taps
While power distribution terminal blocks may be used to distribute branch circuits, an alternative is to distribute branch circuits directly from the breaker’s terminals if the terminals have provisions for more than one cable. Terminals for larger breakers may contain one or more conductor openings. The feature is normally used for paralleling cables to feed the full ampacity of the breaker to a load. The multiple terminal openings on the load side of the breaker may also be employed for deriving lower-current branch or feeder circuits from a breaker in accordance with the NEC and UL 508A tap rules .
Short circuit interrupting ratings
Power terminal blocks serve a purpose for distributing circuits within industrial control panels. Their function is to provide a means to tap smaller conductors from a circuit breaker, provided the tapping rules are followed. Conductors equal to the full ampacity of the circuit breaker must be extended to the power terminal block (PTB). Properly sized taps may extend from the PTB to an OCPD.
Even when the tapping rules are followed, the short-circuit current rating (SCCR) of the circuit may be limited. UL 508A, Table SB 4.1, assigns an SCCR of 10,000 A to an unmarked, untested PTB. This may severely limit the SCCR for an industrial control panel. The use of standard, multiple-conductor terminals for circuit breakers can overcome this limitation. Listed and approved terminals take on the same SCCR as the breaker to which they are connected.
It is important to understand misapplication of the tap rule to avoid design and field errors. Figures 1 and 2 illustrate a tap not permitted by the NEC or UL 508A. Note that the 4/0 cable is a correct cable size based on the tapping rules (minimum 10% of the breaker rating and less than 10 ft in length). The 4/0 cable terminates on a PTB. After that, 1 and 10 AWG wires are tapped from the PTB. These latter taps are not allowed. The 4/0 cable must terminate on a circuit breaker of the correct ampacity, namely 225 A or smaller. A tap cannot be made from the 4/0 cable prior to terminating the cable on a properly sized overcurrent protective device.
The argument here regards “tapping a tap.” The 4/0 conductor is a tap from the 1,200 A circuit breaker. Note that connecting the conductor to one terminal of a multi-equipped breaker terminal is the same electrically as if four 350 KCMIL conductors were extended from the circuit breaker and the 4/0 cable tapped from those. The 4/0 cable is required to terminate at a 225 A or smaller circuit breaker in order for the wire to be considered protected. In the example, instead, the 4/0 cable is further tapped with a conductor as small as 10 AWG. Though the 1 AWG conductor can be tapped from the 1,200 A circuit breaker, the 10 AWG conductor cannot. Neither conductor can be tapped from the 1,200 A circuit breaker through the use of a PTB. In both of these scenarios, 1 and 10 AWG conductors are not considered to be properly protected.
Figure 2 illustrates the example in Figure 1 taken to the extreme. In Figure 2, one sees that the 4/0 conductor can now be overloaded by the addition of the 250 A breaker. By adding this breaker to the circuit, a total load of 405 A may be imposed on the 4/0 conductor. Under a full load scenario, the 4/0 conductor would be overloaded, but the 1,200 A breaker may never see sufficient current to trip, leading to failure of the 4/0 conductor.
Creating branch or feeder circuit taps from circuit breakers using circuit breaker terminals
Figure 3 illustrates a 2,500 A breaker frame with a 1,600 A trip unit (higher ampacity trip units are available to the full-frame rating). Various breaker sizes are tapped from the breaker. While breakers with smaller trip units may be used, cable sizes 2/0 AWG and larger must be used to tap from the 2,500 A breaker. Note that the terminals shown are 2,000 A terminals because provision is available to terminate six cables. Terminals for 1,600 A could be used, saving some cost, but they allow termination of only four cables.
Figure 4 illustrates 1,200 A breaker frame with an 800 A trip unit. The drawing is intended to depict the range of breakers that may be used. Because the minimum cable size for the 1,200 A frame is 3/0, the 125 A breaker is fed with 3/0 AWG, though the trip unit required may be smaller.
A 1,200 A breaker with a 1,200 A trip unit can be provided. Since the terminals are the same for either trip unit, the same cable-size limitations apply.
The 600 A circuit breaker contains terminals capable of handling two wires, each for sizes 2 AWG to 500 KCMIL. Figure 5 illustrates a 400 A breaker, as well as a 125 A breaker tapped from the 600 A circuit breaker. Taps are shown at full capacity for each breaker. While trip units could be smaller, the smallest cable size that may be used is 2 AWG due to the lower limit of the upstream or main breaker terminals; here, cable size is the limiting feature. In practice, the 600 A circuit breaker may contain a lower-ampacity trip unit to avoid the necessity to oversize the incoming cable.
Figure 5 illustrates two 250 A circuit breakers tapped from the 500 A breaker. In practice, the 500 A circuit breaker is used to avoid the necessity to oversize the incoming cable.
Figure 6 provides an additional example of taps to serve lower-ampacity loads from higher-ampacity circuit breakers, only from a different series of circuit breakers. While terminal configurations differ from the first series, the rules for tapping remain the same.
The main circuit breaker in Figure 6 has five smaller breakers tapped from its terminals. One may actually tap six breakers because the terminals for the 2,500 A breaker will accept six conductors. The terminal capacity is for 6-2 AWG to 600 KCMIL. Consequently, a circuit breaker as large as 450 A can be tapped from the 2,500 A breaker using a single, 600-KCMIL conductor. NEC 240.4 (B) permits sizing to the next higher standard rating shown in 240.6 if the conductor rating is not equal to a standard rating; the maximum OCPD cannot exceed 800 A. UL 508A does not offer a similar exemption.
PTBs are often used in UL 508A-listed industrial control panels and NEC installations for tapping circuits. However, economies can be gained, and higher short-circuit ratings achieved, through the use of standard, multiple-conductor terminals on circuit breakers. The engineer should review all loads carefully, including tap loads, to ensure that the main and branch circuit protection devices are serving their primary function to protect the wire.
|Bredhold is an application engineer at Eaton Corp. in Louisville, Ky. Bredhold is a member of IEEE, UL 508A Standards Technical Panel, and NFPA 99 Healthcare Standards committee. Matwijec is a product manager for molded case circuit breakers at Eaton in Pittsburgh.|
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