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What is a bus stab in an electric panel?
Sunday, July 7, 2019
Most electric panels have two vertical bus bars. Each bus bar has bus stabs, also called “bus fingers” or “connector fingers,” that are bent out to receive breakers. Each one can serve two full-size breakers, connected from left and right. In the example shown above, stabs #1 and #3 are part of the right bus bar. Stab #2 is connected to the left bus bar, and they alternate left and right as it goes down the pair of bus bars.
Many manufacturers specify a maximum total amperage allowed to be connected to one stab. So, for example, if a panel is rated for maximum of 150 amps per stab, then a 100 amp breaker on both sides of the same stab would exceed the max rating, but two 60 amp breakers would not. According to the manufacturer Square D, “the restrictions are applied only when we must limit the total amperage to pass applicable UL heat rise tests.” Here’s an example of a rating below.
A tandem breaker, also called a “half-high,” which allows two breakers to attach to one side of a stab in what would ordinarily be a single breaker slot, doubles the amount of circuits on a stab to four when used on both sides, and the total of all four must be under the rating max. To learn more about tandems, see our blog post What is a tandem circuit breaker? And for info on a panel detail to keeps too many tandem breakers from being installed in a panel, go to What is a CTL breaker?
Another situation where the maximum stab rating may come into play is when a panel is backfed, which is also why the two slots opposite a backfeed breaker are often unused. See our blog post What is a "backfeed" main breaker at an electric panel? for more on this.
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