**Question 6. **When are twin circuit breakers legal to use in a panelboard? Does the panelboard need to be listed for application? Do the panel cover and directory need to be listed? — M.V.
Answer 6. First, we should clarify the term twin circuit breaker, which is a standard full size (single-pole frame) circuit breaker that incorporates two overcurrent devices, each switched independently. The overcurrent devices connect to the same ungrounded phase bus in the panelboard. Twin circuit breakers are typically limited to 120/240 volt, 15 and 20 amperes, and 10,000 amperes interrupting capacity. Twin circuit breakers are also known as dual, tandem, piggyback and two-pole thin. As an aside, there are single-pole overcurrent devices that are half the width of a standard full size single-pole circuit breaker. The single-pole devices are known as half size and single-pole thin.
Twin circuit breakers are legal for use in panelboards if used and installed per the manufacturer’s instructions and minimum code requirements. The panelboard should be marked to indicate the acceptable use and installation of these circuit breakers. A panelboard may bear the marking of 8-16 circuit, meaning any combination of eight twin circuit breakers or sixteen full size circuit breakers. The panelboard and/or circuit breaker may have a mechanical feature to limit the use and installation of twin circuit breakers to specific panelboards too. From a Code standpoint, 408.15 limits the number of overcurrent devices on a lighting and appliance panelboard to 42. This section also requires that “A lighting and appliance branch-circuit panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed rated, and approved.” A panelboard marked as Class CTL is a circuit-limiting panelboard that meets the requirements of this section.
As a general rule, all electrical equipment and materials should be listed and labeled for the application as it would be difficult or impossible to determine the suitability of electrical equipment and materials for use and installation without listing and labeling. Naturally, most installations require the use of various passive devices, such as nuts, bolts, washers, straps, and so forth. These passive devices can easily be determined as acceptable for use and installation without listing and labeling. This usually requires some simple common sense.
I hope this provides a reasonable response to your questions. However, I would encourage you to contact the appropriate manufacturer’s representative as he or she may have more specific information and possible limitations on the use and installation of twin circuit breakers.— Lanny McMahill, CMP-1