Because tandem circuit breakers allow for two circuits to be installed on a panelboard in a one circuit breaker space, they're typically used after a panelboard has been filled to capacity with standard circuit breakers. Because of this, they're often referred to as "cheaters."Is this really 'cheating'? No, it's not. The use of tandem circuit breakers is a perfectly acceptable practice, as long as the panelboard is designed for tandem circuit breakers and they're installed in locations within the panelboard where they're allowed.

How does a home inspector determine whether tandem circuit breakers are allowed on the panelboard they're inspecting? There are a number of ways to do so.

Class CTL Panelboards

Panelboards must follow UL Standard 67, which requires all lighting and appliance panelboards to be Class CTL (Circuit Total Limiting). Here's an old formula for determining how many circuits are allowed in the Class CTL panelboard being inspected; this formula is helpful to use when inspecting older electric panels without clear labeling inside the panel. Take the amperage of the panelboard, multiply by the number of poles, and divide by 10. It sounds complicated, but it's not — let's use a 100-amp panelboard as an example:

100 Amp x 2 Poles = 200200 / 10 = 20

Based on this formula, the maximum number of circuits allowed in a 100-amp 120/240-volt panelboard is 20. For panelboards not manufactured as "lighting & appliance" panelboards, there is no limit to the number of circuit breakers allowed. This formula is also no longer applicable for today's panels.

The 2008 National Electrical Code (NEC) makes it a little confusing. Previous editions of the NEC limited the maximum number of circuits in a lighting and appliance panelboard to 42. The 2008 version of the NEC removed the “lighting & appliance” panelboard designation, however, NEC 408.54 says, “A 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 listed.” Manufacturers still list the maximum number of circuit breakers allowed, and must provide a rejection feature to help prevent the use of tandem circuit breakers where not allowed.

Class CTL panelboards have different methods of preventing class CTL tandem circuit breakers from being used in locations where they’re not allowed. This is referred to as a “rejection feature”. In the photo below, the highlighted bus stabs are notched to allow the use of tandem circuit breakers; the bus stabs which don’t have this notch won’t allow tandem breakers.