Re: tandem breakers
From the NEC Handbook. "Class CTL panelboards incorporate physical features that, in conjunction with the physical size, configuration, or other means provided in Class CTL ciruit breakers, fuseholders, or fusible switches, are designed to prevent the installation of more overcurrent protective poles than the mumber for which the panelboard is designed and rated."
I have a Murray Type MH-T double 20 amp breaker that fits in a single position (one individual phase slot) of a panel, and it has a large label (no glasses necessary to read it
) on the side that says 'FOR REPLACEMENT USE ONLY. NOT FOR CTL ASSEMBLIES'. I also have a "two 1 pole" Sq D breaker that has the same large label on the side.
If you have a panel enclosure that is listed as CTL, these circuit breakers (even though they can be made to fit) are not permitted in the enclosure.
Pierre