Can I replace two Siemens QP type, handle tied, single pole 15 amp breakers with a tandem QT type in the same configuration?

Question

Our Siemens electrical panel has two single pole 15 amp QP type breakers, with a handle tie that are together powering a mini-split HVAC system. One breaker has the white neutral line into it (wrapped in black tape as appears to be expected) and the other has black hot line.

Can I replace these with a tandem 15 amp QT type Siemens breaker (edit: specifically, model Q1515U)? Assuming I use a handle tie (model ECQTH2, which it looks like qualifies as a common trip for those breakers) and wire it with the same neutral line in one and hot line in the other.

Edit: fwiw, this panel does support tandems in every slot

Photo of the current breaker if it's helpful:

Answer 1

NO. Your current configuration puts each hot on a different leg, resulting in 240 volts (120+120). The tandem breaker you’re proposing puts both hots on the same 120 volt leg. This will result in zero volts across the hots. If this were a multi-wire branch circuit, you’d end up with double current on the neutral.

What you could use is a compatible quad breaker (4 breakers in the space for two) with appropriate handle ties to produce two 240 volt circuits or one 240 and two 120s.

Answer 2

For a primer on how spaces in a panel work (vis-a-vis double-stuff breakers), read my Q/A here. Besides the fundamental problem that DoxyLover mentions, it is also impossible to fit handle-ties between the throws of a 1-pole duplex breaker. There would be no reason to do that since they are on the same pole.

Handle ties don't provide common trip. Handle ties provide common maintenance shutoff only. Handle-ties are mainly for MWBCs, which do not require common trip. And are also permissible on 240V-only loads in most cases, e.g. A mini-split.

Some do make handle ties designed to go between two double-stuffs, so you would fit two 15A duplex breakers and form a 240V/15A common-maintenance-shutoff breaker in the middle positions.

Internal mechanisms within the breaker cause common trip, and two separate breakers just can't do that thing. Since 120/240V (w/neutral) loads generally require common trip, you must use a 2-pole or a quadplex.