Are GE and Siemens breakers interchangeable?

Not just because they fit. A breaker is only code compliant in a panel if the panel is listed for that breaker, and NEC 110.3(B) requires listed equipment to be used per its listing and labeling. A Siemens QP will often snap onto a GE bus and appear to work, but unless documentation says otherwise, that combination was never tested together and violates 110.3(B).

Where to look first

The panel label inside the door lists every breaker type the panelboard is listed to accept. GE load centers list GE THQL and THQP types. Siemens panels list QP, QT, and related Siemens types. If the label does not name it, the factory answer is no.

UL classified breakers are the legal path

There is one legitimate route for crossing brands: UL classified breakers. These are breakers that a manufacturer submitted for testing in specific competitor panels, and they carry paperwork listing the exact panel catalog series they are classified for. Eaton sells its CL and CHQ classified lines covering many GE, Siemens, Square D, and other panels, and Siemens has offered classified types as well. If you use one, match your specific panel catalog number to the classified breaker cross reference and keep the documentation, because the classification is panel specific, not brand wide.

Why it matters beyond the inspection

Bus stab tension, plating, and heat rise were all tested in the listed combinations. A mismatched breaker can seat loosely, arc at the stab, and cook the bus over time; that damage takes out the panel, not just the breaker. There is also an insurance angle after a fire when an investigator finds an unlisted combination. With GE residential breakers still widely available, and classified alternatives covering discontinued panels, there is no good reason to gamble. When a legacy panel has no listed or classified breaker available at all, that is the signal the panel is due for replacement rather than another improvised swap.