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Regarding this, will a Square D breaker fit a Siemens panel?

Yes, Siemens makes what is called a "QD" Series breaker that is UL listed and "classified" to be used in a very specific list of Sq. D load centers. The list comes in the box with the breakers, but you CANNOT use them willy-nilly in place of QO breakers.

Furthermore, what breakers are compatible with Square D Homeline? The Homeline breakers are made to fit in most "interchangeable" panels, such as GE, Bryant, Murray, ITE, etc. However, Square D added a small protrusion on their bus bars to reject those other breakers from being installed in the Homeline panels.

Also to know, are homeline and Siemens breakers interchangeable?

Siemens manufactures a CLASSIFIED breaker that is suitable for replacement ONLY in certain Square D panels as indicated on the breaker literature. So I can install a SQ D Homeline in a Siemens panel.? There is no such thing as an interchangeable circuit breaker.

Are Murray and Square D breakers interchangeable?

They are not interchangeable. They will fit but they are not listed for use in either panel.