Do you have to use Square D breakers in a Square D panel?

Yes, you generally have to use Square D breakers in a Square D panel to maintain the UL listing, comply with the National Electrical Code, and ensure safe operation. Square D panels are designed, tested, and UL listed to be used only with Square D breakers that belong to that specific series. Square D Homeline panels are listed only for Square D Homeline breakers, and Square D QO panels are listed only for Square D QO breakers. You cannot use Siemens, Eaton BR, GE, or other brands unless they are specifically UL classified as compatible. The reason is that breaker dimensions, bus bar contact shape, spring tension, and trip characteristics are engineered to match the panel's bus bars. Using the wrong brand can cause poor contact, overheating, arcing, and failure to trip properly during an overload. This violates NEC 110.3 which requires equipment to be used according to its listing and labeling, and it can void your homeowner's insurance or cause a home inspection failure. There is an exception. Some manufacturers make UL classified breakers that are tested and listed for use in competitors' panels. For example, Eaton makes CL classified breakers that are UL classified for use in Square D QO and Homeline panels. These classified breakers will be marked on the breaker itself as classified for use in specific panels. Standard non-classified breakers from other brands should never be used in a Square D panel.