Square D is one of the most common electrical panel brands in North America because it has been a trusted name in circuit breakers and safety devices for more than a century. Founded in 1902 in Detroit, the company introduced the enclosed safety switch and later became a leader in residential and commercial panelboards. After Schneider Electric acquired it in 1991, Square D kept its reputation for reliable, code-compliant equipment and stayed widely available through electrical suppliers. That long history, consistent quality, and deep installer familiarity are why so many homes and businesses still run Square D panels today, and why matching a generator interlock kit to one comes up so often.

Square D sells two main residential panel lines, QO and Homeline, and they exist to serve different markets and budgets. QO is the premium line, with a visible trip indicator on each breaker handle, higher interrupting ratings, and a lifetime warranty; electricians tend to prefer it, and it shows up frequently in light-commercial work. Homeline is the value-focused residential line, built to be cost-effective and quick to install while meeting the same safety standards. The two lines use different breaker designs and mounting, so a QO breaker will not fit a Homeline panel or the reverse. Interlock kits follow the same logic, though not as rigidly: several of our Square D kits work with both QO and Homeline breakers, while others are built for one series only. The K-5310, for example, is Homeline-only, and the K-5210 and K-5610 are QO-only, which is why confirming your series before ordering matters.

Your main breaker amperage is the next thing to establish. Open the panel door and look for the large two-pole breaker at the top, or occasionally the bottom, labeled “Main” or “Service Disconnect”; the rating, typically 100, 125, 150, or 200, is printed on the handle. The panel’s interior label or your meter can confirm the service size, but the main breaker itself is the most direct source. Worth knowing before you start searching by amperage alone: it narrows the field rather than deciding it. Our K-5010 covers the entire 150–200 amp range by itself. Usually the main breaker will be a large black rectangle at the top or bottom of the panel. Also, two panels that are both 200 amp, can need entirely different kits. What usually picks the exact kit is the gap between the bottom of the main breaker and the top of the branch breaker columns, along with whether the main throws horizontally or vertically. Those two details take seconds to check and eliminate most of the catalog.

The majority of Square D panels fall into a few common configurations, each with a corresponding interlock kit. The K-5010 is our most common Square D kit, fitting 150 to 200 amp panels with a large rectangular main breaker at the top or bottom and a gap of 1¼ to 1½ inches down to the breaker columns. The K-5110 covers 125 amp services with a QO main centered directly above the columns, and because the kit interfaces with the main, the branch breakers in that panel can be either QO or Homeline. The K-5210 fits older QO panels with a centered main and a vertical throw with a 3-4in gap between the breaker columns, and the K-5310 handles a centered four-pole main with a horizontal throw and a 2½-inch gap. Panels where the main sits beside the branch breakers rather than above them, typically meter-main combinations, take the K-1315, which works with a two-pole main next to either a two-pole or four-pole breaker. For panels with stacked two-pole breakers, the K-9011 fits a main in the top left and the K-9010 a main in the top right for Homeline breakers, while the K-9110 is the match for stacked QO breakers. Where the breakers sit adjacent rather than stacked with a half-inch between columns, the K-9410 and K-9310 cover those layouts. K-9310 will be for QO breakers and K-9410 is for Homeline.