If you've ever opened an older electrical panel and found six breakers across the top instead of one main breaker, you've found a split-bus panel. It's not a defect and it's not a brand. It's a wiring configuration that used to be standard practice, built on a rule the electrical code has been walking back for over 40 years.
Where it came from
The exception that made split-bus panels possible entered the National Electrical Code in 1965, allowing a panel to skip a single main breaker when used as service equipment for a single home. It built on an even older principle: the NEC began permitting up to six service disconnects back in 1937. This became known as the "Rule of Six" or the "six disconnect rule," and it was standard practice through most of the mid-20th century.
The honest reason it existed wasn't safety. It was cost. High-amperage single main breakers used to be expensive to manufacture. Six smaller switches ganged together were cheaper. Once breaker manufacturing advanced enough to bring single main breaker costs down, the economic argument for split-bus disappeared.
How the code changed
The NEC restricted new split-bus installations starting around 1981. Existing panels were grandfathered in, which is exactly why this design still turns up in older King County homes today. The bigger change came with the 2020 NEC: Section 230.71 now requires a single service disconnect by default. Up to six disconnects are still permitted, but only if each one sits in its own separate enclosure, not sharing one box the way a classic split-bus panel does.
The actual safety reason
Here's the part that doesn't get explained well: even with all six switches in a shared enclosure switched off, the incoming service lugs and bus bars inside that same box stay energized. Flipping every switch on the panel doesn't get you to a truly de-energized, safe-to-work-on state. That still requires the utility to disconnect power upstream at the meter. That's the specific hazard the 2020 code change addressed, and it's a much more precise reason than "it's old" or "it's against code."
A common real-world problem
By design, the top section of a split-bus panel is meant to hold only double-pole (240V) breakers: these are the six main disconnects, one of which, often rated around 60A, feeds the entire lower section. The lower section, in turn, is meant to hold only single-pole (120V) breakers for everyday lighting and receptacle circuits. Finding a single-pole breaker mixed into the top section, or anything other than that 60A feed supplying the bottom, is a sign the panel has been altered from its intended configuration at some point.
This is harder to spot in the field than it sounds, because these breakers aren't always labeled as part of a "main" group. In the panel below, the dryer, range and an unlabeled breaker sits under a printed "SERVICE DISCONNECTS" header, and only the 60A breaker actually carries a "MAIN" tag, even though all four are functioning as part of the same service disconnect group.
Only the 60A breaker is labeled "MAIN," even though the dryer, range and an unlabeled breaker are part of the same service disconnect group
Real-world labeling can be even less consistent than that. In the panel below, someone wrote "RANGE MAIN" directly on one breaker, while the dryer breaker right next to it, doing the same job as part of the same disconnect group, has no "main" label anywhere on it.
"RANGE MAIN" is handwritten on one breaker; the dryer breaker beside it, part of the same group, isn't labeled as a main at all
That matters because that lower section depends entirely on the single feed breaker above it. Adding a large breaker for a new heavy load into the lower section, such as a subpanel or a big appliance circuit, can put more demand on that section than the 60A feed was ever sized to carry, even if the new breaker's own rating looks perfectly fine sitting in the panel by itself.
Washington's current code
Washington State currently enforces the 2023 NEC, in effect since April 2024, which kept this requirement in place. The next update, the 2026 NEC, is expected to take effect in Washington by the end of 2026.
A practical, present-day complication
If a standby generator is part of your plans, split-bus panels create a real mechanical problem: standard generator interlock kits work by locking out a single main breaker before generator power flows. Without one main breaker, there's no clean point to lock out, and each split-bus panel is configured differently enough that one-size-fits-all interlock solutions aren't safe or practical.
Sources
- National Fire Protection Association, NFPA 70: National Electrical Code, Section 230.71 (Maximum Number of Disconnects) and Section 230.85 (Emergency Disconnects), 2020 and 2023 editions.
- Washington State Department of Labor & Industries, WAC 296-46B (Electrical Safety Standards, Administration, and Installation), adoption of the 2023 NEC effective April 1, 2024. See L&I's Electrical Laws, Rules & Policies page.
- International Association of Electrical Inspectors, "The 6-Disconnect Rule," IAEI Magazine, March/April 2020 edition.
This is general educational information, not a substitute for an in-person evaluation of your specific panel by a licensed electrician.
You're welcome to quote this article elsewhere with attribution and a link back to the original at hancoelectric.com.