Interconnection methods in plain English

The word describes two different things and that is where the confusion starts. There is the utility process, the application and the approval, and there is the physical decision about where the solar conductors connect to the building. This article is about the second one.

The choice is constrained. A breaker that physically fits into the panel does not mean the connection is allowed, because the limit is not the breaker, it is the busbar it clamps onto.

The three methods

Backfeed breaker, sometimes just called the 120 percent rule connection: solar lands on a breaker in the panel like any other circuit, just running the other direction. It is the simplest and the most common. Note that the method list also carries a separate option named Load Side Tap, which is a different thing covered below.

Supply side: solar connects on the utility side of the service disconnect, between the meter and whatever the first shut-off is. On a service with a separate disconnect that means between the meter and the disconnect, not between the disconnect and the panel. The busbar limit does not apply because no busbar is involved, but other limits do: the output is bounded by the service rating, the tap conductors have their own sizing rules, and the connection brings its own service disconnect for the solar, with its own bond and grounding electrode connection. It always means an electrician working on conductors that stay live until the utility pulls the meter.

Load side tap, also called a feeder or feed-through tap: solar connects to the conductors running between the service and a downstream panel rather than landing on a busbar. The busbar rules are replaced by a conductor test, not removed. The feeder has to be rated for the main breaker plus 125 percent of the solar output, and an existing dwelling feeder sized under the 83 percent allowance often is not. Check it before promising it.

What the 120 percent rule bounds

For a backfeed breaker, the main breaker rating plus 125 percent of the inverter's continuous output current cannot exceed 120 percent of the busbar rating. On a 200A busbar that is 240 amps of total supply, and a 200A main leaves 40. The solar breaker itself is then sized at the next standard size at or above that same 125 percent figure. The allowance also requires the solar breaker at the opposite end of the bus from the main, plus a permanent label saying so.

An MLO service panel is a different animal. With the service conductors landing straight on the lugs there is no overcurrent device ahead of the bus at all, and the code lets each breaker in that panel be one of the service disconnects. A solar breaker there is therefore on the supply side by definition, so the 120 percent rule does not govern it and the limit is the service ampacity instead. Do not force the busbar math onto it.

That exemption is about what is ahead of the bus, not about the missing main. An MLO subpanel, or an MLO panel fed from a meter-main, does have a breaker protecting its busbar, and there the arithmetic runs normally with that feeder breaker as the first term.

When the arithmetic does not leave enough room, there are four moves: shrink the main breaker to buy headroom, upgrade the panel to a larger busbar, move to a supply-side connection where the busbar limit does not apply, or add a listed power control system that actively limits the combined current onto the bus. Reducing the solar is the fifth and the one nobody wants. Each has its own test to pass.