Spotting a feedthrough panel
A feedthrough panel is fed through the service equipment lugs rather than through a breaker, and which interconnection methods are legal depends on knowing the difference.
Answers the Feedthrough panel question during setup.
Reading this one means looking inside service equipment, which is qualified-person work. Behind the cover the busbar and the line-side lugs are energised, and the line-side lugs stay energised with the main breaker switched off. Have a licensed electrician open it and photograph what you need.
“That second panel inside is just a subpanel.”
The common misreading. Here is what is actually true.
Two panels on a job usually means a main panel and a subpanel. Sometimes it means something else: a feedthrough. The service equipment at the meter has a second set of lugs below the main breaker, conductors leave from those lugs, and they feed the panel indoors directly.
The difference is not cosmetic. A subpanel is fed from a breaker, and that breaker protects it. A feedthrough panel is fed from lugs, so its conductors are protected by the main breaker upstream. That changes what the panel is in code terms, and it changes which ways you are allowed to connect solar.
Two separate questions the screen asks
You may have been told that backfeeding a feedthrough panel needs an MLO panel. It does not. What changes with the panel type is which number bounds the 120 percent math, not whether the connection is allowed.
The screen asks two different things about this panel. Panel type is about the enclosure: Convertible means it has a slot that can take a main breaker, MLO means it physically cannot. Feedthrough main is about whether a breaker is actually installed in that slot. A convertible panel with an empty slot is a real and common state: pick Convertible and leave the main rating blank. Do not record it as MLO.
Getting that pair wrong is what produces the belief. On MLO the methods that use the panel's own main breaker are withdrawn and the software offers a new enclosure instead, which looks like a requirement rather than a consequence.
What it changes
Interconnection options depend on the busbar math at each panel, and a feedthrough arrangement gives you two busbars in series fed from one main. The obvious tie-in point may be ruled out at one panel and available at the other, which is why the software asks instead of assuming.
How to get the answer
Confirming the feed lands on lugs rather than a breaker means looking inside service equipment, which is qualified-person work. There is an exterior test that gets you most of the way.
- 1Count the conduits leaving the service equipment and see where each one lands. A feedthrough shows up as a conduit running to the second panel with no breaker in the first panel sized to feed it, and the circuit directory will not list it.
- 2Where the service equipment is a disconnect with no branch breakers, its load lugs ARE the feedthrough. Any panel fed from lugs rather than from a breaker is a feedthrough panel by definition, which is why both disconnect service styles imply one.
- 3A breaker in the first panel that feeds the second: ordinary subpanel, and that breaker is its protection. Read its rating off the handle.
- 4Where a breaker feeds it, read the rating off the handle. Where lugs feed it, there is no rating on the lug: what protects those conductors is the main breaker upstream, so record that rating instead.
- 5To confirm from inside, ask the licensed electrician on site to look and photograph it. Do not open service equipment yourself.
What to photograph
- The service equipment with its door open and all inner covers in place, plus every conduit entering and leaving it.
- The conduit run between the two enclosures.
- The second panel with its door open and its circuit directory readable.
This is guidance for reading equipment that is already visible and for describing it on a drawing. It is not instruction for electrical work. Any work on service equipment, panels, or conductors is for a licensed electrician, and local code and utility rules govern.
Every one of these questions is asked once, in setup.
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