Does a back fed breaker need a hold down kit?

Yes. NEC 408.36(D) requires plug in back fed devices terminating field installed ungrounded supply conductors to be secured by an additional fastener.

Can any 2 pole breaker be back fed?

No. Breakers marked LINE only, including many GFCI, AFCI, and electronic trip devices, cannot be reverse fed. Check the label.

Does a main lug subpanel require a main breaker?

Not if the upstream feeder overcurrent device is rated no greater than the panelboard, per NEC 408.36.

Can a main lug panel use a back fed 2 pole breaker as the disconnecting means?

Yes, a back fed circuit breaker can serve as the disconnecting means in a main lug only panelboard, but there are three conditions and skipping any one of them is a failed inspection.

Condition one: the panel has to be listed for it

Section 110.3(B) requires equipment to be installed per its listing and labeling instructions. Manufacturers publish main breaker conversion kits for specific main lug only panelboard catalog numbers. The kit includes the mounting bracket, the hold down, and often a filler or relabeled dead front. If the manufacturer does not list a kit for that enclosure, you cannot invent one by dropping a two pole breaker into the top slots.

The panelboard bus rating also has to be at least the rating of the breaker you are calling the main. Section 408.36 requires a panelboard to be protected by an overcurrent device rated no greater than the panelboard.

Condition two: the hold down fastener

Section 408.36(D) is the one people miss. Plug in type overcurrent devices that are back fed and used to terminate field installed ungrounded supply conductors must be secured in place by an additional fastener that requires something other than a pull to release the device from the panel. A friction clip is not enough. Bolt on breakers in a bolt on panel already satisfy this because the breaker is screwed to the bus.

The reason is straightforward. On a back fed device the load side stabs are energized from the utility side, so if the breaker rocks off the bus while someone is working in the panel with the main open, the exposed stabs are still live.

Condition three: the breaker has to be suitable for back feeding

Most standard molded case breakers are marked line and load only for convenience and are listed for reverse feed. Some are not. GFCI and AFCI breakers, breakers with integral ground fault protection, and some electronic trip units are marked LINE and cannot be reversed. Read the breaker label before you land supply conductors on the load lugs.

Do you even need one?

Often not. Section 408.36 does not require a main breaker in a subpanel where the panelboard is protected by the feeder overcurrent device ahead of it and that device is rated no greater than the panelboard. A 100 ampere main lug only subpanel fed from a 100 ampere breaker in the main panel is already protected. Installers usually add a back fed main for a local disconnecting means, not because the code demands it.

Where it is required is a service. Section 230.71(A) allows up to six service disconnects, and for a one and two family dwelling 230.85 in the 2023 NEC calls for an emergency disconnect at a readily accessible outdoor location. A back fed main in a service panelboard also has to satisfy 230.70 for location and 408.36(A) for the panelboard rating.

Field takeaway

Use the manufacturer main breaker kit, install the hold down bracket that comes with it, and verify the breaker is not marked LINE only. If you are just looking for a subpanel disconnect and the upstream feeder breaker already matches the panel rating, you may not need the conversion at all.