What is a Backfeed Main Breaker in an Electric Panel?

Electrical Technology

Backfed Main Breaker on a Bus Stabs in a Load Center

Backfeed Main Breaker

A backfeed main breaker is a standard branch circuit breaker installed in an electric panel and used as the main disconnect by feeding power “backward” into the bus bars rather than receiving it from top-mounted main lugs.

In other words, the breaker is back-fed when the incoming supply (service entrance) conductors land on the breaker terminals, and the breaker then energizes the panel’s busbars. It is commonly used for solar interconnections or generators.

Backfeed breakers usually placed at the opposite end of the bus bar from the main service entrance (often the top left) and must be labeled “MAIN”. It is used to allows power from a secondary source (like a solar inverter or generator) to energize the entire panel.

Main and Backfeed Main Breaker in a Panel

How a Backfeed Main Breaker Works

In a typical residential load center, utility power is supplied to the main breaker through the service entrance conductors. The main breaker is usually located at the top (or sometimes the bottom) of the panel and is directly connected to the panel’s busbars. The branch circuit breakers then snap onto these energized busbars and distribute power to the individual branch circuits through branch circuit conductors.

In a backfeed configuration, the utility power is supplied directly to a branch circuit breaker installed among the other branch circuit breakers in the panel. In this arrangement, that breaker functions as the main disconnect. It connects to the panel busbars and energizes them, which in turn supply power to the remaining branch circuit breakers and their associated circuits.

In this configuration, the breaker feeds the busbars from what is normally considered the load side of the breaker. Because the busbars are energized from the load terminals rather than the line terminals, which is why it is called backfeeding.

Good to Know: A hold-down kit is mandatory to mechanically secure the backfeed breaker to prevent it from being accidentally pulled out while energized.

Where a Backfed Breaker Used

Instead of a factory-installed main breaker, a two-pole breaker is installed at the top of the panel and backfed to act as the main disconnect.

A backfeed breaker can also be used in a typical panel that already has a regular main breaker located above the busbars. In this case, the backfeed breaker is used to supply power to the panel during a power outage, usually from an alternate source such as a generator.

To ensure safe operation, a special protective device, such as a mechanical interlock (Using ATS with generator) or hold-down kit, is installed. These devices prevent the main breaker and the backfeed breaker from being switched ON at the same time. This safety mechanism ensures that only one power source can supply the panel at a time. Hence, it prevents backfeeding into the utility lines and protecting both equipment and utility workers.

According to NEC 408.36(D), when a plug-in type breakers or main lug used as backfed device, it must be secured in place by an additional fastener.

As the NEC 408.36(D) states that back-fed devices must be fastened with a hold-down kit. It is because if the breaker is not secured, It could pull out of the bus stab and the live conductors would still be energized. This dangerous case creates a serious shock and arc-flash hazard.

To prevent this issue, manufacturers provide a retaining clip or hold-down screw kit to lock the breaker to the panel.

Similarly, for equipment over 1000V AC / 1500V DC, a permanent sign using the words “Danger: Contact either side of this device may be energized by backfeed” and a single-line diagram within sight clearly showing the connection of high voltage end of the backfed device – NEC 495.25(A) & (B), and 495.44(B).

Moreover, in an interconnected electric power production sources (generators, solar panels, batteries), any overcurrent protective device such as fuse or breaker with or without “Line” and “Load” marking may suitable for backfeeding or reverse current when rated for the applications. NEC 705.30(D).

Good to Know: When a backfed breaker is used for generator interlocks, a mechanical interlock kit must be used to ensure the main grid breaker is OFF before the generator backfeeds the panel.

Main Backfeed Breaker on a Bus Stab & Its Rating

When a backfeed main breaker is snapped onto a bus stab, make sure its rating does not exceed the “maximum sum of breaker ratings per stab” specified by the manufacturer. The allowable bus stab rating can be found on the printed label or nameplate located inside the main panel.

Max Amps per Stab Rating in a Panel

For example, if the maximum sum of breaker ratings per stab is 200A, the rating of the backfeed main breaker should not exceed 200A, assuming the opposite side of the stab is blank and unused.

Similarly, if the backfeed breaker used as the main breaker is rated 150A and installed on one side of the bus stab, the breaker installed on the opposite side of the same stab should not exceed 50A.

When installing a backfeed breaker on a bus stab, the best practice is to leave the opposite side of that stab blank. It means, no additional branch circuit breaker should be installed on it. This ensures the stab rating is not exceeded and helps maintain the producing heat within limits.

Precautions:

Disclaimer: Electrical work is dangerous. The author assumes no responsibility for any loss, injury, or damage resulting from the use or misuse of this information, including improper circuit installation.

Resources:

Related Posts:

Sizing & Rating

Wiring Tutorials

Related Articles
↑
Exit mobile version