Panel Interlock Kits: When They Apply and What Makes Them Safe

What a Panel Interlock Kit Does (and What It Doesn’t)

A panel interlock kit is a mechanical barrier installed on the front of a home’s main electrical panel that forces an either/or choice between two breakers:

• the main breaker (utility power feeding the panel), and
• a generator backfeed breaker (power coming from a generator through a generator inlet).

When installed correctly, the interlock physically prevents both breakers from being ON at the same time. This is the core safety function: it blocks a condition where generator power could energize the utility service conductors.

What an interlock kit does not do: it does not automatically manage loads, it does not “clean up” generator power quality, and it does not provide automatic transfer. It is a manual method that relies on correct hardware, correct installation, and a clear operating procedure.

When Interlock Kits Apply

Interlock kits are typically used when you want to energize many or all branch circuits in an existing panel from a generator, but you will manually choose which loads to run. They are commonly paired with a generator inlet (an outdoor inlet box) and a dedicated generator breaker in the main panel.

An interlock is appropriate only when the panel model supports a listed interlock kit and the generator breaker can be positioned so the interlock can reliably block simultaneous operation.

Continue in our app.

You can listen to the audiobook with the screen off, receive a free certificate for this course, and also have access to 5,000 other free online courses.

Or continue reading below...

Download the app

What Makes an Interlock “Safe”: Required Conditions

1) Listed/Approved Hardware for the Specific Panel

The interlock kit must be listed/approved for the exact panel manufacturer and model (and often the specific panel series). Interlocks are not universal. The geometry of the deadfront (panel cover), breaker spacing, and handle positions must match the kit design.

• Use the panel’s labeling (inside the door) to identify the exact model.
• Use only the interlock kit part number specified by the panel manufacturer or a kit explicitly listed for that panel model.

2) Correct Breaker Positioning and Handle Relationship

Interlocks depend on the physical relationship between the main breaker handle and the generator breaker handle. Most designs require the generator breaker to be in a specific location (often at the top of the breaker stack near the main, depending on panel style), so the sliding plate or bracket can block one handle when the other is ON.

If the generator breaker is installed in the wrong position, the interlock may appear installed but won’t actually interlock the two sources.

3) Proper Labeling and Identification

Clear labeling reduces operator error during an outage and helps anyone servicing the panel understand the interlock arrangement. At minimum, labeling should identify:

• the generator backfeed breaker as “Generator Backfeed / Interlocked” (or equivalent),
• the required operating steps (often provided as a placard with the kit), and
• the generator inlet location and circuit identification if applicable.

Labels should be durable, legible, and placed where the operator will see them while operating the breakers (typically on or near the panel door and near the generator breaker).

4) A Dedicated Generator Inlet with Correct Wiring

A safe interlock setup uses a dedicated generator inlet (inlet box) feeding the panel through a dedicated cable/conduit run to the generator breaker. Key points:

• The inlet must be the correct type for the generator connection (e.g., 120/240 V inlet for typical split-phase portable generators).
• Conductors must be correctly sized and terminated: hot/hot/neutral/ground as required by the system.
• The generator breaker must be a proper 2-pole breaker for 120/240 V systems (when applicable) and installed per the panel’s breaker compatibility rules.

Practically, the inlet and wiring are what make the interlock usable and repeatable. “Temporary” wiring through doors/windows, improvised cords, or non-dedicated connections undermine the safety intent and create physical hazards.

Limitations You Must Plan For

Generator Breaker Sizing Limits the Maximum Feed

The generator breaker in the panel sets an upper bound on how much current can be fed into the panel from the generator. This is not a “performance upgrade” knob; it must match the inlet rating, wiring ampacity, and generator output capability. Oversizing the breaker to “get more power” is unsafe and defeats overcurrent protection.

Also note that the generator breaker is not there to protect the generator itself; it primarily protects the panel feeder conductors from the inlet and the panel bus from being fed beyond the intended rating for that interconnection method.

Manual Load Management Is Required

With an interlock, the panel is energized like normal, meaning any branch circuit that is ON can draw power. You must manually manage loads to avoid:

• overloading the generator (stalling, tripping, or voltage sag),
• starting too many motor loads at once,
• running large heating loads unintentionally.

In practice, many homeowners use a “critical loads” approach by turning off nonessential breakers before switching to generator power, then adding loads back one at a time.

You Need a Clear, Repeatable Operating Procedure

Interlocks are safe when operated correctly. A written procedure reduces mistakes under stress (darkness, cold, time pressure). The procedure should be short, posted, and match the actual hardware arrangement.

Example operating procedure (generic—must match your panel and kit):

1. Turn OFF or unplug sensitive/high loads (HVAC, electric range, dryer) at their breakers.
2. At the panel: switch OFF the main breaker.
3. Slide/engage the interlock mechanism to allow the generator breaker to turn ON.
4. Turn ON the generator backfeed breaker.
5. Connect generator to inlet with the correct cord; start generator; let it stabilize.
6. Turn ON selected branch breakers one at a time, prioritizing essential circuits.

Return-to-utility procedure should also be posted, typically reversing the sequence: shed loads, turn OFF generator breaker, move interlock, turn ON main, then shut down and disconnect generator.

Common Mistakes (and Why They’re Dangerous)

Wrong Interlock Kit (Not Listed for the Panel)

A kit that “almost fits” can leave enough handle travel to allow both breakers ON, or it can interfere with the deadfront in a way that encourages removal or modification. Using the wrong kit also risks loose mounting, misalignment, and unreliable operation over time.

What to do instead: verify the panel model and use the exact listed kit. If no listed kit exists for that panel, use a different approved interconnection method rather than improvising.

Missing Breaker Hold-Down / Retainer (Where Required)

Some panels require a breaker retaining clip/hold-down for backfed breakers so the breaker cannot lift or unseat under fault conditions. If the hold-down is missing, the breaker may not remain properly secured to the bus, which can lead to arcing, overheating, or failure during a fault.

What to do instead: install the manufacturer-specified hold-down hardware for the backfed generator breaker position.

Incorrect Breaker Type for the Panel

Using a breaker that is not listed for the panel (even if it “snaps in”) can create poor bus contact, overheating, nuisance tripping, or failure to trip under fault conditions.

What to do instead: use only breaker types listed on the panel labeling for that panel model/series, and match the required pole configuration (often 2-pole for 120/240 V generator feeds).

Bypassing or Defeating the Interlock Mechanism

Removing the sliding plate, leaving screws loose, bending parts, or operating the breakers with the deadfront removed defeats the safety function. The interlock must be able to do its job every time, not only when someone is careful.

What to do instead: keep the deadfront installed, keep the interlock hardware intact, and replace damaged parts with the correct kit components.

How to Verify the Interlock Works: Step-by-Step Functional Check

This functional check verifies the mechanical interlock action. It does not replace electrical inspection/testing, but it is a practical confirmation that the mechanism prevents simultaneous ON states.

Preparation and Safety

• Ensure the panel cover (deadfront) is installed and secured. Interlocks are designed to work with the deadfront in place.
• Do not connect a generator for the mechanical check.
• Stand to the side of the panel when operating breakers and use one hand when practical.

Functional Check Steps

1. Start with utility mode: Main breaker ON, generator backfeed breaker OFF. Confirm the interlock plate is positioned so it blocks the generator breaker from turning ON (as designed).
2. Attempt prohibited action #1: With the main breaker ON, try to turn the generator breaker ON. The interlock should physically prevent the handle from moving to ON.
3. Transfer to generator position (mechanically): Turn the main breaker OFF. Move/slide the interlock mechanism to the generator position.
4. Attempt prohibited action #2: With the interlock in generator position, try to turn the main breaker ON. The interlock should physically prevent the main handle from moving to ON.
5. Confirm allowed action: With the main OFF and interlock in generator position, turn the generator breaker ON. It should move freely to ON without forcing or binding.
6. Return to utility position: Turn the generator breaker OFF. Move/slide the interlock back to the utility position. Turn the main breaker ON. Confirm the generator breaker is now blocked from turning ON.

What “Fail” Looks Like

• Both breakers can be turned ON (even partially) with the deadfront installed.
• The interlock binds, requires excessive force, or can be bypassed by normal breaker operation.
• The interlock only works when screws are loose/tight in a specific way (misalignment).

If any fail condition occurs, stop using the setup until the correct kit, breaker position, and required retaining hardware are verified and corrected.

Quick Reference: Interlock Safety Checklist