Interlock Kits for Generators: How They Work, What You Need, and Common Mistakes
An interlock kit is one of the simplest and most affordable ways to connect a portable generator to your home safely— but only if it’s installed correctly. This guide explains how interlocks prevent backfeed, what parts you need, what a safe operating sequence looks like, and where DIY installs go wrong.
What is an interlock kit?
DefinitionAn interlock kit is a mechanical safety device installed on the front of your main electrical panel that forces a safe choice between two power sources:
- Utility power (main breaker ON, generator breaker OFF)
- Generator power (main breaker OFF, generator breaker ON)
It does not automatically switch power. It does not select circuits for you. You’re effectively running a “whole panel” but you must manually choose which breakers you turn on during generator operation to keep the load within your generator’s capacity.
How an interlock prevents backfeed
“Backfeed” is when generator power accidentally energizes your service drop and the utility grid. That can kill line workers and can damage equipment.
The interlock physically blocks one breaker handle when the other is in the ON position. If installed correctly, you cannot have main breaker ON and generator breaker ON at the same time.
NORMAL (Utility):
MAIN = ON
GEN BREAKER = OFF (interlock blocks it)
OUTAGE (Generator):
MAIN = OFF (interlock blocks it)
GEN BREAKER = ON
Interlock vs transfer switch (which is better?)
Neither is “always better.” They solve the same core safety problem (no backfeed) in different ways.
| Feature | Interlock kit | Manual transfer switch |
|---|---|---|
| Isolation method | Blocks main and generator breaker from being ON together | Switch routes power to selected circuits |
| Which circuits run | You choose by turning breakers ON/OFF in the panel | Pre-selected circuits (clean and simple) |
| Cost | Often lower | Often higher (switch + wiring) |
| Ease during outage | More “manual management” | More “set it and run essentials” |
| Great fit for | People who want flexible whole-panel choice | People who want simple, pre-defined essentials |
If you want simplicity, a transfer switch with labeled essential circuits is hard to beat. If you want flexibility and understand load management, an interlock can be a great value.
What you need (parts list)
A safe interlock setup is more than the plate on the panel.
| Component | What it does | Notes |
|---|---|---|
| Panel-specific interlock kit | Prevents main and generator breaker from being ON together | Must match your panel model and breaker positions |
| Generator inlet box (30A or 50A) | Outdoor generator connection point | Match to your generator output and cord |
| 2-pole generator backfeed breaker | Receives generator power into panel bus | Must be correct type for your panel (brand/model compatible) |
| 4-conductor feeder (L1/L2/N/G) | Connects inlet to panel/breaker | Correct sizing and protection required |
| Generator cord | Connects generator to inlet | Rated for amperage; keep as short as practical |
| Labels / operating steps sticker | Prevents operator error during outages | Put the sequence right on the panel door |
Installation overview (panel → inlet)
Interlock installs involve live-panel hazards and code-sensitive details. The safe approach is to follow your kit’s instructions and local requirements (or hire a licensed electrician). This section is a high-level overview so you understand the layout and can verify the work.
Typical layout
[ PORTABLE GENERATOR OUTSIDE ]
|
(generator cord)
|
[ INLET BOX ]
|
(4-wire feeder: L1, L2, N, G)
|
[ MAIN PANEL ]
- MAIN breaker
- GEN backfeed breaker (2-pole)
- Interlock plate blocks MAIN vs GEN
Key install concepts
- The generator inlet feeds the 2-pole generator breaker.
- The interlock must physically prevent MAIN ON + GEN ON.
- Neutral and ground must be handled correctly for your system design.
- Breaker and conductor sizes must match the inlet rating and conductor ampacity.
A male-to-male cord feeding an outlet is unsafe and illegal. A safe interlock setup uses an inlet box, correct breaker, and a listed interlock device.
Safe operating steps during an outage
Put these steps on a label inside your panel door.
- Turn OFF large loads (water heater, range, dryer, AC, etc.).
- Main breaker OFF (utility disconnected).
- Slide interlock to allow generator breaker ON.
- Generator breaker ON.
- Start generator outside, let it stabilize, plug into inlet.
- Turn ON only the circuits you need, one at a time (manage load).
Returning to utility power
- Turn off branch circuits you enabled for generator (reduce load).
- Generator breaker OFF.
- Slide interlock back to allow main breaker ON.
- Main breaker ON.
- Shut down generator, unplug cord, store safely.
Turn on loads slowly and deliberately. Generators hate sudden overloads. Staged loading reduces trips, brownouts, and cord heating.
Common mistakes (and why they matter)
- Wrong interlock kit (not panel-specific) → unsafe bypass, inspection failure.
- Wrong breaker type (not correct for panel) → poor connection, overheating risk.
- Loose lugs / poor torque → heat, arcing, failures.
- Operator error (steps not followed) → overloads or unsafe sequencing.
- No load management plan → generator bogging, nuisance trips, brownouts.
- Bad labeling → confusion when it’s dark/stressful.
Transfer switches “force” you to run only selected circuits. Interlocks give you flexibility—but you must actively manage loads.
When an interlock may NOT be the right choice
- You want a simple “essentials only” system with zero breaker juggling.
- Multiple people may operate the system and you want less room for user error.
- You need an automatic solution (interlocks are manual).
- Your panel layout doesn’t support a proper interlock position for the generator breaker.
Interactive Quiz: Interlock Kits
Focus on the “why” in each explanation—this is where safety lives.