How to Install a Generator Interlock Switch

Running a portable generator into your Toronto or GTA home safely means connecting it to your electrical panel, not stringing extension cords across the living room. An interlock switch is the cheapest, cleanest way to do that legally. This guide covers what an interlock kit actually is, how it installs, and why it beats a manual transfer switch for many portable generator setups.

How to install a generator interlock switch

A generator interlock switch is a mechanical device installed on your electrical panel that physically prevents the utility main breaker and the generator backfeed breaker from both being on at the same time. Installation requires a licensed contractor to add a generator inlet outside the home, run a cable to a dedicated backfeed breaker in the panel, and install the interlock kit specific to your panel brand. Typical GTA install cost: $500–$1,500 CAD depending on inlet location and panel work.

How the interlock works

The physical logic is simple:

• The main utility breaker sits at the top of the panel.
• A dedicated generator breaker (backfeed) sits in the panel, wired to an outdoor generator inlet.
• The interlock kit is a metal slide-plate that blocks one breaker from being on while the other is on.
• To use the generator: turn off main, slide the interlock, turn on generator breaker. Impossible to have both simultaneously.
• Prevents back-feeding grid, protects utility workers, and ensures your generator only powers your home.

Interlock vs manual transfer switch

• Interlock kit: covers your whole panel. Any circuit can run on generator (up to generator capacity). Cheaper.
• Manual transfer switch: pre-selected circuits only (usually 6–10). More expensive. Simpler operation.

Interlock is better for homeowners who want flexibility. Transfer switch is better for households where different people might operate it and simplicity matters. Related: how to install a transfer switch.

What installation involves in Ontario

1. Load calc to determine generator inlet size (typically 30 amp for smaller portables, 50 amp for larger).
2. Choose generator inlet location on exterior wall near where you plan to park the generator.
3. Purchase panel-brand-specific interlock kit (Siemens, Eaton, Square D, etc.).
4. Licensed contractor installs the outdoor inlet box (weatherproof, typically NEMA 3R).
5. Runs cable from inlet through wall to the electrical panel.
6. Installs backfeed breaker in panel (double-pole 30 or 50 amp).
7. Installs interlock kit on panel cover.
8. Files ESA notification.
9. ESA inspection.
10. Certificate of Inspection issued.

Choosing the right interlock kit

Interlock kits are panel-specific. Wrong-brand kits will not fit properly and will fail ESA inspection.

• Panel-manufacturer kits (Siemens, Eaton, Square D): OEM guaranteed to fit. Typically $50–$150 CAD.
• Third-party kits from brands like GenInterlock, InterLockKit.com: specific to certain panel models, similar cost.
• Must be listed for the panel model — no universal kits.

Your licensed contractor sources the correct kit for your specific panel.

Using the interlock during an outage

1. Power goes out.
2. Roll portable generator outside (min 6 metres from windows, doors, vents).
3. Start the generator, let it warm up.
4. Turn off main breaker at the panel.
5. Slide the interlock plate to expose the generator breaker.
6. Plug generator cord into outdoor inlet.
7. Turn on generator breaker.
8. Turn on only the circuits you need (start small; add loads incrementally).
9. Do not exceed generator capacity.

Reverse the process when utility power returns.

Load management with interlock

Because the interlock exposes the whole panel to generator power, you are responsible for not overloading the generator. Best practices:

• Turn off all circuit breakers before starting generator.
• Turn on essential circuits first (furnace, fridge, some lights).
• Add loads slowly, watching for generator strain.
• Avoid running electric range, dryer, and central AC simultaneously (unless generator is oversized).
• Do not turn on the entire panel at once.

Safety rules for portable generators with interlock

• Never run indoors. Carbon monoxide kills. Outdoors only, 6+ metres from any opening.
• Weatherproof cover on the inlet when not in use.
• Do not modify the interlock to bypass its safety function.
• Verify utility disconnect before starting generator (main breaker fully OFF).
• Test annually to verify everything works before the next outage.

Improperly used interlock setups can back-feed the grid — lethal to utility workers.

Cost breakdown in the GTA

• Interlock kit: $50–$150 CAD.
• Generator inlet box (30 or 50 amp): $80–$200 CAD.
• Cable, breaker, weatherproof conduit: $100–$250 CAD.
• Licensed contractor labour: $300–$800 CAD.
• ESA permit: included in contractor quote typically.
• Total installed: $500–$1,500 CAD.

Much cheaper than a full manual transfer switch install and legal for portable generator use.

Expert tip from our ESA-licensed electricians

If you own a portable generator and plan to use it more than once a decade, install the interlock. In our GTA calls, the number of homeowners we see running extension cords through partially-open patio doors during outages is disturbing. Interlock kits are $500–$1,500 installed, legal, safe, and take you from crawling behind furniture to flipping breakers in your utility room. One weekend of investment for years of clean outage recovery.

Frequently Asked Questions

Is an interlock legal in Ontario?

Yes when installed by a licensed contractor with ESA notification and a listed panel-specific kit.

Can I install an interlock myself?

Panel work is licensed contractor territory in Ontario in most cases. The homeowner exemption has limits.

How big a generator can I run on interlock?

Depends on the inlet and cable size. Typical residential is 30 or 50 amp, matching common portable generator outputs.

What if my panel does not have room for the breaker?

Sometimes a subpanel or panel replacement is needed. Get a load calc first.

Are interlocks weatherproof?

The interlock plate is inside the panel; the outdoor generator inlet has a weatherproof cover.

Related reading from ezsmart.ca

• How to Install a Transfer Switch
• Portable vs Standby Generator
• What to Do in a Power Outage
• How to Size a Standby Generator
• Does a Generator Auto Turn On?

Contact us

Ready to add a generator interlock to a Toronto or GTA panel? Call ezsmart at 416-838-9006 or use our contact page. An ESA-licensed electrician will source the right kit, install the inlet, and file the notification.