EZSMART Corporation, ESA/ECRA #7012690 , North York , Ontario
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A power outage hits your neighbourhood and your portable generator is running — but connecting it safely to your home panel without an interlock kit risks backfeeding live voltage onto the utility grid, which can electrocute hydro workers restoring power. An interlock kit is the code-compliant, lower-cost alternative to a full transfer switch for most Ontario homes.
A generator interlock kit is a mechanical slide or bracket that mounts on your existing panel and physically prevents the main breaker and a dedicated generator breaker from being switched on at the same time. This eliminates backfeed risk without requiring a separate transfer switch enclosure. Interlock kits typically cost $75–$150 CAD versus $500–$2,000 CAD for a manual transfer switch — a significant saving on a solution that meets the same safety requirement when installed correctly.
In Ontario, wiring a generator interlock kit to your electrical panel requires an ESA permit and work by a licensed electrician. The Ontario Electrical Safety Code (OESC 28th Edition, Rule 84-018) governs generator connections to premises wiring, and the Electrical Safety Authority enforces this. An interlock kit installation involves working inside a live panel, adding a 240V double-pole breaker, and running a heavy-gauge cable — none of which are homeowner-DIY exceptions under Ontario rules.
This guide explains how the wiring process works so you understand what your electrician will do, can purchase the right materials in advance, and can ask informed questions. For a broader overview of what work triggers permit requirements, see our guide on what electrical work requires an ESA inspection.
Your electrician will need three things from you before the job starts: the brand and model number of your electrical panel (printed on a sticker inside the panel door), the interlock kit matched to that panel, and the generator’s rated amperage output. Interlock kits are panel-specific — a Siemens kit will not fit a Square D QO panel, even though they look similar. Purchase the kit from the panel manufacturer’s own line when possible. Generic kits are available but must be verified to fit your panel model before purchase.
Your generator’s output determines the breaker size. A 5,500-watt generator at 240V draws roughly 23 amps peak — a 30A double-pole breaker is the standard choice for most portable generators in that range. Larger generators (7,500W and up) may need a 40A or 50A breaker, and the wiring gauge must match. Use a 10 AWG copper wire for a 30A circuit and 8 AWG for a 40A circuit — Ontario Electrical Safety Code specifies minimum conductor sizing for generator circuits.
The interlock kit itself installs on the outside of the panel door using the existing knockouts or screws. A metal slide plate connects over both the main breaker handle and the generator breaker handle so they cannot both be in the ON position simultaneously. The mechanical interlock is the safety device — the wiring is standard 240V breaker wiring.
The electrician installs a new 30A or 40A double-pole breaker in a slot near the main breaker — typically at the top of the panel so the interlock slide reaches both handles. From that breaker, a dedicated circuit runs to a generator inlet (also called a power inlet box), which mounts on the exterior of the home. The generator’s output cord plugs into this inlet. When the power fails, the homeowner switches off the main breaker, flips on the generator breaker, plugs in the generator, and starts it. The interlock prevents the main from turning back on while the generator is feeding the panel. See our detailed walkthrough in how to install a generator interlock switch.
The generator inlet must match your generator’s output plug. Most portable generators use a NEMA L14-30 four-prong twist-lock connector (30A, 250V) or a NEMA L14-50 for larger units. Do not use a dryer outlet as a makeshift generator inlet — it is not rated for outdoor use, lacks weatherproof protection, and violates OESC Rule 84-018 requirements for permanent generator connections.
The inlet box must be installed on the exterior wall in a weatherproof enclosure with a cover rated for wet locations. The cord running from the generator to the inlet (the generator extension cord or “suicide cord” if homemade — never use a homemade version) should be a matched NEMA L14-30 cord, rated for outdoor use, at least 10 AWG conductors, and no longer than 25 feet to limit voltage drop. According to the Electrical Safety Authority (ESA), improperly connected generators are one of the leading causes of electrocution during and after major Ontario ice storms and power outages. Read the ESA generator safety guide before any generator hookup. The difference between a portable and a whole-home standby generator is covered in our guide on portable vs. standby generators.
In our experience installing generator interlocks across the GTA, the most common homeowner mistake is using an extension cord rated for indoor use — or rated for 15A on a 30A circuit. We have seen cords melt inside the interlock inlet during extended outages because the cord AWG was undersized for the generator load. When you are running your furnace, refrigerator, sump pump, and a few lights simultaneously, a 5,500W generator pulls close to its rated current continuously for hours. Buy a cord rated at minimum 10 AWG, 30A, outdoor-rated, with a proper NEMA L14-30 plug on both ends. Also: when the power comes back on, always disconnect the generator and turn off the generator breaker before flipping the main back on — the interlock prevents both from being on simultaneously, but the sequence still matters for protecting your generator from the inrush when utility power returns. For automatic switchover without any manual steps, see our guide on standby generators that turn on automatically.
Ready to have a generator interlock kit installed by an ESA-certified electrician in the GTA? Call us at 416-838-9006 or visit our contact page — we will get back to you the same day.
Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp
