Fil Chauffant Toiture: 120V or 240V? The Circuit to Plan

A roof heating cable works just as well at 120 V as it does at 240 V, but the voltage determines how many feet of cable you can connect to a single circuit breaker. At 240 V, a single 15-amp circuit breaker powers about 1.6 to 1.7 times as much cable as at 120 V, which affects the number of circuits that need to be run to the panel.

Key points to remember

• For a self-regulating cable rated at 6 watts per foot in a wet location, a 15-A circuit breaker can handle approximately 150 feet at 120 V versus 255 feet at 240 V (start-up at 4 °C, according to the manufacturer’s technical data sheet).
• The maximum length per circuit depends on only three factors: voltage, circuit breaker rating, and ambient temperature at the time of startup.
• Starting the motor in extremely cold weather reduces the allowed length: the same 120-volt cable drops from 150 to 125 feet on a 15-amp circuit breaker.
• Regardless of the voltage, every heating cable circuit requires protection against equipment ground faults. This is not an optional feature.
• 120 V is suitable for most bungalows and cottages. 240 V makes sense once you have more than 200 feet of roofs, gutters, and downspouts to protect.
• Operating 150 feet of heated cable costs between $25 and $40 in electricity for one winter at Hydro-Québec’s Rate D, depending on the number of hours it is in use.

Voltage isn’t just a technical specification. It determines how many circuits your panel must supply, and thus a significant portion of the project’s actual cost. Here’s how to make that choice, backed by numbers, for a residential installation in the Greater Montreal area.

The cable is laid in a zigzag pattern across the overhang and secured with roof clips.

120 V or 240 V: What the Voltage Actually Means

A self-regulating heating cable produces the same amount of heat per foot, regardless of the voltage. A 6-watt-per-foot model delivers 6 watts per foot at both 120 V and 240 V. What changes is the current drawn from the circuit.

At the same power, doubling the voltage cuts the current in half. One hundred feet of cable rated at 6 watts per foot consumes 600 W, which is 5 amps at 120 V but only 2.5 amps at 240 V. The circuit breaker therefore handles half the current and can power much more cable before reaching its limit.

In reality, the gain isn’t exactly double, because the maximum length is calculated based on the inrush current at startup, not on the steady-state current. A cold self-regulating cable draws significantly more than its rated power during the first few minutes, then reduces its power output based on the ambient temperature. Manufacturers’ tables take this startup surge into account.

Maximum cable length per circuit

The following are the published specifications for a 6-watt-per-foot self-regulating heating cable designed for wet locations—that is, the type used on roofs, gutters, and downspouts.

There are two key points to consider. First, the -18 °C column is the one that matters in Quebec: if the system is powered on when it’s already very cold, this maximum length in feet must be adhered to. Second, the 30 A at 120 V is capped at 200 feet because the limit is then set by the cable itself, not the circuit breaker. Increasing the amperage without increasing the voltage eventually becomes pointless.

The calculation is always based on the total length of cable, never on the roof area. You add up the roofline to be protected, the zigzags along the overhang, the total length of the gutters, and the height of each downspout. This follows the same logic used to determine the price of a Fil Chauffant Toiture cable.

The three areas to add together before selecting the voltage.

A concrete example of a single-family home

Let’s take a house with 60 feet of roof edge, 60 feet of gutters, and two 12-foot downspouts. With a round trip through the gutters and 24-inch zigzags under the overhang, you can easily end up with 210 feet of heating cable.

At 120 V, this length cannot be powered by a single circuit. It must be split: one 15-amp circuit for the gutters and downspouts, and a second circuit for the roof edge. Two circuit breakers, two branch circuits, two junction boxes—which means more work for the electrician.

At 240 V, those same 210 feet can be powered by a single 15-amp circuit. That’s where the math changes. The cable and accessories cost about the same; it’s the wiring to the panel that’s no longer needed.

Ground-fault protection: Non-negotiable

A damaged heating cable can produce a sustained electrical arc that a standard circuit breaker will not trip. That is why manufacturers and codes require equipment ground-fault protection on every branch circuit supplying heating cable, whether 120 V or 240 V.

This protection comes in two forms: a specialized circuit breaker in the panel, or a snow and ice controller with built-in protection. Pre-assembled plug-in connection kits for 120 V often include the protection inside the enclosure itself. For 240 V systems, it is more commonly found in the panel or in the controller.

Another requirement to keep in mind: the power supply for a stationary heating system must be sized for 100% of the load, without the margin typically applied to other appliances. The Régie du bâtiment du Québec publishes its technical interpretations on this specific point.

The external connection must be watertight and protected against ground faults.

Gutters, downspouts, and roof clips

The circuit doesn’t just power the roof edge. A complete heating cable system covers three zones, and each adds a few feet to the total.

• The roof edge trim has a zigzag pattern, and its height depends on the depth of the overhang. Roof clips are sold in sets; a set of 10 clips covers approximately 7 linear feet of trim.
• Gutters. According to the manufacturers’ technical specifications, a gutter that is 5 to 6 inches wide requires two cable runs, not just one.
• Rainwater downspouts, whether plastic or metal, with a downspout bracket to prevent the cable from rubbing and wearing out.

A common mistake is to install the roof drainage system without downspouts. Meltwater collects at the top, flows back down, and then freezes in the clogged downspout. The system is working for nothing. Our “How It Works ” page explains this path of the meltwater in detail.

Accessories, Kits, and Connections

The cable alone does not make a de-icing system. Products available on the market are sold as kits or by the foot, and the connection accessories vary depending on the voltage selected.

• Connection kit: plug-in version for 120 V, often with built-in protection, or hard-wired version in an outdoor junction box for 240 V.
• Roof clips and downspout brackets, to keep the cable away from areas of friction inside the downspouts.
• Splicing kit and end sealant, required whenever a cable is cut to the desired length.
• Snow and Frost Controller—the only device that prevents the system from consuming energy when there is nothing to melt.

These accessories account for part of the difference between two quotes for comparable residential installations. A high-quality protective sleeve or splicing kit is inexpensive to install but costly to repair in February.

Operating costs, in dollars

Voltage has no effect on the electricity bill: 900 W is still 900 W. What affects the cost is the number of hours of operation—in other words, the controller.

As of April 1, 2026, Hydro-Québec’s Tariff D charges 7.065¢ per kilowatt-hour for the first 40 kilowatt-hours consumed each day, and 11.142¢ for the remainder. For 150 feet of cable at 6 watts per foot, or 900 watts:

• 400 hours of operation in the winter, which corresponds to a controller that detects both cold and humidity: 360 kWh, or about $25.
• 800 hours, which is equivalent to a simple thermostat that activates based solely on temperature: 720 kWh, or about $51.
• A system left plugged in continuously from December through March consumes more than 2,500 kWh and costs more than $175.

These amounts are based on calculations using the published rate schedule, not on meter readings. Above all, they illustrate one thing: the meter reader gets paid back faster than the cable does.

Who can perform the connection in Quebec?

Laying the cable on the roof and making the electrical connection are two different trades. The connection to the panel must be performed by an electrical contractor licensed by the Régie du bâtiment, and the installation must comply with Chapter V of the Quebec Building Code, which mandates adherence to the Canadian electrical installation standard.

In practical terms, this means a dedicated circuit, a weatherproof outdoor junction box, ground fault protection, and a notice of work when required. A heating cable system that is properly installed but improperly connected is still considered noncompliant.

How to decide between the two voltages

The rule of thumb can be summed up in one sentence: add up the length of your cables before choosing the voltage.

For roofs under 150 feet, on a single-story home with one gutter and one downspout, 120 V is sufficient and simplifies everything. The pre-assembled plug-in kit is available in this configuration; the panel requires only a single outlet, and the controller is more affordable.

Between 150 and 200 feet, the decision depends on the available space on the panel. For lengths over 200 feet—on a gable-roofed house, a multi-unit apartment building with multiple downspouts, or a commercial building—240 V becomes the rational choice: fewer circuits, less wiring, and a single control zone. In fact, almost all types of heating cables are available in both voltages.

One last point that’s often overlooked: you need to decide on the voltage before buying the cable, not after. A 240-volt cable plugged into a 120-volt outlet will operate at one-quarter of its rated power and won’t melt just about anything.