Floor Heating Electrical Requirements: When to Use 120V vs. 240V

When it comes to voltage, most of us don’t have to give it much thought until we travel overseas or shop for major appliances. But having a basic understanding of voltage is important when choosing floor-heating mats or cables so you can find the right product to help ensure years of trouble-free enjoyment. But how do you choose between 120V and 240V? You might turn to an online forum and get well-meaning but bad advice that could lead to added costs. Instead, read this guide to help you select the right voltage for your floor-heating project.

Is voltage the same as power?

No. A product’s voltage rating doesn’t indicate how much power it uses. Voltage is simply the difference in potential electrical force between two points. But what does that mean? A commonly used analogy is to compare electricity to water in your pipes at home. Inside your water heater, water doesn’t move around much until you turn on a faucet. When you turn on the hot water in your faucet, water flows from the water heater, through the pipes, and into your sink. The water pressure is the voltage (V), controlling the speed and force at which the water flows out. The amount of water flowing would be the amperage (I). And power (P) in watts is the voltage multiplied by the amperage, which tells you how much power is being used. This useful formula lets you calculate Watts : P = V x I .

Electrical requirements for radiant floor heating systems

240V vs 120V floor heating products

Is it true that 240V products generate more heat, heat up faster, cost less to run, and work more efficiently than 120V?

No, no, no and no. This is a big misconception that pops up in Internet forums. Let’s say you take two mat systems of the same square footage, one 120V and the other 240V. If they are designed with the same watts per square foot (the industry standard is 12 W/sf), both products will use the same amount of watts and use the same amount of power. This is because a 120V system draws twice the amps per square foot of a 240V system, and that a 240V system draws half the amps per square foot of a 120V system. Ohm’s law tells us that when voltage decreases, amps increase proportionally. For example, let’s say you want to heat a 100 square foot area. According to the manufacturer’s specs, we know both systems use about 1200 watts, and we know the voltage, so we find the amps by using the formula : V x I = P, or P / V = I. (This formula is part of Ohms law, which requires a Resistance value also):

So while the 240V sounds like it has twice the power, it generates the same amount of heat (watts) per square foot as the 120V system, heats up at the same rate, and would cost about the same amount per square foot to heat the floor. And for standard power supplies of 120V and 240V, matching products are equally efficient (120V to 120V vs. 240V vs. 240V).

Do cables or mats need to match the voltage of the power source?

Yes. We’ll get into the why below, but if there’s one important takeaway about voltage and floor heat, it’s that the voltage of your mat or cable product should match as closely as possible the voltage of your power source. Almost all homes in the USA and Canada have 120 and 240 VAC available in their electrical panel. If you want to use an existing circuit, choose from what is available. If you are running a new circuit and the heated floor is smaller than 120–150 square feet, we always recommend 120V . You can choose 240V, for a new circuit for small areas, but it will cost you more and take up an extra slot in your panel. In general, the square footage of the area you want to heat is one of the main factors in choosing between 120 and 240V products, which we’ll get to next.

So why choose a 240V over a 120V system, and vice versa?

At Warm Your Floor, we recommend 120V systems for heated areas less than 150 square feet (at 12 W/sf), and 240V systems for heated areas more than 150 square feet. The reason for this is a single thermostat can control 15 amps. Using the numbers in the previous question, 120V systems draw more than 15 amps over 150 square feet, so manufacturers offer 240V systems for larger areas. Using the example above, a 240V system could heat up to 300 square feet and still be controlled by a single thermostat.

Sometimes, a customer’s power supply is rated with a nonstandard voltage. A few of the outdoor products Warm Your Floor carries can be special ordered from the manufacturer (SunTouch and Nexans are two) to match specific requirements. Which brings us to the next question…

What happens when a product’s voltage rating doesn’t match the voltage of the power supply?

We’ve covered the fact that you’ll want to match your power source voltage to your mat, but what would happen if a 240V mat is connected to a 120V power source?

A 12 watt per square foot SunTouch mat would output only 25% of its heat. This would be not at all effective in heating your floor.

In the reverse case, connecting a 120V product to a 240V power source will overload the system y increasing the heat output to 4 times the normal, causing damage to the mat and thermostat, premature failure, and costly reinstallation.

But in special cases, you’ll need to mismatch voltages between the power source and the floor-heating products. The standard voltages Warm Your Floor carries are 240V and 120V, but some customers have indoor power supplies rated at 208V. In this case, a higher voltage product rated at 240V can be connected to a lower voltage power supply at 208V, but the heat (watts per square foot) generated will be reduced by 25 percent, running at 75% power. To find this number, we divide the lower voltage by the higher voltage and square the result:

(208V / 240V)² = .75 = 75%

To make up for this 25% reduction, some manufacturers recommend spacing cables slightly closer together, so more cable would be needed to heat a smaller area. But this method is limited by the 15 amp max load for the thermostat. And it doesn’t work with all products, as some manufacturers require a set distance between cables. We can recommend floor-heating products that provide more watts per square foot to offset this reduction. But be sure to consult a qualified electrician to help you plan a layout that meets your needs safely and effectively.

Getting your project started with Warm Your Floor

Have more questions? Contact us today and we’ll help you find the right products for your floor-heating project.