Heated Floor Calculator

Because amperage is total wattage divided by voltage, and this calculator switches the recommended voltage from 120V to 240V once heated area passes 150 sq ft. At that exact crossover, wattage is barely higher than it was just below the threshold, but doubling the voltage roughly halves the current draw — so the required breaker can actually shrink even as the system gets slightly bigger.

It follows the National Electrical Code's rule for continuous loads (NEC Article 210): the calculator takes the raw circuit amperage and multiplies it by 125% before sizing the breaker, since a continuous load isn't allowed to exceed 80% of a breaker's rating. That 125%-adjusted amperage is then rounded up to the next standard overcurrent device size listed in NEC 240.6(A) — 15, 20, 25, 30A, and so on — with a 15A floor, since no standard breaker comes in a smaller size. A calculated load that lands at, say, 22A doesn't get a custom-sized breaker; it gets bumped to the next standard size up.

One thermostat/relay zone per 150 sq ft of heated area, rounded up — a 400 sq ft heated kitchen-and-hallway run needs 3 zones, not one oversized one. Each zone typically needs its own thermostat and relay rated for that zone's load, so larger installations split naturally into multiple independently controlled areas rather than one big circuit.

Because cabinets, vanities, tubs, and other permanent fixtures don't get heating mats installed underneath them — there's no benefit to heating floor you'll never feel warmth through, and running mat under a fixed cabinet base is a fire-safety and installation problem besides. The fixture percentage input subtracts that unheated area before any wattage or mat-length math runs.

Proportionally and directly — operating cost scales linearly with daily hours, so doubling run time from 8 to 16 hours a day roughly doubles the monthly electricity cost. Many installations use a programmable thermostat to limit run time to specific morning and evening hours rather than running continuously, which keeps this cost down without giving up comfort when the floor is actually in use.