The short answer

A 15 A circuit legally carries 12 A of continuous load, and a 20 A circuit carries 16 A — not the breaker’s face value. That single derate, combined with NEC 426.28’s square-footage trigger and the HeatTrak HR-P power unit’s 13 A per-unit ceiling, is what actually decides how many mats you can run, not how many you can afford. None of this shows up on a typical retail listing. It comes from HeatTrak’s own PRO electrical installation guide and from the National Electrical Code section that guide is built around.

Why 15 A means 12 A, not 15 A

Snow-melting mats run for hours at a stretch during a storm, which puts them in the NEC’s “continuous load” category — anything drawing full current for three hours or more. Continuous loads are not allowed to use the full rating of a branch circuit; they’re limited to 80% of it, which is the same as saying the breaker has to be sized at 125% of the load it carries.

Worked out in amps rather than percentages:

  • 15 A circuit × 0.80 = 12 A available for continuous equipment.
  • 20 A circuit × 0.80 = 16 A available for continuous equipment.

That 3-4 A gap is not slack you get to spend on an extra mat. It’s the margin the code requires so the breaker isn’t run at its trip point for hours at a time.

Sizing on the top of the amp band, not the middle

HeatTrak’s published amp draws — like most of the industry’s — carry a stated tolerance of ±10%. That’s a manufacturing spread, not a marketing footnote, and it changes which number you’re allowed to plan against. A mat rated at 9 A can draw anywhere from about 8.1 A to 9.9 A; a mat rated at 13 A can draw up to about 14.3 A.

If you size a circuit on the nameplate number — or worse, on the midpoint of the tolerance band — you’ve sized it against a figure the manufacturer never actually promised not to exceed. The number that matters for a breaker calculation is the top of the stated band: nameplate amps × 1.10. Anything else is optimistic rounding wearing the clothes of an engineering calculation.

What NEC 426.28 actually requires

This is the part almost no retailer surfaces, because it isn’t a wattage spec — it’s a permitting rule, and it comes from HeatTrak’s own PRO electrical installation guide citing the code directly:

  • 40 sq ft or less: a Class A ground-fault circuit interrupter (4-6 mA trip threshold) on the receptacle is acceptable protection.
  • 41 sq ft or more: the installation needs Ground Fault Equipment Protection (GFEP) on a dedicated branch circuit, feeding a secured receptacle labelled “For Snow-Melting Equipment Only.”

That threshold is about total heated area, not mat count. Two 20-sq-ft mats laid end to end on the same walkway cross 40 sq ft exactly like one 41-sq-ft mat would, and the code doesn’t care how you got there. Anyone laying out a driveway apron or a wide set of steps needs to add up square footage across mats before deciding what kind of circuit protection the job needs — a Class A GFCI cord-cap plug that was fine for a single 20-sq-ft stair mat is not the protection the code calls for once a second mat pushes the combined footprint past 40 sq ft.

The HR-P power unit’s 13 A ceiling

Multi-mat layouts on HeatTrak’s residential line typically daisy-chain through the HR-P power unit rather than plugging each mat straight into a wall receptacle. HeatTrak’s PRO electrical installation guide caps that power unit at 13 A per unit for residential mats. That’s a hard ceiling on the unit itself, independent of what the branch circuit could otherwise carry — a 20 A circuit with 16 A of continuous-load headroom does you no good if the power unit feeding the mats is only rated to pass 13 A through it. Once a combination’s top-of-band draw would exceed 13 A on one power unit, the answer is a second HR-P unit on its own circuit, not a bigger breaker on the same one.

The worked refusal case

Take a mat published at 13 A, ±10%. Top-of-band draw: 13 × 1.10 = 14.3 A.

A 15 A circuit allows 12 A of continuous load. 14.3 A is above that, full stop. It doesn’t matter whether you run one of these mats or plan to add a second circuit somewhere else in the layout — the draw of that single mat, sized correctly at the top of its tolerance band, already exceeds what one 15 A circuit is allowed to carry. No quantity of additional 15 A circuits fixes a mat that doesn’t fit on the first one. The only legal paths are a 20 A dedicated circuit (16 A of headroom, which clears 14.3 A) or a smaller mat.

This is the calculation a “how many mats fit on a 15 A breaker” search is actually asking, and it’s also the one that gets skipped, because the retail answer (“check the amp rating on the box”) stops one step short of applying the 80% rule to it.

Where to check your own layout

Because the arithmetic depends on the specific mats in a specific footprint — square footage for the 426.28 threshold, top-of-band amps for the circuit derate, and whether one HR-P power unit or two is needed — we built a layout planner that runs these numbers against published product data rather than a rule of thumb. It won’t tell you whether your panel has an open slot or whether your local jurisdiction has amended 426.28; that’s a question for the permit desk and the person doing the wiring.

Consult a licensed electrician for anything hardwired

Everything above describes what the code and the manufacturer’s guide say, not how to pull a permit or land a wire on a lug. Dedicated 240 V branch circuits, GFEP breakers, and hardwired power-unit feeds are electrical work: consult a licensed electrician for the installation itself, and have them confirm the local jurisdiction hasn’t amended NEC 426.28’s thresholds.

Two things this math doesn’t solve

First, no portable-mat manufacturer publishes a combined-mat wattage total, so a whole-layout load calculation still has to be built amp-by-amp from each unit’s published draw rather than read off a single spec line — that’s slower and more error-prone than it should be for a product category this common. Second, the 41-sq-ft trigger for a dedicated GFEP circuit means a layout that grows past a single stair mat can turn into an electrician’s invoice partway through a project, which is a real added cost nobody advertises up front.

Who should not use this as a DIY sizing guide

Anyone planning a footprint at or near 40 sq ft should treat the GFEP threshold as a design constraint from the start, not a surprise at inspection. Anyone in a zone with heavy, frequent snowfall — the Sierra Nevada & Cascade communities or the Great Lakes snowbelt, for instance — is generally better served by an embedded electric or hydronic system sized for continuous winter duty than by chaining multiple portable mats and power units across a single service panel. And anyone who isn’t prepared to have a licensed electrician verify the branch circuit, the GFEP device, and the receptacle labelling should treat this page as background for that conversation, not a substitute for it.

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