Seat heater · relay · fuse

Heated seat wiring diagram: relay, fuse and switch

A seat heater pad is a resistor of about 50 W — 4 A per seat. Two seats are 8 A, too much for a small dashboard switch and its thin wires, so the switch drives a relay and the relay feeds the pads from their own fuse.

Open the editor · ▶ Run the live demo

The circuit

Battery + → 15 A fuse → relay 30; relay 87 → both heater pads; pads − → ground under the seat.
Switch fed from an ignition-switched fuse → relay 86; 85 → ground. The seats cannot be left on with the car off.
Two 50 W pads: 8.3 A — a 15 A fuse and 1 mm² wire (calculator, 2 m run).
Most kits have a thermostat in the pad and a High/Low switch; on Low a resistor or the series connection halves the power.

Under-seat wiring

Route the wires along the seat frame with slack for the seat to slide and tilt.
Do not pierce or splice into the seat’s airbag or seat-belt sensor wiring (usually yellow connectors).
Ground to a bolt on the floor, not to the seat frame alone.

Heat them in the simulator

Run the live demo: a relay feeding two 50 W heaters from a 15 A fuse, and the switch from ignition.
Ignition on, switch on: both seats heat, about 4.2 A each.
Click the switch wire: only the coil current flows through it.

Questions

Can I wire the heater straight through the switch?

For one pad, with a switch rated for 5 A and more, yes. For two seats use a relay.

Why does the heater cut out after a while?

The thermostat in the pad opens at its set temperature and closes again as it cools — that is normal.

What fuse for one seat?

One 50 W pad draws about 4 A: a 7.5–10 A fuse.

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