Wiring diagram for Chinese carbon fiber seat heater kit
IF THE IMAGE IS TOO SMALL, click it.
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LOW (series) draws about 1A per chair (2 pads)
HIGH (parallel) draws just under 5A per chair (~60W)
The relay contacts, the switch contacts, its R wire, its Y wire (at least up to the splice), & the Bk ground wire must be rated for more than the SUM of the elements' current draws. The switch's Bk wire must be rated for the bulb's draw, but it's more convenient to use the same size as the other Bks. The other wires must be rated for more than ONE element's current draw.
The illumination side of the switch (to the Right of the dashed link) is optional - it's not necessary for the rest of the circuit to work.
The fuse mentioned at the top Left must be sized for SUM of all the heating elements in the circuit PLUS the relay's coil & the switch's lamp; in this case, 4.2A plus ~300mA per chair at the absolute minimum (so a 5A fuse for 1 chair, or a 10A for 2).
This diagram was created from this set of symbols using MSPaint: