Which wire is positive 12V?

The positive 12V wire is the red one in most DC systems, with black as negative. Ribbed or striped insulation on zip cord marks the positive side when both wires are black.

On a barrel plug, the center pin is usually positive; on a battery, the terminal stamped with a plus and often a red cap.

Vehicles are the exception zone. Yellow may be constant hot, red switched, and black is not reliably ground, so check the diagram.

When wires are unmarked, set a multimeter to DC volts and touch both leads across the pair: a positive reading means the red probe is on positive, a negative reading means it is on the other wire.

With polarity settled, the DC wire size calculator picks the gauge your amps and distance need.