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At 12 Vdc, a 300 mA camera at 100 feet on standard 18 awg will have a voltage drop of 0.38 volts. Industry standard is +/- 10% which is 1.2 volts. In this example, you are within limits.
12 Vdc, 0.8 amps or 800 ma (not unreasonable for an IR camera) at a distance of 150 feet on 18 awg would give you a drop of 1.54 volts, which is greater than acceptable limits. The way to get around this is to have a shorter cable run, or increase the size of your power wire. In this example, increasing the power wire to 16 awg would decrease the voltage drop to 0.96 volts, which is within limits.