Wiring Trailer Lights with a 7-Way Plug (It's Easier Than You The black battery wire should be same size as the white ground wire - 10 gauge in chart - faq311-7-way-trailer-diagram-rv_2_800 The picture below this chart shows correct wiring for RV plugs. As a professional RV transporter I have seen to many trucks wired with those 2 wires to small and cause a fire from overheating.