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I have a new truck bed cap. I need to wire the cap's dome light and third brake light. As such, the cap has four wires (supply to dome, ground from dome, supply to brake light, ground from brake light).

I plan to power the lights from the tail light harness (drive side). My question is, can I splice the two grounds (from the dome light and from the brake light) together, and tie to a single ground (i.e. to the ground wire in the tail light harness), or do I need to tie them to separate ground sources (i.e. multiple points on the frame). Thanks!

  • In one system, all grounds must be connected together to prevent problems by difference in ground level. It doesn't matter if you connect them on separate places on a frame or all on one point; the frame connects them together. The thick wires will probably carry large currents. Make sure they don't have to run through a thinner wire to arrive at the power source. I'd advice you to run a thicker wire from the power source's negative side to the frame, and connect all grounds to that frame. Doesn't matter where. – Bart Feb 12 '18 at 10:01
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There is no problem with joining the two Grounds together.

Just make sure that the ground you connect it too has sufficient current capacity but nearly every ground wire in the vehicle will be able to carry the current required for two lights.

Assuming both bulbs draw 21w so 42w in total (which is an over estimation) and your truck is 12v then the current would be only around 2.8A. 0.35mm2 cable which is the smallest used in most vehicles can handle around 7A. So providing the ground cable to the tail lights has sufficient capacity it will be fine.

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    We basically said the same thing ... you posted just before I did, so +1 to you. – Pᴀᴜʟsᴛᴇʀ2 Feb 9 '18 at 19:57
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With only the two lights, you should be just fine attaching them to a single ground point. You can ground quite a few wires before you need to worry about overloading a ground point ... and when you do it's because you have too many on a bolt and cannot fit anymore. If you are running two wires into one wire, then attaching the single wire to the ground point, you need to ensure you don't overload any single wire. For the most part, that's pretty hard to do, especially considering you only have two lights. Depending on the light involved, you could tie in several more lights to the single ground wire prior to having issues. Regular lights (other than headlights) just don't draw that much juice.

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