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I am putting a 50W 3.4A max 12V solar panel on my camper van. I want the option of removing it from my roof racks & mounting it remotely (so I can park in shade, but still have the panel in the sun).

I am planning on just putting a replacement extension cord socket on the two MC4 leads, and then cutting the socket off of an extension cord (16 gauge) & wiring that to the charge controller, then running 2 wires (16 gauge) to the battery.

My question is since usually the total wire length will be around 6-7' can I use 16 gauge wire and then use a 14 gauge extension cord for when I want the panel remote or do I need everything to be 14 gauge (planning on 25-30' extension cord)?

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    \$\begingroup\$ Yes you can as long as the total power loss on this setup is not a concern. \$\endgroup\$ – Eugene Sh. Feb 12 '19 at 17:10
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Think of wires as "bad" (or weak) resistors. For every foot of wire, you incur some amount of resistance, which imparts a voltage drop (read: power loss) to the system.

Extension cord wire gauge should be chosen based on the amount of current that you intend to draw, and the total distance needed. There are charts which provide this information, such as this one at the Engineering Toolbox:

Engineering Toolbox Extension Cord Capacity Chart

Such charts are derived by using the wire resistance, distance, insulation type, allowed increase in temperature, etc.

A thicker wire (lower gauge) has less resistance, and thus less power loss. If you first determine that your application can use a particular gauge for the entire length needed, you are safe to substitute a heavier gauge wire entirely or in part.

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