When wiring in a new project, you might feel tempted to save some cash by purchasing your wire from Home Depot or Lowes. After all, electric wire is simply electric wire! Electrons flow over the surface and power the device all the same. Why pay more for "Marine Grade" wire?
While most electric wire is copper, land or marine grade, marine wire is "tinned copper" and this is where all the difference is. Tinned copper wire is much more resistant to the corrosion promoting marine environment. If you wire something up with regular household copper wire, the wire will corrode and break off of the terminals in under a year! Using marine grade wire will ensure that your properly performed connections will last a very long time.
Since the whole problem with wiring on a boat is the moisture level and exposure to the elements, shouldn't "outdoor wire" hold up to the task? Outdoor wire has a much stronger insulative casing, allowing it to resist the harmful UV rays from the sun. After that, there is little difference between indoor and outdoor wiring. Yes, outdoor wires will be exposed to rain. After the rain clears, the wires will be nice and dry for many days to come. On a boat, the wires are going to get wet from time to time, and spend the rest of their days in a very humid environment.
Boats that are kept in salt water have it even harder, as salt in the air will expedite the corrosion process and lead to an even earlier death to any substandard wiring. Regular copper wiring will begin to spew green powder out of the electrical connections as it corrodes at an alarming rate!
When you buy wire, make sure that the spool clearly states that it is marine grade and made out of tinned copper wire. If a friend offers you some spare wire and you can't find any markings on the wire stating if it is marine grade or not, there is a simple test that is usually pretty accurate.
When you strip the plastic cover off the wire, household copper wire will be copper colored. Tinned copper wire will look silver colored. If your friendly gift wire doesn't have silver colored strands, consider reserving that wire for other less critical tasks like fishing good wires through bulkheads or tying up coils of good "marine grade" wire.