Marine Electrical Systems, Part 1b: ELECTRICAL CONNECTIONS (Continued)by Cameron Clarke
There are many types of wire, many of which have little use in marine applications. Sometimes it is impossible to buy the right types or better grades in foreign ports. That is ok, as long as you recognize that and can deal with other problems later. Anchor Marine makes a very good wire for marine use and is highly recommended when available. It is composed of many fine strands, each tinned (coated with tin to prevent salt corrosion of the underlying copper) and the insulation resists oil and saltwater much better than household or automotive wire. If you consult a wire resistance table, you will notice there is less resistance in multi-stranded tinned wire than other types.
Table 1, Copper Wire Table @ 95oF, 10% Max voltage drop
Ohms per foot Max Ampere load
00 0.0000811 200 0 0.000102 125 2 0.000162 90 4 0.000253 70 6 0.000403 50 8 0.000641 30 10 0.00102 25 12 0.00162 20 14 0.00258 15 16 0.00409 7 18 0.00651 3
Use the table above to compute resistance of wire runs, include length both to and from device. If you use 25 feet of #14, two-conductor wire for a cockpit lamp, then you have 50 feet of #14 wire. Multiply 50 feet times 0.00258 ohms per foot from table above to determine 0.129 Ohms of wire resistance, without allowance for any connection or terminal resistance, just wire itself. I would use at least #14 wire for a bilge pump, if the run were over 100' I would consider using #12.
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The major concern is not so much the wire, but the connections! If you possess a very accurate ohmmeter, you can directly measure the electrical resistance of each connection, expressed in ohms. Don't worry about this for now. My analysis over the years says the average connection (even crimped after it is used six months) measures a mere 0.03 ohms. The average connection that was crimped and soldered measures 0.01 ohms. What does this mean?
Take our two preceding short pieces of wire. There were three connections, right? That is what I counted. I would estimate the resistance in that wire after six months service to be about 0.09 ohms (3 x 0.03). Is this not much resistance? It would be much higher if corroded by salt water.
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