T&T: Wire sizes

[Mark Richter]

Hans,
There are marine wire sizing graphs and tables in the 2009 West catalog, 
P283.  #18 wire has 6.5 ohms per 1000'.  Lets say your run (round trip) is 
100'.  That makes 0.65 ohms.  E=IR, or the voltage drop is proportional to 
the resistance and the current.  I'll round the 10 w load to one amp (12 
watts at 12v).  Our voltage drop will be 1 amp times .65 ohms or 0.65 volts. 
That's about 5% of our 12v source, more than the 3% recommended for 
navigtion lights.  If the run is 100', use #16 wire minimum.  The LED anchor 
light will probably use less than 0.1 amp, so even #20 wire would be large 
enough, electrically.  But ABYC requires minimum wire size of #16 for single 
wires, or #18 for sheathed multi-wire cables (for strength reasons).
A 1 amp load (12w at 12v) with 60' (round trip) of #18 wire would result in 
3% voltage drop, acceptable for navigation lights.  But mast wires are often 
not well supported for long distances, so physical strength becomes 
important, too.  I'd probably run a pair of #14/2 marine cables, and try to 
suport it as well as you can inside the mast.  You may want to run a 14/3 
cable to serve both lights if they are close together, as they can share the 
ground wire.  One good method of support is to install the wires in PVC 
conduit, using some stiff shipping foam scraps to jam the wires in the PVC. 
Then use pop-rivets to install the conduit inside the mast.

Mark Richter, Winnie the Pooh, Ortona, FL on the Okeechobee Waterway
Mark's Mobile Marine
General boat repairs at my dock,
specializing in electrical system design, installation and repair

>I need to wire the new alu mast I have had made, so I have a question to 
>the
> gurus on the list: How do I calculate the wire gauge needed for the anchor
> light (10W or LED) and the steaming light? In other words can I do with a 
> 20
> gauge wire or do I need to go to 18, 16 or 14 gauge?