Articles: Power

Wire Sizing

What's wrong with this picture? It was taken inside an upper helm console. On a positive note, all the loads are fused. Messiness aside, take a look at the possible load on this fuse block. There are 170 amps worth of fuses. (You'd never draw max amps from each, but it's a theoretical possibility.) Now try to spot the feeder wire. It's about a size 14 gauge, which is rated for about 25 amps max before potentially overheating. This is only one factor of wire size, the other being voltage drop. Looking at the 3% drop table for a 15 to 20-foot run, this cable should take a max of 5 amps for best performance and reasonable voltage drop. A theoretical 170 amps from a cable best rated for 5 amps -- yikes!

Not only is this a safety issue, it is a surprise that the upper helm navigation equipment is still running. The voltage must be very low. This wire run should probably be a 2 gauge, approximately 16 times bigger. This problem is commonly seen on older boats. Originally the boat probably had very little on the upper helm, maybe a VHF or a sounder that ran fine off a smaller wire. But over time, more and more equipment is added onto the existing power wire and overloading it. When you add more things, you need to look at the whole picture.


Comments (1)

  1. interesting article:
    Jun 07, 2011 at 10:01 AM

    Thank you for a great post.


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