A Little Confused About the Best Way to Wire My DC

[goboatingnow]

Originally Posted by albergsailor

so what happens if there is a short before the breaker( in between the battery and the breaker) will the breaker still trip? i just read it wont. so is there like a big fuse that i should have right before the battery?

Yes there are battery fuses to do just that. Useful things to fit.

[nitpik]

One solution is to install a Blue Seas terminal fuse block #5191 directly on the battery terminal. - available 30A to 300A

[Ex-Calif]

Originally Posted by albergsailor

so what happens if there is a short before the breaker( in between the battery and the breaker) will the breaker still trip? i just read it wont. so is there like a big fuse that i should have right before the battery?

Once again it is about the wire not necessarily the load. If a wire is rated to say 30amps, it should be protected at or below 30 amps as close to the
power supply as practicable.

Now it would be cumbersome on a boat with 18 salon and berth lights to have 18 switches and fuses. So you will see them grouped. A 12v 40w light will draw 3.3 amps. If I group 6 on a panel I might choose a 20 amp fuse, but have I done the right thing? It depends on the wire run to the lights. See linked chart.

http://sailboatstuff.com/images/Imtr...mendations.pdf

For an averrage 15 foot run with 10% desired voltage drop (acceptable for lighting) I might choose an 18 guage wire to each light as each light is less than 5amp. Big mistake as the 20 amp fuse will not protect what is now a 5 amp wire. In this case I could and probably would choose a 14 guage wire to make the light runs as each one can take 20 amps. Remember we aren't protecting the 40w light bulbs we are protecting the wire run.

Where a dilemma comes in is this. I have a nice 14 guage wire leading to my current 40w incandescent light. I buy a wallmart led light for cheap. Heck, why not it only draws 15w for the same lumens. However the pigtail on the light is 18 guage and when you strip it back and pigtail it in to the 14 guage it looks like cat whiskers. That pigtail clearly cant take 20 amps (likely only 5) and you have increased the potential for that fixture pigtail to melt and potentially catch fire if something happens to the circuit. I may be being alarmist as what really might happen on a conductor that small and short is that it will melt through, including the conductor (wire) and the circuit will be cut before enough heat could really start a fire. However in heavier circuits like starters, windlass, charging circuits the potentials are very real.

The other temptation is adding stuff. Most of us have seen enough older boats to know that people are going to add stuff and our experience is that most of it is a bodge job. The problem is that there are no spare protected switches left on my panel! Do I double up something? My answer is not normally and I am guilty as many on adding stuff.

I have added a bilge pump, a couple of fans and an inverter to my boat. I bought a nice switch panel for the bilge pump with a built in fuse holder. In this case, because the pump needs power even with the master off I took power from the house switch on the unswitched side. The run to the fuse is about 3 feet. Should it be protected at the house switch, ideally but I compromised with a 3 foot run to the fuse.

The inverter was mounted at the electrical panel on a shelf. I decided it did not need a separate switch. I sized the wire appropriately and used a 5 amp inline fuse - the inverter itself draws about 4 amp- and tied in to the 12v buss. I dont love it that way but the inverter is accessible as is the fuse. A future panel will have a better solution.

The fans each draw about 1 amps. Again I drew from the 12v bus inserted a 3 amp inline fuse and sized the circuits(wires) for 5 amps.

If you are doing a new build or rebuild there is a great opportunity to fix the mistakes and bodges, even one's own...