Fuse/Breaker question

So I'm finishing up my fiberglass duck boat and am about to run the electrical. I'm no means an expert, however I'm pretty comfortable with doing it. I just am unsure about:

I'm running a short distance of 30' round trip. I'm adhering to the ABYC standards for voltage drop based on wire gauge. My question is does the fuse/breaker amperage need to be different than what the smallest wire in the run can tolerate?

For example, say my nav lights have 18awg wire and I run 16awg from the 12v battery to it. If I use a 5amp breaker, will I still be ok? Or am I just being dumb?

 

Your fuse should trip before the wire does. The whole idea of the fuse is to protect the wire from overheating and melting. But the fuse can be rated at up to 150% of the amperage in the circuit (or circuits under 50 volts) and it will still trip before the wire melts. No you are not being dumb. You are being smart to ask.

 

I use a different approach; I put fuses close to the items like lights, electronics etc and a general breaker at hand of the skipper at the helm and another one close to the battery.
The size of the breakers is:
-double main breaker at the battery + and - size 150% of the general maximal current drain (and the wire is accordingly sized for that). It's the catastrophe breaker in case of short circuit of the main line. It serves also to isolate the battery, very useful also when an unidentified low loss drains out the battery in a few days when the boat is at the mooring or marina. It's not very funny to find that the battery is empty a Sunday morning...
-breaker at the helm on the +; it can serve as general switch also. Something around 125 % of the main drain in amps. It protects all the lines before the division between the different electric items.
- fuses on the lines (always the +) sized accordingly the drain of each electric item. So each wire is protected. It makes also far easier to identify the source of an electrical problem like a short circuited external light (that happens generally a Saturday night during the holidays far away from civilization).
Most electrical problems occur at the junctions and connections, so I have tendency to tin weld (only electronic tin with non acid flux) and protect with "liquid vinyl". This stuff seals completely.
Tapes and retractable tubes are useless as water and salt can go under. They are not reliable on long term as the connections can be destroyed by corrosion.
I do not use any connection by screw as they have tendency to corrode and become impossible to unscrew. It's far easier to un-weld that to struggle with screws or corroded connectors (the automotive plastic things that become as brittle as glass, splendid 25 k-ohms resistors when corroded as they are made with low quality brass which dissolves in marine atmosphere in contact with pure copper wires). If you are obliged to use such things, there is a beginning of solution; silicone jelly, the same as use on the phone lines.
Last thing, forget the home depot ordinary automotive cables. They will dry and break in marine atmosphere in a very short time and they are not waterproof because the low quality plastic insulation may be porous. But on a small boat you are not obliged to use the overly priced special marine cables with gold plated connectors.
Use silicone insulated high temp cables, expensive but you'll love the 20 years of tranquility with total reliability... The automotive ones are simply excellent if you tin weld and protect the copper strands with liquid vinyl at the connections. No water, no salt, no oxygen=zero corrosion.

 

Nothing particularly wrong with this approach. The breaker is protecting te wire run and the fuse is in reality protecting the lamp.

Fuses or breakers to protect the wire run are required to be close to the source of power; the battery in this case. It's supposed to be seven inches, or 40 inches if the wire from the battery to the fuse is in a sheath. So your aprach is ok, it's jsut a bit redundant on a simple system like hardcore's

 

Ike it's just common sense and has to be adapted to each case.

Unhappily common sense and simple proven recipes are not always used in boat electricity (and house electricity also) as I've seen many times. I have even seen a complete circuit in one color (black) big gauge because this very ordinary wire was cheaper in one big roll with just one too big used breaker...You can imagine that all that burned nicely with a very black smoke in the middle of a trip with 20 divers aboard. They ended with no more electricity, no radio, a boiling battery, and no more engines (all wire commands are nice but need electricity). Happily they were close to the coast and could use a cell phone for the mayday call to the dive shop. I'm ready now to see any and everything, even the foolishest able to win the Darwin awards.

I prefer to divide by sections and size the breakers or fuses following the max drain in each section. A bit redundant but far simpler to repair and to identify the source of the problem. It permits also most times to isolate the problem in minutes and to keep the remaining of the electric items working. On working boats that can save thousands of bucks. On leisure boats that makes your day.

 

I agree. And, that is exactly why there are regulations and industry standards for electrical systems whether they are on boats, in house, rv's, autos or airplanes. Each is a little different based on the use and circumstances. But they are there to prevent fire and shock. When people don't follow the standards then bad things happen.

By that I do not mean exceeding the standard. For instances many marine professionals will tell you, you have to use tinned wired. The truth is you don't. It is not required but it is better and exceeds the standards so electricians use it and designers spec for it and surveyors look for it. And that's good.

The standards are the minimum for a safe installation. Anything less is hazardous to your health.

 

My boat an Egg Harbor 37 made in 1970 was made with untinned copper wire.
I have yet to see a problem with the factory wiring in regards to being untinned.

The copper wiring inside some of the insulation is in some cases turned dark brown with a surface oxidation, but it is still functional. In places where i modified the wires I used either phosphoric or white vinegar to clean the bare copper ends prior to soldering and crimping on new connectors.

I also have gotten rid of a few wire nuts. I have various sized pieces of copper tubing. I cut off a 1/2 to 3/4 inch piece, solder the bares ends of the wires to be joined . Then copper crimp them together. Then I take a piece of heat shrink tubing and seal it up. Done that with AC and some DC and works well.

A few sections of black SO type wire has oddly rotted the interior insulation for the white and green wires, it got dried and cracked. The black wire the insulation is still perfect.

All the other wire, the vinyl or PVC insulation is perfect. The boat wiring is fairly well protected from rain and salt spray being a closed in type cabin cruiser. My old boat trailer, the wiring for the lights disintegrated in some places due to being dunked under salt water over and over and it was untinned.

I recently added 2 rule 3700 crash pumps. I used a 10 foot section of yellow shore power cable. I noticed the wire was not tinned. I ran a fourth 8 gauge wire for helm switch operation. I put the wires in black ENC flex conduit.The light duty helm switch flips on two 40 amp cube relays so they can run independently so that the float switch wont turn them both on at the same time. The breaker size I used was 40 amps. Under pumping water load each 3700 draws about 12 to 14 amps.

 

Voyager, your first post reads like you have many remote fuses and 2 (only) master breakers (in series).

I'm with Ike on that, fuses must be as close as practical to the distribution point.
A damaged +ve could short to ground on the wiring run which would be before the fuse so would trip the breaker, then you lose all electrics until the circuit is identified and disconnected.

The fuse is not there to protect a circuit or device which goes faulty, it's to protect the other circuits FROM that fault. It is common for electronic devices to have an inline fuse in the power line very close to the device, but the misconception is that it is to protect the device - it protects the circuit. e.g GPS, Radar and Radio on one circuit;- Your GPS fries, that fuse ensures your Radio and Radar are still online.

It may be that you missed some detail on your post, but to me your reliability sounds like it is the product of good electrical engineering practices rather than your chosen design.