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Author Topic:   Small Boat Re-wiring
WaterWorldBoating posted 04-24-2015 07:01 AM ET (US)   Profile for WaterWorldBoating   Send Email to WaterWorldBoating  
I'm in the process of rewiring my Boston Whaler 13-foot Sport model boat. The battery is on the Port side and the console is on the Starboard. Currently the wiring is not marine grade hence one reason for the upgrade. The boat currently has a terminal block at the stern for the running lights and a open fuse block with glass tube fuses in the console. There is no circuit breaker or in-line fuse at the battery.

The boat currently has 40-HP outboard engine run directly from battery (with no fuse inline), navigation light lamps, a bilge pump, a compass light, a marine radio, and a fish finder. At a later date I may add a couple LED lamps under the thwart seats.

From what I read in other re-wire post by LHG there should be an inline fuse at the battery for auxiliary items. Is there a particular that folks prefer and what size? Should I use 30-Ampere or 20-Ampere? Seems from what I've seen the 20-Ampere in-line fuse holder I saw uses 12-gauge wire and I was thinking of using Ancor 10/2 duplex marine grade wire from the battery to a Blue Seas 6-spot fuse holder with negative bus. [I am] open to suggestions.

From the console back to bilge pump in the splash well should I use 12/2-duplex wire? Is a special kind needed? I don't want to end up buying the wrong sizes or types.

Also. What connectors and shrink wrap do folks recommend and where do you buy them from? I looking into ordering the wire sleeve wrap from the site jimh posted a while back once I decide on color and size.

Many thanks!

jimh posted 04-24-2015 10:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Engine cranking circuits are generally not fused because of the very high currents that flow during engine starting.

The purpose of a current limiting device in an electrical circuit is:

--to protect the wiring from carrying too much current so the wire does not become too hot, melt, or otherwise create a hazard; and

--to protect the load from damaging itself should there be an internal fault, if there is only one load; if there are many loads, then no single fuse can protect all of them as the fuse for the branch circuit would be larger than the current of any one load.

Distribution of electrical power in a 12-Volt system generally requires that the voltage drop in a circuit will dictate the size of the conductor, not the current rating in the circuit. Selecting wire size for 12-Volt power distribution can be easily done by recognizing a simple relationship, which I first pointed out some time ago, and have given the name Hebert's Rule , as I have never seen it mentioned by anyone else. Hebert's Rule for conductor sizing is a very simple:


The maximum length (in feet) of a conductor that can be used for two-way power distribution is equal to the system voltage (in Volts) if the current is to be at the maxim rated current of that conductor for power distribution as figured according to the 1-Ampere/700-circular mils rating system.

The rule is easier to understand in an example. Let us say we have a conductor whose circular copper 700-mil rating is 15-Amperes. This is the rating for a copper conductor of size 10-AWG. The rule says we can use this conductor to carry 12-Volts at a current of 15-Amperes for a maximum of 12-feet if we want to maintain a 3-percent or less voltage drop.

The absolute current rating for 10-AWG is much higher. It can carry 30-Amperes safety without becoming too warm. What should be the fuse or circuit breaker size in such a circuit? Using a 30-Ampere current protection device is appropriate to protect the conductors.

Power is usually distributed from secondary power distribution centers. The main power feed to such a secondary power distribution center is usually sized according to the aggregate electrical load that will be using the secondary power distribution center as a source.

Not every load will be in use simultaneously, so it is common practice that the total fused current for all the branch circuits may exceed the rating of the fuse in the feed to the secondary load distribution center.

I do not believe that LHG was the first to come up with the notion that there ought to be some over-current protection in the feed to a secondary power distribution center from the main power source. That has been a practice in electrical power distribution since the first day of electrical power wiring.

I very strongly recommend to not use in-line fuses which have to be spliced into the conductors carrying the power distribution. Splices in wiring have no place on a boat and should be avoided as much as possible. Particularly on a small boat, a 13-foot boat, there should never be any reason to make a splice in an electrical conductor. Perhaps if you must put a fuse at the battery, an in-line, spliced-in fuse would be unavoidable. At least put in under the battery box cover.

Also avoid like the plague use of spiced-in pre-made electrical connectors with pig-tails, particularly those awful trailer-wiring connectors.

And never use a sealant that leeches acetic acid to make any sort of electrical connection seal.

And never use wire nuts for any purpose in a marine environment.

I also recommend AGAINST using any sort of shrink wrap. You should not be wrapping any electrical connections in order to insulate them. If you need to add extra insulation, you should use heat-shrink tubing, not wrapping.

If you really need to insulate or protect an exposed connection, use a paint-on insulation like a liquid vinyl electrical tape product. Or, use a quality electrical vinyl tape, like SCOTCH 33+ vinyl electrical tape. Beware of poor quality tape from Asia. Use the real stuff.

As for using a two-conductor cable with an outer insulation sheath over both conductors--a "duplex cable--this can sometimes be advantageous on a really small boat where the cable will be running completely exposed to the weather. I don't use these types of cables on my boat, as the electrical wiring is not exposed to splash, spray, and weather; for the most part all the electrical wiring runs under the decks.

The general practices of distribution electrical power on a small boat are described in

Boat Electrical Circuits and Wiring Practices

jimh posted 04-24-2015 02:21 PM ET (US)     Profile for jimh  Send Email to jimh     
On a boat that is 13-feet long it is unlikely any conductor in a 12-Volt power system will be longer than 12-feet, so you don't have to worry about voltage drop being a consideration. The conductor can be sized according to its 700-mils circular cross section current rating. The rating of wire according to American Wire Gauge and Amperes is as follows:

00-AWG = 190-Amperes
0-AWG = 150-Amperes
1-AWG = 119-Amperes
2-AWG = 94-Amperes
3-AWG = 75-Amperes
4-AWG = 60-Amperes
5-AWG = 47-Amperes
6-AWG = 37-Amperes
7-AWG = 30-Amperes
8-AWG = 24-Amperes
9-AWG = 19-Amperes
10-AWG =15-Amperes
12-AWG = 9.3-Amperes
14-AWG = 5.9-Amperes
16-AWG =3.7-Amperes

For more about calculating conductor size with consideration of voltage drop, see

Wire Conductor Size for Power Distribution

Tom Hemphill posted 04-24-2015 05:52 PM ET (US)     Profile for Tom Hemphill  Send Email to Tom Hemphill     
Please see an earlier thread on this topic:

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