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-   -   Fusing the wire or the object? (http://www.sailnet.com/forums/electrical-systems/65408-fusing-wire-object.html)

SanDiegoChip 06-08-2010 04:30 PM

Fusing the wire or the object?
 
We have three lights and a cigarette lighter plug on a 16 gauge wire.
They are wired in a series.
There is one wire then a soderless connector which goes to the first light and a wire from it then goes to the next and so on.
We want to fuse them.

Two lights are LED.
One light is a incandescent.
Then the cig lighter plug for a low draw light we want to hang off the bow at anchor at times.
How do we fuse them?
Would we put one fuse for all?
One fuse for each?
If one fuse for each what size fuse or how to know what size?Thank you,
Chip

btrayfors 06-08-2010 04:42 PM

Chip,

Although one wire runs from one thing to another, they're no doubt wired in parallel, not series. There must be another wire for the return.

Figure the total possible draw for all items and install a fuse which is about 25% more than the highest possible draw.

Be absolutely sure not to use the cigarette lighter plug for anything substantial. Your AWG16 wire is none too large.

Bill

SanDiegoChip 06-08-2010 05:10 PM

http://i922.photobucket.com/albums/a...a/navlight.jpg
This is the light we would like to hang from the bow at times.
Also am considering replacing bulb with LED.
Yes there is a ground wire for the return. It also runs in yes you are right parallel.
So just add them up and put in a 25% larger fuse. Where do you get the numbers to add up? Off the bulbs?
Thanks,
Chip

btrayfors 06-08-2010 05:28 PM

Quote:

Originally Posted by SanDiegoChip (Post 611038)
http://i922.photobucket.com/albums/a...a/navlight.jpg
This is the light we would like to hang from the bow at times.
Also am considering replacing bulb with LED.
Yes there is a ground wire for the return. It also runs in yes you are right parallel.
So just add them up and put in a 25% larger fuse. Where do you get the numbers to add up? Off the bulbs?
Thanks,
Chip

Yeah, sometimes it's hard to find.

The LEDs are insignificant in terms of amperage draw....very, very little. The incandescents could be as much as, say, 25W or a little over 2 amps each. Say, 5 amps for the two incandescents and another amp or so for the LEDs. A 7A fuse should be more than enough.

Note that the adequacy of wire size depends not only on amperage draw, but also on the total length of the circuit. For lights and non-critical equipment, a 10% voltage drop is considered OK. For critical equipment, a 3% drop is preferable, or even less. There are lots of voltage drop tables on the Web, and in West Marine Catalogs. You can do the math yourself.

For what you describe, AWG16 should be OK for runs of 20' or less, round trip. If it's much more than that, you should use AWG14.

Bill

mitiempo 06-08-2010 07:45 PM

I agree with Bill. I would only use 16 ga for a LED only run, 14 ga for normal lighting and 12 ga or larger for higher draw items like pumps. 14 ga is normally protected with a 15 amp breaker which is less than the ampacity of 14 ga and not less than the lights, etc on the circuit.
Except for sensitive electronics, you should always be fusing for the wire. The purpose of the fuse is to be sacrificial before the wire burns. Some electronics require small fuses in line.

erps 06-08-2010 08:36 PM

Quote:

How do we fuse them?
It's my understanding that the fuse protects the wire should a short develop anywhere in the circuit. The fuse should be sized to protect the smallest wire in the circuit and as close to the power source as possible. There are tables on the internet that shows how much current a given size wire is rated at for a given distance.

mitiempo 06-08-2010 08:48 PM

That is correct, it protects the wire in case of a short circuit. But wire is rated for its amp carrying ability, as this link at Blue Seas shows.
Electrical Properties of Standard Annealed Copper Wire - Resources - Blue Sea Systems
Distance doesn't relate directly with current carrying ability of the wire which is not variable. But voltage drop does vary greatly with distance, most of which I try to keep to 3% or less, even for lighting. The fuse or breaker for a given wire size should not vary with distance on a given gauge of wire. The wire gauge should certainly vary with length however for a specific load.


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