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DIY: Choosing correct wire size

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"What size wire do i need for..." This is a very common question and one a lot of people guess at but there is a meathod to choose best for safety and reliability. It is easy to do with a little bit of multiplication and a chart.

First you have to start with how much power you have to handle. Weather it is a single item or rigging up a fuse panel everything has a max amperage rating just add it all up. For this example lets say your rigging up a fuse panel and want it to be able to handle 30 amps.

Now you have to figure length. They call it a circuit for a reason the engery flows to and from the power source so if you just measure from the battery to the fuse panel one way and its 10 feet you have to double it to 20 feet to take into account the entire circuit there and back.

So you have 30 amps and 20 feet of wire. Multiple the two and you get 600.

Then you go to the chart.

WireChart.jpg

The rows that you are looking at are labels Famps and in most cases the 12 volt section. Now when your deciding weather to follow the 3% or 10% collums take into account what this circuit is being used for. This area is left up to some degree of interpritation. The 3% column is very critical items that immediately affect the safety of the vessal like running lights, bilge pumps, or the fuse panels that power them. The 10% column is for most other loads.

So say this fuse panel your wiring is powering your entire dash which include the switches for running lights and bilge pumps you follow down the 3% column until you get a number over 600 used in our example and you will need to use 6 gauge wiring. But say this fuse panel is only powring a stereo, fishfinder, some spreader or courtasy lights, nothing critical to your safety, then follow the 10% column and looks like 12 gauge is enough. Now this is a minimum for consideration just because the chart says 12 doesnt mean you cant step it up to 10 gauge.

Also after you have chosen your wire alway remember to fuse it, in this example we figured on 30 amps so i would put a 30 amp breaker in the line to protect against shorts.

I hope i didnt complicate it to bad and everyone understands, let me know if you have any questions.

Edited by EdB

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This chart was published by Blue Sea Systems

There are many ways to figure which wire size to use I just like this on cause how easy and fast it is.

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This is great reference so I made it a sticky.

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I LOVE it.

We always wonder what size wire to pull for this or that.

The big question I have is why the electronics companies all run tiny tiny wires into their units - 16, 18 or smaller.

We always want to pull 10 from the batteries, but they you are hooking up to 18 from 10. Doesn't seem to make sense.

So, can someone please explain the advantage to me of running big wire back from the battery if the device you are hooking up only has tiny wires on it?

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From what I was told and have learned, with dc it has alot to do with voltage drop. A #14 will carry the amps ok but will have a big voltage drop running say 20', so the bigger wire will not get as much voltage drop in that same distance. So the marine radio pigtail can be small because it is only a foot long or so. Just my 2 cents

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That's right the leads are short and most of the time they are low power, no more than 5 amps or so.

It is generally not recommended to run everything back to the battery. You should have some sort or distribution block or fuse panel with one adequately size set of wires going to the battery.

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That's right the leads are short and most of the time they are low power, no more than 5 amps or so.

It is generally not recommended to run everything back to the battery. You should have some sort or distribution block or fuse panel with one adequately size set of wires going to the battery.

So does that mean you have to add up the amps of all the devices running into the fuse panel to determine your wire size from the panel to the battery? I've got to redo my wiring, it looks like my battery is growing hair right now so I guess I've got the add the panel so I can have just one battery connection for everything.

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that is correct mrhook, I also am doing this in our boat. I bought a fuse block and ran #4s to a fuseblock protected by a 70A inline fuse at the batt.

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Be very generous with your planning. Give yourself room for future equipment add ons!

That way you do not have to go back and rewire.

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