For this example we will use 16ga wire, it will work best with the MPPS-16ES add-on 12-volt outlet by MarinePro. Whether you start “feeding” from the power source or from the install site is entirely up to you. The higher location is usually much easier. So we will trim our positive and negative wires to the appropriate length and tape the ends together side by side. This will provide a nice clean end to begin feeding through the wall, cabinet, body panel etc. Work these wires gently around existing parts and pieces that might be in the way. Once you have the wires run it’s a good idea to go over and wire tie them into place or secure them somehow.  Do not connect them to live power just yet!

12 Volt 6' Extension Power Cord with Cigarette Lighter Socket

Work these wires gently around existing parts and pieces that might be in the way. Once you have the wires run it’s a good idea to go over and wire tie them into place or secure them somehow.  Do not connect them to live power just yet!

An inline fuse is not necessary but we recommend that you use one. Make sure that you can get to it easily in the future. For 16ga wire the fuse size should be no more than 22amps.

We are using a 12-volt blade fuse holder by Bussmann which can easily handle our 22amp max draw. Rather than crimping we prefer to use solder and heat shrink tubing when connecting our inline fuse. We will also use solder and heat shrink tubing when we connect the 16ga wire we have run to the fuse block or battery.

Once you have the wires and fuse holder soldered up it’s time to hook it up to live power. Using the appropriate crimp ends for your fuse block or 12-volt power distribution block prepare the positive wire end. The positive wire end should be plugged into an available auxiliary fuse slot in the fuse block or distribution block. The negative wire should be connected last and should be hooked up to the vehicles chassis or another location that is grounded to the chassis.

It’s now time to fuse this set up and test it! Choose a fuse close to 20 amps to use here. The socket and wiring can handle it. Once the fuse is in test the new 12-volt outlet with a multi-meter. The center is positive and the outer walls of the outlet are the negative. Once you verify that there is good power there, plug something in! Try something small and inexpensive first like a cellphone charger.

Congratulations! You now have an additional 12-volt auxiliary power outlet.

Well it’s not always that easy. Let’s say that we want to hook up a 12 volt heater to the same 12 volt outlet that we are running a 12 volt coffee maker from. Each of these items need about 20amps. On their own they’re fine, together they will draw around 40amps. This is a lot of power through a standard 12 volt outlet. Most cigarette lighter sockets are fused for just 20amps.

Some might think that inserting a larger fuse in the vehicles fuse box will let them draw more power. They would be right but this is not recommended. Vehicle manufacturers only use wire large enough in diameter (wire gauge) to handle the power allowed by the fuse size.

If we simply insert a larger fuse we will likely melt the wires supplying power to the outlet. There is also a very great danger of fire.

The proper way to “insert a larger fuse” would be to first figure out how much power you need. Do this by looking up the items power specifications and doing a little math.

Wire size required for amps desired is as follows:
12Ga wire up to 41amps
13Ga wire up to 35amps
14Ga wire up to 32amps
15Ga wire up to 28amps
16Ga wire up to 22amps
17Ga wire up to 19amps
18Ga wire up to 16amps
19Ga wire up to 14amps
20Ga wire up to 11amps

So after we have a roll of the proper size wire and a fuse we can get to work!

We must replace the positive and the negative wires powering the 12 volt outlet with the new heavier gauge wire. Cut a length of wire that will reach the vehicles fuse box from the 12 volt outlet. On the back of the outlet we’ll find a two wires attached to the 12 volt outlet. The center wire is Positive(+) and the outer wire is Negative(-). We will replace both with our new heavier gauge wire.

Once that is complete we would insert our new larger fuse into the fuse block slot that will fee the outlet. We need to test the outlet now with our digital multimeter to make sure that it has power. On verification we would try something small and inexpensive like a cell phone charger. Once we see that it works we can begin trying out our larger items.

We must keep an eye on the temperature of the 12 volt outlet now. We’re drawing more amps than originally intended by the manufacturer so we just need to make sure that it’s not getting hot. A hot 12 volt outlet is a bad sign.

You should always have an automotive professional perform the tasks outlined here. If you do not have a strong background in automotive wiring and electrical equipment you should not attempt the tasks outlined here.