#67 What Size Wire

Good Morning!

I got a question a couple days ago: “If I’m moving my battery to the trunk of the car, can I use the sub / amp cable as the positive lead ?  The previous owner had the car wired for subs but I need space for turbo things.”

To answer this question we need to figure out if the size of the wire that the previous owner of the car used for the stereo is adequate to run the entire electrical system of the vehicle.

Sizing the wire required for relocating the main battery in the car depends on how long the wire is, how much current is drawn from battery, and how much current your alternator will produce.

Testing the Current Requirement

What I usually recommend to do is to measure how much current is drawn from the battery after starting the engine 4 times in a row.   The repeated engine starts is to apply electrical stress to the battery and the starter so that the worst case current load the battery will typically experience under normal conditions can be measured.

You an use a current clamp meter to measure the current at the battery positive wire.  If you don’t have or have access to a clamp meter, you can get one on Amazon though this link. https://amzn.to/2KJkYAv

Testing the output of the alternator for maximum output is a bit more difficult because most OEM alternators are sized to output more than the vehicle requires at full load, so that even if you have everything turned on in the car, the alternator can still change the battery. 

What I typically do is use the rating of the alternator published by the manufacturer. In a standard passenger vehicle, this rating is normally less than the current drawn by the starter mentioned above.

We will use either the tested current at the battery or the current rating of the alternator, whichever is higher.

Calculating the Voltage Drop

Once we know what the current requirement is and the length of the wire, we can size the wire based on it’s voltage drop. The rule of thumb for sizing wires is to have less than 0.5V (3-4% of the total voltage) voltage drop over the length of the wire at the current we need to use it at.

The following page has a great a calculator you can use to calculate the voltage drop and an explanation of physics behind the calculations if you want to read up on it.  https://www.omnicalculator.com/physics/voltage-drop

For example, if your alternator is rated at 160 amps and you measured 200 amps at the battery, we will use 200 amps for the calculation. For a 200 Amp load and the length of the existing stereo wire in the car is 15 feet and is also 00 (2/0) AWG wire, then you’re good to go. If the existing stereo wire is only 4 AWG, then you’ll have to upgrade your wire size.

I hope that helps!

Have a great day!

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Author: Karsy

Your average writer, maker, robot programming, gaming, tig welding, foodie, car jock, who loves to travel and is obsessed with coffee and lifting heavy things. Cheers!