What Gauge Wire to Connect 12v Batteries (Parallel & Series)

Graham Bogie

Picking the correct wire gauge for 12V batteries is vital to keep your electrical system safe.

The wire gauge, or diameter, plays a significant role in determining:

  • the amount of current a wire can safely carry, 
  • the potential voltage drop, and 
  • the overall performance of your battery system. 

An incorrect choice can lead to issues like:

  • excessive heat, 
  • voltage problems, 
  • and even cable burnout.

There are up to three wired connections in a battery setup:

  1. Battery bank links (connecting multiple batteries)
  2. Charger(s) to battery bank
  3. Battery bank to inverter & other load devices

So, what gauge wire do you need to connect your batteries?

At a Glance: Battery Cable Size Chart

If you know the current and lengths of your system, use the chart below to pick the correct wire gauge for your batteries.

Battery Cable Size Chart for picking what Gauge Wire to Connect 12v Batteries

How to Choose the Right Wire Gauge for 12V Batteries

Calculating the correct wire gauge for your electrical system is crucial to ensure it operates safely and efficiently. 

The wire gauge can be calculated using the following steps:

  1. Determine the Current: You first need to know the maximum current (amps) that will flow through the wire. 
  2. Calculate the Wire Length: Measure the total length of the wire you will be using. It should include the distance from the power source to the device and back, as the current travels both ways.
  3. Consider Voltage Drop: Some voltage will be lost in any electrical system as the electricity travels along the wire due to resistance. For most 12V systems, keep the voltage drop below 3%.
  4. Use a Wire Gauge Chart or Calculator: Refer to a wire gauge chart or calculator to determine the correct wire size for your system.

Our battery cable size chart above assumes no more than a 3% voltage drop.

For a custom size, use our wire size calculator, where you can amend your desired voltage drop.

Remember that these calculations provide a minimum wire size. 

Using thicker gauge wire is generally safe, but an undersized wire could lead to overheating and potentially a fire.

To link two batteries, you will wire them in either series or parallel

For example, you may have two 100ah 12v batteries:

  • wired in parallel to provide 200ah at 12v, or
  • wired in series to provide 100ah at 24v.

Regardless of how they’re configured, each battery can only ever pass 100 amps to the next. 

So, the battery link wire gauge must cope with at least that current. 

  • 100Ah batteries need cable to cope with 100a
  • 200Ah batteries need cable to cope with 200a
  • and so on.

Wire length is still important when calculating the correct size. However, batteries are usually located next to each other, and the cables are usually a foot or 250mm at maximum. 

Read the battery cable size chart above to check what gauge wire you need to connect batteries in your battery bank. 

We recommend 4|0 AWG, 120mm² for all battery sizes up to 200ah. Thicker wires will help avoid any excessive voltage drop. 

Here’s a quick summary of common battery sizes & their minimum wire gauges:

Battery SizeMinimum Wire Gauge
50ah8 AWG, 10mm²
75ah4 AWG, 25mm²
100ah2 AWG, 55mm²
175ah2|0 AWG, 70mm²
200ah4|0 AWG, 120mm²

What Gauge Wire to Connect 12v Batteries to Chargers?

You could have multiple chargers in your camper’s electrical system. For example:

  • MPPT controller to charge from a solar power system
  • B2B charger to charge the battery while you drive
  • Converter (battery charger) to use shore power to charge

Follow the manufacturer’s recommendations instead of measuring wire gauges. They know their products better than anyone.

Remember, though, these recommendations are for the cable size from the charger to the battery. 

If you’re using a bus bar between the battery and charger, the cable connecting the bus bar to the battery must be big enough to handle the total current rating of all the chargers on the bus bar.

If you're using a bus bar between the battery and charger, the cable connecting the bus bar to the battery must be big enough to handle the total current rating of all the chargers on the bus bar.

What Gauge Wire to Connect 12v Batteries to Devices? 

The cable from the battery bank to each electrical device must be sized to cope with the maximum current that device can pull. 

You may have a cable from the battery bank to a bus bar or fuse block. That cable must be sized to cope with the combined maximum draw of all the devices connected to the fuse block or bus bar.

If you also have a separate circuit connecting say an inverter through a fuse, that cable must be sized to the maximum draw of the inverter.

Follow the same steps outlined above for each cable from the battery bank:

Calculate the Maximum Current

Working out how much power you need is easy. 

All your RV’s devices usually have a power and current rating. 

If they all run on 12 volts, like your battery system, you add up the current ratings of each device to find out your total power use.

You’ll need an appropriately sized inverter for any devices that use AC power (230v or 110v). 

If using an inverter, use the manufacturer’s recommendation. As an indication, a 3000w inverter will be rated at about 250 amps.

Calculate the Wire Length

Measure the length of cable you need from the battery bank to the fuse block or bus bar. 

If you’re still planning the layout and cable runs, aim for as short a run as possible.

Consider Voltage Drop

The shorter the cable, the better. But you also want to minimize voltage drop.

The chart above is based on no more than a 3% voltage drop. 

Pick the largest wire size that meets your current requirements for the cable length you need. 

Graham Bogie

Graham Bogie - Expert in RV & Campervan Electrics - Mowgli Adventures

Graham is a seasoned marine electrical engineer with two decades of experience designing customized electrical systems for plant machinery and converting campers and overland vehicles. His expertise has led him to author the reputable Campervan Electrics Handbook and become the chief designer of our Campervan Electrical Design Service. As a knowledgeable figure in the field, his YouTube channel, blog, Facebook group, and newsletter, offering electrical advice and product reviews, reach more than a million users each year.

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