What Battery Cable Size Should I Use?
Now that summer is almost upon us, one upgrade RV'ers and boaters often make is to their battery systems. Whether you’re adding an additional battery or a whole new solar power system, choosing the correct battery cable size for your system is critical. Let’s talk about why it’s so important to select the right cable size and, more importantly, how to do it!
What Size Wire Is A Battery Cable?
Cables coming directly from your battery are the main artery of your RV electrical system. Since they come directly from the battery, they typically carry more current (measured in amps) than any other cables or wires in your RV. As a result, your battery cable size will need to be rated for the highest current and ultimately the thickest.
What size wire you need for your battery cabling depends on how much power your RV requires. There isn’t one correct answer to this question.
Below we will discuss how to figure out how much power your RV uses and how to use that information to select the proper cable size for your batteries
What is Wire Gauge?
Wire gauge is the measurement of a wire’s diameter or thickness. The US standard for measuring wire gauges is the American Wire Gauge scale, or AWG for short.
In the AWG system, the higher the number of the cable rating, the thinner the wire and, therefore, the less current it can carry.
For example, if you look at the chart below, you will see that 12 AWG, which has a diameter of 2.05 mm, can carry 20-25 amps up to 4 feet. 14 AWG, which has a diameter of 1.62 mm, can only carry 15-20 amps the same distance.
Wire Size Requirements: Determining Factors
Thicker wires can carry more current for longer distances. Without getting into the math behind it, the reason for this is that a cable’s resistance increases as its diameter decreases or the length increases.
Therefore, the size cable you need depends on two things: how much current you need to carry and how long your cable runs need to be. This is why the AWG sizing chart lists the different current capacities at various lengths. As the cable length increases, so does the required cable thickness.
Wires have a maximum voltage rating as well. However, since your RV battery cables will only be 12 volts, you do not need to worry about the voltage rating when determining which battery cable size to use.
What Happens If The Battery Cable Size Is Too Small?
As we mentioned earlier, thicker wires have lower resistance. Resistance in a wire causes two main things to happen as current passes through it.
Voltage Drop
The first is that a voltage drop occurs. This means that the voltage at the end of the wire is lower than the voltage at the battery. If you have too much drop in voltage, your electronics will not work.
The voltage drop in a wire is calculated using Ohm’s law, V=I*R. V is voltage drop, I is the current passing through the wire, and R is the wire’s resistance. As you can see, if you increase the current, the resistance, or both, you will increase your voltage drop.
Resistance in a wire is dependent on both the thickness (the gauge) and the total length of the wire. If you undersize your battery cables, one issue that can occur is an excessive voltage drop that may prevent your electronics from working.
Wires Get Hot
The second thing that happens as current passes through a wire is that heat is generated. Much like voltage drop, more resistance in the wire results in more heat being generated. If wires are undersized, they can get so hot that the casing melts and can cause a fire. Fires are much more catastrophic than too much voltage drop and are the main risk in choosing too small of a battery cable.
RV fires often lead to the complete loss of not only the RV but also its contents. Having wire that is overrated for the amperage helps protect wires from overheating and potentially igniting. While it’s better to be safe than sorry when it comes to wire gauge, going too big has some drawbacks as well.
What Happens If The Battery Cable Size Is Too Big?
There are three main drawbacks to choosing a battery cable wire gauge that is too big: cost, weight, and ease of use.
Cost
Probably the most significant consideration is cost. Thicker wire gauges cost more. If you are only running a few feet of battery cable, the additional cost will be insignificant. As cable runs get longer, cost becomes more of a consideration.
Weight & Ease of Use
Similar to cost, as the wire gauge increases, so does the weight. Again, if your cable runs are short, the added weight will be negligible.
The last drawback to using thicker cable is that working with it is more challenging. Trying to bend and manipulate overly thick cabling in an RV’s small cramped compartments is not a fun time.
The drawbacks to oversizing your battery cabling are much less risky than choosing cables that are too small. However, choosing excessively thick cables can add unnecessary cost, weight, and frustration to your project. While it’s smarter and safer to choose too big rather than too small, just picking the thickest cable you can find isn’t a great strategy either.
How Do You Figure Out How Many Amps An RV Will Be Using?
Calculating your current requirements is pretty straightforward. Most appliances and electronics in your RV will have a current and power rating. If all of your electronics run on 12 volts (the same as your battery system), you simply add up the current ratings for each to determine your total current draw.
If you have appliances and electronics that run on 120 volts, the same as the power available in your home, you will need an inverter. An inverter converts DC power (from the battery) to AC power (like in your house). The process for calculating your current requirements with an inverter is simple as well.
You first need to add up the total power requirements (in watts) of each appliance in your RV to determine what size inverter you need. For example, if the combined power requirement of all your appliances and electronics is 2,500 watts, you probably want a 3,000-watt inverter.
Once you know your inverter size, the calculation to figure out the current draw is easy. Simply divide the watt rating of the inverter by the input battery voltage. In our example above, you divide 3,000 watts (the inverter rating) by 12 volts (the battery voltage), giving you a maximum current draw of 250 amps.
What Gauge Wire Size Should Be Used For Battery Cables?
Remember that choosing the correct wire gauge for your battery cable size is based on two factors: current and distance.
Now that you know how to calculate your current requirement, you just need to figure out how far you need to run your cables. Remember, shorter is always better. Less cable means less weight and lower cost.
After you know both the cable length and the current, you can quickly look up what size battery cable to use. The wire sizing chart below helps you choose the correct wire gauge for your RV batteries. From this table, it’s easy to see that lower current and shorter distance allow for smaller cables.
You’ll also see that as current or distance increases so does the required cable thickness. Reach out to an expert if something becomes confusing; guessing which wire gauge is not the solution to your problem.
Picking The Correct Battery Cable Size
RV battery cables are a small but essential part of a complex and integral system in your RV. Choosing the wrong size battery cable can lead to extra cost, frustration, and potentially even a fire.
However, picking the correct battery cable size for your system doesn’t need to be stressful. Use the tips above or reach out to a Stevens Battery expert with any questions to help make your RV or Boat battery upgrade project a success!
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