12V circuit is a low-voltage electric circuit. It’s primarily used in most batteries. To produce larger amounts of electric power (wattage), we usually need quite a lot of amps. That means we need relatively big 12V cable sizes.
Now, how do you figure out what size wire you need for a 12V circuit?
Example: Let’s say we want to connect a 200W device to a 12V battery. That means we have to use a 12V wire size that can handle at least 16.67 amps (200W/12V = 16.67A). Accounting for the 80% NEC rule (we will explain this later on), you need a wire with at least 13.34A ampacity. In this case, we will need a 18 AWG wire size because it has a 14A ampacity.
Further on, if the battery is located some distance from the device (say 20 feet, 50 feet, or even 80 feet away), we need to account for voltage drop (big impact). This increases the amps we need which, in turn, increases the 12V wire size (we need more ampacity). We might have to use 14 AWG wire (20A), 12 AWG wire (25A), or even 10 AWG wire (35A).
We have to be quite careful when sizing 12V wires. If we choose a wire with too low an ampacity, the circuit can go up in flames (as well as the battery).
To not get the size of the 12V cable wrong, you can use two key resources further on:
- 12V Cable Size Calculator. This calculator estimates the minimum ampacity a wire should have. You just input the total wattage of the device you want to run on a 12V wire, and you will get the min. wire amps (accounted for the 80% NEC rule as well).
- 12V Wire Size Chart. This wire chart tells you what 12V wire size to choose if the device you want to run with the 12V voltage is some distance away (from 15 feet to 90 feet).
You can use both of these tools to adequately size a 12V wire. Let’s first look at the 12V cable size calculator before we proceed to the all-encompassing 12V wire size chart:
How To Calculate Needed 12V Wire Size (Theoretically)?
We calculate a theoretical 12V wire size in two steps:
- Calculate the amps (based on wattage). First of all, we have to figure out how many amps will run in a 12V circuit if we run the device at 100% wattage. That means we take the max. wattage and divide it by 12 volts to get the amps. We use this basic electric power equation:
P (Watts) = I (Amps) × V (Volts)
Now, we have to express the electric current (I, measured in amps), and plug in ’12V’ because we have a 12-volt circuit:
I (Amps) = P (Watts) / 12V
With this formula, we can calculate how many amps are likely to run in the 12V circuit. Example: If we want to run a 150W device, we will need I = 150W / 12V = 12.5 Amps.
- On top of the calculated amps, we have to apply the 80% National Electric Code (NEC) rule. This states that the calculated amps should represent at most 80% of the ampacity of the wire we use. This is a safety measure; choosing a bigger wire will prevent the 12V circuit from catching on fire. To account for this rule, we simply multiply the calculated amps by a 1.25 factor like this:
Min. Ampacity = Calculated Amps × 1.25
In our 150W case, this results in:
Min. Ampacity (150W) = 12.5A × 1.25 = 15.63A
That means we need a wire size with an ampacity of 15.63A or higher. To choose the correct AWG wire size for a 12-volt circuit, consult the complete AWG wire size chart here.
From this chart, we see that 18 AWG wire has an ampacity of 14A (copper wire, at average 75°C). This wire is too small in our case; if you choose the 18 AWG wire, you will likely fry the circuit.
The correct wire size for a 150W 12V circuit is the 16 AWG with 17A ampacity. This 17A ampacity is bigger than the minimum required 15.63A ampacity we calculated.
Using this calculation and consulting the AWG wire size chart you can adequately choose a 12V wire size for devices that are 0 feet away (this is a theoretical calculator, for practical use, you have to consult the 12V wire size amp ampacity chart you find further on).
To help you out with this calculator, we have created a calculator that does all of this automatically (including accounting for the 80% NEC rule):
12V Cable Size Calculator (At 0 Feet Distance)
Here you simply input the max. wattage of the device you want to run with a 12V battery, and you will get the minimum ampacity the circuit wire should have:
Here is how to use this calculator:
You can use our example for a 150W 12V device. Slide the wattage slider to ‘150’ and you will get the calculated minimum required ampacity: 15.63A.
Based on this min. ampacity, you can pick the correct size wire for this 12V circuit by consulting the AWG wire size chart.
Accounting For Voltage Drop (12V Wire Sizes Used In Practice)
Now, this is how you theoretically calculate the required size of 12V wire for any device. However, in practice, we need bigger (thicker) wires.
Why is that?
Because we have to account for voltage drop. All wires are at least some feet long (10 ft, 20 ft, 30 ft, and so on). When the electricity is running through this wire, the voltage (that was at first 12V) drops to 10V, 8V, 6V, and so on. In a 12V system, the voltage drop should not exceed 3% per foot of wire length.
If the voltage drops and we still want to keep the max. wattage our device needs, we have only one choice (according to the electric power equation):
Increase the amps.
The circuit will increase the amps by itself when it detects the voltage drop. This happens automatically. What you need to do is to recognize that your circuit needs more amps.
Example: If you theoretically calculate that your 12V device requires 15A of current (and account for 80% NEC rule), you theoretically use a 14 AWG wire with 20A ampacity.
Practically, however, you have a 25 ft long wire. That will reduce the voltage from 12V to let’s say 8V, and you now require 22.5 amps to deliver the same amount of power (wattage). That also means that a 14 AWG wire will not be enough. Instead, you will have to use a 10 AWG wire with 35A ampacity.
Voltage drop calculation is quite complex. Nobody really does it by hand. Instead, electricians consult the 12V wire size chart.
This chart tells you what gauge wire you have to use for a 12V circuit that requires certain amount of amps at a certain distance. The easiest to explain this is just to look at the 12-volt wire size chart here:
12V Wire Size Chart
(scroll to right, and down)
|Length (Feet):||5 Amp 12V Wire (60W):||10 Amp 12V Wire (120W):||15 Amp 12V Wire (180W):||20 Amp 12V Wire (240W):||25 Amp 12V Wire (300W):||30 Amp 12V Wire (360W):||40 Amp 12V Wire (480W):||50 Amp 12V Wire (600W):||60 Amp 12V Wire (720W):|
|15 Ft||16 AWG||12 AWG||10 AWG||10 AWG||8 AWG||8 AWG||6 AWG||6 AWG||4 AWG|
|20 Ft||14 AWG||12 AWG||10 AWG||8 AWG||8 AWG||6 AWG||6 AWG||4 AWG||4 AWG|
|25 Ft||14 AWG||10 AWG||8 AWG||8 AWG||6 AWG||6 AWG||4 AWG||4 AWG||2 AWG|
|30 Ft||12 AWG||10 AWG||8 AWG||6 AWG||6 AWG||4 AWG||4 AWG||2 AWG||2 AWG|
|40 Ft||12 AWG||8 AWG||6 AWG||6 AWG||4 AWG||4 AWG||2 AWG||2 AWG||1 AWG|
|50 Ft||10 AWG||8 AWG||6 AWG||4 AWG||4 AWG||2 AWG||2 AWG||1 AWG||1/0 AWG|
|60 Ft||10 AWG||6 AWG||6 AWG||4 AWG||2 AWG||2 AWG||1 AWG||1/0 AWG||2/0 AWG|
|70 Ft||10 AWG||6 AWG||4 AWG||2 AWG||2 AWG||2 AWG||1/0 AWG||2/0 AWG||2/0 AWG|
|80 Ft||8 AWG||6 AWG||4 AWG||2 AWG||2 AWG||1 AWG||1/0 AWG||2/0 AWG||3/0 AWG|
|90 Ft||8 AWG||4 AWG||4 AWG||2 AWG||1 AWG||1/0 AWG||2/0 AWG||3/0 AWG||3/0 AWG|
Here is how you can read the 12V wire size table:
Let’s say we have a 300W device we want to run with a 12V battery. The distance between the device and the battery (wire length basically) is 30 feet. What AWG gauge wire do we need for this 12V circuit?
You just check the chart and see that you would need a 6 AWG wire. 6 AWG wire has an ampacity of 65A; that’s quite a lot and gives you an idea of what huge a factor the voltage drop is in 12V circuits.
You can use this chart to pick the correctly sized wire for any 12V circuit.
Note: As a general rule, if you are in doubt about the 12V wire size, go for a thicker wire. Thicker wire has higher ampacity and will much less likely catch flames than a thinner wire.
Hopefully, now the choice of 12V wire sizes is a bit clearer. If you need any help, you can use the comment section below and we’ll try to help you out.