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.
Thank you.
Can I use same rating wire for 12V 1A & 6V 2A?
Or Can I use same wire, if wattage is same but voltage is different?
Hello Yogesh, the rating wire is connected with ampacity. That means amps, not watts. You have to check just the amps; volts and watts are irrelevant. 1A wire is different than 2A wire. Hope this helps.
As the voltage decreases, the amperage increases to give the same wattage output. You need to do the calculations as given to determine the cable sizes required.
I have a 12 v “cooler” for my Jeep it has a 12 v sump pump pumping water thru a small radiator with a 12 v 120 watt radiator fan. I am using a 14 gauge wire I noticed it was getting hot. I believe I need to go up a size the distance is less than ten feet. Would I be correct I’m rewiring it using the next gauge up. Thanks
Hi there. Alright, a 120W 12V radiator fan draws 10 amps when operating at 100% output. 14 AWG wire has an ampacity of 20A at median temperature. That means you can – applying the NEC 80% rule here – put at most 16 amps on that wire. So, in theory, 14 AWG wire is adequately sized. Now, if the wire gets hot, that’s not a good sign. It might be best to switch the 14 AWG wire with 12 AWG wire just to be safe. Hope this helps.
OK, I think I got it, but can you confirm for me: I have an RV water pump I’m using for a drip irrigation system (12V, 7.5 AMP). Battery will be approx. 75 feet away from the pump due to location of water tote, and best sun exposure to solar charge the battery. What gauge of wire would I need to connect the water pump to the battery?
Hello John, alright, you need 7.5 amps. Accounting for NEC 80% rule, you need a wire with an ampacity of 9.375A. Accounting for 75 feet voltage drop, you get up to 14 AWG wire (it has 20A ampacity). Hope this helps.
Hi. Thanks for the information.
I have the following scenario.
I need to connect a 12V 90Amp deep cycle battery to a 62lb trolling motor with a 58amp max draw for a 10ft distance. This is pre-wired with a 10 awg wire and based on the chart, the recommendation would be 4 awg approx. what would be the effects of keep using the original 10 awg instead of replacing it by the recommended.
Hello Daniel. Well, 10 AWG wire has a median temperature ampacity of 35A and can carry, accounting for NEC 80% rule, up to 28 amps. 4 AWG wire has an ampacity of 85A and can carry up to 68 amps. If you have a 58 amp draw, you would kind of need a wire that can handle that current, and 10 AWG doesn’t seem sufficient. It’s interesting to see that 10 AWG wire was added here. Maybe there are 2 10 AWG wires? they would have a combined ampacity of 70A and could carry 56 amps; that would be closer to what you actually need.
Well, in practice, if you put too much current on too small a wire, the wire can catch on fire. That’s just the 101 of how current works and we have NEC code to prevent this.
Thanks for taking your time to reply. Two 10 awg wires (black + red) are preinstalled, but I will follow your recommendation and use a 4 AWG to avoid issues. Thanks!
Hello. I need to run 15 feet of cable from the batteries to the 3000w inverter in the semi truck. What size of cable do I need?
Hi Ben, if that is a 12V battery, we first need to calculate the amp draw like this: 3000W / 12V = 250 Amps. Alright, you need a wire that can handle a little bit more than 250 amps (to account for small voltage drop due to 15 feet cable length). 250A is quite a lot; AWG wires can’t handle that, you will need MCM or kcmil wires.
If you are using copper wires, you can consult this copper kcmil wire chart. You can see that 250 kcmil copper wire has a median ampacity of 255A. This is perfect for your setup.
If you are using aluminum wires, you can consult this aluminum kcmil wire chart. You can see that 300 kcmil aluminum wire has a median ampacity of 255A as well.
In short, you need a 250 kcmil copper or 300 kcmil aluminum wires. Hope this helps.
if can go with a higher voltage then that will lower the amps and allow you to use a smaller wire size. watts divided by voltage gives you amps
Hi Mac, this line of thinking is correct; higher voltage equals lower amps and smaller wires.
Great information Can I run a 20′ run with 6 LED lights At 3 W a piece with 18 gauge wire
According to the AWG wire gauge chart here, the 18 AWG copper wire can handle 14 amps. You have 6x3W = 18W. At 12V, the max. current will be 1.5 amps. So, the 18 AWG wire will be more than enough to handle this amp load even if you have a 20′ run.
So if I am running two 20 watt led lights, total of 40W in a 12v system. that would equal 3.333 amps and with the NEC rule would come to 4.17a. If I need to run 25′ I would say 16 AWG is good? I assume 18 AWG would be to small for the distance.
Hi Fred 18 AWG copper wire has a median ampacity of 14 amps (here is the full AWG copper wire ampacity chart). Even if you are run the wire for 25′, the 18 AWG will be more than enough. 16 AWG would be overkill. Hope this helps.
If I am running a wire from my starter battery (12V) approx 12′ to a 50A DC-DC charger to charge auxilliary batteries, what size wire would that need to be?
Also about 4′ from the 50A DC-DC charger to (2) 140Ah AGM batteries (total 280Ah), what would that wire size need to be? Thanks
Hi Marti, for a 50 amp current at 12 feet distance, you would need 6 AWG copper wire. It has 65A ampacity; this is more than enough for a 50A current. The same is true for your 2nd question; you might be able to facilitate that amp draw with 8 AWG copper wire but 6 AWG copper wire is the safe bet. Hope this helps.
hi
im wiring a camper van next weekend. i want to wire a switch panel to the battery then have my appliances running from there
fridge around 5amp
diesel heater 10amp (on start up)
led lights 2amp
water pump 6amp
total 23amp
what size cable will i need ? it will be 10ft to the switch panel then 6ft back to the appliances
will i need to upgrade the cable in the chinese switch panel bought off amazon ?
Hi Adam, summing up these amps we get a total of 46 amps, and a short distance from the switch panel. So, you would need a wire that can handle about 50 amps or more. If you check this copper wire gauge ampacity chart, you can see that 8 AWG copper wire has 50A ampacity and 6 AWG copper wire has 65A ampacity. The 8 AWG should be OK in theory, for added safety, you can go with a thicker 6 AWG wire.
About the switch panel: You have to check how many amps it allows. If it’s less than 50 amps, you should upgrade it. Hope this helps.
Can I run my 120 Watts 12V, 10A Tire Inflator using my 360 watts 30A, DC 12V Switching power supply( AC input 110/220V 50/60Hz
Hi Brix, yes. The tire inflator top wattages is 120W, while the power supply can provide 360W. That’s more than enough.