12V Voltage Drop Calculator + Chart (0-200 Feet Away)

12V voltage is massively prone to voltage drop. If you run a 12V wire for 10, 20, 50, 100, or even 200 feet, the voltage drop will be significant (since 12V is one of the lowest voltages; compared to 120V, or 220V, for example). Calculating the 12V voltage drop is not exactly easy, and you do need this calculation if you want to adequately size a 12V wire.

Here is the deal with the 12V voltage drop:

It has a significant impact. A 10-volt voltage drop is not all that significant in a 120V circuit; you end up with a 110V circuit. In a 12V DC battery current, a 10V drop will result in only a 2V voltage. That’s why we have to use much thicker wires (low AWGs, even kcmil copper wires) if we want to use a significant 12V battery current (50 amps, 100 amps, 200 amps) some distance away.

To help you figure this 12V DC battery voltage drop out, we will first explain how to calculate a 12V voltage drop, but then you will get these two very easy-to-use resources:

• Two 12V Voltage Drop Calculators. In the 1st calculator, you insert the wire gauge, specify copper or aluminum wire, 12V wire length, amps, and the calculator will tell you the voltage drop in the circuit (V), voltage drop percentage (%), and the voltage at the end of the wire. The 2nd calculator is even more useful; you specify 12V wire length, amps, and allowable voltage drop (2%, 3%, 5%, 10%). The 2nd calculator will tell you which gauge wire you should use to facilitate that current (copper wire in AWG or kcmil).
• Two 12V Voltage Drop Charts. We have calculated the 12V voltage drops at 0-200 feet distances for 0-200 amp services and used the NEC 310.316 and NEC Chapter 9 Table 8 to determine the copper wire size (in AWG or kcmil) you need for that circuit, based on 3% critical voltage drop (1st chart) and on 10% non-critical voltage drop (2nd chart). All the results are summarized in a neat 12V voltage drop and 12V wire size chart further on.

Note: If the manual calculation we are going to show seems a bit complex, don’t worry. The 2 calculators below will make all these calculations very easy. For summarized 12V wire sizes based on the voltage drop at a 0-100 feet distance, you can just check the chart near the end.

Alright, here is a typical example of what we are dealing with:

Let’s say you are using a 12V DC battery current and want to get 50 amps at the end of a 20 feet wire using 6 AWG copper wire. What is the voltage drop over this distance? Is 6 AWG copper wire the adequate wire size for this 50 amp current? Let’s first learn how to calculate this step-by-step:

How To Calculate 12V Voltage Drop And Determine Wire Gauge

For all 12V DC currents, we can use this 12V DC voltage drop formula:

V_D = 2 × L × K × I / CM

We see that voltage drop (V_D) depends on the length of the wire (L), specific wire resistivity (K), and current (I). From our example, we know that the wire length is 20 feet, and the current is 50 amps.

What we need to figure out is the specific wire resistivity (K). We know that we have a 6 AWG copper wire. The specific wire resistivities and cross-sections in terms of circular mils (CM) can be found in NEC Chapter 9 Table 8. We are going to calculate a 12V voltage drop for copper wires; these have a specific resistivity of 12.9 ohms per circular mil foot at 75°C (167°F).

Our example: We have a 12V battery, want 50 amps, we have a 6 AWG copper wire (that has a 26,260 CM cross-section, according to Table 8), and a 3% allowable voltage drop. Can we use this setup for an electric device that is 20 feet away from the source? Let’s do some math:

We know that the allowable voltage drop is 0.36V because 12V × 0.03 = 0.36V. Now, we express the wire length (L) from the equation above and input the numbers like this:

Max. Wire Length L (3% Vd) = V_D × CM / (2 × K × I) = 0.36V × 26,260 / (2 × 12.9 × 50A) = 7.3 Feet

We see that the 6 AWG copper wire is too thin for this service 20 feet away. At most, we could produce a 50 amp 12V service with a 3% allowable voltage drop 7.3 feet away.

If we are not so strict and we use a 10% non-crucial voltage drop (that would be a 1.20V drop), we get this:

Max. Wire Length L (10% Vd)  = 1.20V × 26,260 / (2 × 12.9 × 50A) = 24.4 Feet

That means that we could use 6 AWG copper wire if we have a 10% allowable voltage drop to deliver 50 amps at a distance of 20 feet (max. distance = 24.4 feet). For 12V voltage drops for copper and aluminum wires, you can use the 1st Calculator further on.

Now, I see that 6 AWG copper wire is not sufficient for a 3% allowable voltage drop over 20 feet for a 50 amp circuit. The next question comes quite naturally; which gauge wire we should be used to facilitate this current? 4 AWG? 3 AWG? 2 AWG?

To calculate that, we have to find the right AWG or kcmil copper wire. All AWG and kcmil wires have a specific thickness or number of circular mils. So, we are going to calculate how many CM the wire should have, and then match that with AWG or kcmil wire. Here is how we express the CM from the equation:

CM = 2 × L × K × I / V_D

Alright, let’s insert out 20 feet (L), 12.9 ohms resistance (K), 50 amp current (I), and 0.36V allowable voltage drop (this is 3% for 12V circuit) like this:

Wire Size (In CM) = 2 × 20 × 12.9 × 50 / 0.36 = 71,667 CM

We see that we need a wire with a cross-section of at least 71,667 CM. We consult Table 8 or this CM to AWG chart, and we see this (screenshot from CM to AWG chart):

To facilitate our 50 amp 12V current 20 feet away with a 3% allowable voltage drop, we need a copper wire with at least 71,667 circular mils. That means that:

• We can’t use 8 AWG, 7 AWG, 6 AWG, 5 AWG, 4 AWG, 3 AWG, or 2 AWG copper wires because they have less than 71,667 CM.
• We can use 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG, or kcmil copper wires because they have more than 71,667 CM.
• In practice, we will use the thinnest wire that can facilitate our current; in this case, this is 1 AWG copper wire with 83,690 CM.

These calculations and cross-referencing can be quite complex. That’s why the 2nd Calculator will help you calculate the wire size automatically. Based on this 2nd Calculator, we have also created two 12V voltage drop charts for 3% and 10% allowable voltage drops.

Let’s now look at the 1st Calculator, where you can calculate the voltage drop. For 12V wire selection, use the 2nd Calculator and corresponding 12V voltage charts:

12V Voltage Drop Calculator (1st Calculator)

Here is how this 1st calculator works:

Let’s use our example. We have a 12V DC circuit, 20 feet long 6 AWG copper wire, and are using a 50 amp current. What is the voltage drop? Just slide the 1st wire length slider to ’20’, choose the ‘6 AWG copper wire’, and slide the 3rd amps slider to ’50’. We get the result:

For this circuit, the voltage drop is 1.02V. In the 12V circuit, this is an 8.50% voltage drop, and we can expect for the whole circuit to have a 10.98V voltage. These are the same results we get with all that complicated manual calculations.

Now, let’s look at how we can determine the wire size for a 12V DC circuit at some distance (accounting for voltage loss):

12V Wire Size Accounted For Voltage Loss Calculator (2nd Calculator)

Here we usually have to consult what is the allowable voltage drop percentage. Usually, we use a 3% acceptable 12V voltage drop for panel feeders, electronics, and specialized lights (voltage can drop by 0.36V, minimum allowable voltage here is 11.64V). 10% allowable voltage drop is used for general lighting and general appliances (voltage can drop by 1.20V, minimum allowable voltage here is 10.80V).

Example how to use this calculator: Let’s say we have a simple LED light that requires a 30 amp current. We want to power it via a 12V DC battery that is 50 feet away. What size wire do we need if we account for a 10% acceptable voltage drop? Just slide the 1st wire length calculator to ’50, the 2nd amps calculator to ’30’, and the 3rd acceptable voltage drop percentage slider to ’10’.

We get this result: For this setup, we will need a 4 AWG copper wire. This wire has an 85A ampacity; it can handle 85 amps. That means that, to keep voltage drop under 10%, we need to use quite a thick wire that can handle 85 amps for 30 amp current.

This seems overkill at first, but 4 AWG wire has a lot less resistivity than thinner wires, and we need that very low resistivity to say above the minimally acceptable 10% voltage drop voltage of 10.80V.

To help you out, we have calculated the 12V voltage drop wire sizes needed at 3% and 10% allowable voltage drops. Here are both the 12V voltage drop charts you can freely use:

12V Voltage Drop Chart (Wire Sizes At 3% Allowable Drop)

As you can see, with a low 3% allowable voltage drop, we need thick wires quickly. If we have a 12V current up to 50 feet away from the source, we have to use:

• 4 AWG copper wire for 10 amp current.
• 2 AWG copper wire for 15 amp current.
• 1 AWG copper wire for 20 amp current.
• 2/0 AWG copper wire for 30 amp current.
• 3/0 AWG copper wire for 40 amp current.
• 4/0 AWG copper wire for 50 amp current.

For 100 amp, 150 amp, and 200 amp 12V currents 50 feet away, we already need kcmil wires (400 kcmil, 600 kcmil, and 750 kcmil copper wires, respectively).

Let’s look at a looser 10% acceptable voltage drop chart as well:

12V Voltage Drop Chart (Wire Sizes At 10% Allowable Drop)

We see that for a 10% allowable 12V voltage drop, we can use thinner wires than for a 3% allowable voltage drop.

For example, if we have a 12V current 50 feet away and we allow for a 10% voltage drop, we can use:

• 8 AWG copper wire for 10 amp current.
• 8 AWG copper wire for 15 amp current.
• 6 AWG copper wire for 20 amp current.
• 4 AWG copper wire for 30 amp current.
• 3 AWG copper wire for 40 amp current.
• 2 AWG copper wire for 50 amp current.

We hope that these charts and calculators illustrate well how to calculate a 12V voltage drop. If you have any question or would like for us to do some math for you, you can use the comment section below, and we will help you out.