Voltage Drop Calculator12V wire size calculator
Wire size and voltage drop for 12 V and 24 V DC circuits in vans, RVs, boats and small solar systems. Enter the one-way length from the battery; the return wire is counted for you.
Result
Copper, 75 °C basis| Wire size | 12 AWG |
| Area | 3.31 mm² |
| Voltage drop | 0.36 V |
| Percent drop | 2.96 % |
| At the load | 11.64 V |
Within 3% target
12 AWG. Drop 0.36 volts, 2.96 percent. Within 3% target.
How this is worked out
V = 2 × K × I × L ÷ CM
2 × 12.9 × 6 A × 15 ft ÷ 6,530 CM = 0.36 V
0.36 V ÷ 12 V × 100 = 2.96 %. At the load: 12 V − 0.36 V = 11.64 V.
- L is the one-way length; the 2 counts the return conductor.
- K is 12.9 for copper and 21.2 for aluminum, in ohm·CM per foot on a 75 °C basis.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
What voltage drop does on a 12 V system
A few tenths of a volt matter at 12 V. 0.36 V is 2.96 % of a 12 V battery, and the loads in a van or an RV notice it:
- Compressor fridges measure the voltage at the fridge. A long, thin feed can make the fridge switch off early or fail to start, even with a charged battery.
- Inverters draw the most current, so their cables lose the most. Under a heavy load the drop can trip the inverter’s low-voltage alarm or shutdown.
- Lights can dim or flicker at the end of a long run.
- Pumps and fans run slower and weaker.
Worked example: a fridge 15 ft from the battery
A 72 W compressor fridge draws 6 A from a 12 V battery. The cable runs 15 ft one way, so the current travels 30 ft there and back. The target is 3%.
| Wire | Drop | At the fridge | Against 3% |
|---|---|---|---|
| 14 AWG | 0.57 V (4.71 %) | 11.43 V | Over |
| 12 AWG | 0.36 V (2.96 %) | 11.64 V | Within |
So 12 AWG is the smallest size that keeps this run within 3%. The calculator above starts with these numbers. To see the smaller wire fail, check 14 AWG for this run.
In metric: with the fridge 5 m from the battery, 4 mm² is the smallest size within 3% (0.32 V, 2.68 %), and 2.5 mm² loses 0.51 V (4.29 %). Choose mm² and metres in the calculator to work in metric.
The drop is 2 × K × I × L ÷ CM with K = 12.9 for copper: the 2 is the return wire. The formula guide works through each step.
24 V halves the current
The same 72 W fridge on a 24 V battery draws 3 A. Over the same 15 ft, 14 AWG is enough: 0.28 V of drop, 1.18 %. This is one reason larger vans, trucks and boats use 24 V.
AWG, B&S and mm²
US and Canadian cable is sized in AWG, UK and European cable in mm², and Australian auto cable is often sold in B&S. B&S uses the same gauge numbers as AWG: 6 B&S is 6 AWG, about 13.3 mm². Automotive cable is often sold by a nominal size, such as “6mm” or “6 B&S”, that is not its copper area. Check the real area on the cable or its datasheet, and choose the largest size here that is not bigger than that area.
Campervans and caravans
The long runs in a campervan or caravan are usually from the leisure battery to the fridge, the inverter and the lights. UK cable is sold in mm², often written “6mm” for 6 mm². Choose mm² and metres above. Mains hook-up wiring is a 230 V circuit with its own rules and is not covered by this page.
4WD dual battery systems
In a dual battery setup the longest run is usually from the starting battery to the second battery or its DC-DC charger. Enter the charger’s maximum current as the load current and the cable length to it, one way. Anderson plugs and extension leads to a camper trailer add their own length to the run.
Boats
Marine DC wiring follows the ABYC E-11 standard, which sets its own voltage-drop limits for different circuits and its own rules for conductors. Use the calculator to check a drop, then check the run against E-11 before you wire a boat.
Fuses and ampacity
On long 12 V runs voltage drop usually sets the size, but the wire must also carry the current safely. Fit a fuse or breaker close to the battery, rated for the wire, and check the wire’s ampacity in your code book or standard. The ampacity guide explains which check governs, and the calculator’s optional ampacity field checks the value you enter.
Frequently asked questions
Do I enter the round-trip length or the one-way length?
The one-way length: the distance from the battery to the load along the cable route. The calculator doubles it for the return wire. If you measured both wires together, divide by two.
Why does a 12 V circuit need a bigger wire than a 120 V one?
For the same power a 12 V load draws ten times the current, and each volt lost is a ten times larger share of the supply. A 72 W fridge on 15 ft of 12 AWG loses 0.36 V at 12 V (2.96 %) but only 0.036 V at 120 V (0.03 %).
How do I measure a run from the battery?
Measure along the route the cable will take, from the battery or the fuse block or busbar that feeds the circuit, to the load, with bends and slack included. The calculator assumes the negative wire is the same length and size as the positive.
Can I work in mm² and metres?
Yes. Choose mm² and metres under Inputs. The sizes are the standard metric series from 0.75 to 300 mm², on the same 75 °C basis as the AWG sizes. The calculator remembers your choice.
Which drop target should I use?
The calculator starts at 3%, a common choice for 12 V circuits. Some builders choose a tighter target, such as 1% or 2%, for electronics that are sensitive to voltage. That is your choice, not a rule. Boats follow ABYC E-11, which sets its own limits.
Related: voltage drop calculator for AC and DC, how to calculate voltage drop, ampacity vs voltage drop, how this calculator works.