DC Voltage Drop Calculator
Calculate voltage drop for 12V, 24V, and 48V DC circuits — solar, battery banks, RVs, boats, and automotive wiring. Enter your current, distance, and wire size to see the drop in volts and percent, plus the minimum AWG that passes. For 120V/240V AC circuits, use our AC voltage drop calculator.
DC Voltage Drop Formula
DC voltage drop is calculated as: VD = 2 × K × I × L ÷ CM.The factor of 2 accounts for the round trip — current flows to the load on the positive conductor and returns on the negative, so both lengths of wire add resistance.
VD = voltage drop in volts
K = 12.9 for copper, 21.2 for aluminum (ohms per circular mil-foot)
I = current in amps
L = one-way distance in feet
CM = circular mil area of the wire (e.g. #10 AWG = 10,380 CM)
Worked Example: 12V Refrigerator Circuit
A 12V compressor fridge in an RV draws 10 amps and sits 20 feet (one-way) from the battery. On #10 AWG copper:
VD = 2 × 12.9 × 10 × 20 ÷ 10,380 = 0.497 V
0.497 V ÷ 12 V = 4.1% — fails the 3% limit
Stepping up to #8 AWG (16,510 CM):
VD = 2 × 12.9 × 10 × 20 ÷ 16,510 = 0.31 V
0.31 V ÷ 12 V = 2.6% — passes
Note that #10 AWG has plenty of ampacity for 10 amps — it fails purely on voltage drop. This is the defining problem of low-voltage wiring: ampacity sets the floor, but voltage drop almost always demands the bigger wire.
12V Wire Size Chart (3% Voltage Drop, Copper)
Minimum copper wire gauge to keep voltage drop at or below 3% (0.36 V) on a 12-volt system. Distances are one-way from source to load; the formula already accounts for the return conductor.
| Current | 10 ft | 15 ft | 20 ft | 25 ft |
|---|---|---|---|---|
| 5 A | #14 | #12 | #10 | #10 |
| 10 A | #10 | #8 | #8 | #6 |
| 15 A | #8 | #8 | #6 | #4 |
| 20 A | #8 | #6 | #4 | #4 |
| 30 A | #6 | #4 | #2 | #2 |
Every value is computed with VD = 2 × 12.9 × I × L ÷ CM, choosing the smallest gauge whose drop stays at or under 0.36 V. Example check: 15 A at 15 ft on #8 AWG = 2 × 12.9 × 15 × 15 ÷ 16,510 = 0.35 V — just inside the limit. Always verify the gauge also meets ampacity and fusing requirements for your installation.
Why Low-Voltage Systems Need Bigger Wire
Two effects gang up on 12V, 24V, and 48V systems, and both push you toward heavier wire than an AC electrician would ever expect for the same load.
1. Far less headroom. A 3% drop budget on a 120V circuit is 3.6 volts — you can lose almost 4 volts and still be fine. On a 12V system, 3% is just 0.36 volts. Ten times less margin means the same length of the same wire eats through your budget ten times faster.
2. More amps for the same power. Power = volts × amps, so delivering a fixed wattage at low voltage requires proportionally more current. A 240-watt load draws 2 amps at 120V but 20 amps at 12V. Since voltage drop scales directly with current, the low-voltage circuit drops ten times as many volts in identical wire — while having ten times less room to spare.
Combined, a 12V circuit effectively needs on the order of 100 times the “drop performance” of a 120V circuit carrying the same power over the same distance. This is why RV, marine, and off-grid installers routinely pull #4, #2, or even 4/0 cable for runs that would be #12 or #14 in household AC wiring, and why doubling system voltage (12V → 24V or 48V) is the cheapest way to shrink copper costs: it halves the current and doubles the drop budget at the same time.
Solar Panel Wire Sizing
A solar installation has several distinct DC wire runs, and they do not all deserve the same treatment:
- Panels to charge controller.PV strings operate at the array's maximum power voltage (Vmp), often 18–150V depending on how many panels are in series. Higher string voltage means lower current and gentler voltage drop, so this run is usually the easy one. Size it at Vmp and the array's operating current.
- Charge controller to battery — the critical segment. This run operates at the low battery voltage (12/24/48V) and carries the full charge current. Target 1–2% drop here, not 3%: the controller regulates absorption and float voltages by what it senses at its own terminals, so every tenth of a volt lost in this cable becomes a charging error that can chronically undercharge the bank.
- Battery to inverter or DC loads.Inverter feeds can carry very high current (a 2,000W inverter on 12V pulls roughly 180–200 amps), so keep this run brutally short and thick.
Example: Controller to Battery
A 20-amp charge controller sits 8 feet (one-way) from a 12V battery bank, with a 1.5% target (0.18 V). On #8 AWG copper: VD = 2 × 12.9 × 20 × 8 ÷ 16,510 = 0.25 V = 2.1% — too much. On #6 AWG: VD = 2 × 12.9 × 20 × 8 ÷ 26,240 = 0.16 V = 1.3% — passes. Set the max drop field in the calculator above to 1.5 to size runs like this. For panel counts and battery bank capacity, see our solar system calculator.
Frequently Asked Questions
How do I calculate DC voltage drop?
Use the formula VD = 2 × K × I × L ÷ CM, where K is 12.9 for copper (21.2 for aluminum), I is the current in amps, L is the one-way distance in feet, and CM is the wire's circular mil area. The 2 accounts for the round trip — current flows out on the positive conductor and back on the negative. For example, 10 amps over 20 feet on #10 AWG copper: VD = 2 × 12.9 × 10 × 20 ÷ 10,380 = 0.50 volts, which is 4.1% of a 12V system.
What wire size do I need for 30 amps at 12 volts?
It depends on distance. To stay within a 3% drop (0.36V) on a 12V system with copper wire, 30 amps needs #6 AWG at 10 feet one-way, #4 AWG at 15 feet, and #2 AWG at 20 to 25 feet. Ampacity alone would allow #10 AWG for 30 amps, but voltage drop forces much larger wire at 12 volts — always size low-voltage wire for drop, not just ampacity.
Is voltage drop worse for DC than AC?
The physics is the same — resistance drops volts regardless of AC or DC. What makes DC systems feel worse is that they usually run at much lower voltage. A 3% limit on a 12V system is only 0.36 volts of headroom, while 3% at 120V is 3.6 volts — ten times more. And because power equals volts times amps, delivering the same wattage at 12V requires ten times the current of 120V, which multiplies the drop. That is why 12V circuits need dramatically thicker wire than equivalent 120V AC circuits.
Why is the distance doubled in the DC voltage drop formula?
Current must complete a full circuit: out to the load on the positive conductor and back to the source on the negative conductor. Both conductors have resistance, so the electrical path is twice the one-way distance. That is the 2 in VD = 2 × K × I × L ÷ CM. Always enter the one-way distance in this calculator — the doubling is handled for you.
What is an acceptable voltage drop for a 12 volt system?
3% is the common general-purpose limit (0.36V on a 12V system) for loads like lighting, pumps, and refrigerators. For critical circuits — solar charge controller to battery, battery monitoring, or sensitive electronics — aim for 1 to 2%, because even small drops distort charge voltage sensing and can leave batteries chronically undercharged. Some non-critical loads like resistive heaters can tolerate up to 10%.
Can I use this calculator for solar panel wiring?
Yes. Use it for every DC segment of a solar installation: panel strings to the charge controller (at the array's operating voltage, Vmp), controller to battery bank, and battery to loads or inverter. The controller-to-battery run is the most critical — it operates at the low battery voltage and carries full charge current, so target 1 to 2% drop there. For the AC output side of an inverter, use our AC voltage drop calculator instead.
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