📉 DC Voltage Drop Calculator
Enter the run length, current, wire gauge and system voltage to get the voltage drop, the voltage at the load and whether it's within limits.
Voltage drop
5.3%
0.64 V
Voltage at the load
11.36 V
Power lost in wire
12.8 W
Too high — over 3%. Use thicker wire, a shorter run, or a higher system voltage.
Drop = 2 × length × current × copper resistivity ÷ wire area (the 2 is the round trip out and back). Low-voltage DC is very sensitive to drop, which is why thick wire and short runs matter — and why higher system voltages (24/48 V) cut the current and the loss. NEC allows up to 3%; solar best practice targets ≤2% on the main run to preserve harvest. Assumes copper at ~room temperature. 🔒 In your browser.
How the dc voltage drop calculator works
Every wire has resistance, so some voltage is lost pushing current along it. For a DC run the drop is 2 × length × current × the wire's resistance per length (the 2 accounts for the current going out and coming back). The tool computes the drop in volts and as a percentage of the system voltage, the voltage left at the load, and the power wasted as heat — and flags whether the drop is within recommended limits.
Low-voltage DC is very sensitive to voltage drop, which is why solar and battery wiring uses thick cable and short runs. NEC allows up to 3%, but solar best practice targets 2% or less on the main array run to avoid losing harvest. If the drop is too high, use thicker wire, shorten the run, or step up to a higher system voltage (24 or 48 V), which carries the same power at lower current.
Frequently asked questions
How do I calculate DC voltage drop?
Drop = 2 × one-way length × current × the wire's resistance per metre (from copper resistivity ÷ cross-section area). The factor of 2 is the round-trip out and back. Divide by the system voltage for the percentage.
What is an acceptable voltage drop?
The NEC treats 3% as the informational limit for a branch circuit and 5% total. For solar, best practice is stricter — 2% or less on the main DC run — because any drop there is lost production. Aim for ≤2–3%.
Why is voltage drop worse on 12V systems?
The same power at a lower voltage means much higher current, and drop is proportional to current. So a 12 V system carrying 100 A drops far more over the same wire than a 48 V system carrying 25 A — which is why bigger systems use higher voltages.
How do I reduce voltage drop?
Use thicker wire (lower gauge number), shorten the run, reduce the current, or raise the system voltage. Thicker wire and higher voltage are the usual fixes for a long run between panels, controller and battery.
Does voltage drop waste energy?
Yes — the lost voltage becomes heat in the wire (power lost = drop × current). A few percent drop is a few percent of your energy gone as heat, plus the load runs on lower voltage, which is why keeping drop low matters in off-grid systems.