Voltage Drop Explained: the 3% Rule and Formula
Voltage drop is the voltage a wire loses to its own resistance between the source and the load. Keep it at or below 3% on a branch circuit and your equipment sees enough voltage to run correctly and efficiently.
What voltage drop actually is
No conductor is perfect. Copper and aluminum both have resistance, and when current flows through that resistance some voltage is consumed in the wire itself instead of reaching the load. That loss is voltage drop. The longer the run and the higher the current, the larger the drop. At the far end your light is dimmer, your motor runs hotter and your battery charger delivers less power than the nameplate promises.
The accepted target is to keep voltage drop at or below 3% on an individual branch circuit, and about 5% for the combined feeder-plus-branch path. The NEC mentions these figures in informational notes (210.19 and 215.2) as recommendations rather than hard requirements — but exceeding them wastes energy and can cause equipment to misbehave, so good practice treats 3% as the design limit.
The formula
For a single-phase circuit the drop in volts is:
VD = (2 × K × I × L) / CM
For a three-phase circuit the factor 2 becomes 1.732 (the square root of 3):
VD = (1.732 × K × I × L) / CM
In both, K is the resistivity constant in circular-mil·ohms per foot — 12.9 for copper and 21.2 for aluminum at about 75°C — I is the current in amps, L is the one-way length in feet, and CM is the conductor cross-section in circular mils. The percentage is simply VD divided by the source voltage:
%VD = VD / Vsource × 100
Worked example: 12 AWG, 20 A, 100 ft
Take 12 AWG copper (6,530 circular mils) carrying 20 amps over a 100-foot one-way run on a 120 V single-phase circuit:
VD = (2 × 12.9 × 20 × 100) / 6,530 = 51,600 / 6,530 = 7.90 V.
As a percentage that is 7.90 / 120 = 6.6% — more than double the 3% guideline. The voltage at the load is only about 112 volts. Step up to 10 AWG (10,380 cmil) and the drop falls to (2 × 12.9 × 20 × 100) / 10,380 = 4.97 V, or 4.1%. To get under 3% you would move to 8 AWG. The Voltage Drop calculator shows the volts, the percentage and the end-of-run voltage instantly.
The five levers
Only five things change voltage drop, and understanding them lets you fix any drop problem:
- Conductor size (CM): bigger cross-section, less drop. This is the usual fix — go up a gauge.
- Length (L): drop is directly proportional to the one-way distance. Halving the run halves the drop.
- Current (I): drop scales with load current. A lightly loaded circuit drops little.
- Metal (K): copper drops less than aluminum for the same gauge.
- Voltage (V): the same watt-load at 240 V draws half the current of 120 V, so the percentage drop falls dramatically. This is why long runs are often wired at the higher voltage.
Why 12 V and DC runs are so sensitive
Low-voltage DC systems — RV, van and solar 12 V circuits — suffer the worst voltage drop because the same power means very high current. One thousand watts is about 8 A at 120 V but roughly 83 A at 12 V. Since drop scales with current, a short 12 V run can lose a damaging percentage. That is why 12 V wiring uses surprisingly fat cable; see the 12V DC Wire Size calculator and the guide on 12V wiring for vans and RVs.
From drop limit to wire size
To turn the 3% rule into a gauge, rearrange the formula for the minimum circular mils: CM = (2 × K × I × L) / (V × %max). Pick the smallest standard AWG that meets or exceeds that number and also satisfies ampacity. The voltage-drop reference table gives the drop per gauge at 20 A over 100 ft so you can eyeball a starting point, and the Wire Resistance calculator lets you check the raw ohms of any run.
Why under-volting hurts equipment
Voltage drop is not just wasted energy; it changes how equipment behaves. Resistive loads such as heaters and incandescent lamps put out power in proportion to the square of the voltage, so a six percent drop means a heater delivers roughly twelve percent less heat and a lamp visibly dims. Motors are worse off: a motor starved of voltage draws more current to produce the same torque, which raises its temperature and shortens its life, and a motor that cannot start cleanly can stall and trip. Electronics and chargers compensate by drawing still more current at the lower voltage, which deepens the drop in a vicious circle. This is why holding the run within the three percent guideline is about reliability and longevity, not merely the power bill.
Where the loss actually goes
The voltage consumed in the conductor turns into heat in the wire. On a long, heavily loaded run that heat is real and measurable, and it is one reason ampacity and voltage drop are related concerns rather than separate ones. A conductor sized generously for voltage drop also runs cooler, ages more slowly and leaves headroom for the future. Conversely, a conductor pushed to its limits on both counts will be the hottest, most stressed part of the circuit. Thinking of voltage drop as heat in the wire makes it intuitive why a bigger conductor, a shorter run or a higher voltage all help: each one reduces the current density or the resistance that produces the heat.
Two-percent and combined budgets
The three percent figure is a branch-circuit guideline, but many designers split a tighter budget across the system. A common approach allows about two percent on the feeder and three percent on the branch for roughly five percent total from the service to the load. On critical runs, on solar and battery cabling, and on long low-voltage circuits, builders frequently tighten the branch target to two percent or less because the consequences of under-volting are more severe. The calculator lets you set any target, so you can see exactly how the required gauge changes as you move from three percent to two percent, which is often the difference of a single gauge.
Quick mental checks
A few rules of thumb help you sanity-check any result. Doubling the length doubles the drop; halving the current halves it; going up three gauge sizes roughly doubles the cross-section and therefore roughly halves the drop. Switching from one hundred twenty to two hundred forty volts for the same power roughly quarters the percentage drop, because the current halves and the source voltage doubles. If a calculated result violates one of these proportions, you have probably entered a length, a current or a voltage incorrectly. Carrying these proportions in your head lets you estimate a gauge before you ever open the calculator and catch typos the moment a number looks wrong.
Voltage-drop results are planning estimates. Final conductor sizing must comply with the NEC edition adopted by your jurisdiction and be confirmed by a licensed electrician.