Free voltage drop calculator
Size your wire run with confidence — see the volts lost, the voltage reaching your load, and the NEC percentage drop, updated live, as you type.
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Electrical calculations are for reference only. Always consult a licensed electrician for wiring work. Follow applicable electrical codes.
Results are estimates. Consult a professional.
How the voltage drop calculator works
Voltage drop is the voltage a wire loses to its own resistance on the way from the source to the load. Every conductor has resistance, and the longer the run and the higher the current, the more voltage disappears as heat before it reaches the equipment. This voltage drop calculator estimates that loss using the National Electrical Code (NEC) Chapter 9 method: you enter the wire size (AWG), the conductor material, the one-way distance, the load current, and the source voltage, and it returns the volts lost, the voltage that actually arrives at the load, and the drop as a percentage you can check against the NEC recommendation.
The NEC voltage drop formula explained
Each term in the formula maps onto a physical reason a wire loses voltage. Once you see what each one does, you can predict which way the drop moves before you run a single number.
K — the resistivity constant
K is how much resistance a conductor material offers per circular mil of area per foot of length. Copper is the better conductor at roughly 12.9 Ω·cmil/ft; aluminum is worse at roughly 21.2, which is why an aluminum wire of the same size always drops more voltage than copper. K bundles the material's resistivity into one constant so the formula stays simple.
CM — circular mils from the AWG size
Circular mils (CM) is the conductor's cross-sectional area: the diameter in mils (thousandths of an inch) squared. A bigger conductor has more circular mils, more room for current, and therefore less resistance and less drop. Because the American Wire Gauge (AWG) scale runs backwards, a smaller AWG number means a thicker wire — 10 AWG is fatter than 12 AWG. The calculator looks up the circular mils for the AWG you pick.
L and I — length and current
Length (L) is the one-way distance from the source to the load, not the round-trip total — the formula's leading 2 already doubles it to count both conductors. Current (I) is the load in amps. Voltage drop is directly proportional to both: double the run or double the current and you double the drop. That linear relationship is why long runs and heavy loads are where voltage drop bites.
AWG to circular mils reference table
Voltage drop depends on the conductor's circular mils, and that value is fixed by its AWG size. These are the copper building-wire sizes this calculator supports, straight from NEC Chapter 9, Table 8. Notice how each three-gauge step roughly doubles the circular mils — and roughly halves the voltage drop.
| Wire size (AWG) | Circular mils (CM) | Typical use |
|---|---|---|
| 14 | 4,110 | 15 A lighting circuits |
| 12 | 6,530 | 20 A general receptacles |
| 10 | 10,380 | 30 A water heaters, dryers |
| 8 | 16,510 | 40–50 A ranges, feeders |
| 6 | 26,240 | 55 A feeders, sub-panels |
| 4 | 41,740 | Large feeders |
| 2 | 66,360 | Service / large feeders |
| 1/0 | 105,600 | Service entrances |
| 2/0 | 133,100 | Service entrances |
| 4/0 | 211,600 | Large service runs |
Circular-mil values from NEC Chapter 9, Table 8 (Conductor Properties). Typical-use column is general guidance only; the actual ampacity for a circuit depends on insulation type, temperature, and conduit fill.
A worked voltage drop example
Marcus is running a dedicated 120 V circuit out to a workshop 100 ft away. He plans to pull 12 AWG copper and expects a steady 15 A load. Before he buys the wire, he wants to know whether the voltage drop stays inside the NEC limit.
Step 1 — Look up the constants
Copper gives K = 12.9. From the table above, 12 AWG has CM = 6,530 circular mils. The run is single-phase, so the multiplier is 2.
Step 2 — Plug into the formula
V_drop = (2 × 12.9 × 15 × 100) ÷ 6,530 = 38,700 ÷ 6,530 = 5.93 V.
Step 3 — Convert to a percentage and check the limit
%drop = (5.93 ÷ 120) × 100 = 4.94%. The voltage that actually reaches the workshop is 120 − 5.93 = 114.07 V.
The NEC 3% and 5% voltage drop recommendation
The NEC does not flatly prohibit voltage drop, but its informational notes recommend keeping it within limits so equipment runs the way it was designed to. The headline figures are 3% and 5%.
- ≤ 3% on a branch circuit — the wiring from the last panel out to the receptacles, lights, or motor it serves.
- ≤ 3% on a feeder — the wiring from the service to a sub-panel.
- ≤ 5% total — the combined drop across the feeder and the branch circuit added together, measured from the service to the final outlet.
These percentages are recommendations in the informational notes, not hard code requirements, but designers and inspectors treat them as the working standard. Staying inside them keeps motors at full torque, incandescent and LED lighting at full brightness, and resistive heaters at full output — and it keeps the wasted energy that voltage drop represents to a minimum.
How to fix excessive voltage drop
If the calculator returns a drop above the limit, every fix works by lowering the conductor's resistance or by lowering the current it carries. In rough order of how often they are used:
- Use a larger wire (lower AWG number). This is the standard fix. Each three-gauge step roughly doubles the circular mils and roughly halves the drop — going from 12 AWG to 10 AWG to 8 AWG.
- Shorten the run. Voltage drop is directly proportional to length, so relocating the panel or the load closer cuts the drop one-for-one.
- Switch copper for aluminum where you were using aluminum. Copper's lower K means about 39% less drop for the same size — useful on long runs where upsizing is expensive.
- Raise the system voltage. The same load at 240 V instead of 120 V draws half the current, which halves the volts dropped and quarters the percentage drop.
- Split the load. Two circuits each carrying half the current each drop half as much voltage.
To size the wire from the load and current first, start with Ohm's law, and to see what a long, lossy run costs you over a year, run the numbers through the electricity cost calculator.
Copper vs aluminum, single-phase vs three-phase
Two choices change the formula's constants: the conductor material sets K, and the number of phases sets the leading multiplier.
Copper vs aluminum
Aluminum's resistivity constant (≈21.2) is about 64% higher than copper's (≈12.9), so the same AWG size in aluminum drops about 64% more voltage. Aluminum is lighter and cheaper, which is why it is common on large feeders and service entrances, but it usually has to be one or two gauge sizes larger than copper to hit the same drop.
Single-phase vs three-phase
Single-phase circuits use the multiplier 2, because current travels out and back on two conductors. Balanced three-phase circuits use √3 (about 1.732) instead, because the return currents in the three phases partly cancel. For the same line current and wire size, a three-phase run therefore drops noticeably less voltage than the single-phase equivalent.
| Variable | Copper | Aluminum |
|---|---|---|
| Resistivity constant K (Ω·cmil/ft) | ≈ 12.9 | ≈ 21.2 |
| Relative voltage drop, same size | Baseline | ≈ 64% more |
| Single-phase multiplier | 2 | 2 |
| Three-phase multiplier | √3 ≈ 1.732 | √3 ≈ 1.732 |
K values are the standard DC resistivity constants used in the NEC Chapter 9 circular-mil method. This calculator computes single-phase copper and aluminum; for three-phase, substitute √3 for the 2 in the formula.
How accurate is this voltage drop calculator
The arithmetic is exact: for the AWG, material, length, and current you enter, V_drop = (2 × K × I × L) / CM is computed to full floating-point precision and reported alongside the percentage drop and the voltage at the load. The circular-mil values match NEC Chapter 9, Table 8.
The simplification is that this is a DC-resistance estimate. For ordinary residential and commercial circuits at 60 Hz, conductor resistance dominates and this method is what electricians use every day. Where it diverges from reality: large conductors and high frequencies add inductive reactance that this formula ignores — for those, the NEC's Chapter 9, Table 9 gives effective impedance figures. The K constants also assume conductors near 75 °C; a hotter conductor has slightly higher resistance and drops a little more. Temperature, power factor below unity, and bundling several conductors together all nudge the real drop upward. Treat the result as a sound design estimate, size conservatively, and consult NEC Chapter 9 Table 9 when the run is large or the frequency is high.
Frequently asked questions about the free voltage drop calculator
About this voltage drop calculator
This voltage drop calculator runs entirely in your browser using the NEC Chapter 9 circular-mil method. Enter the wire size (AWG), conductor material, one-way distance, load current, and source voltage, and it returns the voltage lost to the conductor's resistance, the voltage that actually reaches the load, and the drop as a percentage you can check against the NEC's 3% branch / 5% total recommendation. Nothing is uploaded — every figure is computed locally and updates as you type.