Voltage Drop Calculator
Voltage drop across copper or aluminum conductors — for reference.
How to use
Type the circuit values; the calculator returns voltage drop, percent drop, and a flag if the drop exceeds the NEC-recommended limits.
- Enter the source voltage (120, 240, 277, 480 etc.) and the current in amps.
- Enter the one-way distance from the source to the load in feet.
- Pick the conductor gauge (AWG), material (copper or aluminum), and phase (single or three).
- Read the answer. The NEC recommends a maximum 3 % drop on a branch circuit and 5 % total (feeder + branch). Above that, step up one or two gauges.
Reviewed 3 June 2026 · methodology cited
About this calculator
Voltage drop is the loss in voltage as current flows through a conductor — every wire has some resistance, and the longer the wire and the higher the current, the more voltage is lost between the source and the load. Too much drop and a motor stalls, a lamp dims, or a heating element delivers less heat than it was rated for. The National Electrical Code (US) and Canadian Electrical Code both publish recommended limits.
This calculator uses the standard K-value approximation common in industry training material: K equals 12.9 Ω·circular-mils per foot for copper and 21.2 for aluminum at 75 °C. It is a reference utility for sanity-checking a circuit; actual installation sizing is your electrician's call based on the load, the routing, the ambient conditions, and the local code.
The formula
The single-phase voltage-drop formula is V_drop = 2 × K × L × I ÷ CM, where K is the conductor K-value, L is the one-way length in feet, I is the current in amps, and CM is the conductor cross-section in circular mils. The factor of 2 accounts for the current path traveling both directions (source to load and back). For three-phase, the multiplier is √3 instead of 2, because the line-to-line drop accounts for the phase angle.
A worked example: 40 amps on 60 feet of 10 AWG copper, single phase 240 V. CM for 10 AWG ≈ 10,380. V_drop = (2 × 12.9 × 60 × 40) ÷ 10,380 = 61,920 ÷ 10,380 ≈ 5.96 V. Percent drop = 5.96 ÷ 240 ≈ 2.5 %. Within the NEC's 3 % branch-circuit recommendation, so 10 AWG passes. If the run were 100 feet, the drop would be about 4.1 % — over the branch limit, and the prudent move is 8 AWG.
Recommended drop limits
| Circuit | Branch drop | Total drop |
|---|---|---|
| Branch circuit (NEC FPN) | ≤ 3 % | — |
| Feeder (NEC FPN) | — | ≤ 2 % |
| Total (branch + feeder) | — | ≤ 5 % |
| Sensitive electronics | ≤ 1 % | ≤ 3 % |
| Motor load (start) | ≤ 5 % | ≤ 10 % |
Common installation notes
The K-value formula is an approximation good to within a few percent for most residential and light-commercial work. It assumes the conductor temperature is at 75 °C and the power factor is unity (resistive load). For motor loads at lower power factor, the reactive component of the impedance adds to the drop — the NEC publishes Chapter 9 Table 9 for full impedance values, and motor-load calculations should use those rather than this K-value shortcut.
NEC recommendations are not enforceable code by themselves — they live in informational notes (210.19(A) FPN No. 4 for branch circuits, 215.2(A)(1) FPN No. 2 for feeders). The 3 percent / 5 percent limits are best-practice guidance. Some local codes adopt them as mandatory; most do not, but every utility company and many large facilities require them by contract. Always size to the stricter of the local code and the equipment manufacturer's installation instructions. And again: this is a reference calculator. Real installation sizing requires a licensed electrician.
Frequently asked questions
Why multiply by 2 for single phase?
Current flows from the source to the load and back through the neutral, so the conductor length the current actually travels is twice the one-way distance you measure. Both legs contribute resistance and both contribute voltage drop, so the formula multiplies the one-way drop by 2.
Why √3 for three phase instead of 3?
In a balanced three-phase system the current returns through the other two phases rather than a neutral, and the phases are 120° apart. The voltage drop between any two line conductors is √3 times the per-phase drop because of the phase angle — the same √3 that appears in the relationship between line and phase voltage.
Is the 3 % branch / 5 % total limit code or guideline?
It is informational guidance in the NEC, not enforceable code by itself. NEC 210.19(A) FPN No. 4 and 215.2(A)(1) FPN No. 2 recommend these limits but leave actual maximums to the local jurisdiction. Some local codes adopt them as mandatory; most utilities and large building specs require them as a minimum. Always check with the authority having jurisdiction (AHJ).
Why does aluminum drop more than copper at the same gauge?
Aluminum has about 60 % of the conductivity of copper by volume, so its K-value (21.2) is about 1.65 times that of copper (12.9). Same gauge, same length, same current — the aluminum conductor drops 1.65 times more voltage. To match copper performance, aluminum is typically sized one to two AWG gauges larger.
Does this account for power factor or temperature?
No. The K-value formula assumes 75 °C conductor temperature and unity power factor (purely resistive load). Real installations with motor loads (lagging PF), high ambient temperatures, or harmonics-heavy electronics need the full NEC Chapter 9 Table 9 impedance values rather than this approximation. For sanity checks on resistive loads it is accurate to within a few percent.
Can I use this to size a circuit for my own work?
No. This is a reference utility to verify the math behind a value an electrician quoted you, or to learn the relationship between gauge, length, and drop. Real installation sizing requires a licensed electrician working from the local electrical code, equipment manufacturer instructions, and the full set of derating factors (ambient temperature, conduit fill, continuous load multipliers, etc.).