Voltage Drop Calculator
The voltage lost along a copper or aluminium cable for a given current and run length, single phase, three phase or DC, and what reaches the load.
Result
Voltage drop: 11.364 VEstimate
At 20 °C and ignoring reactance. It does not size a conductor or check it against any electrical code; local codes vary.
- As a share of the source voltage
- 9.47%Estimate
- Voltage at the load
- 108.636 VEstimate
- Power lost in the cable
- 170.47 WEstimate
- Conductor resistance per 1,000 ft
- 2.525 ΩEstimate
https://calcgallery.com/electronics/voltage-drop-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the Voltage Drop Calculator
- Choose single phase, three phase or DC, and enter the source voltage and the load current.
- Enter the one-way length of the run, from the supply to the load.
- Choose the conductor size, in AWG or mm², and copper or aluminium, and read the drop and the voltage left at the load.
The formulas
The resistance of a metre of conductor is its resistivity divided by its cross-sectional area. Then:
single phase and DC drop = 2 × length × current × resistance per metrethree phase drop = √3 × length × current × resistance per metreThe percentage is the drop divided by the source voltage. The power lost in the cable is the current times the drop, and √3 times that for three phase.
Where the numbers come from
Resistivity is taken at 20 °C for copper and for aluminium, from the site’s register of cited values; the worked steps show the resistance per 1,000 ft it gives. Conductors run warmer under load, which raises their resistance and the drop, so treat the result as a lower bound. AWG areas are calculated from the formula that defines the gauge, not copied from a table, and metric sizes use their nominal areas.
What is left out
Reactance, which adds to the drop in large AC cables, and the temperature of the conductor. Neither changes the result much for ordinary branch circuits.
With your numbers
- 2 × 45.72 m × 15 A × resistance per metre
- Drop = 7.147 V, 5.96% of the source
Quick reference
At 20 °C and ignoring reactance. It does not size a conductor or check it against any electrical code; local codes vary.
| Example | Voltage drop |
|---|---|
| 120 V, 15 A, 150 ft of 12 AWG copper | 7.147 V |
| Same run in 10 AWG copper | 4.495 V |
| 240 V three phase, 30 A, 200 ft of 8 AWG | 6.528 V |
| 12 AWG aluminium, 150 ft | 11.716 V |
| 230 V, 10 A, 45 m of 2.5 mm² | 6.207 V |
| 12 V DC, 10 A, 20 ft of 10 AWG | 0.4 V |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
NBS Handbook 100, Copper Wire Tables (1966), p. 2: "0.017 241 ohm-mm2/meter at 20 °C, 1.7241 microhm-cm at 20 °C", https://nvlpubs.nist.gov/nistpubs/Legacy/hb/nbshandbook100.pdf
Used for
- Resistivity of annealed copper at 20 °C, the International Annealed Copper Standard: 1/58 Ω·mm²/m, 1.7241 µΩ·cm.
NBS Handbook 109, Aluminum Wire Tables (February 1972), Table 1, p. 5: EC-H19, 61% IACS, 0.028264 ohm-mm2/meter at 20 °C, https://nvlpubs.nist.gov/nistpubs/Legacy/hb/nbshandbook109.pdf
Used for
- Resistivity of EC-H19 aluminium conductor (61% IACS) at 20 °C: 0.028264 Ω·mm²/m.
Common questions about the Voltage Drop Calculator
- Why is single phase multiplied by two?
- Because the current goes out along one conductor and comes back along another, so the resistance in the circuit is that of twice the run length. Forgetting it halves the answer. Three-phase circuits use the square root of three instead, because the three currents are out of step and partly cancel in the return path.
- How much voltage drop is acceptable?
- The US National Electrical Code has an informational note suggesting no more than 3% on a branch circuit and 5% in total. It is a note, not an enforceable requirement, and other countries use other figures. This page reports the percentage and does not judge it; local codes vary and must be checked.
- Does this tell me what wire to use?
- No. Choosing a conductor depends on its ampacity, insulation, installation method, ambient temperature, conduit fill and terminal ratings, set by electrical codes that vary by place. Voltage drop is one consideration among several. This page calculates the drop for a size you choose and does not size anything.
- Why does aluminium drop more?
- Aluminium has roughly 60% more resistivity than copper, so the same size of aluminium conductor drops roughly 60% more voltage. That is why aluminium conductors are usually a size or two larger than copper for the same job.