Voltage Drop Calculator
Voltage drop for copper or aluminium cable by AWG, kcmil or mm² size, length and load current, for DC, single-phase and three-phase circuits, with 3% and 5% checks.
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About this toolWhat is the Voltage Drop Calculator?
Every cable has resistance, so some of the supply voltage is lost along the way before it reaches the load. This voltage drop calculator works out how much: choose DC, single-phase or three-phase, copper or aluminium, a wire size in AWG/kcmil or mm², and enter the one-way length, load current and supply voltage. You get the drop in volts and as a percentage, the voltage left at the load, and the power the cable turns into heat.
It also answers the question people usually start with: what size wire do I need? Under the result it names the smallest conductor that keeps the drop within 3% and within 5%, and a short table compares the sizes either side of your choice so you can see what one size up buys you.
Resistance values come from NEC Chapter 9 Table 8 for AWG and kcmil and from IEC 60228 for metric cable, corrected to the conductor temperature you set.
- DC, single-phase and balanced three-phase
- Copper and aluminium, AWG/kcmil (NEC Table 8) and mm² (IEC 60228)
- Temperature correction and parallel conductor sets
- Smallest size for 3% and 5%, with a size comparison table
- Shows power lost in the cable; works offline in your browser
How to use the Voltage Drop Calculator
- Choose DC, Single-phase or Three-phase, and copper or aluminium.
- Pick the size system (AWG/kcmil or mm²) and the wire size. Switching systems picks the nearest equivalent size for you.
- Enter the one-way length from the source to the load, the load current and the supply voltage (line to line for three-phase).
- Adjust the conductor temperature if needed: 75 °C matches NEC tables, 70 °C suits PVC-insulated cable at full load, 20 °C is a lightly loaded cable.
- Read the drop, the percentage, the voltage at the load and the smallest size that meets 3% or 5%. Click a size in the table to switch to it.
Worked examples
20 A kitchen circuit, 100 ft run
Input
Single-phase, copper, 12 AWG, 100 ft, 16 A, 120 V, 75 °C
Output
6.336 V drop (5.28%), 113.66 V at the load; 8 AWG needed for 3%
2 × 16 A × 1.98 Ω/kft × 0.1 kft = 6.34 V. 10 AWG gets to 3.3%; 8 AWG brings it to 2.07%.
12 V DC solar or RV run
Input
DC, copper, 10 AWG, 20 ft one way, 15 A, 12 V
Output
0.744 V drop (6.2%), 11.26 V at the load; 6 AWG for 3%
Low-voltage systems lose a much larger share of their voltage over the same cable, which is why 12 V wiring is often heavier than mains wiring for the same current.
Metric socket circuit
Input
Single-phase, copper, 2.5 mm², 20 m, 16 A, 230 V, 70 °C
Output
5.674 V drop (2.47%), 224.33 V at the load
IEC 60228 gives 7.41 Ω/km at 20 °C; corrected to 70 °C it is about 8.87 Ω/km. Moving to 4 mm² cuts the drop to 1.53%.
Three-phase feeder, copper vs aluminium
Input
Three-phase, 1/0 AWG, 250 ft, 100 A, 480 V
Output
Copper 5.283 V (1.10%); aluminium 8.704 V (1.81%)
√3 × 100 A × 0.122 Ω/kft × 0.25 kft for copper. Aluminium has about 1.6 times the resistance of the same size of copper.
Background and insight
For DC and single-phase AC the current flows out on one conductor and back on the other, so the drop is Vd = 2 × I × R × L, where R is resistance per unit length and L is the one-way length. In a balanced three-phase circuit the return currents largely cancel and the line-to-line drop is Vd = √3 × I × R × L. The calculator uses DC resistance, which is accurate for DC, small AC cables and resistive loads. On large AC conductors (roughly 1/0 AWG or 50 mm² and above) reactance and skin effect add a few percent, so treat the result as a slight underestimate there.
NEC Chapter 9 Table 8 lists DC resistance at 75 °C per 1,000 feet. Stranded 12 AWG copper is 1.98 Ω per 1,000 ft and 4/0 is 0.0608 Ω. IEC 60228 gives maximum DC resistance at 20 °C per kilometre for class 2 stranded conductors: 2.5 mm² copper is 7.41 Ω/km and 16 mm² aluminium is 1.91 Ω/km. Resistance rises with temperature (about 0.393% per °C for copper and 0.403% for aluminium at 20 °C, equivalent to the 0.00323 and 0.00330 coefficients the NEC quotes at 75 °C), so the conductor temperature setting matters: a cable carrying its rated current runs warm.
The 3% and 5% figures come from informational notes in the NEC (under 210.19 for branch circuits and 215.2 for feeders): about 3% on a branch circuit or feeder and 5% overall is suggested for reasonable efficiency. They are advice rather than enforceable rules in the US, while BS 7671 in the UK sets 3% for lighting and 5% for other uses on a public supply. Voltage drop is also separate from ampacity: a cable must be large enough to carry the current safely regardless of how small the drop is.
Common use cases
Sizing a long branch circuit
Check whether a run to a garage, shed or outdoor outlet needs a larger wire than ampacity alone would suggest.
Low-voltage DC systems
Size cable for solar panels, batteries, RVs, boats and LED strips, where a volt of drop is a large share of the supply.
EV chargers and pumps
Make sure a motor or charger at the end of a long run still gets enough voltage to start and run properly.
Comparing copper and aluminium
See how much larger an aluminium feeder must be to match a copper one on voltage drop.
Tips and limitations
- Enter the one-way distance. The formula already doubles it for the return path on DC and single-phase circuits.
- Use the actual load current, not the breaker rating, when you want the real drop. Use the breaker rating for a worst-case check.
- Motors draw several times their running current when starting, so the starting voltage dip is much bigger than the running drop.
- For three-phase, enter the line-to-line voltage (208, 400 or 480 V) and the current in each line.
- This is a planning aid. Final conductor sizing must also meet ampacity, insulation temperature, derating and local code requirements.
Frequently asked questions
How do you calculate voltage drop in a cable?
For DC and single-phase: Vd = 2 × current × resistance per unit length × one-way length. For three-phase: Vd = 1.732 × current × resistance × length. Divide by the supply voltage and multiply by 100 for the percentage.
What is an acceptable voltage drop?
NEC informational notes suggest no more than 3% on a branch circuit or feeder and 5% in total from the service to the farthest outlet. UK wiring rules use 3% for lighting and 5% for other circuits.
What size wire do I need for 100 feet at 20 amps?
At 120 V and the full 20 A, 12 AWG copper drops about 6.6% over 100 ft one way. 8 AWG keeps it near 2.6%, 10 AWG near 4.1%. Enter your actual load to see the smallest size that meets your target.
Does voltage drop depend on the voltage?
The volts lost depend only on current and cable resistance, but the percentage depends on the supply. The same 6 V drop is 5% of 120 V but 50% of 12 V, which is why low-voltage systems need heavier cable.
Why is aluminium wire sized larger than copper?
Aluminium has about 61% of the conductivity of copper, so a conductor of the same size has roughly 1.6 times the resistance. Going up about two AWG sizes gives similar performance.
Should I use one-way or round-trip length?
Enter one-way length, the distance from the panel or battery to the load. The calculator applies the factor of 2 (or √3 for three-phase) itself.
What happens if voltage drop is too high?
Lights dim, heaters lose output, motors run hotter and may struggle to start, and electronics can reset. The lost voltage is also wasted as heat in the cable.
Is there an in-depth guide to the Voltage Drop Calculator?
Yes. “Ohm's Law, Resistor Color Codes and Voltage Drop Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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