Load voltage formula: the quick answer
K is the conductor-material constant, I is current in amperes, L is the one-way source-to-load distance and CM is conductor area in circular mils. Do not double L before using this formula; the factor of 2 already represents the outgoing and return path.
How do you calculate voltage drop?
Every metallic conductor has internal electrical resistance. As current flows through a run of copper or aluminum wire, part of the source voltage is lost as heat along the cable path. Using Ohm's law (V = I × R) and circular-mil resistivity at 75°C, the voltage lost across a two-wire circuit is:
where:
- VD – Total voltage drop between the supply panel and the load terminals, measured in volts (need a target gauge instead? Use the wire size calculator);
- K – Conductor resistance constant at 75°C (K = 12.9 for copper and K = 21.2 for aluminum in Ω·CM/ft);
- I – Load current flowing through the circuit in amps (estimate from wattage with our watts to amps calculator);
- L – One-way wire distance in feet (the multiplier of 2 already accounts for the return conductor); and
- CM – Conductor cross-sectional area in circular mils from NEC Chapter 9, Table 8.
Three-phase AC voltage drop equation
In a balanced three-phase system, the return currents cancel in phase opposition, so the round-trip factor of 2 is replaced by √3:
Subtract VD from the source voltage (V_load = V_source − VD) to find the actual voltage delivered to the equipment terminals. Explore all related tools in our electrical calculators hub.
Worked example: 120 V, 20 A, 75 ft, 12 AWG copper
| Step | Calculation | Result |
|---|---|---|
| Volts lost | 2 × 12.9 × 20 × 75 ÷ 6,530 | 5.93 V |
| Percentage | 5.93 ÷ 120 × 100 | 4.94% |
| Load voltage | 120 − 5.93 | 114.1 V |
What changes the result
- Double current and volts lost doubles
- Double distance and volts lost doubles
- Use a larger circular-mil area and volts lost decreases
- Use a lower source voltage and the same volts lost becomes a larger percentage
- Change conductor material and the K constant changes
Use real operating inputs
Nameplate or calculated design current is more useful than an arbitrary breaker value. Sensitive equipment, motors and circuits with distributed loads may need a method beyond the simple lumped-current formula.
Formula guide or calculator?
Use this page when you want to check each equation step. Use the single-phase voltage drop calculator when you need an interactive estimate for a chosen AWG, voltage, current, material and distance.
Keeping these jobs separate prevents the formula article from pretending to be another calculator page: this guide explains the method, while the calculator compares live inputs and output voltage.
Single phase compared with balanced three phase
| Circuit | Multiplier | Voltage input | Distance input |
|---|---|---|---|
| DC / single phase | 2 | Source voltage | One-way |
| Balanced three phase | √3 | Line-to-line | One-way |
Frequently asked questions
Why does the single-phase formula multiply by 2?
The factor of two represents the outgoing and return conductor path when the entered distance is one-way.
Should I enter one-way or round-trip distance?
Enter one-way source-to-load distance. Do not double it before using the formula because the factor of two already accounts for the return path.
How do I find estimated load voltage?
Subtract calculated volts lost from source voltage. For example, a 5.93 V loss from 120 V leaves about 114.1 V.
What is the load voltage formula?
Estimated load voltage equals source voltage minus calculated voltage drop. In this simplified single-phase model, calculate voltage drop as 2 × K × current × one-way distance divided by conductor circular-mil area.
Does this formula select a safe wire size?
No. It estimates voltage drop only. A qualified professional must still verify load, ampacity, protection, terminals, adjustment factors and the installation method.