Start with the question you are trying to answer
Run a wire-size-by-voltage-drop calculation when you do not yet know the conductor gauge and want to stay within a target percentage loss. Run a voltage-drop check when you already have a specific AWG in mind and want to see volts lost, percentage drop, and delivered load voltage.
These two tools check the same formula from opposite directions. Neither tool completes a full electrical design on its own. Even when a conductor meets your voltage-drop target, you must still verify NEC ampacity, terminal temperature limits, ambient derating, conduit fill, and overcurrent protection.
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.
The inputs that change voltage drop
- 01
System voltage
The same voltage loss represents a much larger percentage drop on a 12 V or 120 V circuit than on a 240 V or 480 V circuit.
- 02
Load current
Higher current increases voltage drop proportionally when distance and wire gauge stay fixed.
- 03
One-way distance
Enter the one-way source-to-load length. The calculator applies the return-path or three-phase multiplier automatically.
- 04
Conductor material and area
Copper and aluminum use different resistance constants, and larger circular-mil area lowers resistance.
- 05
Circuit type
Select DC/single-phase or balanced three-phase explicitly because each uses a different multiplier.
Worked example: 240 V, 30 A, 100 ft copper run
Set a 3% planning limit on a 240 V circuit. Three percent of 240 V equals 7.2 V. Using the single-phase copper formula, the circuit requires 10,750 circular mils. Because 10 AWG offers 10,380 circular mils, the calculator steps up to 8 AWG at 16,510 circular mils.
Enter 8 AWG into the voltage-drop calculator with the same 240 V, 30 A, 100 ft inputs and you get 4.69 V lost, or 1.95%. That second check confirms 8 AWG stays under the 3% target in this simplified resistance model.
Passing the voltage-drop check does not finish the job. You still have to check continuous-load rules, insulation rating, conduit fill, ambient temperature, equipment lugs, and breaker sizing.
| Check | Circular mils / basis | Result at 3% target (7.2 V) |
|---|---|---|
| Required area | 2 × 12.9 × 30 × 100 ÷ 7.2 | 10,750 CM minimum |
| 10 AWG copper | 10,380 CM | Below required area |
| 8 AWG copper (step-up) | 16,510 CM | 4.69 V lost (1.95%) |
What these calculators do not determine
- Allowable ampacity for the actual conductor and installation conditions.
- Breaker or fuse size and required overcurrent protection.
- Terminal temperature ratings, conductor insulation or equipment listing requirements.
- Derating for ambient temperature, raceway fill, cable bundling or multiple current-carrying conductors.
- Motor starting, nonlinear loads, power factor, harmonic effects or unbalanced three-phase systems.
- Local amendments, inspection requirements or whether the work requires a permit and licensed installer.
Use the result as a documented planning check
Write down your voltage, current, one-way length, conductor material, phase, drop target, and output gauge. When an electrician recommends a different wire size, compare those inputs first. Most differences trace back to continuous-load multipliers, 60°C terminal limits, ambient derating, or motor inrush rules rather than basic arithmetic.
On low-voltage DC runs, long subpanel feeders, and hard-starting motor circuits, real-world operating conditions govern the final gauge. Use the calculator for fast preliminary checks, then have a licensed electrician verify the complete circuit design before buying wire.