One of the most common questions when using a voltage drop calculator is:
“Do I enter the full wire distance or the distance from the panel to the load?”
For a normal single-phase AC circuit, measure the distance from the panel to the load in one direction.
That is the distance you enter.
Measure the One-Way Distance
Measure from the panel, disconnect, or source to the equipment being supplied.
For example, if a load is 160 feet away from the panel:
One-way distance = 160 feet
Do not measure the distance there and back yourself.
Why the Calculation Doubles the Distance
Even though you measure the run in one direction, the circuit has a conductor path going out to the load and back.
For a single-phase AC circuit, the voltage-drop math accounts for both conductors.
Using the 160-foot example:
160 feet × 2 = 320 feet total conductor path
That total path is used in the voltage-drop calculation.
Quick Example
Say you have a 240V single-phase circuit supplying a load 160 feet from the panel.
You would enter:
- Voltage: 240V
- One-way distance: 160 feet
- Load current: 44 amps
- Selected conductor size
The calculator uses the one-way distance you enter and accounts for the full conductor path in the calculation.
Why Distance Matters
A short circuit run may have very little voltage drop.
As the distance increases, conductor resistance affects the circuit more. That can reduce the voltage available at the load.
That is why voltage-drop calculations are especially useful for:
- Detached buildings
- Long outdoor equipment runs
- Pool equipment
- Large properties
- Remote panels and subpanels
- Long feeder runs
How Sparky Toolbox Helps
The Sparky Toolbox Voltage Drop Calculator uses the one-way distance from the source to the load.
Enter the voltage, amps, selected conductor, and one-way distance in feet.
The calculator shows:
- Voltage drop in volts
- Voltage drop percentage
- Resistance in ohms/kft
That makes it easier to check long runs before choosing wire size or ordering material.
Important Reminder
Voltage-drop calculations are planning tools. Actual results can vary based on conductor material, conductor size, temperature, connection quality, system type, and installation conditions.
Always confirm project requirements, equipment instructions, approved plans, and local requirements before installation.