Voltage drop becomes more important as a conductor run gets longer.
A feeder to a detached building, a remote panel, site lighting, or equipment at the far end of a building may need a quick voltage-drop check before wire is ordered.
Quick Voltage Drop Example
Example: a 240V single-phase circuit supplying a 44A load. The load is 160 feet from the panel using #6 copper conductors.
Measure the distance from the panel to the load in one direction.
One-way distance = 160 feet
Because this is an AC circuit, the calculation doubles the distance to include the path out to the load and back.
160 feet × 2 = 320 feet total conductor path
Get the conductor resistance from NEC Chapter 9, Table 8.
For this example:
#6 copper resistance = 0.491 ohms/kft
Now find the total circuit resistance:
0.491 ohms/kft × 320 feet ÷ 1,000 = 0.157 ohms
Now calculate voltage drop:
44 amps × 0.157 ohms = 6.9 volts dropped
Next, calculate the percentage of voltage drop:
6.9 volts ÷ 240 volts × 100 = 2.9% voltage drop
The load will receive approximately:
240 volts − 6.9 volts = 233.1 volts
Final Results for This Example
- System voltage: 240V single-phase
- Load current: 44 amps
- One-way distance: 160 feet
- Total conductor path used in the calculation: 320 feet
- Conductor: #6 AWG copper
- Conductor resistance: 0.491 ohms/kft
- Voltage drop: 6.9 volts
- Voltage-drop percentage: 2.9%
- Voltage at the load: approximately 233.1 volts
How Sparky Toolbox Helps
Instead of finding conductor resistance, calculating the total circuit length, and working out the voltage-drop percentage by hand, enter the circuit details into Sparky Toolbox.
The Voltage Drop Calculator shows:
- Voltage dropped over the run
- Voltage-drop percentage
- Conductor resistance in ohms/kft
That gives you a quick starting point before ordering wire or planning a long run.
Important Reminder
This is one simple single-phase example. Voltage drop can change based on system voltage, phase, conductor material, conductor size, load current, circuit length, conductor temperature, and installation conditions.
Always verify project details, equipment requirements, approved plans, and local inspection requirements before installation.