Voltage Drop Calculator
A long wire run can deliver noticeably less voltage to the equipment than the voltage measured at the power source. This voltage drop calculator estimates how much voltage is lost along a circuit and shows the remaining voltage at the load.
Enter the circuit voltage, current, conductor resistance, and cable length to calculate the expected voltage drop. You can also use the result to compare conductor sizes and see how increasing cable length or load current affects the circuit.
Voltage Drop Calculator
Calculate voltage loss, voltage at the load, and voltage drop percentage for DC, single-phase AC, and three-phase circuits.
Circuit Details
Calculation Result
How to Read the Result
Voltage drop is the difference between the voltage at the source and the voltage available at the load under the conditions you entered. A longer cable, higher current, or higher conductor resistance generally produces more voltage drop. For AC circuits, impedance and power factor can also affect the result.
What Is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor.
Every conductor has some electrical resistance. When current flows through it, part of the electrical energy is dissipated as heat. The voltage measured at the load can therefore be lower than the voltage available at the source.
For example, a circuit may start at 120 volts, but the equipment at the end of a long cable might receive less than 120 volts when the circuit is carrying a significant load.
The amount of voltage drop depends mainly on:
- Current flowing through the circuit
- Conductor resistance
- Cable length
- Conductor material
- Conductor size
- Circuit configuration
- AC power factor and reactance, where applicable
- Conductor operating temperature
Longer conductors generally produce greater voltage drop because the circuit has more conductor resistance or impedance. Schneider Electric similarly notes that conductor impedance increases with conductor length.
How to Calculate Voltage Drop
For a simple DC circuit, the basic relationship is:
Voltage Drop = Current × Resistance
or:
Vd = I × R
Where:
- Vd = voltage drop in volts
- I = current in amps
- R = total circuit resistance in ohms
For a two-wire circuit, remember that the current travels to the load and returns to the source. Therefore, the resistance of both conductors may need to be included.
A simplified two-wire calculation can be written as:
Vd = 2 × I × R × L
when R represents conductor resistance per unit length and L represents the one-way distance.
The exact formula depends on how conductor resistance is specified and whether the circuit is DC, single-phase AC, or three-phase AC.
Voltage Drop Percentage
Voltage drop is often more useful when expressed as a percentage of the source voltage.
Use:
Voltage Drop % = (Voltage Drop ÷ Source Voltage) × 100
For example, if a 120-volt circuit loses 3 volts:
Voltage Drop % = (3 ÷ 120) × 100 = 2.5%
The load voltage would be:
Load Voltage = Source Voltage − Voltage Drop
So:
120 − 3 = 117 volts
This lets you see both the actual voltage loss and how significant that loss is relative to the circuit voltage.
Example Voltage Drop Calculation
Suppose a circuit has:
- Supply voltage: 120 V
- Current: 10 A
- Total circuit resistance: 0.30 Ω
The voltage drop is:
Vd = 10 × 0.30
Vd = 3 V
The load voltage is therefore:
120 − 3 = 117 V
And the voltage drop percentage is:
(3 ÷ 120) × 100 = 2.5%
This is a simplified resistance-based example. Real-world AC cable calculations can require conductor impedance and power factor rather than resistance alone.
What Causes High Voltage Drop?
1. Long cable runs
A longer conductor has more resistance, so voltage drop generally increases as the distance from the source to the load increases.
This becomes particularly important for detached buildings, pumps, outdoor equipment, lighting circuits, agricultural installations, and other applications where the load is far from the electrical panel.
2. High current
Voltage drop increases as current increases.
This is why a cable that appears to perform normally with a small load can experience a much larger voltage loss when a high-power device is connected.
3. Small conductors
A smaller conductor generally has greater resistance than a larger conductor of the same material and length.
Increasing conductor size can therefore reduce voltage drop.
4. Conductor temperature
Copper and aluminum resistance changes with temperature. For detailed installations, conductor operating temperature can therefore affect the calculated voltage drop. Southwire’s technical material specifically accounts for temperature when determining conductor resistance.
5. AC circuit characteristics
For AC circuits, resistance isn’t always the whole story. Reactance and power factor can contribute to voltage drop.
Southwire describes an AC approximation using resistance, reactance, current, power factor, and cable length.
Single-Phase Voltage Drop
Single-phase voltage drop calculations commonly involve the complete circuit path, including the outgoing and returning conductors.
For a simplified resistive circuit:
Vd = 2 × I × R × L
where L represents the one-way conductor length and R is the resistance per unit length.
This approach is useful for basic calculations, but an actual installation may require more detailed conductor data.
Three-Phase Voltage Drop
Three-phase circuits use a different relationship from a simple two-wire circuit.
A commonly used approximation for a balanced three-phase AC circuit is:
Vd ≈ √3 × I × (R cos φ + X sin φ) × L
where:
- I = line current
- R = conductor resistance
- X = conductor reactance
- φ = load power-factor angle
- L = applicable conductor length
Schneider Electric documentation also gives three-phase voltage-drop calculations using resistance, reactance, current, and power factor.
Because the appropriate equation depends on the circuit configuration, a calculator should clearly identify whether the calculation is for DC, single-phase, or three-phase power.
Voltage Drop Calculator for Long Wire Runs
Long-distance circuits are one of the most common situations where voltage-drop calculations become important.
Consider a circuit supplying equipment 200 feet from the panel. The one-way distance may look reasonable, but the electrical circuit can have substantially more conductor length once the complete circuit path is considered.
This is especially relevant for:
- Detached garages
- Workshops
- Water pumps
- Well pumps
- Outdoor lighting
- Agricultural equipment
- RV connections
- Solar installations
- Battery systems
- Remote control circuits
- Long feeder circuits
For these applications, entering the correct one-way cable distance, circuit configuration, current, and conductor characteristics is important.
Voltage Drop Calculator for 12V and 24V Systems
Voltage drop becomes particularly noticeable in low-voltage systems.
A 2-volt loss represents:
- 16.7% of a 12-volt system
- 8.3% of a 24-volt system
- 0.83% of a 240-volt system
This illustrates why the same absolute voltage drop can have very different consequences depending on the system voltage.
Low-voltage DC systems such as batteries, automotive accessories, RV equipment, solar components, and control systems can therefore require careful attention to cable length and conductor size.