Voltage Drop Calculator

Calculate voltage drop and ensure your wire size meets electrical requirements

Calculate Voltage Drop

Wire Size (mm²)

Current (A)

Cable Length (m)

System Voltage (V)

Understanding Voltage Drop

What is Voltage Drop?

Voltage drop is the reduction in voltage as electrical current flows through a conductor's resistance. Every wire has inherent resistance that causes some voltage to be lost as heat when current passes through it. Excessive voltage drop can cause equipment malfunction, reduced performance, or safety hazards.

Voltage Drop Formula

Voltage Drop (V) = 2 × I × R × L / 1000

Where:

I = Current in Amperes (A)
R = Resistance per kilometer (Ω/km)
L = One-way cable length in meters (m)
2 = Factor for round trip (positive and negative conductors)

Acceptable Voltage Drop Limits

NEC Recommendation: ≤3%

The National Electrical Code recommends maximum 3% voltage drop for branch circuits and feeders.

Combined System: ≤5%

Total combined voltage drop (feeder + branch) should not exceed 5%.

Critical Loads: ≤2%

Sensitive equipment and critical applications often require <2% voltage drop.

Factors Affecting Voltage Drop

Wire Size: Larger wire (lower AWG) has less resistance and lower voltage drop
Current: Higher current increases voltage drop proportionally
Length: Longer cables have more resistance and higher voltage drop
Temperature: Higher temperatures increase resistance (~0.4% per °C)
Material: Copper has lower resistance than aluminum

Solutions for High Voltage Drop

Increase wire size (use lower AWG number)
Reduce cable length if possible
Use multiple parallel conductors
Increase system voltage (e.g., 24V instead of 12V)
Install transformer or power supply closer to load

Voltage Drop Design Targets

Circuit Type	Typical Limit	Why It Matters
General branch circuits	3%	Balances efficiency and conductor cost
Combined feeder plus branch	5%	Keeps whole-system loss within a practical range
Sensitive electronics	1-2%	Protects signal integrity and startup margin

Authoritative References

American Wire Gauge

Conductor sizing context used in many voltage-drop calculations.

Wire harness overview

Useful when branch routing and bundle length drive delivered voltage.

Frequently Asked Questions

What is voltage drop in practical terms?

Voltage drop is the difference between supply voltage and the voltage that reaches the load after current flows through conductor resistance.

Why is 3% used so often?

Three percent is a common design target because it balances efficiency, equipment performance, and conductor cost in many low-voltage systems.

Can a harness branch have a different limit?

Yes. Sensitive sensors, motors, and communication circuits may use tighter limits than general power distribution branches.

Does material type matter?

Yes. Copper and aluminum have different resistivity, so voltage-drop results must align with the actual conductor material in the design.

Should this result replace testing?

No. It is an engineering estimate that should be confirmed with prototype measurement, temperature review, and final validation in the full assembly.

Related Calculators

AWG to mm² Converter

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