How the Voltage Drop Calculator Works

1. Understanding Voltage Drop

Voltage drop is the reduction in electrical potential as current flows through a conductor. Every wire has resistance, and that resistance converts a portion of the supply voltage into heat. The longer the wire run and the higher the current, the greater the voltage lost before reaching the load.

Why Voltage Drop Matters

• Equipment performance: Lights dim, electronics malfunction, and sensitive equipment may shut down or produce errors.
• Energy waste: Voltage lost as heat in the wire is energy you pay for but never use at the load.
• Safety risk: Motors and compressors draw more current to compensate for low voltage, causing overheating and premature failure.

Code Limits by Standard

All major electrical codes set recommended limits on how much voltage can be lost in a circuit:

2. Choosing Your Standard

The calculator supports three international electrical standards. Choose the one that applies to your installation location and regulatory requirements.

NEC (NFPA 70)

The National Electrical Code covers residential, commercial, and industrial installations in the United States, Canada, and many other countries that adopt NFPA standards. Uses AWG wire gauges and imperial measurements (feet).

Try NEC standard →

IEC 60364

The International Electrotechnical Commission standard is used across Europe, Asia, Africa, and South America. Uses metric wire sizes (mm²) and measurements (meters).

Try IEC standard →

BS 7671

The British Standard (IET Wiring Regulations) governs electrical installations in the United Kingdom. Uses metric wire sizes (mm²) and measurements (meters) with UK-specific voltage levels.

Try BS 7671 standard →

3. Voltage Drop Mode

Use Voltage Drop mode when you already have a wire installed (or a specific wire gauge in mind) and want to check whether it meets code requirements for your circuit.

When to use:

• Verifying an existing installation
• Checking if a planned wire gauge is adequate
• Comparing voltage drop across different wire sizes

Steps:

1. Select your electrical standard (NEC, IEC, or BS 7671)
2. Set phase, conductor material, and insulation type
3. Enter system voltage, load current, and one-way cable length
4. Select the wire size you want to check
5. Click Calculate to see voltage drop results

Example

Check a 20A load on #12 AWG copper wire, 120V single-phase, 100 feet one-way:

Try this example →

4. Wire Size Mode

Use Wire Size mode when you need to determine the correct wire gauge for a new circuit. The calculator considers both ampacity (current-carrying capacity) and voltage drop, then recommends the smallest wire that satisfies both requirements.

When to use:

• Designing a new circuit from scratch
• Finding the minimum wire size for code compliance
• Getting ampacity and voltage drop checked together

Steps:

1. Select your electrical standard
2. Set phase, conductor material, and insulation type
3. Choose branch or feeder circuit type
4. Enter system voltage, load current, and one-way cable length
5. Click Calculate to get the recommended wire size

Example

Find the right wire for a 20A branch circuit, 120V single-phase, 75 feet copper:

Try this example →

5. Advanced Settings

The Advanced Settings panel lets you apply derating factors, use parallel conductor sets, and enable motor circuit sizing. These settings affect both ampacity calculations and wire size recommendations.

Ampacity Derating

Ampacity derating reduces the current-carrying capacity of a wire based on installation conditions. Two factors apply:

• Ambient temperature: When the surrounding air temperature exceeds 30°C (86°F), wire insulation cannot dissipate heat as effectively. Higher temperatures require larger wire gauges.
• Conduit fill: When 4 or more current-carrying conductors share a conduit, mutual heating reduces each conductor's ampacity. The more conductors, the greater the derating.

Try a hot-environment example →

Parallel Conductor Sets

For very high-current circuits, running multiple sets of smaller conductors in parallel can be more practical than a single large conductor. Each set shares the total current equally.

Motor Circuit Sizing

Motor circuits require conductors sized at 125% of the motor's full-load ampere (FLA) rating per NEC 430.22. Enabling this option automatically applies the 125% multiplier to the current value.

Try a motor circuit example →

6. Reading Your Results

The results panel shows everything you need to evaluate your circuit design. Here is what each part means.

Pass / Fail Status

The calculator compares your voltage drop percentage against the applicable code limit. A green "Pass" means your wire meets requirements. A red "Exceeds Limit" means you need a larger wire or a shorter run.

Controlling Factor

In Wire Size mode, the result shows whether ampacity or voltage drop determined the final recommendation:

• Ampacity controls: The wire needs to be large enough to safely carry the current. This is common for short, high-current runs.
• Voltage drop controls: The wire needs to be large enough to keep voltage drop within limits. This is common for long runs at lower voltages.

Alternatives Table

The alternatives table shows nearby wire sizes with their voltage drop percentage and pass/fail status, so you can compare options and select the most cost-effective compliant size.