Voltage drop is a common issue in electrical installations, and if you’ve ever dealt with dim lights, weak motors, or mysterious performance problems, voltage drop is often one of the first things to check. The good news is that it’s predictable—and with the right tool, you can calculate it in seconds.

This voltage drop calculator gives you clear, real-world results for DC, single-phase, and three-phase systems. You can calculate voltage drop, find the maximum safe cable length, or determine the minimum cable size you need for your project.

Unlike basic calculators that use simplified assumptions, this tool includes power factor, copper vs aluminium cable resistances, temperature adjustments, and AC impedance. The goal is to give you results that closely match what electricians and engineers see in actual installations.

What Is Voltage Drop?

Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor. Cables are not perfect—they have resistance (and in AC systems, reactance). As current travels through the cable, voltage is lost along the way.

A few key points:

• In DC circuits, voltage drop depends mainly on cable resistance.
• In AC circuits, both resistance and reactance play a role.
• The longer the cable and the higher the current, the larger the voltage drop.
• Smaller cables and aluminium conductors also increase drop.

Why does this matter? Because electrical equipment is designed to run at, or close to, its rated voltage. If the voltage falls too far, equipment performance drops, efficiency suffers, and in some cases motors may fail to start or overheat.

Most industry guidelines—including IEC, BS and NEC—recommend keeping voltage drop under 3% for branch circuits (such as lighting and small loads) and 5% overall (feeder + branch). These limits help ensure safe, reliable operation.

Why Voltage Drop Matters in Real Installations

Voltage drop isn’t just a technical number. It directly affects how your equipment behaves. Excessive drop can cause:

• Dim lighting, especially on long lighting circuits
• Slow or weak motors, which may fail to start or draw more current
• Reduced efficiency, which increases energy bills
• Overheating, as motors compensate for low voltage
• Shortened equipment lifespan

Engineers, electricians, and installers size cables not just for ampacity (current-carrying capacity) but also to keep voltage drop within acceptable limits. This helps protect the installation and ensures stable performance.

How to Use This Voltage Drop Calculator

This voltage drop calculator is designed to be simple while still giving professional-grade accuracy. Here’s how to use it step by step.

1. Choose Your Calculation Mode

You can pick one of three modes depending on what you want to calculate:

• Voltage Drop:
  Enter cable size and length to calculate voltage drop (in volts and percent).
• Maximum Cable Length:
  Enter allowable % drop and cable size to find the longest cable run permitted.
• Minimum Cable Size:
  Enter allowable % drop and cable length to find the smallest cable that keeps drop within limits.

2. Select the System Type

Choose from:

• DC
• Single-phase AC
• Three-phase AC

The calculator automatically adjusts the equation for each type. For example, DC and single-phase use a route factor of 2, while three-phase uses √3.

3. Enter Voltage and Current

Input your system voltage (e.g., 12 V, 24 V, 230 V, 400 V, etc.) and the current drawn by the load.

4. Set the Power Factor (AC only)

For AC circuits, voltage drop depends partly on the angle between current and voltage. This is expressed using power factor (PF).

• Motors typically run at PF 0.8–0.9
• Resistive loads like heaters run close to PF 1
• For DC, PF doesn’t apply and is automatically set to 1

5. Select Cable Material

Choose between:

• Copper (Cu) – lower resistance, lower voltage drop
• Aluminium (Al) – cheaper, lighter, but higher resistance

The calculator automatically adjusts resistance values based on the chosen material.

6. Choose Operating Temperature

Cable resistance increases with temperature. You can choose:

• 20°C
• 75°C
• 90°C

Higher temperatures result in higher voltage drop.

7. Enter Cable Details

Depending on your chosen mode, enter:

• Cable size
• Cable length
• Allowable voltage drop
• Set Parallel Runs to the number of conductors per phase

Then click Calculate, and the tool gives an instant result.

What This Voltage Drop Calculator Takes Into Account

Many simple calculators only consider basic resistance. This one uses a more complete model:

AC Impedance Formula

For AC circuits, impedance (Z) considers both resistance (R) and reactance (X):

Z = R × PF + X × sinφ

This produces more accurate real-world results.

Material & Temperature Corrections

Copper and aluminium have different resistivities. Also, as cables heat up, resistance increases. The calculator adjusts automatically based on your selected conditions.

Circuit Type Adjustments

• DC & single-phase use a route factor of 2
• Three-phase uses √3

These match standard engineering formulas.

Real Examples

Example 1: Voltage Drop in a Single-Phase Circuit

• 230 V
• 10 A
• PF = 0.85
• Copper 2.5 mm²
• 50 m length

Result:

• Voltage drop ≈ 7.8 V
• Percentage drop ≈ 3.4%

This is right on the boundary for lighting, where 3% is often recommended.

Example 2: Maximum Cable Length in a Three-Phase System

• 400 V
• 50 A
• PF = 0.9
• Copper 16 mm²
• Allowable drop = 5%

Result:

• Maximum length ≈ 120 m (tool-dependent)

This tells you how far you can safely run the cable while staying within voltage drop limits.

Common Voltage Drop Limits & When They Apply

Different standards give different values, but most installers follow this simple rule:

• 3% voltage drop for lighting and sensitive loads
• 5% voltage drop total (feeder + final circuit)

Always check your local electrical code, because limits vary between IEC, NEC, and national standards.

Does Voltage Drop Also Need to Consider Motor Starting Current?

Yes — motor starting current (inrush current) is a major factor in voltage drop, and it’s even more critical than the running current for many installations. While most voltage drop calculators (including this one) use the running current for normal operation, motors behave very differently when they start. During start-up, they draw 5 to 8 times their rated current, and this surge causes a much larger temporary voltage drop.

This short-term drop won’t damage the cable, but it can affect whether the motor actually starts.

Why Motor Starting Current Matters for Voltage Drop

When a motor starts:

• Current can jump to 500–800% of rated current
• Voltage drop can spike above normal design limits
• Lights can flicker, contactors can chatter, and equipment may fail to start
• Motors may stall or overheat if the voltage dips too low

If the voltage drop during start-up is too high, a motor may:

• Struggle to accelerate
• Take longer to reach speed
• Trip overload protection
• Fail to start entirely

This is why motor circuits often require larger cables than what load current alone suggests.

How to Consider Motor Starting Current With This Calculator

The calculator gives accurate results for running (steady-state) voltage drop, but for motors you should also check voltage drop at start-up.

Here’s how:

1. Enter starting current instead of running current

If your motor is 10 A running current and has a starting multiplier of 6x:

• Use 60 A in the voltage drop calculator
• Keep all other parameters the same (system type, PF, cable size, etc.)

This gives you the starting-condition voltage drop, which is the real limiting factor on many installations.

2. Compare the result to motor starting requirements

Most motors require voltage during start not to fall below:

• 80% of rated voltage (typical)
• 85% for many small motors
• 90% for sensitive or electronically-controlled equipment

If your start-up drop exceeds ~10–15%, you may need:

• A larger cable
• A shorter cable run
• A soft starter
• A VFD
• A transformer tap change
• Reduced-voltage starting method

What Power Factor Should You Use for Motor Starting Voltage Drop?

Motor starting PF is much lower than running PF.

Typical values:

• Running PF: 0.80–0.90
• Starting PF: 0.20–0.45

Lower PF increases voltage drop because more of the impedance comes from reactance.

A good guideline:

• Use PF between 0.25 and 0.35 when modelling start-up voltage drop

This gives a realistic snapshot of what the motor “feels” during inrush.

Tips for Getting Accurate Results

To make sure your voltage drop calculation matches real installation conditions, follow these tips:

• Use realistic power factor values, especially for motors
• Check cable construction (single-core vs multi-core) since reactance changes
• Choose the correct operating temperature—most cables run at 75°C or 90°C
• Remember that long runs almost always need upsizing
• For DC systems, keep in mind that reactance doesn’t apply, so PF is always 1

FAQs (Voltage Drop Calculator)

When to Use Each Calculation Mode

Voltage Drop Mode

Use when you already know the cable size and length, and you want to check if drop is acceptable.

Maximum Cable Length Mode

Use when you know the cable size and allowable percentage drop, and you want the maximum run length.

Minimum Cable Size Mode

Use when you know the required length and drop limit and want the smallest cable that fits.

Best Practices for Cable Sizing

• Keep lighting circuits under 3% drop for best performance
• For motors, always use the correct PF and consider starting currents separately
• When in doubt, upsize the cable—it reduces heat, drop, and long-term costs
• Always cross-check against your local electrical code

Try the Voltage Drop Calculator

Use the calculator now to size cables correctly, avoid overheating, reduce energy loss, and ensure your installation performs the way it should. Whether you’re working on DC circuits, residential wiring, industrial three-phase systems, or renewable energy projects, this tool gives fast, practical, real-world results.