Voltage Drop Calculator Formula
Understand the math behind the voltage drop calculator. Each variable explained with a worked example.
Formulas Used
Voltage Drop
vdrop = factor * current * cable_length * resistance / 1000Voltage Drop %
vdrop_pct = factor * current * cable_length * resistance / 1000 / supply_voltage * 100Variables
| Variable | Description | Default |
|---|---|---|
current | Load Current (I)(A) | 30 |
cable_length | One-Way Cable Length(m) | 50 |
resistance | Cable Resistance(ohm/km) | 1.15 |
supply_voltage | Supply Voltage(V) | 230 |
phases | Phase Configuration | 1 |
factor | Derived value= (phases == 1) ? 2 : sqrt(3) | calculated |
How It Works
Cable Voltage Drop
Voltage drop occurs as current flows through the resistance of cable conductors.
Formulas
Single phase: V_drop = 2 I L R_cable / 1000
Three phase: V_drop = sqrt(3) I L R_cable / 1000
where I is current, L is one-way length in metres, and R_cable is resistance in ohms/km. The factor of 2 in single-phase accounts for the return conductor. Most codes limit voltage drop to 3-5% of supply voltage.
Worked Example
Single phase, 30 A load, 50 m cable run, cable resistance 1.15 ohm/km.
- 01V_drop = 2 x 30 x 50 x 1.15 / 1000 = 3.45 V
- 02Percentage = 3.45 / 230 x 100 = 1.50%
- 03This is within the typical 3% limit.
Frequently Asked Questions
What is the maximum allowable voltage drop?
Most electrical codes allow 3% for branch circuits, 2% for feeders, and 5% total from service entry to final outlet. Some critical equipment may require tighter limits (1-2%).
How do I find cable resistance per km?
Cable manufacturers publish tables of resistance per km for each cable size and conductor material. Copper is about 60% less resistive than aluminium. Resistance increases with temperature.
What if voltage drop is too high?
Increase the cable size (lower resistance per km), reduce the cable length (relocate the panel closer), or increase the voltage (use 400V three-phase instead of 230V single-phase).
Learn More
Guide
Understanding Electrical Circuits: Fundamentals for Engineers
Learn the fundamentals of electrical circuits including Ohm's law, Kirchhoff's laws, series and parallel circuits, AC vs DC, power calculations, and three-phase systems.
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