Voltage Drop Calculator

Created: 18/03/2026Updated: 03/06/2026By: Mert
Voltage Drop Calculator
Vdrop = I × Rcable
R = ρ × 2L / A
% drop = Vdrop/Vs × 100
Voltage Drop in Cables
V Source Voltage
I Current
L One-Way Length
Single direction (calculator doubles for return)
A Conductor CSA
ρ Conductor Material
Enter voltage, current, length and CSA
Voltage Drop Results
Voltage Drop
—
V
% of Source
—
%
Voltage at Load
—
V
Cable R (total)
—
Ω
Cable Power Loss
—
W
R per metre
—
Ω/m
Voltage Drop Along a Cable Vs ← Vdrop/2 → Vload ← Vdrop/2 → I Vload = Vs − Vdrop  |  Vdrop = I × 2 × ρ × L / A

Figure 1: Voltage drops equally in the outgoing and return conductors. The load receives the source voltage minus the total cable drop. UK regulations limit drop to 3% for lighting and 5% for other circuits.

Table of Contents
Fundamentals
  1. What Is Voltage Drop?
  2. The Formula
Worked Examples
  1. UK Ring Main Circuit
  2. 12V LED Strip Run
  3. Long Industrial Feeder
Deep Dive
  1. UK Wiring Regulations (BS 7671)
  2. How to Reduce Voltage Drop
  3. Temperature Effects
Reference
  1. Frequently Asked Questions
  2. Related Circuit Analysis Calculators

What Is Voltage Drop?

Voltage drop is the reduction in voltage along a conductor carrying current. Every wire has resistance, and by Ohm’s law (V = IR), current flowing through that resistance creates a voltage drop. The load at the far end of the cable receives the source voltage minus the cable’s voltage drop.

Excessive voltage drop causes dim lighting, motor overheating, unreliable electronics, and wasted energy as heat in the cable. It is particularly important for long cable runs, low-voltage systems, and high-current circuits. The Series Circuit Calculator demonstrates the same voltage-drop principle across discrete resistors.

The Formula

Vdrop = I × Rcable
Rcable = ρ × 2L / A
Where ρ = resistivity (copper: 0.0172 Ω·mm²/m, aluminium: 0.0282), L = one-way length, A = conductor cross-section area (mm²).
The factor of 2 accounts for both the outgoing and return conductors.

Worked Example — UK Ring Main Circuit

Given: 230 V supply, 10 A load, 20 m copper cable, 2.5 mm²

Rcable = 0.0172 × 2 × 20 / 2.5 = 0.275 Ω

Vdrop = 10 × 0.275 = 2.75 V (1.2%)

Vload = 230 − 2.75 = 227.25 V

At 1.2%, this is well within the BS 7671 limit of 5% for power circuits. The Power Dissipation Calculator shows that 27.5 W is wasted as heat in the cable.

Worked Example — 12V LED Strip Run

Given: 12 V supply, 5 A, 10 m copper, 1.5 mm²

Rcable = 0.0172 × 2 × 10 / 1.5 = 0.229 Ω

Vdrop = 5 × 0.229 = 1.147 V (9.6%)

9.6% drop is too high for 12V LED strips. LEDs are sensitive to voltage — even 1 V drop on a 12 V system causes noticeable dimming at the far end. Solutions include using thicker cable, injecting power from both ends, or switching to 24 V strips.

Worked Example — Long Industrial Feeder

Given: 400 V three-phase, 30 A, 100 m aluminium, 6 mm²

Rcable = 0.0282 × 2 × 100 / 6 = 0.94 Ω

Vdrop = 30 × 0.94 = 28.2 V (7.1%)

At 7.1% this exceeds the 5% limit. The cable must be upsized to at least 10 mm² (which gives 4.2%) or the run shortened. For three-phase calculations, the Three Phase Power Calculator provides the full power analysis.

UK Wiring Regulations (BS 7671)

BS 7671 (18th Edition) specifies maximum voltage drop from the origin of the installation to the load. For public supply (230 V nominal): lighting circuits are limited to 3% (6.9 V) and other circuits to 5% (11.5 V). These limits ensure equipment operates within its rated voltage range and prevent excessive heat generation in cables.

How to Reduce Voltage Drop

There are four main strategies: increase the conductor cross-section (thicker cable), shorten the cable run, reduce the current (use a more efficient load or higher voltage), or switch from aluminium to copper. For very long runs, increasing the supply voltage (e.g. 24 V instead of 12 V for LED lighting, or 400 V three-phase instead of 230 V single-phase) is often the most cost-effective solution.

Temperature Effects

Copper resistance increases by about 0.39% per °C above 20°C. In hot environments or heavily loaded cables, the actual resistance (and therefore voltage drop) is higher than the room-temperature calculation. For critical installations, the Circuit Current Calculator can be used with temperature-corrected resistance values.

Frequently Asked Questions

Does voltage drop waste energy?
Yes. The voltage dropped across the cable appears as heat (P = I²R). This is wasted energy that heats the cable instead of powering the load. Higher drop means more waste.
Is voltage drop the same for AC and DC?
For DC and purely resistive AC loads, the calculation is the same. For AC with significant reactive loads, the impedance of the cable (including inductance) affects the drop. This calculator covers the resistive component, which dominates for most installations.
What cable size do I need?
Rearrange the formula: A = ρ × 2L × I / Vdrop(max). Enter your maximum acceptable drop (e.g. 3% of 230V = 6.9V) and the calculator gives you the minimum CSA. Then choose the next standard size up.
Does the neutral carry current back?
In single-phase circuits, yes — the neutral carries the full return current, which is why the formula uses 2×L. In balanced three-phase, neutral current is theoretically zero, so the drop is calculated per-phase with just L (not 2L).
Why is low-voltage drop more critical than high-voltage?
On a 12V system, a 1V drop is 8.3% — seriously affecting load performance. On 230V, a 1V drop is only 0.4%. Lower voltage systems are much more sensitive to cable resistance, which is why automotive and LED installations require careful cable sizing.
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Last updated: March 2026