Voltage drop calculator

What voltage drop actually is

Every metre of cable has some resistance, so pushing current through it uses up a little of the supply voltage before that current ever reaches the load. The load ends up seeing less voltage than the source is putting out, and the difference is lost as heat in the cable itself rather than doing anything useful. That heat is also a real cost: it is electricity paid for and never received as light, torque or charge, for the whole life of the installation.

Left unchecked, an undersized cable shows up in different ways depending on what is at the end of it. A motor runs hotter and starts less reliably, since starting current is many times the running current and the drop is worst at exactly that moment. LED fittings dim or flicker once the supply to the driver falls outside its working range. A grid tied solar inverter can trip on overvoltage on a hot, high output afternoon, because it has to push its own terminal voltage even higher than the grid to export current through a cable that resists it. None of these show up as a fault on day one. They show up as a slow loss of performance and a shorter equipment life, which is why voltage drop is checked at design time rather than fixed after the fact.

The formula this calculator uses

This follows the standard IEC voltage drop method used across most of the world, and referenced directly by France's NF C15-100, in SI units:

ΔU = b × (ρ₁ × L ÷ S × cosφ + λ × L × sinφ) × I_b

Where ΔU is the voltage drop in volts, b is 2 for single phase and DC circuits or 1 for three phase, ρ₁ is the conductor resistivity at its actual operating temperature, L is the one way cable length in metres, S is the cross section in mm², λ is the reactance per metre, and I_bis the design current. Resistivity itself is temperature dependent: ρ₁ equals ρ₂₀times 1 plus α times the temperature rise above 20 °C, using the IEC 60228 constants for annealed copper and aluminium, the same constants that most national wiring codes point back to whether or not they publish their own local voltage drop formula.

How to use this calculator, step by step

Here is the whole process on one example: a single phase 230 V circuit carrying 16 A through 4 mm² copper cable, 40 metres one way, at 70 °C, power factor 0.90.

1. Pick the AC circuit tab for mains wiring, or the DC tab for a battery or solar PV string.
2. Enter the supply: system type, nominal voltage, and power factor for AC. Use 230 V single phase or 400 V three phase here.
3. Enter the load as either a current or a power. 16 A is the design current in this example.
4. Enter the cable: material, size, one way run length and conductor temperature. Here that is copper, 4 mm², 40 m, 70 °C.
5. Read the results card: this example comes out at about 6.0 V dropped, 2.6 percent, and a Pass badge because it is inside the 3 percent target.
6. Check the sizing note underneath. It tells you the smallest standard size that still meets your target, so you know if you have room to go smaller or need to go larger.
7. Check the current carrying capacity note. It compares your design current against a typical reference rating for the size and installation method you picked, separate from the voltage drop result above it.
8. If the result does not meet your target, either raise the target, choose a different size from the dropdown, or use the chart to see how drop falls as cross section increases.

Resistivity by material

Supply voltage by region

Enter whatever your own supply actually is. As a starting point, most of the world outside North America, including Nepal, India, the whole of Europe, the United Kingdom, Australia and most of Africa, standardised on 230 V single phase and 400 V three phase, phase to phase, under IEC 60038. North America runs 120 V single phase with a 240 V split phase supply to large appliances, and commercial buildings are commonly fed at 208 V three phase or 480 V three phase in industrial settings. Japan is the outlier, at 100 V single phase and 200 V three phase. None of this changes the formula, only the number in the voltage field.

How different wiring codes treat the same problem

Every major wiring code cares about voltage drop, though they differ in how strictly they enforce it. IEC 60364-5-52, used as the base standard across Europe, Nepal and most of Asia, gives 3 percent for a final circuit and 5 percent overall as recommended limits. The UK's BS 7671 adopts the same IEC figures directly. The US National Electrical Code gives 3 and 5 percent as an informational note rather than a hard requirement, so it is treated as best practice rather than something an inspector fails a job over. Canada's CSA C22.1 and Australia's AS/NZS 3000 both reference broadly similar figures. Whichever code applies to a given job, a cable that clears roughly 3 percent on a final circuit and 5 percent overall will satisfy essentially all of them.

Current carrying capacity: the check most drop calculators skip

A cable can pass every voltage drop target and still be dangerous, because voltage drop and current carrying capacity are two separate limits. Drop is about the length of the run and how much of the supply you can afford to lose along it. Capacity is about how much current the conductor and its insulation can carry continuously before the cable itself overheats, and it barely cares about length at all. Most free voltage drop tools, including the one that inspired parts of this page, stop at the drop calculation and leave the capacity check as a separate step you have to do yourself, usually in a different tool entirely. This calculator folds a first pass at that second check into the same result, using the installation method you select next to the cable material.

The table below gives typical current ratings for PVC insulated copper cable at 70 °C, in the same format used across most IEC based wiring codes: reference method A1, enclosed in conduit or trunking in an insulated wall, and reference method C, clipped direct or run in free air. For aluminium, this calculator applies a commonly used approximation of 0.78 times the copper rating for the same size and method, since aluminium's higher resistance sheds less heat for the same cross section.

Treat these as typical, indicative figures rather than a rating you can build to. Real current carrying capacity depends on the specific cable construction, ambient temperature, how many other loaded cables it runs alongside, and whether it touches thermal insulation, and manufacturers publish correction factors for all four. Use this table to catch an obviously undersized cable early, then confirm the final size against the manufacturer's datasheet, such as a cable maker's own catalogue, and the wiring code that applies to your installation.

Two worked examples

A single phase 230 V circuit carrying 16 A through 4 mm² copper cable, 40 metres one way, at 70 °C, power factor 0.90. Resistivity at that temperature works out to about 0.0206 Ω·mm²/m, and the voltage drop comes to roughly 6.0 V, about 2.6 percent, comfortably inside the usual 3 percent target for a final circuit.

A common North American case tells a different story. A 120 V, 20 A circuit run 30 metres, about 100 feet, one way, is often wired in 10 AWG copper on the assumption that the size is already generous for the current. At 60 °C that cable actually comes out to about 3.8 percent drop, past the usual 3 percent target, and the next size up, 8 AWG, brings it back to about 2.4 percent. The lesson holds everywhere: a cable sized only for current rating, with no separate check on length, quietly fails on longer runs. Run the same current through a solar water pump instead and the story changes again: an undersized DC string cable can lose several percent of the array's output before it ever reaches the inverter, which is exactly why the solar pump sizing guide treats cable sizing as a real design step, not an afterthought.