What this calculator does
Voltage drop is the loss of voltage that happens along the length of a cable as current flows through its resistance, leaving less voltage available at the far end than was supplied at the source. This calculator uses the standard metric formula, based on the cable's cross-sectional area in mm² rather than an AWG gauge number, to work out that drop for a copper or aluminium conductor.
The formula is voltage drop = (2 × ρ × L × I) / A, where ρ is the resistivity of the conductor material, L is the one-way length of the cable run, I is the current it carries, and A is its cross-sectional area. The factor of two accounts for the fact that current travels out along one conductor and returns along another of the same length, so the resistance of the full circuit is double that of a single one-way run.
The formula
Multiply the conductor's resistivity by 2, by the one-way cable length, and by the current, then divide by the cross-sectional area. Copper has a lower resistivity than aluminium, so a copper cable of the same size carries the same current with less voltage drop.
| Term | Meaning |
|---|---|
| Resistivity (ρ) | A fixed property of the conductor material: about 0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminium. |
| One-way length | The length of cable from source to load in one direction, not the total length of both conductors. |
| Cross-sectional area | The area of the conductor itself, in mm², as printed on the cable or found in its data sheet. |
The inputs explained
| Field | What to enter |
|---|---|
| Conductor material | The conductor material the cable is made from. Copper has lower resistivity and so less voltage drop for the same size and current. |
| One-way cable length (m) | The one-way distance from the supply to the load, not the round-trip length of cable used. |
| Current (A) | The current the cable is expected to carry, in amps. |
| Cross-sectional area (mm²) | The cross-sectional area of a single conductor, in mm², as stated on the cable. |
| Supply voltage (for % drop) (V) | The nominal supply voltage, used only to express the drop as a percentage. |
When to use it
Checking a long run to an outbuilding
A cable feeding a shed, pump or workshop some distance from the main switchboard can suffer a meaningful voltage drop over that extra length, which this calculator estimates before the cable is bought and installed.
Comparing conductor sizes before installation
Running the same length and current through a few candidate cross-sectional areas shows how much a larger cable actually reduces the voltage drop, which is one of the trade-offs against the larger cable's higher cost.
Comparing copper against aluminium
Aluminium cable is lighter and often cheaper than copper of the same current rating, but its higher resistivity means a larger cross-sectional area is usually needed to hold the voltage drop to the same level.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does voltage drop change with cable length?
A fixed 16 A load on 2.5 mm² copper cable, over a range of one-way cable lengths.
| One-way length | Voltage drop | Drop as % of supply |
|---|---|---|
| 5 m | 1.12 V | 0.49% |
| 10 m | 2.24 V | 0.97% |
| 20 m | 4.48 V | 1.95% |
| 30 m | 6.72 V | 2.92% |
| 50 m | 11.20 V | 4.87% |
| 75 m | 16.80 V | 7.30% |
How does voltage drop change with cross-sectional area?
A fixed 16 A load over a fixed 20 m one-way copper run, across a range of cable sizes.
| Cross-sectional area | Voltage drop | Drop as % of supply |
|---|---|---|
| 1.5 mm² | 7.47 V | 3.25% |
| 2.5 mm² | 4.48 V | 1.95% |
| 4 mm² | 2.80 V | 1.22% |
| 6 mm² | 1.87 V | 0.81% |
| 10 mm² | 1.12 V | 0.49% |
| 16 mm² | 0.70 V | 0.30% |
Questions
What voltage drop is acceptable?
Wiring rules in most jurisdictions set a maximum acceptable percentage drop, often around 3 to 5 percent depending on the circuit type, but the exact figure depends on the applicable code. Check the wiring rules that apply to the installation rather than relying on a single rule of thumb.
Why does the formula use 2 × length instead of just length?
Current has to travel out to the load along one conductor and back to the source along another of the same length, so the total resistive path is twice the one-way distance, even though you only measure and enter that one-way distance.
How is this different from the wire gauge (AWG) calculator?
The wire gauge calculator converts an AWG size to a physical diameter and cross-sectional area but does not calculate voltage drop. This calculator takes a cross-sectional area in mm² directly and works out the resulting voltage drop for a given length and current.
Should I use this instead of consulting an electrician?
No. This is a general reference calculation using standard resistivity values, useful for estimating and comparing options. Actual cable selection for a real installation should follow the local wiring rules and be checked or carried out by a qualified electrician.
For converting between an AWG wire size and its physical dimensions, see the wire gauge calculator. For the cost of running a circuit once it is sized, the electricity running cost calculator covers ongoing running costs.