Voltage Drop Calculator
Percentage voltage drop with a 5% compliance check.
Quick answer
With the default inputs: Load 5 kW · Nominal voltage 380 V · Phase Three phase 380 V etc.Voltage drop 0.76%
Edit the inputs below to recalculate.Inputs
Result
Voltage drop
0.76%
| Design current | 8.94A |
| Loop resistance | 0.2188Ω |
| Voltage drop | 2.88V |
| Voltage at load | 377.1V |
| Limit | 5% |
| Verdict | Pass |
| Max length at this size | 330.1m |
| Min size for 5% | 0.61mm² |
| Drop per 100 m | 1.51% |
The simplified forms ΔU = √3·I·R·cosφ for three phase and 2·I·R·cosφ for single ignore reactance, which is fine below 120 mm². Guidance limits: 5% for lighting and power, relaxed to 8% for temporary site supplies. Excessive drop makes motors hard to start, overheats them and shortens life.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Three-phase current | P ÷ (√3 × U × cosφ) |
| Resistance | ρ × L ÷ S |
| Three-phase drop | √3 × I × R × cosφ |
| Single-phase drop | 2 × I × R × cosφ |
| Percentage | ΔU ÷ U × 100 |
| Max length | L × 5 ÷ percentage |
5 kW at 380 V on 50 m of 4 mm² copper at 0.85 power factor: 8.94 A, 0.2188 Ω, a 2.88 V drop and 0.76% — a pass. The same cable at 30 kW drops 4.55%, close to the limit.
How to work it out
- Compute the current first: three-phase I = P ÷ (√3 × U × cosφ), single-phase I = P ÷ (U × cosφ). Rated current is not operating current — full load and light load differ widely.
- Work out conductor resistance R = ρ × L ÷ S, with ρ = 0.0175 Ω·mm²/m for copper and 0.0283 for aluminium. L is the one-way length.
- Three-phase drop is ΔU = √3 × I × R × cosφ. A single-phase circuit carries current out and back through two conductors, so ΔU = 2 × I × R × cosφ. Dropping that factor of 2 is the classic error.
- Divide by nominal voltage for the percentage. Limits: ±5% at motor terminals, ±5% for other utilisation equipment, -10% to +5% for lighting in general workplaces, and up to 10% momentarily during motor or fire-pump starting.
- If it fails there are three remedies: larger cross-section, shorter run, or a substation moved closer. Copper cost rises with the square of the size while rerouting is nearly free, so check the route first.
- Run it backwards: with a 5% allowance, the maximum feed length is Lmax = L × 5 ÷ drop%, which sets the service radius of a distribution board.
Where it goes wrong
- Counting resistance on one conductor only in a single-phase circuit. Current flows out and back, so omitting the factor of 2 halves the calculated drop and passes a circuit that actually fails.
- Treating 5% as a universal allowance. Lighting ends at -10%, motors get 10% during starting, medical equipment is tighter still. Judging everything against 5% misclassifies real problems.
- Using 20°C resistivity for continuous operation. A fully loaded conductor sits at 60-70°C where copper resistance is about 20% higher, which costs another percentage point on runs beyond 200 m.
- Watching only the percentage and not the absolute volts. A 5% drop on a 24 V control circuit is 1.2 V, enough that a relay will not pull in — control and ELV circuits must be checked in absolute terms.
Material and cost
One size up costs roughly 50% more at 4→6 mm², 60% at 6→10 mm² and 55% at 10→16 mm². On runs beyond 100 m, adding an intermediate distribution board is usually cheaper than upsizing: 800-2000 CNY per board including enclosure, breakers and mounting, at the price of an extra level of discrimination to design.
On site
- Villas, factories and temporary supplies are where this bites: the main board sits at one end, the load 80 m away, and the symptoms are hard-starting motors, contactors chattering, and lights dimming visibly.
- For welders and compressors, sizing on rated current understates the drop. Check the transient with starting current at 4-7 times rated, otherwise every start makes the lights flicker across the whole circuit.
- In a three-phase four-wire system with unbalanced single-phase loads, heavy neutral current shifts the neutral point and pushes terminal voltage further off than the phase-conductor calculation suggests.
- Field rule of thumb: aluminium 16 mm², 100 A over 100 m drops about 8% and definitely fails; copper 16 mm² under the same conditions is about 5%, right on the limit.
Basis
- GB 50052-2009 Code for design of electric power supply systems, voltage deviation and fluctuation
- GB 50054-2011 Code for design of low voltage distribution, protection of distribution circuits
- GB/T 12325-2008 Power quality — deviation of supply voltage
FAQ
Why 2 for single phase and √3 for three?
Because a single-phase circuit has an out and a return leg, so there are two lengths of resistance and the drop doubles. In three phase the currents are 120° apart and vector summation gives √3 ≈ 1.732, not 2 or 3. Swapping these factors is the commonest calculation error and shifts the answer by over 15%.
Why not just use a bigger cable on long runs?
Because cost turns sharply once the size grows: capital rises linearly with area while losses fall with the square of the inverse, so there is an economic size. Beyond it the saved energy no longer pays for the copper. The usual answer for loads over 200 m is a local distribution board, or distributing at higher voltage and transforming on site.
What does low voltage actually do?
Motors suffer most: a 10% drop cuts torque by about 19% since torque goes with the square of voltage, while starting current rises, starts take longer, windings run hotter and life halves. Lighting shows it as dimmed incandescent lamps, reluctant fluorescents and unstable LED drivers. The code limit protects equipment, not just brightness.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Voltage Drop Calculator. 2026-10-25. https://jianzhujisuan.com/en/voltage-drop
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.