What voltage drop limits mean in a design
Voltage drop is the difference between the voltage at the origin of an installation and the voltage at the point of use, caused by the impedance of the conductors carrying the load current. A limit on it exists for a simple reason: equipment at the far end of a long circuit still has to work within its rated voltage range.
The limit is expressed as a percentage of the nominal voltage. A 5% limit on a 230 V single-phase circuit means the voltage at the load must not fall more than about 11.5 V below the supply voltage.
Voltage drop is a separate concern from current-carrying capacity. A cable can be perfectly safe from overheating and still fail the voltage check, especially on long runs. That is why the two are always checked together.
Where the limits come from in IEC 60364
IEC 60364-5-52 gives guidance values for voltage drop between the origin of the installation and any point of use. When the installation is supplied directly from a public low-voltage network, the usual figures are 3% for lighting circuits and 5% for other uses.
Where the supply comes from a private low-voltage source — for example a dedicated transformer serving one building — the guidance values are wider, in the region of 6% for lighting and 8% for other uses. These are recommendations rather than absolute rules, and a national standard may set a different figure.
Two practical points follow. First, the limit is measured from the origin of the installation, not from the distribution board, so a feeder to a sub-panel counts towards the total. Second, where no specific figure is given, a stricter project rule or a manufacturer's minimum supply voltage governs.
| Supply | Lighting | Other uses |
|---|---|---|
| Public low-voltage network | 3% | 5% |
| Private low-voltage source | 6% | 8% |
The voltage drop formula for single and three phase
The general form includes resistance and reactance: for a single-phase circuit the drop is 2 × I × L × (R cos φ + X sin φ), and for a three-phase circuit it is √3 × I × L × (R cos φ + X sin φ), with R and X expressed per unit length and L the length of the run.
For the small cross-sections and moderate lengths typical of final circuits, reactance can often be neglected and the formula reduced to resistivity. For copper that gives 2 × L × I × cos φ / (γ × S) single-phase, and √3 × L × I × cos φ / (γ × S) three-phase, where γ is the conductivity — about 57 m/(Ω·mm²) for copper and 34 for aluminium — and S is the cross-section in mm².
Convert the result to a percentage by dividing it by the nominal voltage and multiplying by 100. Keep units consistent: L in metres, I in amperes, S in square millimetres.
Power factor, reactance and when the simple form is enough
The cos φ term matters. A circuit with a low power factor draws more current for the same power, and that current is what produces the drop. Assuming cos φ equals 1 for a motor circuit will understate the drop and can hide a failing design.
Reactance becomes significant on large cross-sections and long runs. Above roughly 25 mm², and at distribution level, the X sin φ term is no longer negligible and the full formula should be used rather than the resistivity simplification.
Temperature also shifts resistance. Conductor resistance rises with temperature, so a cable warm under load has slightly more resistance than its cold value. For final circuits the effect is small, but on heavily loaded feeders it is worth keeping in mind.
How to check every circuit in a project
Check voltage drop circuit by circuit, from the origin to the most remote point. For a circuit fed through a sub-distribution board, the total is the sum of the drops in each section: the feeder plus the final circuit.
The worst case is usually the longest circuit carrying a substantial current, not simply the highest-power circuit. A modest load at the far end of a building can fail while a heavy load near the board passes easily.
- Determine the design current of each circuit.
- Take the route length from the model, not from a straight-line estimate.
- Compute the drop for the section and convert it to a percentage.
- Add the contribution of any upstream feeder.
- Compare the total with the applicable limit.
Building the check into the model is more reliable than doing it by hand at the end, because the length and the load are already known there.
What to change when a circuit fails the check
The first remedy is a larger cross-section. Resistance falls in proportion to area, so the drop falls with it, and this is usually the cheapest fix on a final circuit.
The second is to shorten the electrical route — move a board closer to the load, or split one long circuit into two. The third is to reduce the current by rebalancing the loads, which also helps the heating calculation.
Changing only the protective device does not fix voltage drop. A larger breaker does not reduce the drop, and it may break the coordination with the cable's current-carrying capacity. The device has to stay matched to the conductor.
Common mistakes in voltage drop checks
Measuring from the distribution board instead of the origin of the installation understates the total, because the feeder drop is left out. On a building with sub-panels that omission can be the whole margin.
Using the breaker rating as the current overstates the drop when the real design current is lower; the check should use the load current of the circuit, not the rating of the device protecting it.
Ignoring power factor on motor and transformer circuits understates the drop, and rounding a bare pass to the nearest standard size without rechecking can tip it back into a fail.
FAQ
What voltage drop limit should I use?
IEC 60364-5-52 guidance is 3% for lighting and 5% for other uses from a public low-voltage network, wider for a private supply. A stricter national or project rule governs.
Which formula applies to a three-phase circuit?
The drop is √3 × I × L × (R cos φ + X sin φ). For short final circuits reactance can be neglected and the resistivity form √3 × L × I × cos φ / (γ × S) used instead.
Do I measure voltage drop from the distribution board?
No. It is measured from the origin of the installation, so the feeder to a sub-panel counts. The total is the sum of the drops in each section.
Does a bigger breaker fix voltage drop?
No. The breaker rating does not change the drop and may conflict with the cable's current-carrying capacity. Increase the cross-section or shorten the route instead.
When is voltage drop the deciding factor?
On long runs and circuits with a low power factor. A cable can pass the heating check yet fail the voltage check, so both criteria are always evaluated.