Voltage Drop Calculation: How Long Can Your Cable Be?
Calculate the voltage drop of your cable and see at which length a larger cross-section is needed. Formula, length table and free calculator for Belgium.
AuthorStephan BehmHow long a cable can be is decided by voltage drop: a 2.5 mm² copper cable carrying 16 A at 230 V reaches the usual 5% at about 54 m, and a 1.5 mm² lighting circuit at 10 A reaches the 3% lighting value at about 31 m. Beyond that, you need a larger cross-section, a shorter route or a sub-distribution board closer to the load. The circuit breaker does not answer this question: it protects the cable against overload but says nothing about the voltage that reaches the appliance. So always check both: does the cable suit the protective device, and does voltage drop stay within the target? The stricter requirement sets the cross-section.
When does voltage drop matter?
Typical cases are long runs to a garage, garden room or outbuilding, outdoor lighting, feeders to sub-distribution boards, heat pumps and EV charging points. Besides distance, current, single- or three-phase supply, conductor material, cross-section, nominal voltage and power factor all count.
Always calculate the complete electrical route from the board. If it consists of several segments, such as the feeder to a sub-board plus the final circuit, their voltage drops add up. Calculating only the last section to the appliance understates the result.
What the AREI/RGIE requires
Section 5.2.5 of AREI/RGIE Book 1 ("Voltage drop") requires voltage drop to be limited to the values described in the rules of good practice. It sets no percentage. The familiar 3% for lighting and 5% for other final circuits come from the IEC/NBN HD 60364-5-52 series and are the usual way those rules of good practice are applied.
Subsection 5.2.1.2 b additionally requires voltage drop under normal operating conditions to be compatible with safe operation of the supplied equipment. The second check, cross-section against protection, is governed by Table 4.11 in subsection 4.4.1.4 for domestic installations. It limits the protective device per cross-section but does not prove current-carrying capacity for every installation method. The complete table with the fuse and circuit-breaker columns is in cable cross-section by circuit breaker.
The formula
L is the one-way cable length, not there and back:
- single-phase:
ΔU = 2 × ρ × L × I × cos φ / S - three-phase:
ΔU = √3 × ρ × L × I × cos φ / S - percentage:
ΔU% = ΔU / Un × 100
ρ is resistivity, I current and S the cross-section in mm². PlanElec calculates with 0.0175 Ω·mm²/m for copper and 0.029 Ω·mm²/m for aluminium, a default power factor of 0.95 and the nominal voltage of the circuit: 230 V for single-phase final circuits, and for three-phase circuits the voltage between line conductors (230 V or 400 V). The resistance model leaves out reactance and temperature correction, which makes it a sound preliminary design for homes and small commercial installations.
Rearranged for length, it gives the longest permitted one-way run for a chosen limit: L_max = ΔU_max × S / (2 × ρ × I × cos φ) for single-phase circuits.
Maximum cable length per cross-section
The table below uses copper, power factor 0.95 and the full rated current of the breaker as the load. If the actual load is lower, the run can be proportionally longer.
| Circuit | Cross-section | Current | Limit | Max. one-way length |
|---|---|---|---|---|
| Lighting, 230 V | 1.5 mm² | 10 A | 3% | approx. 31 m |
| Lighting, 230 V | 1.5 mm² | 16 A | 3% | approx. 19 m |
| Lighting, 230 V | 2.5 mm² | 16 A | 3% | approx. 32 m |
| Sockets/appliances, 230 V | 2.5 mm² | 16 A | 5% | approx. 54 m |
| Sockets/appliances, 230 V | 2.5 mm² | 20 A | 5% | approx. 43 m |
| Appliance circuit, 230 V | 4 mm² | 25 A | 5% | approx. 55 m |
| Hob/EV charger, 230 V | 6 mm² | 32 A | 5% | approx. 65 m |
| Three-phase 3N~400/230 V | 2.5 mm² | 16 A | 5% | approx. 108 m |
| Three-phase 3N~400/230 V | 6 mm² | 32 A | 5% | approx. 130 m |
The values apply to a single segment from the board. If the final circuit is fed from a sub-board, subtract the feeder's voltage drop first.
Which voltage applies to your circuit?
The most common mistake is confusing the network type with the circuit. The 400 V of a three-phase connection exists only between two line conductors. An ordinary socket or lighting circuit is connected between a line conductor and neutral even in such a home, and therefore sees 230 V.
| Supply network | Circuit | Nominal voltage | Factor |
|---|---|---|---|
| 1×230 V | single-phase | 230 V | 2 |
| 3×230 V without N | single-phase, L–L | 230 V | 2 |
| 3×230 V without N | three-phase | 230 V | √3 |
| 3N~400/230 V | single-phase, L–N | 230 V | 2 |
| 3N~400/230 V | three-phase | 400 V | √3 |
Because the factor and the reference voltage change together, the difference is large: the same 16 A over 30 m with 2.5 mm² gives 2.78% single-phase, but 1.38% as a three-phase circuit at 400 V. Calculating a single-phase circuit as three-phase produces a result roughly twice as favourable and leads to an undersized cable.
In a network without a neutral, a single-phase circuit runs between two line conductors. The voltage there is also 230 V, and because both conductors carry the full current, factor 2 applies. To identify your network type, see Single-phase or three-phase in Belgium.
Two worked examples
Lighting in a garden room
A 45 m copper run supplies a lighting-only load of 8 A at 230 V. With 1.5 mm² and power factor 0.95:
ΔU = 2 × 0.0175 × 45 × 8 × 0.95 / 1.5 = 7.98 V
That is 3.47%, above the 3% for lighting. With 2.5 mm², the loss falls to 4.79 V or 2.08%. For this criterion, 2.5 mm² is the first standard size that fits.
Garage socket circuit
A 30 m copper run carries 16 A at 230 V. With 2.5 mm²:
ΔU = 2 × 0.0175 × 30 × 16 × 0.95 / 2.5 = 6.38 V
That is 2.78%. Other final circuits use the 5% value, so 2.5 mm² passes the voltage-drop criterion. Whether the cable carries enough current in its actual installation method is a separate check, covered in Installation methods under AREI.
Calculate online for free
The voltage-drop and cable-sizing calculator checks a selected cross-section or finds a preliminary size, free and without sign-up. It covers the three Belgian network types (1×230 V, 3×230 V and 3N~400/230 V), distinguishes single-phase from three-phase circuits, and takes loads in A, W, kW or kVA for copper or aluminium. The result compares standard sizes and reports voltage drop and protection separately.
The calculation runs directly in the browser, so sliders and result respond instantly; the same calculation core is also available through the PlanElec API. The calculator is built for preliminary design. Installation method, grouping, ambient temperature, insulation type, reactance, short-circuit withstand, disconnection conditions, terminals and manufacturer data belong to the professional design that follows.
What to do if voltage drop is too high
The most direct lever is a larger cross-section. Often a shorter route or a sub-board close to the loads is the better solution. Aluminium pays off on long feeders but needs suitable terminations and its own design checks. For an EV charger or heat pump, a three-phase supply sharply reduces current per phase at the same total power. Load management also keeps the service connection and protective device within their permitted range.
Common mistakes
- Applying 3% everywhere. 3% applies to lighting, 5% to other final circuits.
- Checking only the breaker table. It does not contain length or actual load.
- Entering the return distance too. Enter the one-way length; factor 2 is already in the single-phase formula.
- Forgetting upstream segments. The voltage drops of all segments add up.
- Treating voltage drop as the whole design. Installation method and short-circuit behaviour are separate checks.
Frequently asked questions
Do I enter the cable length one way or there and back?
One way: the distance from the board to the load. In a single-phase circuit, factor 2 covers the return conductor; three-phase uses √3.
Can a lighting circuit use 1.5 mm²?
Yes. Table 5.1 under subsection 5.2.1.2 permits 1.5 mm² for circuits without socket outlets, protected under Table 4.11 by at most a 16 A circuit breaker or a 10 A fuse. A long run can still require 2.5 mm² or more because of voltage drop.
Is a larger cross-section always better?
It reduces losses but costs more, takes up space and must fit the terminals. Choose the first size that passes every relevant check.
What else does the inspection need?
The calculator delivers a traceable voltage-drop calculation. An electrical inspection dossier also contains the single-line diagram, the situation plan and on-site measurements; the assessment is made by the accredited inspection body.
From a preliminary result to a verifiable design record
A good calculation sheet records more than the final percentage. Note the board and circuit, every cable segment, the one-way route length, conductor material, phase connection, nominal voltage, design current and the source of that current: nameplate, manufacturer design sheet or explicit planning scenario. An assumed power factor stays visibly marked as an assumption.
Next, record the acceptance criterion and its source. Because section 5.2.5 refers to the rules of good practice, a project value may come from the applicable standard, a specification, a network operator requirement or equipment limits. The 3% and 5% in this article are the usual starting point; a sensitive load can require a narrower voltage range at its terminals.
Then keep the checks separate. Voltage drop answers one question. The conductor must also carry the current in its actual installation method, be coordinated with overload and short-circuit protection, meet disconnection conditions and thermal withstand, and fit the terminals and cable routes mechanically. Several individually acceptable segments can still add up to too much drop. The main feeder, sub-board feeder and final circuit therefore belong in one balance.
After installation, compare the documented route and conductor with the work on site. Measurements reveal supply conditions or contact resistances that a resistance-only model does not predict. When the route, material or phase assignment changes, update the single-line diagram and cable list. Such a record convinces the electrician and the inspector far more than a screenshot with a green tick.
Tracking changes
Keep inputs and results for each design version. If the cable length changes with the real route, the supply changes from single- to three-phase or the load changes to another appliance, recalculate and note the source. Compare percentage, end voltage and the cumulative segments. A larger conductor improves voltage drop but does not answer any open question on protection, terminals or installation.
Stating the result correctly
Do not write "cable is compliant", but for example: "For the documented load, length, nominal voltage and stated project criterion, the resistance model gives this voltage drop." Add the points that are still open. A designer can then repeat the calculation, change inputs and attach the remaining sizing evidence. Mark a scenario calculation for a load that is not yet known as such, and replace it once the actual appliance is chosen. Also note whether the length comes from the plan, a measurement or a conservative assumption.
Checking voltage drop in your project
In PlanElec, you draw cable routes on the floor plan and their length flows into the circuit. The self-check uses it to calculate voltage drop for every final circuit against the 3% and 5% guide values and flags exceedances alongside the cross-section-versus-protection check. The rule coverage is listed in What the self-check verifies.
Related articles
- Cable cross-section by circuit breaker
- Single-phase or three-phase in Belgium
- Installing an EV charger under AREI/RGIE
- Dedicated circuits for appliances
Official basis: FPS Economy, AREI/RGIE Book 1 V06, in particular section 3.2.2 (types of systems), subsections 4.4.1.4 and 5.2.1.2 and section 5.2.5.