Voltage Drop Calculation: How Long Can Your Cable Be?
Calculate the voltage drop of your cable and see when a larger cross-section is needed. Free tool for Belgium, no sign-up required.
Calculate voltage drop and choose a cable cross-section
Matching a circuit breaker to a conductor is essential, but it does not tell you how much voltage reaches the load. Voltage drop rises with cable length and current. A cross-section that works beside the distribution board may be too small for a garage, garden room or EV charger. Cable sizing therefore starts with two separate checks: can the protective device safely protect the conductor, and is voltage drop kept within the intended range? The stricter result sets the preliminary size.
When does voltage drop matter?
Long runs to outbuildings, outdoor lighting, sub-distribution boards, heat pumps and charging points are typical cases. Distance is only one input. Current, single- or three-phase supply, conductor material, cross-section, nominal voltage and power factor all affect the result.
The complete electrical route from the board matters too. If the supply consists of several cable segments, their voltage drops add up. Calculating only the last section to the appliance understates the total.
What AREI/RGIE says — and what it does not
Book 1, subsection 5.2.5 of Belgium’s AREI/RGIE does not state a numeric percentage. It requires voltage drop in electrical lines to be limited to the values described by the rules of good practice. The familiar 3% for lighting and 5% for other final circuits are the commonly applied interpretation from IEC/NBN HD 60364-5-52, not percentages literally printed in AREI/RGIE.
Section 5.2.1.2 also requires cable selection to be compatible with the safe operation of the supplied equipment. Table 4.11 in 4.4.1.4 limits the protective-device rating for each conductor cross-section in domestic installations. It is not a complete current-carrying-capacity calculation for every installation method. For that first lookup, see cable cross-section by circuit breaker.
The calculation
Here L is the one-way cable length:
- 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 conductor resistivity, I is current and S is cross-section in mm². For this preliminary check, PlanElec uses 0.0175 Ω·mm²/m for copper, 0.029 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). This is a resistance-only model; it does not include reactance or temperature correction.
Which voltage applies to your circuit?
The most common mistake in voltage-drop calculations 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 if it were three-phase yields a roughly twice as favourable result and leads to an undersized cross-section.
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 still 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 circuit at 230 V and 8 A. 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% preliminary value used for lighting. Increasing the conductor to 2.5 mm² reduces the loss to 4.79 V or 2.08%. It is the first standard cross-section that passes this criterion.
Garage socket circuit
For a 30 m copper run carrying 16 A at 230 V with 2.5 mm²:
ΔU = 2 × 0.0175 × 30 × 16 × 0.95 / 2.5 = 6.38 V
The result is 2.78%. The preliminary check uses 5% for this type of final circuit, so 2.5 mm² passes the voltage-drop criterion. That does not prove that the cable has adequate current-carrying capacity under its actual installation conditions.
Use the free calculator
The voltage-drop and cable-sizing calculator can check a selected cross-section or find a preliminary size. It covers all three 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 conductors. The result compares standard sizes and reports voltage-drop and protection checks separately.
The calculation runs directly in the browser so that sliders and result move together without delay; the same core is also available as the PlanElec API. It remains a preliminary assessment, not a complete cable design. The tool does not assess installation method, grouping, ambient temperature, insulation type, reactance, current-carrying capacity, short-circuit withstand, disconnection conditions, terminals or manufacturer data. A competent professional must validate the final design.
What can you do if the drop is too high?
Increasing the cross-section is usually the simplest response. A shorter route or a sub-distribution board closer to the loads may be more practical. Aluminium can be economical for long feeders, but it requires suitable terminals, transitions and its own design checks. For an EV charger or heat pump, three-phase supply can sharply reduce current per phase at the same total power. Load management can also prevent the service connection or breaker from being overloaded.
Common mistakes
- Applying 3% to every circuit. This preliminary model uses 3% for lighting and 5% for other final circuits.
- Checking only the breaker table. It does not account for cable length or actual load.
- Entering the return distance too. Enter the one-way length; the single-phase formula already contains factor 2.
- Ignoring upstream segments. Voltage drops across consecutive sections are cumulative.
- Treating an online result as proof of compliance. Installation and short-circuit checks are still missing.
Frequently asked questions
Is cable length one way or there and back?
Enter the one-way distance from the distribution board to the load. Factor 2 covers the return conductor in a single-phase circuit; the three-phase equation uses √3.
Can lighting use a 1.5 mm² conductor?
AREI/RGIE 5.2.1.2 permits 1.5 mm² for a final circuit without sockets when protection and all other conditions are suitable. 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, occupies more space and must fit the terminals. The sensible choice is the first size that passes every relevant design check.
Is the result ready for inspection?
No. It is a transparent plausibility check, not a complete calculation or inspection certificate. An electrical inspection dossier covers diagrams, protection, bonding and other requirements as well.