Electrical cable sizing according to NIBT 2020
Instant calculation using the ElectroCAD Tools algorithms: permissible current Iz corrected by Kt × Kg, voltage drop with unified formula (ρ corrected to 70 °C PVC / 90 °C XLPE, reactance X in steps), compliant with NIBT chapters 5.1 and 5.2.
Automate this calculation in AutoCAD
ElectroCAD Tools integrates these calculations directly into your diagrams with automatic BOM generation, Iz/ΔU checking on each line and SIA 451 export for the quote. Same algorithms, integrated into the schematic diagram.
How do you calculate the electrical cable cross-section according to NIBT 2020?
In Switzerland, a cable's cross-section is determined by two complementary regulatory verifications, imposed by the Low-Voltage Installation Standards (NIBT) published by the SEV/Electrosuisse in 2020. The first check concerns the current-carrying capacity Iz (chapter 5.1): the cable must not heat beyond its insulation limit at the circuit's rated current. The second concerns the voltage drop ΔU (chapter 5.2): the voltage received by the load must remain close enough to the nominal voltage to ensure proper operation.
Un calcul correct exige donc de connaître le courant nominal (généralement le calibre du disjoncteur amont), la longueur réelle du câble, le mode de pose (A1 à G : en tube, encastré, apparent, enterré ou aérien), la température ambiante maximale, le nombre de circuits adjacents groupés et la nature du conducteur (cuivre ou aluminium, isolation PVC ou XLPE). Les deux contraintes — Iz suffisant et ΔU acceptable — sélectionnent la section minimale conforme. Le calculateur applique automatiquement les facteurs correctifs Kt et Kg issus des tableaux normalisés.
Current-carrying capacity method (NIBT 5.1)
The nominal current-carrying capacity of a cable, denoted Iz, is tabulated for reference conditions: 30 °C ambient temperature and a single circuit installed alone. As soon as these conditions change, two multiplicative correction factors come into play: Kt for the actual temperature, Kg for the grouping of parallel circuits. The corrected current-carrying capacity is written:
Iz_corrected = Iz_nominal × Kt × Kg
The compliance rule is simple: Iz_corrected must be greater than or equal to the nominal current I of the circuit, lui-même au plus égal au calibre du dispositif de protection en amont. Pour le cuivre PVC, à 40 °C avec trois circuits groupés jointifs, Kt=0,87 et Kg=0,70 ; un câble de 2,5 mm² en B1, donné pour 27 A à 30 °C (deux conducteurs chargés), ne porte plus que 27 × 0,87 × 0,70 ≈ 16,4 A. Un disjoncteur de 20 A en aval ne protège alors plus correctement le câble contre la surcharge thermique.
This check is mandatory for every final circuit and every feed. It is performed before the voltage drop check, because it determines the absolute minimum cross-section.
Voltage drop method (NIBT 5.2)
The voltage drop along a cable depends on its length, the current carried and the linear resistance of the conductor. For typical building lengths and a power factor close to 1, the reactance is neglected and the simplified formulas are used:
- Single-phase : ΔU (V) = 2 × L × I × ρ / S
- Balanced three-phase : ΔU (V) = √3 × L × I × ρ / S
- Direct current: ΔU (V) = 2 × L × I × ρ / S (out and return)
where L is the one-way length in metres, I the current in amperes, S the cross-section in mm² and ρ the conductor resistivity. For copper at 20 °C, ρ = 0.0224 Ω·mm²/m; for aluminium, ρ ≈ 0.036 Ω·mm²/m. The percentage drop is calculated as ΔU% = ΔU / U_nominal × 100.
NIBT 5.2.4 caps the voltage drop at 3 % for lighting and signalling circuits, and to 5 % for socket outlets, power and heating, measured between the delivery point and the load. On long runs (scattered villas, farm sheds, charging stations), it is this criterion, more demanding than Iz, that sizes the cross-section.
Common cable cross-sections vs current-carrying capacity (Cu PVC, method B1, 30 °C, two loaded conductors)
| Cross-section (mm²) | Nominal Iz (A) | Recommended protection (A) | Typical application |
|---|---|---|---|
| 1,5 | 19 | 10 / 13 | Lighting, signalling |
| 2,5 | 27 | 16 / 20 | Household socket outlets, spare lighting |
| 4 | 36 | 25 / 32 | Kitchen circuits, washing machines |
| 6 | 47 | 32 / 40 | Water heater, three-phase hobs |
| 10 | 63 | 50 | Cooker, sub-distribution board supply |
| 16 | 85 | 63 | 11 kW EV charging station, sub-board |
| 25 | 113 | 80 / 100 | Supply for dwelling, workshop |
| 35 | 138 | 100 / 125 | Main panel for house, SME |
| 50 | 169 | 125 / 160 | Multi-family house service connection |
| 70 | 214 | 160 / 200 | Commercial building |
| 95 | 260 | 200 / 250 | Residential building |
Values from the tables used by the calculator above (NIBT 2020 chapter 5.2): PVC copper, installation method B1, ambient temperature 30 °C, two loaded conductors (single-phase circuit), without grouped circuits. In three-phase (three loaded conductors), Iz is lower. Always correct with the actual Kt and Kg.
Which installation method to choose?
The installation method determines heat dissipation and therefore the value of Iz. NIBT 2020 (annex 5.1) distinguishes several categories. Choosing the right method is as important as the cross-section itself.
A1 / A2 — In a thermally insulating wall
Insulated conductors or cables in a conduit (A1), or multi-core cable in a conduit (A2), embedded in a thermally insulating wall. Worst case: the heat remains trapped, minimum Iz.
B1 — Insulated conductors or single-core cables in conduit
Insulated conductors or single-core cables in a conduit, trunking or duct — surface-mounted, embedded in masonry, in a suspended ceiling or in a service shaft. The typical case of conduit installations in Switzerland.
B2 — Multi-core cable in conduit
Multi-core (sheathed) cable in a conduit or duct, surface-mounted or embedded in masonry. Iz slightly lower than B1 because the cable sheath limits dissipation.
C — Surface-mounted or embedded in masonry
Cable fixed directly to a wooden or masonry wall or ceiling, on an unperforated tray, or embedded directly in masonry without conduit. Good heat dissipation through contact with the wall.
D1 / D2 — Buried
Cable in buried conduit or cable duct (D1) or directly buried (D2). Typical for feeds between buildings, outdoor EV chargers, public lighting.
E / F — Installation in free air
Multi-core cable (E) or single-core cables (F) on cable ladder, perforated cable tray, self-supporting or spaced more than 0.3 × the diameter from the wall. Very good dissipation, high Iz — typical of commercial and industrial buildings.
G — On insulators
Bare or insulated conductors mounted on insulators, spaced apart. Maximum heat dissipation, reserved for special installations.
An often-forgotten NIBT detail: the factors Kt and Kg are multiplied together. Many online calculators apply only Kt, which undersizes the cable as soon as two circuits are grouped. NIBT 2020 chapter 5.1.4 explicitly requires combining both corrections.
Common sizing errors
- Forgetting the temperature factor: a poorly ventilated technical room can reach 40 to 45 °C in summer. Kt then drops to 0.87 or 0.79, which reduces Iz by 13 to 21 %.
- Ignoring circuit grouping: in a loaded trunking with 5-6 circuits, Kg drops to 0.55-0.60. Many villa distribution boards show this trap at the sub-board feed.
- Underestimating the actual length: you must count the cable length, not the straight-line distance. A vertical drop, two bends and a return quickly add 15 to 20 m.
- Confusing phase-to-neutral voltage with line-to-line voltage: 230 V single-phase between phase and neutral, 400 V three-phase between phases. The ΔU formula changes (factor 2 vs √3).
- Mixing copper and aluminium in the calculation: ρ changes, the 1.6× coefficient on the cross-section must be applied consistently over the entire length.
FAQ — Cable cross-section calculation
What cross-section for 16 A single-phase?
For a rated current of 16 A in single-phase 230 V, the minimum cross-section depends on the installation method. In B1 (conductors in conduit or trunking), a cross-section of 2,5 mm² copper PVC gives Iz=27 A at 30 °C (two loaded conductors), compliant with NIBT 2020. For lengths over 30 m, check the voltage drop with ΔU=2·L·I·ρ/S; beyond 35-40 m, move up to 4 mm² to stay below 3 % for lighting.
How do you calculate voltage drop?
En monophasé : ΔU = 2 · L · I · ρ / S, en triphasé équilibré : ΔU = √3 · L · I · ρ / S, où L est la longueur en mètres, I le courant en ampères, ρ la résistivité (0,0224 Ω·mm²/m pour le cuivre à 20 °C) et S la section en mm². Le résultat est en volts ; ramener en pourcentage de la tension nominale. La NIBT 4.7 et le chapitre 5.2 imposent une chute maximale de 3 % for lighting and 5 % for sockets and power.
Minimum NIBT cross-section for lighting?
NIBT 2020 (chapter 5.2.4) sets a minimum cross-section of 1,5 mm² copper for lighting and signalling circuits. For socket outlets and power circuits, the minimum is 2,5 mm² in copper. These minima do not exempt you from the voltage-drop calculation or from checking the permissible current Iz according to the installation method.
Cu or Alu cable: cross-section difference?
Aluminium has a resistivity of about 0.036 Ω·mm²/m versus 0.0224 for copper, i.e. ~1.6× more. To carry the same current with equal losses, an aluminium cross-section must be about 1,5 to 1,6 times higher. Aluminium is generally reserved for cross-sections ≥ 16 mm² according to several Swiss utilities (NIBT 5.2.1 and DSO rules). Copper remains the standard for residential and commercial buildings.
What safety margin should be applied?
A margin of 20 à 30 % between the nominal current I and the corrected permissible current Iz is recommended to absorb ambient temperature variations, insulation ageing and the future addition of parallel circuits. For voltage drop, targeting 2 % for lighting (rather than 3 %) leaves a reserve in case of extension. On main feeders, plan one cross-section size above the strict calculation.
How can this calculation be automated in AutoCAD?
The plugin ElectroSchema pour AutoCAD intègre le calcul automatique de section et de chute de tension à partir des données saisies dans la nomenclature : courant nominal, longueur de ligne, mode de pose, température. La commande dédiée propose la section conforme NIBT 2020 et met à jour les annotations du schéma. Cela évite les ressaisies et garantit la cohérence entre le calcul, le schéma de principe et le devis transmis via SIA 451.
To go further
- Complete overview of the NIBT 2020
- CAN catalogue and SIA 451 standard for electrical tenders
- Calculating the voltage drop in an installation
- Representing the NIBT 2020 on your electrical diagrams
- All ElectroCAD calculation tools
- ElectroSchema — AutoCAD plugin for Swiss electrical schematics
- ElectroCAD Tools — Electrical CAD tool suite
Disclaimer
Calculateur fourni à titre indicatif. Les valeurs d'Iz utilisées sont conformes aux tableaux types de la NIBT 2020 pour les cas courants. Pour les installations particulières (câbles spéciaux, températures extrêmes, harmoniques, circuits photovoltaïques), se référer à la NIBT complète et au constructeur du câble. Tout dimensionnement doit être validé par un installateur électricien autorisé et contrôlé selon l'OIBT.