Circuit breaker calculation: choosing the rating, B/C/D curve and RCD
Choosing an NIBT 2020-compliant circuit breaker: the complete method
Choosing a compliant circuit breaker NIBT 2020 is not just a matter of defaulting to 16 A. The rating depends on the circuit's rated current, the curve (B, C, D) according to the load type and its inrush peak, the DDR (sensitivity and type A, B or F) on the nature of the likely fault current, and the breaking capacity of the prospective short-circuit current at the installation point. This calculator includes the nine most common circuit profiles in Swiss residential and small commercial buildings, applies the EN 60898 and NIBT 4.3 / 7.22 rules, and returns a complete recommendation ready to transfer to the single-line diagram.
The correct approach follows four steps: (1) determine Iₙ from the nameplate power or the load calculation, (2) choose a standardised rating immediately above Iₙ, (3) adjust the curve according to the nature of the load (resistive, mixed, high inrush), (4) select the DDR depending on the nature of the expected fault current and the circuit's position in the panel. EV charging stations and transformerless PV inverters require necessarily a type B RCD under NIBT 7.22 and 4.1.5.3, with no possible exemption.
Standardised ratings NIBT 2020 — reference table
EN 60898 ratings available in practice from all manufacturers present on the Swiss market (ABB, Hager, Schneider, Siemens, Eaton). Indicative copper cross-section for installation method B1 without Kt/Kg correction.
| Rating Iₙ | Min. Cu cross-section | Typical use | Curve | DDR |
|---|---|---|---|---|
| 10 A | 1,5 mm² | Standard lighting, control circuits | B | A 30 mA if damp |
| 13 A | 2,5 mm² | Dedicated socket outlets (washing machine, dishwasher, oven) | C | A 30 mA |
| 16 A | 2,5 mm² | General socket outlets, 230 V cooking | C | A 30 mA |
| 20 A | 2,5 or 4 mm² | Light 400 V cooking, tumble dryer | C | A or F 30 mA |
| 25 A | 4 mm² | Heat pump, baseboard heaters, three-phase water heater | C or D | F 30 mA |
| 32 A | 6 mm² | 7.4 kW single-phase EV charger, 8-10 kW heat pump | C or D | B 30 mA (EV) |
| 40 A | 10 mm² | Floor sub-panel, workshop | C | S 100/300 mA selective |
| 50 A | 10 mm² | Building sub-panel, 22 kW charging station | C | B 30 mA EV / S 300 mA |
| 63 A | 16 mm² | Main circuit breaker for a detached house, substation | C | S 300 mA incomer |
| 80-125 A | 25-50 mm² | Collective main circuit breaker | C | S 300/500 mA selective |
EV charging stations — sizing examples
Mode 3 EV charging stations (IEC 61851 standard) require an RCD type B under NIBT 7.22, because the rectifier bridge of the vehicle's on-board charger can generate DC fault currents that types A and F do not detect.
| Charging station power | Voltage | Calculated Iₙ | Rating | Curve | DDR | Cu cross-section |
|---|---|---|---|---|---|---|
| 3,7 kW | 230 V single-phase | 16,1 A | 20 A 2P | C | B 30 mA | 2,5 mm² |
| 7,4 kW | 230 V single-phase | 32,2 A | 40 A 2P | C | B 30 mA | 10 mm² |
| 11 kW | 400 V three-phase | 15,9 A | 16 A 4P | C | B 30 mA | 2,5 mm² |
| 22 kW | 400 V three-phase | 31,8 A | 32 A 4P | C | B 30 mA | 6 mm² |
| 44 kW (light DC) | 400 V three-phase | 63,5 A | 80 A 4P | C | B 30 mA | 25 mm² |
Detailed example of a 11 kW three-phase 400 V charging station, cosφ = 1 (calculator reference): Iₙ = 11 000 / (√3 · 400) = 15,88 A → next higher standardized rating 16 A four-pole curve C, RCD type B 30 mA, cross-section 2,5 mm² Cu (installation method B1, length < 30 m). Pour des longueurs supérieures ou des poses moins favorables, vérifier la chute de tension avec le cable sizing calculator.
Curves B, C, D: choose according to the load's inrush peak
Curve B (Im = 3 to 5 × Iₙ) — la courbe la plus sensible. Elle convient aux charges purement résistives ou à très faible pic d'appel : éclairage halogène, plinthes chauffantes électriques sans ventilateur, panneaux rayonnants, résistances de chauffe-eau accumulation, câbles chauffants au sol. Elle améliore la sélectivité avec l'amont mais déclenche sur le moindre transitoire d'une charge à inrush, ce qui la rend inadaptée aux prises générales modernes (chargeurs à découpage, drivers LED bon marché).
Curve C (Im = 5 to 10 × Iₙ) — le choix par défaut domestique et tertiaire courant. Elle absorbe les pics d'appel modérés des prises générales et des charges électroménagères classiques : lave-linge avec moteur universel, lave-vaisselle, four électrique à résistance, congélateur, télévision, ordinateurs, éclairage LED avec drivers correctement dimensionnés, climatiseurs split inverter, pompes à chaleur en démarrage progressif. La courbe C est aussi compatible avec les bornes VE et les onduleurs PV résidentiels, dont les pics d'enclenchement sont limités électroniquement.
Curve D (Im = 10 to 20 × Iₙ) — pour les pics d'appel élevés et soutenus. Elle est obligatoire pour les moteurs en démarrage direct (rotor cage absorbant 6 à 8 Iₙ pendant 0,5 à 3 secondes), les transformateurs dont le courant d'enclenchement (inrush) atteint 8 à 12 Iₙ sur les premières alternances, les ballasts ferromagnétiques d'éclairage à décharge (HQI, sodium), les machines à souder, les compresseurs à piston. La courbe D demande en contrepartie une section de câble vérifiée pour la protection thermique et un Icc présumé suffisamment élevé pour que le déclenchement magnétique reste effectif en cas de court-circuit en bout de ligne.
RCDs — types AC, A, F, B and sensitivities
The type AC only detects pure sinusoidal alternating currents. It is in practice prohibited in Switzerland on new installations since NIBT 2020 (article 4.1.5.3), because modern loads (LEDs, chargers, induction, variable speed drives) almost all generate pulsating or smooth DC fault currents that this type does not detect.
The type A detects alternating current and the pulsating DC current from simple rectifiers (linear power supplies, half-wave). It is the minimum acceptable for general socket outlets and lighting in new installations.
The type F is a reinforced type A for mixed frequencies and harmonics. Recommended for residential variable-speed drives, induction hobs, inverter air conditioners and washing machines with electronic speed control.
The type B also detects the smooth DC fault currents produced by three-phase rectifiers. It is mandatory for EV charging stations (NIBT 7.22) and transformerless photovoltaic inverters (NIBT 4.1.5.3). More expensive (CHF 200-400 vs 40-80 for a type A), it is only justified on the prescribed circuits.
As for sensitivities: 10 mA (medical locations, special wet rooms), 30 mA (reference for protection of persons, mandatory for all socket outlets and circuits accessible to the public), 100 mA (intermediate selectivity), 300 mA (fire protection at the head of the distribution board, selectivity with 30 mA downstream).
Selectivity: residual-current and short-circuit
The residual-current selectivity garantit qu'en cas de défaut sur un départ, seul le DDR aval déclenche, sans couper tout l'étage ou tout le bâtiment. Elle repose sur deux critères cumulatifs : une sensibilité amont supérieure à 3 fois la sensibilité aval (ex. 300 mA amont sur 30 mA aval) et un retard de déclenchement amont (type S, sélectif). Un DDR S 300 mA en tête de tableau associé à des 30 mA classiques par circuit constitue le schéma de référence en villa et petit immeuble locatif.
The short-circuit selectivity (between circuit breakers) relies on the coordination of the curves: an upstream 63 A curve C circuit breaker with Im = 630 A and a downstream 16 A curve C with Im = 160 A is selective for Icc < 630 A. Au-delà, les deux déclenchent. Pour la sélectivité totale, certains fabricants publient des tables de coordination (back-up) qui exploitent la limitation d'énergie du disjoncteur amont. À vérifier au cas par cas selon le constructeur.
ElectroCAD tools for the electrical designer
The circuit breaker calculator completes the ElectroCAD tool ecosystem for pre-sizing and detailed design of Swiss electrical installations. See also the NIBT 2020 cable sizing calculator, the electrical power calculator, the guide to single-line diagrams and protective devices, as well as the page NIBT 2020 and Swiss electrical diagrams. Back to the full list: all the tools.
FAQ — Questions from designers
What rating for an 11 kW three-phase EV charging station?
For a charging station 11 kW at 400 V three-phase with cosφ = 1 (the on-board charger regulates its cosφ to 1), the rated current is Iₙ = 11 000 / (√3 · 400) = 15,88 A. Next standard rating up: 16 A four-pole (4P) curve C. RCD type B 30 mA mandatory under NIBT 7.22. Minimum cross-section 2,5 mm² Cu in installation method B1 up to about 30 metres. For 22 kW three-phase, the rating rises to 32 A 4P, cross-section 6 mm² Cu.
Why is a type B RCD mandatory on an EV charging station?
The on-board charger of an electric vehicle contains an active PFC rectifier bridge which, in the event of an internal fault, can pass to earth a smooth direct current. Type A and F RCDs do not detect this type of current: their magnetic core is blind to pure DC because it generates no flux variation. Type B uses an additional Hall-effect or flux-gate sensor that also detects DC. The NIBT 7.22 makes this type mandatory for any mode 3 charging station in a new installation.
Curve C or curve D for a motor?
The curve D is required for a motor in direct-on-line starting (Iₐ = 6 to 8 Iₙ for 0,5 to 3 s) because curve C, which trips between 5 and 10 Iₙ, may trip unwantedly on the inrush peak. The curve C remains acceptable if starting is star-delta or soft-start (Iₐ ≈ 2,5 Iₙ), or if the motor is controlled by a variable frequency drive. In all cases, check selectivity with the upstream device and the breaking capacity for the prospective Icc.
What is the difference between Icn 4.5 kA, 6 kA and 10 kA?
The ultimate breaking capacity Icn is the maximum short-circuit current the circuit breaker can interrupt without being destroyed. In single-family housing connected downstream of a transformer substation, 4,5 kA (EN 60898) is generally sufficient. In semi-industrial close to a substation, aim for 6 kA. In industrial or at the head of a distribution board fed by a short cable from a transformer, allow for 10 kA or more (EN 60947-2 — the so-called “industrial modular” range). To be calculated precisely from the short-circuit power Scc provided by the utility.
Can a type AC RCD still be installed in Switzerland?
No in practice on new installations. The NIBT 2020 article 4.1.5.3 requires an RCD suited to the nature of the likely fault current. As the vast majority of modern domestic loads generate pulsed or DC currents (LEDs, switch-mode power supplies, induction, variable-speed drives, power electronics), type AC is unsuitable and therefore prohibited. The minimum is now the type A, type F is recommended for mixed-frequency loads, and type B is mandatory for EV and PV.
How do you annotate the circuit breaker on the single-line diagram?
According to the standard SIA 451 and IEC 60617, an RCBO is annotated with: the rating in amperes, the curve (B/C/D), the RCD sensitivity in milliamperes and the type (A/B/F). Complete example: 16 A — C — 30 mA — type B for an 11 kW three-phase EV charging station. On the diagram, the combined circuit-breaker + RCD block symbol is shown as a single module with a star indicating the residual-current function. Our plugin ElectroSchema automatically generates this notation from the panel load summary.
Automate these calculations in AutoCAD?
ElectroSchema integrates automatic selection of the breaker rating, curve and RCD directly into the EC_LIGNES command: NIBT 2020-compliant recommendation reported in real time on the single-line diagram and exported to SIA 451.
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