Electrical Cable Cross-Section Calculator (French NF C 15-100)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
The minimum cable cross-section depends on the circuit type: 1.5 mm² for lighting (10 A breaker), 2.5 mm² for a 12-outlet circuit (20 A), 6 mm² for a cooktop (32 A). These values are set by the French NF C 15-100 standard.
Explanation
In France, the NF C 15-100 standard sets the minimum conductor cross-section and the maximum protection rating for each type of circuit in a domestic electrical installation, so that the cable never heats up dangerously even under heavy demand, and the breaker trips before the cable reaches its limit. This calculator gives, for each common circuit type, the copper cable cross-section to respect and the associated breaker rating: 1.5 mm² protected at 10 A for lighting (up to 8 light points); 1.5 mm² at 16 A for an 8-outlet circuit or 2.5 mm² at 20 A for a 12-outlet circuit; 2.5 mm² at 20 A for a dedicated circuit (washing machine, dishwasher, oven, freezer, each on its own dedicated circuit); 6 mm² at 32 A for the cooktop; 2.5 mm² at 20 A for the water heater and electric heating up to 4500 W (a power figure our room heating power calculator helps size before choosing the circuit). These cross-sections are regulatory minimums: a long cable run between the panel and the point of use may require a larger cross-section to limit voltage drop, and the lighting circuit's point count should be checked against the load a room actually needs, which our room lighting wattage calculator helps estimate. Any new or modified electrical installation must be inspected by an approved body before being energized: this calculator is a planning aid, not a compliance validation.
Example: an outlet circuit (12 outlets)
Inputs
Circuit type: power outlets, 12-outlet circuit.
Calculation
The NF C 15-100 standard allows up to 12 outlets on a 2.5 mm² circuit protected by a 20 A breaker (or a maximum of 8 outlets on 1.5 mm² protected at 16 A).
Result
This 12-outlet circuit requires 2.5 mm² cable protected by a 20 A breaker.
Frequently asked questions
Why does the cooktop require such a large cross-section?
A cooktop (electric or induction) can draw up to 7000 W at peak, about 30 A at 230 V: the 6 mm² cable and 32 A breaker are sized for this sustained current. It's the only common domestic circuit that requires such a large cross-section; the oven, despite its name, is a standard dedicated circuit at 2.5 mm² / 20 A.
Does the cross-section depend on the cable type (rigid or flexible)?
No, the regulatory cross-section (1.5 mm², 2.5 mm², 6 mm²) is the same regardless of the conductor type, rigid or flexible: it refers to the area of the copper cross-section, not the cable's outer diameter. However, the installation method (buried in a wall, in trunking, in a construction void) affects the cable's actual capacity and can, in unfavorable cases, require a larger cross-section.
Can I plug an outlet into a lighting circuit as a temporary fix?
No, it's prohibited and dangerous: a lighting circuit is protected by a 10 or 16 A breaker and wired in 1.5 mm², sized for a few bulbs. Plugging a high-power appliance (heater, vacuum, kettle) into it heats the cable well beyond its capacity, without the breaker necessarily tripping in time. Each use has its own dedicated circuit type.