How to choose a circuit breaker — curves B, C and D explained

An automatic circuit breaker (CB) is the main protective element in any electrical panel. It trips the circuit in case of overload or short circuit. But how do we choose the right model? The key is the letter on it – it indicates the characteristic tripping curve.

What the characteristic curve means

The curve shows at how many times the nominal current the circuit breaker trips instantaneously (magnetic tripping). Different loads have different inrush currents and therefore require different curves.

Curve B

Instantaneous tripping at 3–5 times the nominal current.
Designed for sensitive loads with no inrush current or very low inrush current.

Typical applications:
- Lighting
- Sockets for domestic use
- Electronic equipment
- Cable protection in residential installations

Curve C

Instantaneous tripping at 5–10 times the nominal current.
The most common curve in domestic and commercial installations. Tolerates moderate inrush currents.

Typical applications:
- Air conditioners
- Washing machines and dishwashers
- Low-power pumps
- Lighting with transformers
- General purpose in domestic panels

Curve D

Instantaneous tripping at 10–12 times the nominal current.
Designed for loads with very high inrush current.

Typical applications:
- Motors and compressors
- Transformers
- Industrial equipment
- High-power pumps

How to choose the current rating (Amperes)

The nominal current of the circuit breaker should be greater than the operating current of the load but less than the permissible current of the cable.

Basic rule:
- Lighting: 10A or 16A, curve B or C
- Sockets: 16A, curve C
- Water heater: 16A or 20A, curve C
- Electric stove: 25A or 32A, curve C
- Air conditioner: 16A or 20A, curve C
- Motor/compressor: calculated by power, curve D

What 6kA means

The number 6kA is the short-circuit current at which the circuit breaker can trip safely. For domestic installations, 6kA is the standard and perfectly sufficient.

Single-pole, double-pole, and triple-pole circuit breakers

- 1P (single-pole) – protects only the phase. Used for individual circuits in residential installations.
- 2P (double-pole) – protects phase and neutral. Mandatory for the main circuit breaker or in installations with TT earthing.
- 3P (triple-pole) – for three-phase installations and industrial panels.

See available automatic circuit breakers in our store [LINK TO STORE].

Important: Electrical installations must be carried out by a qualified electrician. The information in this article is for educational purposes.