Which MCB curve should you use: B, C or D?
B curve trips at 3–5 times rated current and suits resistive loads like lighting and heaters. C curve trips at 5–10 times and is the general-purpose choice for mixed domestic and commercial circuits. D curve trips at 10–20 times, for motors and transformers with high inrush current.
What the curve actually describes
Every miniature circuit breaker has two separate tripping mechanisms inside it. A thermal element - a bimetallic strip - handles sustained overload, and it behaves the same way whatever the curve. A magnetic element handles short circuit, and it trips almost instantly. The curve letter describes only the magnetic element: how many times the rated current has to flow before it operates.
This matters because many loads draw a large current for a fraction of a second when they start. A motor can pull six to eight times its running current on startup; a transformer or a bank of LED drivers can pull far more for a few milliseconds. If the magnetic element is too sensitive, the breaker trips every time the equipment starts, and the installation gets blamed for a specification error.
| Curve | Magnetic trip | Typical use |
|---|---|---|
| B | 3–5 × In | Lighting, heaters, socket circuits with resistive loads. Long cable runs where fault current is low. |
| C | 5–10 × In | General purpose. Mixed domestic and commercial circuits, small motors, most socket outlets. |
| D | 10–20 × In | Motors, transformers, welding sets, X-ray equipment - anything with heavy inrush. |
| K | 10–14 × In | Inductive and motor loads in industrial installations, to IEC 60947-2. |
| Z | 2–3 × In | Highly sensitive electronics and semiconductor protection. |
The mistake that gets made most often
Fitting a higher curve to stop nuisance tripping, without checking that the fault current at the far end of the circuit is still enough to operate it. If a D curve 32 A breaker needs 640 A to trip magnetically, and a fault at the end of a long thin cable can only deliver 400 A, the breaker will not trip magnetically at all. It will fall back to the thermal element and take seconds - during which the cable is carrying a fault current it was never sized for.
This is why the curve, the cable size and the length of the run have to be considered together, not separately. Choosing a curve to stop a nuisance trip can quietly remove the protection you thought you had.
Breaking capacity is a separate decision
Curve and breaking capacity are often confused. The curve is about when the breaker trips; the breaking capacity is about how much fault current it can safely interrupt without being destroyed. It is quoted in kiloamps - 7.5 kA, 10 kA, 15 kA.
The nearer the installation is to the transformer, the higher the prospective fault current, and the higher the breaking capacity needs to be. A domestic consumer unit at the end of a long service is usually fine at 7.5 kA or 10 kA. A distribution board inside a factory close to its own transformer may need 15 kA or an MCCB.
Sources and further reading
Published by Electrolite Systems. Last updated
Last reviewed 3 September 2026. This guide explains how the decision is made; it is not a substitute for a design calculation for your specific installation.