How to Size a Circuit Breaker: A Practical Guide
A field-focused guide to selecting the correct circuit breaker rating and characteristics. Covers design current, device rating, cable capacity, the Ib <= In <= Iz coordination rule, breaking capacity in kA, B/C/D trip curves, and how UAE regulations apply.
Sizing a circuit breaker is one of the most misunderstood steps in electrical design. Many people assume the breaker rating should match the load, but that is not its job. A protective device is selected to protect the cable, not the appliance, ensuring the conductor never carries more current than it can safely dissipate as heat.
Get this wrong and you face two failure modes: an oversized breaker that lets a cable overheat before it trips, or an undersized one that nuisance-trips on normal inrush. This guide walks through the engineering logic the way an MEP designer applies it on site in Abu Dhabi, in line with IEC 60364 and the DoE wiring regulations.
How it works
The process begins with the design current (Ib) — the actual current the circuit is expected to carry under normal conditions. For a single load this is derived from the power rating, voltage and power factor; for a distribution board it is the diversified sum of connected loads. Ib is the starting reference for everything that follows.
Next is the nominal device rating (In) — the standard rating of the protective device, chosen from preferred values such as 6, 10, 16, 20, 32, 40, 63 A. In must be at least equal to Ib, so the breaker does not trip on the normal load it is meant to carry. You always round up to the next standard rating, never down.
The third quantity is the continuous current-carrying capacity of the cable (Iz) — the maximum current the selected conductor can carry continuously after applying correction (derating) factors for ambient temperature, grouping and installation method. The governing rule that ties all three together is: Ib <= In <= Iz. The breaker must be large enough to pass the design current, yet small enough that it trips before the cable is overloaded. This is the heart of overload coordination.
Sizing is not complete with overload protection alone. The device must also withstand a short circuit. Its breaking capacity (Icu/Ics, in kA) must be equal to or greater than the prospective fault current at the point of installation — the maximum current that would flow during a bolted fault. A breaker with insufficient kA rating can fail catastrophically instead of clearing the fault, so the kA rating is a separate, mandatory check beyond the amp rating.
Finally, the trip curve must suit the load's inrush behaviour. Type B trips at roughly 3–5 times In, Type C at 5–10 times, and Type D at 10–20 times. Resistive and lighting loads use Type B/C; motors, transformers and capacitor banks draw a high momentary inrush and need Type C or D to avoid nuisance tripping. Always remember the underlying principle: the breaker protects the cable, and every parameter — In, Iz, kA and curve — is chosen so the conductor is never the weakest link.
Main types
In the UAE
- The Abu Dhabi DoE (formerly ADDC) Electricity Wiring Regulations are based on the principles of IEC 60364 and BS 7671; breaker sizing, cable selection and protection coordination must follow these standards.
- Protective devices and materials must comply with recognised BS/IEC standards and, where applicable, carry conformity marking referencing ESMA / Emirates standards.
- Discrimination (selectivity) between upstream and downstream devices and correct fault-level coordination are required so that only the breaker nearest the fault operates, keeping the rest of the installation energised.
How GPR applies this
At GPR in Abu Dhabi, our electrical teams size every protective device against verified design current, derated cable capacity and the prospective fault level at each board before submitting for DoE approval. We confirm breaking capacity in kA and discrimination across the distribution hierarchy so installations are both compliant and selective — protecting the cable, the equipment and the building, and avoiding costly rejections at inspection.
Frequently asked questions
What size breaker do I need for a circuit?
Start from the design current (Ib), pick the next standard rating (In) at or above it, and confirm the cable's derated capacity (Iz) is equal to or greater than In, satisfying Ib <= In <= Iz.
Does the breaker rating match the load or the cable?
Neither directly — the breaker is sized to protect the cable. In must cover the load current but must never exceed the cable's continuous capacity.
Type B vs Type C breaker — which do I use?
Type B (3–5 x In) suits resistive and lighting loads; Type C (5–10 x In) suits mixed circuits and small motors with moderate inrush.
What does the kA rating of a breaker mean?
It is the breaking capacity — the maximum short-circuit current the device can safely interrupt. It must exceed the prospective fault current at that point in the network.
Why does my breaker keep tripping with a motor?
Motor inrush can be several times the running current; a curve that is too sensitive (e.g. Type B) trips on start-up. Use Type C or D sized for the steady-state current.