Technical guide · 5 min read

What size breaker for an AC unit, and why does it keep tripping?

By Konoz technical team Published 17 October 2026 Last updated 17 October 2026

Short answer

An AC breaker must be above the unit’s running current, no larger than the maximum breaker on its nameplate, and no larger than the corrected cable capacity. With 2.5 mm² in conduit at 45 °C the limit is 16 A; with 4 mm² it is 25 A. Repeated tripping usually comes from overload, an undersized breaker, the wrong curve, earth leakage or loose terminals.

The rule: three numbers that must agree

If your AC trips its breaker every afternoon, or you want the right breaker from day one, it comes down to three numbers: the unit current, the breaker rating and the cable capacity. The Saudi Electrical Code SBC 401 (clause 43-3.2) ties them together:

Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz

Ib: design current, i.e. the unit current from its nameplate.
In: breaker rated current.
Iz: cable capacity after temperature and grouping correction.
I2: the current that ensures tripping within the conventional time; for miniature breakers to IEC 60898-1 it is 1.45 × In, so the second condition is met whenever In ≤ Iz.

On top of that, if the manufacturer prints a maximum breaker or fuse (MOCP / Max fuse) on the nameplate, do not exceed it.

Breaker size by wire size

Once you know the unit current, pick the smallest standard breaker above it, then confirm the cable can carry it. Values below are for single-core PVC copper wires in conduit (B1), one single-phase circuit per conduit, after temperature correction:

Largest breaker the wire can be protected by (single-phase)

Size (mm²)	Iz at 40 °C	Max breaker at 40 °C	Iz at 50 °C	Max breaker at 50 °C
2.5	20.9 A	20 A	17.0 A	16 A
4	27.8 A	25 A	22.7 A	20 A
6	35.7 A	32 A	29.1 A	25 A
10	49.6 A	40 A	40.5 A	40 A
16	66.1 A	63 A	54.0 A	50 A

Calculation: B1 two-loaded column (Table A.52-2) × PVC factor (0.87 at 40 °C, 0.71 at 50 °C); breakers from the standard series 10, 13, 16, 20, 25, 32, 40, 50, 63 A.

B, C or D curve?

A miniature breaker has a thermal element for slow overloads and a magnetic element that trips instantly on high current. The curve sets the multiple of rated current at which the magnetic element trips, per IEC 60898-1:

Instantaneous tripping curves of miniature breakers

Curve	Instantaneous trip range	Typical use
B	above 3 to 5 × In	resistive loads, long cables, protection of persons
C	above 5 to 10 × In	general circuits
D	above 10 to 20 × In	high inrush loads: motors, transformers

Ranges from IEC 60898-1. Some manufacturers publish a narrower D band for their product (e.g. 10 to 14 ×), so check the breaker’s own catalogue.

A compressor draws far more than its running current at start-up, especially non-inverter units. Curve C is the general choice; if the breaker trips only at compressor start and the manufacturer allows it, a D curve of the same rating may suit, provided the project engineer confirms the breaker still clears a fault at the far end (fault-loop impedance). Never “fix” it by raising the breaker above the cable rating.

Why does the AC breaker trip? Common causes

Before replacing anything, note when it trips: instantly at switch-on, after hours in peak heat, or is it the RCD that trips? Each pattern points to a cause:

Gradual overload (trips after a while): a dirty condenser coil, wrong refrigerant charge or poor airflow round the outdoor unit all raise compressor current, and extreme heat raises it further.
Breaker below the unit current: compare the nameplate figure with the breaker rating.
Trips at start: starting current above the magnetic band of a B curve, or a weak compressor start capacitor.
Hot breaker: miniature breakers are calibrated at a 30 °C reference in the IEC standard, and in a crowded panel in the sun the thermal element can trip early. Manufacturers publish temperature derating tables; check your breaker’s catalogue and improve panel ventilation.
Loose terminals: a loose terminal heats the breaker and scorches it; any blackening or burning smell means isolate the circuit now.
Short circuit or earth fault: instant tripping on every attempt means a fault in the cable, compressor or fan. Do not keep resetting it.
Low voltage: motor current rises as voltage falls; measure voltage at the unit while running and review cable length and size.

Residual current devices (RCD) and AC units

SBC 401 requires an RCD of no more than 30 mA on socket-outlet circuits up to 20 A in wet locations, outdoors and similar (clause 41-2.5 of the 2007 edition), and the current edition has a clause on additional protection by RCDs (41-5.1) and an annex on RCD types (G.53). A hard-wired AC circuit follows the project design, and the project specification may require residual-current protection for outdoor units.

If it is the RCD that trips, the likely causes are moisture or water in the outdoor junction box, damaged cable insulation, or leakage in a fan or compressor motor. Some inverter units state the RCD type they need in the installation manual; follow the manufacturer.

Safe diagnosis steps before replacing the breaker

These steps are for a licensed electrician; we list them so you know what to ask for and can read the report:

Log the trip pattern: time, ambient temperature, whether the MCB or the RCD tripped, and whether it trips at start or after a while.
Isolate and prove dead with a tester before opening the panel or the outdoor unit’s terminal box.
Inspect terminals: any blackening or melting on the breaker or terminal block means replacement and re-tightening to the manufacturer’s torque.
Test insulation resistance of the cable and compressor with a suitable insulation tester, following the verification procedures in the code (Chapter 61) and disconnecting electronics as the manufacturer instructs.
Run and measure: running current with a clamp meter and voltage at the unit while running, compared with the nameplate.
Compare the three numbers: unit current, breaker and corrected cable rating. If the cable is smaller than the breaker the unit needs, the fix is a larger cable, not a larger breaker.

Safety note: this guide is engineering background, not a substitute for the project electrical engineer’s design or the consultant’s approval. Installation and termination must be done by a licensed electrician; isolate the supply at source and prove it dead with a tester before any work.

From calculation to supply

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Frequently asked questions

What breaker for a 24,000 BTU AC?

Not from BTU. Take the running current and maximum breaker from the nameplate and pick a breaker above the current, not above the maximum breaker or the cable rating: with 2.5 mm² in conduit at 45 °C the limit is 16 A, with 4 mm² it is 25 A.

Why does the AC breaker trip only in the afternoon?

At midday condensing pressure rises, so compressor current rises, and the panel heats up so the thermal element nears its trip point. Clean the condenser, check airflow and charge, measure the current, and review breaker, cable and panel temperature.

Should I fit a bigger breaker if it trips?

No, unless the cable can carry the larger breaker after temperature correction and the nameplate maximum allows it. A breaker above the cable rating lets the cable overheat instead of tripping, which is a fire risk.

C or D curve for an AC breaker?

Curve C (instant trip at 5–10 × In) is the general choice. Curve D (10–20 × In) is for high-inrush loads where the manufacturer allows it and the engineer confirms faults at the far end still clear.

References

We checked these sources on the last-updated date. Where the code leaves room for engineering judgement, the decision rests with the project engineer or consultant.

Written by: Konoz technical team · Last updated: 17 October 2026

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