18-10 2025
Low-Voltage Circuit Breaker Selection Guide: Matching Breaker Current Ratings to Load Requirements
- Why “current rating” is the first filter
A circuit breaker is a two-faced device. In its “closed” life it must carry the everyday load current for decades without nuisance heating. In its “open” life it must clear the worst short-circuit the utility can deliver within milliseconds. Of the two duties, the first—carrying the load—occupies 99.999 % of calendar time, yet it is the part most often under-engineered. A breaker that is too small trips on hot afternoons and slowly erodes customer trust. A breaker that is too large never trips and quietly sacrifices the cable. Correct current sizing is therefore the foundation of every subsequent decision (trip curve, breaking capacity, poles, accessories). The following text is a field-oriented manual for choosing the rated current I n of low-voltage breakers (≤ 1 000 V a.c.) so that it embraces the real load, tolerates foreseeable overloads, and still protects the conductor.
- Re-cap of basic parameters
- I b : design current of the load (continuous or cyclic).
- I n : rated current stamped on the breaker handle.
- I z : current-carrying capacity (ampacity) of the cable or busbar after all derating.
- I 2t : Joule-integral the breaker lets through during a fault.
- I cu : ultimate short-circuit breaking capacity.
- I cs : service short-circuit breaking capacity (multiple breaks).
Rule zero (IEC 60364-4-43 & NEC 240.4):
I b ≤ I n ≤ I z .
The breaker must sit electrically between the load and the wire.
I b ≤ I n ≤ I z .
The breaker must sit electrically between the load and the wire.
- Step 1 – Compile the load inventory
List every piece of equipment downstream. For each item record:
- Name-plate active power P (W) or apparent power S (VA).
- Load type: resistive, motor, electronic, LED, transformer, EV charger, etc.
- Duty cycle: continuous (> 3 h), intermittent, or short-time.
- Simultaneity factor k (diversity). Table 1 gives conservative values for dwellings.
Table 1 – Diversity for single-family houses (IEC 60364-5-52:2025)
- Lighting & sockets: k = 0.7
- Cooking: k = 0.8
- Water heater: k = 1.0
- Heat-pump compressor: k = 1.0
- EV charger (7 kW): k = 0.9
- Pool pump: k = 0.8
Compute the design current:
Single-phase: I b = P / (U 0 cos φ) · k
Three-phase: I b = P / (√3 U L cos φ) · k
Three-phase: I b = P / (√3 U L cos φ) · k
where U 0 = 230 V, U L = 400 V. If power factor is unknown, use cos φ = 0.9 for motors, 1.0 for heaters, 0.95 for electronics.
- Step 2 – Treat continuous loads first
Both NEC 210.20(A) and IEC 60364-5-52 treat loads operating ≥ 3 h as “continuous”. The breaker must carry 125 % of their current without overheating. Two equivalent ways to satisfy the rule:
a) Increase the load current by 25 % before sizing:
I b,cont = 1.25 I cont + I non-cont
or
b) Select a breaker 25 % larger than the continuous portion, then verify I n ≤ I z.
I b,cont = 1.25 I cont + I non-cont
or
b) Select a breaker 25 % larger than the continuous portion, then verify I n ≤ I z.
Method (b) is more common because cable sizing usually includes the 125 % already.
- Step 3 – Motor and transformer loads
Motors draw 5–7 × full-load current (FLA) at start. The magnetic trip must not operate on inrush, yet the thermal element must protect the windings. Use motor-rated breakers or adjust standard ones:
- Choose I n ≥ 1.15 FLA (service factor 1.15 motor) or 1.25 FLA (SF 1.0).
- Select trip curve D (10–20 I n) so that magnetic pickup ≥ 14 I n > starting peak.
- Provide separate overload relay or built-in thermal adjustment for precise motor protection.
For LV/LV transformers (< 10 kVA) treat the primary side as a motor circuit—use curve D and I n = 1.2 × primary rated current.
- Step 4 – Electronic & LED loads
Switch-mode power supplies draw high-frequency pulses and generate 3rd harmonic in the neutral. Three cumulative effects matter:
- Peak current > rms current → risk of magnetic false trip.
- Higher heating in bi-metal strip → thermal trip drifts low.
- 3rd harmonic adds in neutral → neutral conductor may exceed phase current.
Counter-measures
- Use curve C (5–10 I n) minimum; avoid curve B.
- Specify “full-current” or “HD” breakers tested with non-sinusoidal waveforms.
- Count neutral as current-carrying conductor when sizing cable I z.
- For large LED retrofits, measure inrush with a clamp-on power quality meter; oversize I n by 20 % if peak > 7 I rms.
- Step 5 – Apply ambient-temperature derating
MCBs are calibrated at 30 °C. Above this, the bi-metallic strip needs less current to bend, so the real trip point falls ≈ 0.5 %/°C. A 32 A breaker in a 60 °C roof space becomes effectively 32 × (1 – 0.005 × 30) ≈ 27 A. Simple fix: select the next higher standard I n such that after derating the effective value still ≥ I b. Do not exceed I z of the cable at the actual temperature.
- Step 6 – Cable derating versus breaker derating ------------------------------------------------* Derate the cable first (temperature, grouping, insulation, soil thermal resistivity). The resulting I z is the hard ceiling. Then derate the breaker if installed in a hot panel. Finally pick the smallest standard I n that satisfies:
I b ≤ I n,eff ≤ I z
Table 2 – Standard MCB ratings (IEC 60898-1)
6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A
6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A
- Worked examples
Example 1 – UK kitchen radial (single-phase)
- 4.2 kW cooker, cos φ = 1, continuous.
- I b = 4200 / 230 = 18.3 A → continuous factor 1.25 → 22.9 A.
- Cable: 2.5 mm² twin & earth, clipped direct, 30 °C → I z = 27 A.
- Choice: 25 A MCB, curve C. Check: 22.9 A ≤ 25 A ≤ 27 A ✔
Example 2 – US split-phase HVAC condenser
- Name-plate 240 V, 18 A FLA, continuous.
- NEC 440.32: I n ≥ 1.25 × 18 = 22.5 A → next size 25 A.
- Locked-rotor 96 A → magnetic pickup must be > 96 A.
- 25 A curve-C breaker trips magnetically at 125–250 A → OK.
- Wire: 10 AWG THWN, 75 °C, I z = 35 A @ 30 °C → 25 A ≤ 35 A ✔
Example 3 – Chinese apartment main incomer
- Load diversity total 38 A per phase.
- Cable: 10 mm² PVC, in conduit, 40 °C ambient → I z = 46 A.
- Breaker panel recessed, 35 °C → derate 0.975 → 0.975 × 50 A = 48.8 A.
- Select 40 A MCB (next below 46 A). 38 A ≤ 40 A ≤ 46 A ✔
- Matching the breaker to the cable—graphical method
Plot the time-current curve of the cable (adiabatic line I 2t = k 2S 2) and the breaker thermal curve on log-log paper. The breaker curve must lie below the cable damage line for every point below 5 s. Most LV breakers already satisfy this for copper ≤ 10 mm², but for aluminium or long motor feeders, check the chart or use manufacturer software.
- Selectivity with upstream breakers ------------------------------------------------* For domestic panels selectivity is advisory, not mandatory. Good practice:
- Keep at least two current ratings (≥ 1.6×) between series MCBs.
- Use curve B downstream, curve C upstream.
- Ensure upstream magnetic pickup ≥ 1.4 × downstream instantaneous.
With MCCB/MCB mixtures, insist on manufacturer selectivity tables;
