Size an AC circuit breaker
Reports not just the rating but the calculation that actually comes up on inspection: a B16 requires a loop impedance below 2.875 Ω. Over 45 m of 2.5 mm² the cable already contributes 0.74 Ω — leaving 2.13 Ω for the supply.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-07-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Selected commercial size | I_B <= I_n <= I_z | 16 A | exact |
| Magnetic trip current | k * I_n | 80 A | exact |
| Maximum loop impedance | U_0 / I_a | 2.875 Ohm | exact |
| Cable impedance | 2 * rho(T) * L / A | 0.74264 Ohm | exact |
| Permissible supply impedance | Z_s,max - Z_line | 2.1324 Ohm | exact |
- Formula
I_B ≤ I_n ≤ I_z · Z_S,max = U_0 / (k · I_n) · Z_cable = 2 · ρ(T) · L / A- Valid for
- Rating selection from operating current and conductor ampacity (I_B ≤ I_N ≤ I_Z), IEC or NEC trade series. Disconnection proof as reverse calculation: permissible loop impedance = U₀ / (trip factor × I_N) with the band upper limits from IEC 60898-1 (B 5×, C 10×, D 20×), cable contribution at operating temperature (70 °C PVC), disconnection time selectable as classification (0.4 s final / 5 s distribution — the impedance limit is unaffected, since the proof runs via the magnetic instantaneous trip; deliberately conservative). Reference: B16, 45 m, 2.5 mm² → 2.87 Ω permissible, cable 0.64 Ω, supply budget 2.23 Ω. NO SUBSTITUTE FOR MEASUREMENT on the finished installation.
- Not covered
- Selectivity between breakers, residual-current protection (RCD) and its interplay, breaker short-circuit withstand, manufacturer curves within the standard bands, load distribution in three-phase systems.
- Data sources
- Official publications of the standards bodies and state authorities (NFPA, IEC, DKE/VDE, CEN) · retrieved 2026-07-15
Frequently asked questions
Which circuit breaker rating do I need for my circuit?
The basic condition is I_B ≤ I_n ≤ I_z: the rating must sit above the operating current and below the conductor's corrected ampacity, and the calculator picks the matching size from the regional product series. It then performs the disconnection check via loop impedance — the calculation that actually comes up when an installation is inspected.
What is the maximum loop impedance allowed for a B16 breaker?
Z_S,max = U_0 / (k · I_n) with k = 5 for curve B, 10 for C and 20 for D — for a B16 at 230 V that is 2.875 Ω. Your own cable contributes Z = 2 · ρ(T) · L / A: over 45 m of 2.5 mm² that is already 0.74 Ω, leaving 2.13 Ω for the supply. Crucially, the check is run at the insulation's operating temperature (PVC 70 °C, XLPE 90 °C) — computing at 20 °C makes the impedance about 20 % too small and passes installations that should fail.
Does this calculation apply in a TT system or with an RCD as the protective device?
No — the calculator assumes a TN system with a radial final circuit and balanced load. TT and IT systems, RCDs as the disconnecting device, selectivity across the protection chain, manufacturer-specific tripping curves and the reactive part of the loop are not covered. Loop impedance is measured on the finished installation, not calculated.
What is the disconnection condition — and why is the right rated current not enough?
The rated current protects the cable from overload; the disconnection condition protects people in the fault case: on a short to the protective conductor, enough current must flow for the magnetic instantaneous trip (5× rated for B, 10× for C characteristic). Whether that succeeds is decided by the loop impedance of source and cable. A correctly rated breaker on a too-long cable does NOT trip on a fault — the budget bar shows exactly this limit.
Why is an inverter source more critical than the grid?
An island inverter delivers only 1.5–3× its rated current in a short — far too little for the magnetic trip of a C breaker. What works on the strong grid fails on the same cable behind the inverter. That is why source impedance is an input here, and often the answer is: B characteristic, a smaller rating, or an RCD as additional fault protection.
B or C characteristic?
B trips magnetically at 3–5× rated current, C at 5–10×. C tolerates inrush peaks (compressors) without nuisance tripping but needs HALF the permissible loop impedance — often unattainable on weak sources and long runs. Rule in island grids: B where the loads allow it; C only with a verified disconnection condition.
What does the disconnection time change about the permissible impedance?
It classifies the circuit and checks plausibility: 0.4 seconds applies to final circuits up to 32 A — choose 0.4 s and need a larger breaker, and the page notes that 5 seconds would be permissible for that. The impedance limit itself does NOT change: this page proves disconnection via the magnetic instantaneous trip, which acts in milliseconds and thus satisfies both time limits. That is deliberately conservative — using the softer thermal curves for a 5-second proof requires the manufacturer curve and professional planning.
Why does the disconnection proof use 70 °C conductor temperature?
Because IEC 60364 conducts the proof at the insulation's operating temperature, not room temperature — 70 °C for PVC. The warm conductor has about 20 % more resistance, and computing at 20 °C yields an over-optimistic loop impedance. If you enter less than 60 °C, the page notes that the assumption is unusually soft.
Is the computed loop impedance sufficient as proof?
No — and the page says so in every output: loop impedance is MEASURED on the finished installation. The calculation serves planning: it shows in advance whether the intended combination of breaker, length and cross-section can pass at all, and how much supply-impedance budget remains at the service entrance. It does not replace the measurement.
When do I need C or D characteristics — and what do they cost?
For inrush peaks (motors, transformers, chargers) that would trip a B breaker magnetically. The price shows in the reverse calculation: C trips reliably only at 10×, D at 20× rated current — the permissible loop impedance halves or quarters against B. On long runs the proof with D is practically impossible; the page warns then.
Does the calculation apply to three-phase circuits?
Yes — the phase count is adjustable since 5 August 2026. The disconnection proof itself remains single-phase in nature (line to earth, 230 V), since the fault case is an earth fault of one line conductor; with three-phase mainly the per-conductor operating current changes. Distributing load across phases remains professional planning.