Circuit Breaker Sizing Calculator

Summary

Breaker sizing is often treated as an afterthought to cable sizing — pick the cable, then "match" a breaker to it — but the correct method works both ways at once: the breaker protects the cable from overload, and the cable must be able to carry the breaker's rating without overheating even after correction factors for grouping, ambient temperature and thermal insulation are applied. Get the order wrong and you end up with a breaker that's either too small (nuisance tripping on legitimate load) or too large (the cable overheats under fault conditions long before the breaker trips).

The underlying rule, set out in Regulation 433.1 of BS 7671, is simple to state and easy to apply once you've done it a handful of times: Ib ≤ In ≤ Iz. Ib is the design current — what the circuit will actually draw in normal use, including any diversity allowance for circuits like cookers. In is the breaker's nominal rating — one of a fixed set of standard sizes. Iz is the cable's tabulated current-carrying capacity, adjusted downward for grouping with other cables, high ambient temperature, or being buried in thermal insulation. As long as the breaker sits between the actual load and the cable's real (corrected) capacity, both the appliance and the cable are protected.

This calculator walks through the method with worked, real-number examples for the circuit types a domestic or light-commercial electrician sizes most often — ring finals, radials, immersion heaters, cookers, showers and EV chargers — and cross-references the full cable size table in cable sizing, which this article uses as its source data.

Key Facts

• The governing rule — Regulation 433.1 of BS 7671: for overload protection, Ib ≤ In ≤ Iz must be satisfied for every circuit.
• Standard MCB ratings (domestic) — 6A, 10A, 16A, 20A, 25A, 32A, 40A, 45A, 50A. There is no "in-between" size — always round up to the next standard rating above Ib, never round down.
• Type B, C and D curves — Type B trips at 3–5× rated current (standard for domestic circuits); Type C at 5–10× (used where motor inrush current is significant, e.g. compressors); Type D at 10–20× (industrial motor starting). Type B is correct for the overwhelming majority of domestic circuits.
• Breaking capacity (kA rating) — separate from the current rating, this is the maximum fault current the breaker can safely interrupt without failing; domestic consumer units typically use 6kA-rated devices, verified against the prospective fault current at the installation, not just the load current.
• Correction factors reduce Iz, not In — grouping (Cg), ambient temperature (Ca), and thermal insulation (Ci) all reduce the cable's real-world current-carrying capacity below its tabulated value; a cable rated 27A clipped direct can drop to well under 20A if grouped with several other loaded cables or fully surrounded by insulation.
• Diversity applies to Ib, not to cable/breaker tables — a cooker circuit, for example, uses diversity (first 10A at 100%, remainder at 30%, plus 5A if a socket is fitted to the control unit, per the IET On-Site Guide) to establish a realistic design current lower than the sum of all elements' full-load current.
• No diversity on continuous, single-load circuits — showers and immersion heaters draw their full rated current continuously with no diversity allowance; Ib is simply the appliance's rated current.
• Ring finals are the standard exception — a 32A breaker is permitted with 2.5mm² cable (rated around 27A clipped direct) only because a ring final splits current between two legs feeding the same protective device; this arrangement is unique to ring circuits and does not apply to radials.
• Voltage drop is a separate check, not part of Ib ≤ In ≤ Iz — even where a breaker and cable combination satisfies the overload rule, the circuit still needs a separate check that voltage drop stays within 3% (lighting) or 5% (other circuits) of nominal voltage per Appendix 12, which can force a larger cable on long runs regardless of current rating.
• RCBOs combine both protections — an RCBO provides the same In/Iz overcurrent protection as an equivalent MCB, plus 30mA earth-fault protection in the same device; sizing the overcurrent element follows exactly the same Ib ≤ In ≤ Iz method.
• Never upsize a breaker to "stop nuisance tripping" — if a breaker trips on genuine load, the fix is to investigate the load or upsize the cable and breaker together as a matched pair, never to fit a larger breaker onto an unchanged, undersized cable.

Quick Reference Table

Detailed Guidance

The three-rule method, step by step

1. Establish Ib — the design current. For a fixed appliance, use its rated current (from the data plate or manufacturer spec) directly rather than dividing kW by voltage where the manufacturer states a current figure — appliance current draw often doesn't divide cleanly, and the manufacturer's figure accounts for real-world tolerances.
2. Select the smallest standard In at or above Ib — never round down. If Ib is 13A, the next standard rating up is 16A, not 10A.
3. Confirm Iz (corrected) is at or above In — take the cable's tabulated current-carrying capacity from Table 4D5A (or the relevant table for the installation method) and apply any correction factors for grouping, ambient temperature or thermal insulation. If the corrected Iz falls below In, the cable must be upsized — the breaker rating cannot be reduced below In just to make an undersized cable "fit," because that would drop In below Ib.
4. Check voltage drop separately — confirm the circuit still meets the 3%/5% voltage drop limit over its full length at the design current, using Table 4D5B mV/A/m values. This is an independent check from the Ib ≤ In ≤ Iz rule and can be the limiting factor on long runs even when overload protection is satisfied.

Worked example 1: immersion heater (clean, straightforward case)

A 3kW immersion heater on a 230V supply draws Ib = 3000W ÷ 230V ≈ 13A. The next standard breaker rating at or above 13A is In = 16A. 2.5mm² twin-and-earth clipped direct has a tabulated capacity of Iz = 27A — comfortably above 16A even before considering correction factors. The full chain: 13A ≤ 16A ≤ 27A — satisfied with margin. A dedicated double-pole isolator and heat-resistant flex (3183TQ) at the heater itself complete the installation per manufacturer requirements.

Worked example 2: ring final circuit (the special case)

A ring final circuit is the one situation in UK domestic wiring where a 32A breaker is used with 2.5mm² cable, even though that cable's clipped-direct rating (27A) is below the breaker's rating on paper. This is permitted because a ring topology feeds every point on the ring from two directions simultaneously — current entering the consumer unit splits between both legs of the ring, so under normal, evenly-distributed loading, no single length of 2.5mm² cable actually carries the full 32A. BS 7671 Appendix 15 permits this specific, well-defined arrangement (2.5mm² T&E, 32A protective device, floor area up to 100m², unlimited socket-outlets) precisely because of this current-sharing behaviour — it is not a general exception to the Ib ≤ In ≤ Iz rule, and it does not apply to any other circuit topology. A broken ring (accidentally wired as two radials joined mid-run) removes this current-sharing protection and leaves a single 2.5mm² leg exposed to up to 32A, which is why ring continuity testing is mandatory at commissioning.

Worked example 3: EV charger (equipment-rated current, not a kW calculation)

A 7.4kW single-phase EV charge point is rated by its manufacturer to draw a maximum of 32A — this is a fixed equipment output current, not something to recalculate from the kW figure, since the charger itself limits current to that value regardless of the "7.4kW" marketing figure (7.4kW ÷ 230V would actually work out to roughly 32.2A, essentially the same number here, but always use the manufacturer's stated maximum current where it's given rather than deriving it independently). With Ib = 32A, the matching breaker is In = 32A, and a 6mm² cable clipped direct (Iz = 47A) comfortably covers it — 32A ≤ 32A ≤ 47A. On longer runs, particularly to an outbuilding, voltage drop and cable derating for outdoor/buried installation methods (see outdoor electrics) often push the cable size up to 10mm² well before the breaker rating needs to change.

Correction factors — why the cable's real capacity is rarely the table value

The tabulated current-carrying capacities in Table 4D5A assume a single cable clipped direct to a surface in free air at a 30°C ambient. Real installations rarely match that exactly:

Applying these factors can turn a cable that looks generously sized on the table into one that's marginal or undersized for the breaker actually fitted — always apply the relevant factors for the real installation method before confirming Iz, not just the headline clipped-direct figure.

Breaking capacity — a separate check from current rating

The kA rating on an MCB (commonly 6kA for domestic consumer units) describes the maximum fault current it can safely interrupt without the device itself failing catastrophically — this is entirely separate from its current rating (In) and load-carrying role. It must be checked against the prospective fault current at the installation (available from the supplier's declared Ze/PFC data or measured on site), not derived from the circuit's normal load current.

Frequently Asked Questions

What size MCB do I need for a shower?

It depends on the shower's actual rated current, which the manufacturer states on the appliance — don't rely purely on dividing the kW rating by 230V, since element tolerances mean actual draw can be higher than the simple calculation suggests. As a guide, most 7.5–8kW showers use a 32A breaker on 6mm² cable, while 8.5–9.5kW units commonly need a 40A breaker on the same 6mm² cable, and anything above roughly 10kW typically needs 10mm² cable — see the full breakdown in cable sizing. Always verify against the specific appliance's data plate.

What happens if the breaker is the wrong size for the cable?

If the breaker (In) is rated higher than the cable's corrected capacity (Iz), the cable can be carrying more current than it can safely dissipate without the breaker tripping to protect it — this is a genuine overheating and fire risk, not just a compliance technicality. If the breaker is rated lower than the actual load (In below Ib), it will trip under normal legitimate use, which is a nuisance but not itself unsafe — the fix there is to correctly recalculate Ib and confirm the right breaker and cable pairing for the real load, not simply fit a bigger breaker.

Can I just fit a bigger breaker to stop a circuit nuisance-tripping?

No — this is one of the most dangerous "fixes" in domestic electrical work. If a breaker trips repeatedly under genuine load, the correct response is to establish why (undersized cable for the actual load, a developing fault, or a circuit that's simply been asked to do more than it was designed for) and re-size the cable and breaker together as a matched pair. Fitting a larger breaker onto an unchanged cable removes the protection that cable was relying on, and it will not trip in time to prevent overheating under a real fault.

Is Type B always correct for domestic circuits?

Almost always, yes. Type B (3–5× rated current trip threshold) suits the vast majority of domestic loads, including resistive heating, lighting and general socket circuits. Type C (5–10×) is occasionally needed where a circuit has significant motor inrush current on starting — some pumps, larger compressors, or certain heat pump outdoor units — where a Type B breaker would nuisance-trip on legitimate start-up current. Check the specific appliance manufacturer's installation requirements where inrush current is a concern.

Regulations & Standards

• BS 7671:2018+A4:2026 (IET Wiring Regulations) — Regulation 433.1 sets out the Ib ≤ In ≤ Iz overload protection rule; Regulation 434 covers fault protection and breaking capacity requirements.

• BS 7671 Table 4D5A / 4D5B — current-carrying capacity and voltage drop tables for twin and earth cable, referenced for establishing Iz and checking voltage drop.

• BS 7671 Appendix 4 — installation methods, ambient temperature, grouping and thermal insulation correction factor tables.

• BS 7671 Appendix 12 — permitted voltage drop limits (3% lighting, 5% other circuits).

• BS 7671 Appendix 15 — the specific ring final circuit arrangement permitting a 32A device with 2.5mm² cable.

• IET On-Site Guide — Table 2 diversity allowances for cooker and other circuits with non-continuous full-rated loads.

• IET Wiring Matters — technical commentary on BS 7671 sizing and protection rules

• NICEIC technical guidance — practical cable and breaker sizing guidance for registered electricians

• Electrical Safety First — consumer and trade safety guidance on circuit protection

• cable sizing — the full cable size and current-carrying capacity reference this calculator is built on

• rcd vs rcbo — how RCBO devices combine the overcurrent protection covered here with earth-fault protection

• ring final circuit explained — the full detail behind the ring final's special 32A/2.5mm² exception

• consumer units — consumer unit standards and device selection this breaker sizing feeds into