Breaker Capacity Calculator

In this calculator, MCB means a miniature circuit breaker (list 6~63A) and MCCB a molded-case circuit breaker (list 15~2000A); for 63A and below it picks an MCB first. These names split ratings by range and are not the same as KEC's industrial/residential classes — KEC sets trip characteristics separately for industrial and residential circuit breakers and requires residential ones where ordinary people may touch them, such as household panels (212.3.4). Check with the maker whether a product is a residential type. An ELB (earth-leakage breaker) mainly detects leakage current to prevent electric shock and leakage faults; some also provide overcurrent protection, and in TN systems an ELB used for fault protection must be a combined overcurrent type (211.2.5). Where ELBs are required starts with KEC 211.2.4, and separate clauses add places such as general-use outlets of 20A or less (211.2.3 ④) and bathroom outlets (234.5).

It is not a KEC value — it is this calculator's default for general loads. KEC 212.4.1 requires two conditions, not a multiplier: design current ≤ breaker rating ≤ cable ampacity, and the current that ensures operation ≤ 1.45 × cable ampacity. For a different margin, enter it in Custom Safety Factor.

This calculator sets the default safety factor for motor loads at 1.5 for direct-on-line and 2.0 for Y-Δ starting, higher than general loads (1.25). These are the calculator's defaults, not KEC values, so check them against the motor nameplate and the maker's starting-current data. For protective devices installed indoors on low-voltage circuits, KEC sets no multiplier; it requires the rating to be chosen with the starting current of the starting method in mind (212.6.3 ②). To check for trips during starting, compare the breaker's instantaneous trip curve (for residential circuit breakers: B over 3In up to 5In, C over 5In up to 10In, D over 10In up to 20In — Table 212.3-3; In is the breaker's rated current) with the motor's actual starting current.

General-use outlets rated 20A or less used by ordinary people, and outdoor portable equipment rated 32A or less, must have additional protection by an ELB with a rated residual operating current of 30mA or less (211.2.3 ④, 211.6.1). An ELB alone is not accepted as a protective measure; it is additional protection used together with one of the measures in 211.2 to 211.5 (211.6.1). Outlets in bathrooms or toilets with a bathtub or shower, where people use electricity while wet, must be protected by a shock-protection ELB of 15mA or less with an operating time of 0.03s or less, an outlet with such an ELB built in, or an isolating transformer of 3kVA or less (234.5). Set other sensitivities by the clause for the installation location and the maker's data. This calculator sizes overcurrent ratings only.

If the calculated value falls between ratings, pick the next rating up (e.g. 20.1A → 25A). The calculator rounds up to the smallest rating at or above the minimum required rating, checking the MCB list (63A and below) first and then the MCCB list. When it picks an MCB, it also shows the same value rounded up within the MCCB list — use that one for panels that use MCCBs (e.g. 20.1A → 25A MCB · 30A MCCB). Available ratings and trip characteristics vary by maker, so confirm with the catalog. Going one rating down cuts the margin and can cause nuisance trips.

Under KEC 212.4.1 the breaker rating must not exceed the cable ampacity (I_n ≤ I_Z). Ampacity is corrected for installation method, ambient temperature, and grouping, so size alone does not decide it. Use this site's wire size calculator to get the corrected ampacity and compare it with the chosen rating. This site's wire size calculator uses installation method B1 and corrects only for ambient temperature and grouping (number of circuits), so for other installation methods check the matching ampacity table directly.

Inverters create high-frequency leakage current that can make a standard ELB trip unnecessarily. Even so, on circuits that require an ELB (KEC 211.2.3 ④, 211.2.4, etc.) do not remove it — check the ELB type (Type F or B, IEC 62423) and rated residual operating current recommended by the inverter maker. On such circuits, an ELB may be omitted only where a clause allows it — for example, supply through an isolating transformer with a secondary voltage of 300V or less and an unearthed load-side circuit (211.2.4 ① item 가, exception (5)), or circuits such as emergency lighting, whose interruption would affect public safety, with a ground-fault alarm to a monitoring station (211.2.4 ②).