How to use this calculator
- Choose the device: MCB to IEC 60898-1 or MCCB to IEC 60947-2.
- Enter the design current Ib directly, or the load power, voltage and power factor to work it out.
- Enter the corrected current-carrying capacity Iz of the cable (after ambient and grouping factors). The cable size calculator gives this value.
- For an MCB choose the trip curve; for an MCCB enter the conventional tripping current multiple from the maker's data.
- Read the rating In, the I2 check and, if nothing fits, the largest rating the cable allows.
How it works: the formula
In must be at least the design current so the device does not trip in normal service, and not more than Iz so the cable is protected. I2 is the current that makes the device trip within its conventional time. Requiring I2 ≤ 1.45 Iz limits how long the cable can run overloaded. For MCBs I2 is 1.45 In, so the second condition follows from the first. For other devices it has to be checked with the device's own I2.
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| Ib | Design current of the circuit | A |
| In | Rated current (or overload setting Ir) of the protective device | A |
| Iz | Corrected current-carrying capacity of the cable | A |
| I2 | Conventional tripping current of the device | A |
| k | I2 / In | — |
Worked example
MCB, Ib = 28 A, cable Iz = 34 A, curve C:
If the cable were rated only Iz = 30 A, 32 A would be too large and 25 A would be below Ib, so no rating fits: the cable must be upsized. A 15 kW three-phase load at 400 V and PF 0.85 draws 15000 / (1.73205 × 400 × 0.85) = 25.47 A, which also leads to a 32 A device.
Assumptions
- Overload protection only, by IEC 60364-4-43 cl. 433.1.
- MCB ratings are the IEC 60898-1 preferred values from 6 A to 125 A; MCCB ratings are common manufacturer frame/rated currents.
- The device is at its reference ambient temperature.
Limitations
- Fault protection is not checked: the earth-fault loop impedance must let the device trip within the disconnection time of IEC 60364-4-41.
- The breaking capacity (Icn, or Icu/Ics for MCCBs) must be at least the prospective short-circuit current at the point of installation.
- Selectivity (discrimination) with upstream devices, motor starting and transformer inrush need the maker's time–current curves.
- Trip-curve suggestions are guidance only; the right curve depends on the load's inrush current and the available fault current.
- Motor circuits are normally protected by a motor-protection device or overload relay coordinated with the contactor, which this tool does not cover.
References
Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.
- IEC 60364-4-43, Protection for safety — Protection against overcurrent, clause 433.1 (Ib ≤ In ≤ Iz, I2 ≤ 1.45 Iz).
- IEC 60364-4-41, Protection for safety — Protection against electric shock (disconnection times, loop impedance).
- IEC 60898-1, Circuit-breakers for overcurrent protection for household and similar installations (preferred ratings, B/C/D instantaneous ranges, I2 = 1.45 In).
- IEC 60947-2, Low-voltage switchgear and controlgear — Circuit-breakers (MCCBs and ACBs).
- IEC 60364-5-52, Wiring systems — current-carrying capacity of cables (source of Iz).
- BS 7671, Requirements for Electrical Installations — Regulation 433.1 and Appendix 4 (equivalent rules in UK practice).
Frequently asked questions
Why not simply pick a breaker one size above the load current?
Because the breaker protects the cable, not the load. The rating must also be at or below the cable's corrected capacity Iz. If the next rating above Ib is larger than Iz, the cable is too small for that circuit and has to be upsized.
What does I2 ≤ 1.45 Iz mean?
I2 is the current at which the device is sure to trip within its conventional time (one or two hours). A cable can tolerate about 1.45 times its rated current for that time without serious damage, so the device must trip at or below that level.
Which curve should I choose: B, C or D?
B trips instantaneously between 3 and 5 × In and suits resistive loads and long circuits with low fault current. C (5–10 × In) is the usual choice for general circuits, lighting and small motors. D (10–20 × In) tolerates high inrush from transformers, large motors and welders, but needs a high fault current to trip instantaneously.
Why does the MCCB need a conventional tripping current input?
IEC 60898-1 fixes I2 at 1.45 In for MCBs. MCCBs follow IEC 60947-2, where the value depends on the product and, for adjustable electronic units, on the setting. Enter the figure from the maker's data so the I2 check is valid.
Is a 13 A rating standard?
13 A is one of the preferred rated currents listed in IEC 60898-1 and is common in some countries, but not every manufacturer offers it in every range. Check availability before you specify it.