Short answer: a cable is protected only when four currents line up — the design current of the circuit (Ib), the nominal rating of the protective device (In), the current-carrying capacity of the cable as installed (Iz), and the conventional tripping current of the device (I2). The rule is Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz. For a domestic MCB to IEC/EN 60898-1 that second inequality is satisfied automatically, because I2 is fixed at 1.45 × In. For an MCCB to IEC 60947-2 it is not, because I2 is around 1.3 × In, and the practical limit tightens to roughly In ≤ 1.11 × Iz.
Almost every undersized feeder we are asked to quote around fails on one of those four numbers rather than on the breaker. The device gets chosen from the load, the cable gets chosen from what the wholesaler had, and the installation method that halves the cable’s capacity — trunking, insulation, a hot roof void — never enters the calculation. We write as an independent trading company: not a manufacturer, not an appointed distributor of any brand.
The four currents, and what each one really is
They are often treated as one number with different names. They are four different measurements taken at three different points in the design.
| Symbol | What it is | Where it comes from | What breaks if you get it wrong |
|---|---|---|---|
| Ib | Design current of the circuit | Load schedule — connected load with a diversity factor applied | Too low: the breaker trips on normal load. Too high: every downstream choice is oversized and the fault protection gets slower |
| In | Nominal rating of the protective device | The device’s marking — 16, 20, 25, 32, 40, 63 A on an MCB; frame and trip setting on an MCCB | Above Iz: the cable overheats before the breaker notices |
| Iz | Continuous capacity of the cable as installed | Cable table for the reference method actually used, then derated | Using the free-air number for a cable in insulated trunking overstates capacity by a large factor |
| I2 | Conventional tripping current — the current at which the device trips within the conventional time | Fixed by the product standard: 1.45 × In for 60898 MCBs | Ignoring it on an adjustable MCCB leaves a band where the cable runs hot indefinitely |
The distinction that matters commercially: Ib is a calculation, In is a catalogue choice, Iz is a table lookup plus derating, and I2 is a property of the standard the device is built to. Only In is negotiable in a quote. The rest are constraints.
Why the second inequality exists at all
If In ≤ Iz were sufficient, the rule would end there. It is not, because a breaker does not trip at exactly In. It is allowed to carry a small overload for a long time — one hour or more at the conventional test current — without tripping. That tolerance is what makes nuisance tripping rare, and it is also what puts the cable at risk.
The 1.45 factor in the second inequality is the cable’s own overload tolerance, taken from the same standard family: a cable may run at 1.45 × Iz for the conventional time. The two numbers are matched on purpose, so that a device built to 60898 and a cable built to the same assumption meet exactly at In = Iz. Change the device standard and the match is lost.
| Device | Standard | I2 as a multiple of In | What that means for cable selection |
|---|---|---|---|
| NB1-63 / NXB-63 MCB (B, C or D curve) | IEC/EN 60898-1 | 1.45 × In (fixed) | Check In ≤ Iz, then Ib ≤ In. The 1.45 test is already covered |
| MCB used under IEC 60947-2 (some industrial variants) | IEC 60947-2 | Set by the manufacturer’s trip data | Read the table; do not assume the 60898 factor carries over |
| NM1 / NXM MCCB, thermal-magnetic | IEC 60947-2 | Around 1.3 × In at the thermal setting | Roughly In ≤ 1.11 × Iz; with an adjustable Ir, set Ir against Iz, not against the load alone |
| MCCB with electronic trip unit | IEC 60947-2 | Defined by the Ir, tr and Ii settings | The setting sheet becomes part of the design; a device shipped at factory default is not a design |
That single row explains a recurring field problem: an MCCB chosen at exactly the cable’s tabulated capacity. It is 11% over the limit before any derating is applied, and it will run warm for years before anything fails.
Iz is not the number in the catalogue
The tabulated capacity assumes a reference method — free air, conduit on a wall, thermal insulation, buried, perforated tray. Moving from free air to a bundle in insulated trunking can take a large fraction of the capacity away before any other factor applies.
Then the correction factors stack multiplicatively: ambient temperature, grouping, thermal insulation and, for harmonic-rich loads, a neutral adjustment. The formula is short; the discipline is in applying every factor that is actually present:
Iz = Iz(table) × K(temperature) × K(grouping) × K(installation) × K(other)
Two habits make this reliable. Derate first, then choose the breaker — never the reverse. And write the factors onto the circuit schedule, because the next person to touch the board will otherwise repeat the calculation from the catalogue page and conclude the cable is oversized.
The second half: fault current, not just load current
Ib ≤ In ≤ Iz governs overload. Short circuit is a separate check, and a feeder can pass the first and fail the second.
The breaking capacity of the device has to exceed the prospective fault current at its terminals. On a final circuit in a small board, 6 kA is often enough and the NB1-63 at Icn 6 kA fits; closer to a transformer, or on a sub-main, 10 kA or a moulded-case device is the honest answer, and the NB1-63H at 10 kA is the modular step up. Guessing this from transformer kVA instead of calculating or measuring it is how boards get replaced.
The cable then has to survive the fault for the time the device takes to clear it. The adiabatic check is k²S² ≥ I²t, where S is conductor cross-section in mm² and k is a material constant — 115 for PVC-insulated copper, 143 for XLPE-insulated copper. The I²t is read from the device’s published let-through energy at the prospective fault level, not assumed. A cable that passes the thermal check at the far end of a long run can still fail it at the origin, where the fault current is highest.
A worked example, with the assumptions stated
Take a 25 A three-phase sub-main, 4 mm² copper, in trunking with two other loaded circuits at 35 °C. Suppose the cable table gives 30 A for that size and reference method in free air — quoted as an assumption, not as a claim about any particular cable. Apply a temperature factor of 0.94 and a grouping factor of 0.85: Iz becomes 30 × 0.94 × 0.85 = 24.0 A.
The 25 A MCB now fails the first inequality, because In (25) exceeds Iz (24). Two legitimate answers: step the cable to 6 mm², or step the device down to 20 A if the load allows. Choosing 6 mm² keeps the load and costs copper; choosing 20 A keeps the cable and may trip on start-up. The wrong answer is to keep both and assume it will be fine.
| Check | Numbers in the example | Result |
|---|---|---|
| Ib ≤ In | Ib 22 A, In 25 A | Pass |
| In ≤ Iz | In 25 A, Iz 24.0 A after derating | Fail — step to 6 mm² or drop to 20 A |
| I2 ≤ 1.45 Iz | 1.45 × 25 = 36.25 A against 1.45 × 24.0 = 34.8 A | Same result, same fix — the two inequalities move together on an MCB |
| Breaking capacity | Icn 6 kA against calculated fault level | Verify per board; 10 kA where the fault level is higher |
| Thermal withstand | k²S² against I²t at the fault level | Check at the origin, not only at the far end |
Voltage drop runs alongside, not after
A circuit can pass all four currents and still deliver a voltage at the load that fails the equipment or the local code. Volt drop is a separate calculation, and on long runs it is usually the constraint that sets the size while the ampacity check sits comfortably satisfied. Run both before the bill of materials is fixed; changing the cable after the board is drilled is the expensive version of the same decision.
What to put on the enquiry
Design current and load type; supply voltage, frequency and phases; cable size, insulation and conductor material; installation method and grouping; ambient temperature at the route; prospective fault current if known; and the destination country, because both the installation code and the certificate requirement follow the market. With those, device rating and cable size stop being a preference and become a result.
FAQ
Is In ≤ Iz enough, or do I really need the 1.45 check?
On an MCB to IEC/EN 60898-1, In ≤ Iz is enough, because the standard fixes I2 at 1.45 × In and the inequalities coincide. On an MCCB to IEC 60947-2, or on a device with an adjustable thermal setting, the 1.45 check is what stops the cable running in a protected overload band indefinitely.
Can I use a 10 kA MCB where the fault level is only 4 kA?
Yes. Higher breaking capacity is not a problem; it is unused headroom. The reverse — a 6 kA device on a board with a 9 kA prospective fault — is a device that may not safely clear a fault, and the marking is the only warning you get.
Does a C curve change the cable requirement?
Not the overload requirement, which is set by In. It changes the magnetic trip threshold and therefore the maximum earth-loop impedance for the circuit, which is a separate check. The cable still has to carry the load and survive the fault.
What if the cable is already installed and In ≤ Iz fails?
Either reduce In, add a parallel run, or change the installation method to raise Iz — for example by separating cables that are currently bunched. Leaving it and relying on the load staying below the design figure is not acceptable, because the design figure is by definition the maximum.
Related: MCB range overview · NB1-63 miniature circuit breaker · 6 kA vs 10 kA breaking capacity · Selectivity between MCB and MCCB · How to size a distribution board · MCB trip curves B, C and D · IEC 60898 vs 60947 · Request a quotation