MCB Size Calculator
Calculate Smarter. Work Faster.
Free MCB size calculator — enter full load current and load type to instantly get the correct MCB rating and trip curve type (B, C, or D).
MCB Size Details
Enter the load current, pick a trip curve and a pole configuration — the calculator returns the rating, the actual instantaneous trip band in amps, and which conductors the breaker disconnects.
Iz = derated current-carrying capacity of the cable. Zs = measured or calculated earth fault loop impedance at the far end of the circuit.
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How MCB Size and Curve Type Are Selected
Rule of thumb: choose the smallest standard MCB rating In that is at least the design current Ib — or at least 1.25 × Ib for a continuous load or as a planning margin — and no larger than the cable’s current-carrying capacity Iz: Ib ≤ In ≤ Iz. A 12 A load needs a 16 A MCB either way, on a cable rated at least 16 A.
Formula: IEC 60364-4-43 (and BS 7671) requires Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz, where I2 is the current that ensures the breaker trips within the conventional time. For an IEC 60898-1 MCB, I2 = 1.45 In, so the second condition is met automatically once In ≤ Iz. IEC practice does not add a fixed 25%: the rating only has to reach the design current. The 1.25 factor comes from North American practice for continuous loads (a standard breaker may carry only 80% of its rating continuously) and is also a common planning margin elsewhere — the calculator lets you choose the basis. Whatever the basis, the cable must be rated for the MCB rating actually fitted.
Two elements, one device. The thermal element handles sustained overload and takes seconds to minutes; the magnetic element handles short circuits and operates in milliseconds. The rating sets the thermal behaviour, the curve sets the current at which the magnetic element takes over. That is why a 16 A Type B and a 16 A Type C carry the same load identically but behave completely differently the moment a motor starts.
Standard ratings. The result is rounded up to the preferred series — 6, 10, 16, 20, 25, 32, 40, 50, 63 A and above — so a calculated 15 A becomes a 16 A device, and the cable must be rated for that 16 A, not for the 15 A that was calculated.
Breaking capacity. IEC 60898-1 MCBs are marked with a rated short-circuit capacity Icn — commonly 4.5, 6 or 10 kA — which must be at least the prospective fault current at the board, unless a tested back-up device upstream covers the difference.
Disconnection time. For fault protection the magnetic element must operate on an earth fault, which means the fault current must reach the top of the curve band: 5 × In for Type B, 10 × for Type C and 20 × for Type D. With a 0.95 voltage factor this gives the familiar maximum loop impedance Zs ≤ 0.95 × U0 ÷ (k × In) — for example 1.37 Ω for a 32 A Type B on a 230 V system, and 0.68 Ω for a 32 A Type C. This is why moving to a higher curve on a long circuit can quietly break the disconnection requirement.
Worked example: a small motor circuit drawing 12 A. Rating = 12 × 1.25 = 15 A → 16 A. On a Type C curve that gives an instantaneous band of 80–160 A, which rides through the starting surge but still clears a short circuit in milliseconds.
Summary: rating from the load current, curve from the inrush, poles from the isolation the circuit needs — three independent choices. Whatever the three produce, the rating must not exceed the current-carrying capacity of the cable it protects.
Worked Example
Small motor, FLC=12A, moderate inrush: MCB Rating = 12 × 1.25 = 15A → 16A standard size, Type C curve (trips at 80-160A instantaneous).
MCB rating and curve type selection here uses standard general guidelines — for a final design, confirm the exact starting/inrush characteristics of your specific load against the MCB manufacturer's published curve data, and verify the selected MCB's rating and curve type against the applicable electrical code and any downstream protection coordination requirements. Combine with a residual current device (RCD/RCCB) or use a combined RCBO where earth leakage protection is also required, and have the final panel design reviewed by a qualified electrical engineer.
MCB Trip Curve Types and Their Applications
| Curve | Instantaneous Band | Defined In | Typical Application |
|---|---|---|---|
| Type Z | 2–3× | IEC 60947-2 | Semiconductor and measuring circuits, sensitive electronic equipment |
| Type A | 2–3× | Manufacturer range | Semiconductor protection. Not part of the IEC 60898-1 household set and not widely stocked — check availability before specifying |
| Type B | 3–5× | IEC 60898-1 | Lighting, heating and other resistive loads; most domestic final circuits |
| Type C | 5–10× | IEC 60898-1 | Mixed commercial loads, small motors, air conditioning, fluorescent and LED banks |
| Type K | 8–12× | IEC 60947-2 | Industrial motor and transformer circuits where a tighter band than Type D is wanted |
| Type D | 10–20× | IEC 60898-1 | Transformers, large motors, welding sets and other heavy-inrush equipment |
The band is the only thing separating one curve from another at the same rating. Thermal characteristic, current-carrying capacity and the physical device are identical.
Two points are worth separating, because they are routinely blurred together in curve-type lists. B, C and D are defined in IEC 60898-1, the standard for household and similar circuit breakers — these are the three you will find in any wholesaler. K and Z belong to industrial breakers built to IEC 60947-2, and A appears only in some manufacturers’ semiconductor-protection ranges. Listing all six as if they were interchangeable options on a shelf leads people to specify a curve their supplier does not carry.
The second point is that the bands overlap. A fault at eleven times rated current sits inside the instantaneous region of Type C, Type K and Type D simultaneously — what separates them is behaviour at the lower end, where a Type B has already tripped instantly and a Type D is still waiting on its thermal element. Choosing a curve is choosing how much brief inrush to ignore, not choosing how fast the breaker is.
That has a safety consequence worth stating plainly: a higher curve raises the current needed for an instantaneous trip, so on a long circuit with high loop impedance a Type D may not reach its magnetic threshold during an earth fault at all, and disconnection then falls to the much slower thermal element. Where a curve is raised to stop nuisance tripping, the earth fault loop impedance should be re-checked against the disconnection time the local wiring rules require.
MCB Poles: SP, SP+N, DP, TP, TPN and 4P
The pole count answers a different question from the rating and the curve. Rating asks how much current the breaker carries, curve asks how much inrush it tolerates — poles ask which conductors actually get disconnected when it opens. A 16 A Type C behaves identically as a single-pole or a four-pole device; only the isolation changes.
| Configuration | Conductors Switched | Where It Is Used |
|---|---|---|
| SP (1P) | Line only | Domestic final circuits — lighting, socket circuits, fans |
| SP+N (1P+N) | Line + neutral | Same duty as SP where the neutral must also be isolated, in one module width |
| DP (2P) | Line + neutral | Heavier single-phase appliances, water heaters, and wherever supply polarity is not guaranteed |
| TP (3P) | Three phases | Three-phase loads with no neutral — most three-phase motors |
| TPN (3P+N) | Three phases; neutral on a solid link, not switched | Three-phase distribution boards feeding single-phase circuits from a common neutral |
| 4P | Three phases + neutral | Where the neutral itself must be broken — generator changeover, some TT arrangements |
TPN and 4P are not the same thing
This is the distinction most pole-count lists get wrong by lumping them together. In a TPN device the neutral passes through a solid link: it is brought into the breaker for convenience of wiring, but it is not disconnected when the breaker trips or is switched off. A 4P device has a genuine fourth switched pole. If your reason for wanting the neutral in the breaker is isolation — working safely on the circuit, or separating a generator neutral — a TPN will not do it.
Where a neutral is switched, the order matters: it must not be disconnected before the line conductors, or re-connected after them. Devices designed for the job handle this internally; improvising one by wiring a neutral through a spare pole of an ordinary breaker does not.
Common Mistakes When Sizing an MCB
1. Using Type B on a motor circuit. The 3–5× band is exceeded by ordinary starting inrush, so the breaker trips every time the motor starts.
2. Defaulting to Type D everywhere “to be safe”. It is the opposite of safe: a higher curve needs more fault current to trip instantly, which can leave a long circuit relying on the slow thermal element.
3. Treating the MCB as motor overload protection. A breaker sized to pass starting current cannot also catch a modest sustained overload. That needs a separate overload device.
4. Rounding the calculated rating down. Round up to the next available rating, then size the cable for that rating rather than for the calculated figure.
5. Exceeding the cable’s current-carrying capacity. The MCB protects the cable; a rating above the cable’s ampacity protects nothing.
6. Checking the current rating but not the breaking capacity. A correct rating says nothing about whether the device can safely interrupt the fault current available at that board.
7. Not separating thermal from magnetic nuisance tripping. A trip after minutes of running points at the rating or the load; a trip at the instant of switching points at the curve or a genuine fault. They have opposite fixes.
8. Changing curve without re-checking loop impedance. Going from Type B to Type C halves the maximum Zs for the same rating; a long radial that passed on Type B can fail the disconnection requirement on Type C.
Frequently Asked Questions
What is the formula for MCB sizing? +
Choose the smallest standard rating In that satisfies Ib ≤ In ≤ Iz — at least the design current Ib and no more than the cable capacity Iz. Many designers apply In ≥ 1.25 × Ib for continuous loads (North American practice) or as a planning margin: a 12 A load gives 15 A, which becomes a 16 A device. IEC 60364 itself only requires In ≥ Ib. Where the local wiring rules set a different factor, use theirs.
Which curve type should I use for general lighting and resistive loads? +
Type B. Lighting, heating and other resistive loads draw no meaningful inrush, so the tighter 3–5× band gives faster disconnection on a fault without nuisance tripping.
Which curve type should I use for a motor circuit? +
Type C for most small and medium motors, Type D where inrush is heavy or prolonged — large motors, transformers, welding sets. Type K is the industrial alternative between the two. The motor still needs a separate overload device.
Can I use a higher curve type than necessary just to be safe? +
It stops nuisance trips and weakens protection at the same time. A higher curve raises the current needed for an instantaneous trip, so on a long circuit the magnetic element may not operate during an earth fault at all, leaving disconnection to the much slower thermal element. Re-check the earth fault loop impedance against the disconnection time your wiring rules require.
What is the maximum earth fault loop impedance for an MCB? +
For the magnetic element to disconnect an earth fault, the fault current must reach the top of the curve band: 5 × In for Type B, 10 × In for Type C, 20 × In for Type D. With a 0.95 voltage factor, Zs max = 0.95 × U0 ÷ (k × In). On a 230 V system that gives 1.37 Ω for a 32 A Type B and 0.68 Ω for a 32 A Type C, matching the tables in BS 7671. Enter the measured Zs in the calculator to check it.
What breaking capacity should an MCB have? +
At least the prospective fault current at the board where it is installed. IEC 60898-1 MCBs are marked with a rated short-circuit capacity Icn — commonly 6 kA for domestic and 10 kA for commercial and industrial boards. Near a large transformer the fault level can exceed this, and then an MCCB or a tested back-up combination is needed.
Does MCB rating alone provide motor overload protection? +
No. An MCB sized to let motor starting current through cannot also protect against a modest sustained overload — the two requirements pull in opposite directions. Motor overload protection comes from a separate overload relay or an MPCB with an adjustable thermal element set to the nameplate current.
What is the difference between an MCB and an MCCB? +
Scale and adjustability. MCBs are fixed-rating devices, typically up to about 125 A, with a fixed curve. MCCBs run to far higher ratings with adjustable trip settings and much higher breaking capacities. An MCB suits final circuits; an MCCB suits sub-mains and large feeders.
Should MCB rating exceed the protected cable's ampacity? +
No. The MCB protects the cable, so its rating must be at or below the cable's current-carrying capacity under the actual installation conditions. Rounding the rating up to a standard size means the cable has to be sized for that standard rating, not for the raw calculated figure.
How do I know the actual inrush current of my specific load to pick the right curve type? +
Nameplate data first, then the equipment manufacturer's documentation. Where neither gives a figure, the curve is chosen from the load class — resistive, motor, or transformer — and confirmed in service: a correctly rated MCB that trips only at start-up is telling you the curve is too tight, not that the rating is wrong.
What is the difference between a Type B and a Type C MCB? +
Only the instantaneous band. Type B trips instantly at 3–5× rated current, Type C at 5–10×. The thermal characteristic is identical, so a 16 A of either carries 16 A indefinitely. Type B suits resistive loads; Type C tolerates the brief surge of small motors and LED driver banks.
What is a Type K and a Type Z MCB used for? +
Type K trips at roughly 8–12×, between C and D, for industrial motor and transformer circuits. Type Z trips at 2–3× for semiconductor and measuring circuits. Both come from industrial breakers built to IEC 60947-2 rather than the IEC 60898-1 set that defines B, C and D, so check availability before specifying one.
What is the difference between a TPN and a 4-pole MCB? +
A TPN device has three protected poles and a neutral on a solid link — the neutral is not broken when it opens. A 4-pole device has a genuine fourth switched pole. If the reason for wanting the neutral in the breaker is isolation, a TPN will not provide it.
Does the number of poles change the MCB rating or trip curve? +
No. A 16 A Type C has the same thermal characteristic and the same instantaneous band whether it is single-pole or four-pole. Poles decide only which conductors are disconnected.