What the Circuit Breaker Size Calculator does
This calculator picks a protective device rating for a circuit and then checks the coordination rules rather than just returning a number. It applies the multiplier the load type calls for, selects the next standard rating from the IEC or NEC series, and tests Ib <= In <= Iz and the 1.45 x Iz conventional tripping condition, showing each check as a pass or a failure with the reason.
It also suggests a tripping curve for IEC devices and works out the maximum earth-fault loop impedance that curve implies - the number that decides whether the breaker will actually trip magnetically on a fault at the far end of the circuit, rather than sitting on its slow thermal element for half a minute.
How to use it
- Choose IEC or NEC. The standard rating series differ: IEC runs 6, 10, 13, 16, 20, 25, 32, 40, 50, 63 and so on, while NEC 240.6(A) runs 15, 20, 25, 30, 35, 40, 45, 50, 60, 70.
- Enter the design current Ib - what the circuit actually draws.
- Enter the cable's capacity after derating, Iz, from the cable size calculator. Without it the coordination check cannot be made.
- Pick the load type. A motor branch circuit under NEC 430.52 can take a breaker at up to 250% of full-load current so the starting current passes; a continuous load under NEC 210.19 takes 125%.
- Enter the voltage to earth and the prospective fault level to get the loop-impedance ceiling and the minimum breaking capacity.
- Read every finding, not just the rating. A device that satisfies Ib but exceeds Iz protects nothing.
Reading the results
Ib <= In means the device will not trip on the normal load. Fail this and the circuit nuisance-trips.
In <= Iz means the cable is protected. Fail this and the cable can run continuously above its safe capacity without the device ever noticing - the failure mode that starts fires.
I2 <= 1.45 x Iz is the conventional tripping condition from IEC 60364-4-43. For circuit-breakers it is satisfied automatically whenever In <= Iz, because their I2 is 1.45 x In. For fuses it is not, which is why the fuse page has an extra check.
The curve suggestion is about inrush, not about the load current. A type B trips magnetically at 3 to 5 times In, a type C at 5 to 10 and a type D at 10 to 20; a higher band lets more inrush through, at the cost of needing a lower loop impedance to trip on a fault.
Worked example: a 24 A circuit on a 10 mm² cable derated to 32 A
Design current is 24.06 A and the cable's derated capacity is 32.23 A. With a general load the multiplier is 1, so the target is 24.06 A and the next IEC standard rating is 25 A.
Check one: In (25) >= Ib (24.06). Passes, but only just - a 5% growth in the load would start tripping the breaker, and the next size, 32 A, would exceed the cable's 32.23 A capacity by a whisker. This circuit has no headroom in either direction, which is a design signal rather than an arithmetic result.
Check two: In (25) <= Iz (32.23). Passes comfortably.
Check three: I2 = 1.45 x 25 = 36.25 A against 1.45 x 32.23 = 46.7 A. Passes, as it always does for a breaker once In <= Iz.
For a general load the suggested curve is type B, tripping instantaneously between 75 A and 125 A. With 230 V to earth, the loop impedance must therefore be below 230 / 125 = 1.84 ohm for the magnetic element to operate. If this circuit were a motor needing a type D curve, the band would be 250 to 500 A and the loop impedance would have to be below 0.46 ohm - a fourfold tighter requirement that long circuits routinely fail.
With 11.6 kA of prospective fault current at the board, the device needs a breaking capacity of at least 15 kA from the standard series, and most designers would fit 25 kA.
Formulas and scoring rules
- The coordination rule
Ib <= In <= IzIEC 60364-4-43. Design current, device rating, cable derated capacity.- Conventional tripping condition
I2 <= 1.45 x IzI2 = 1.45 x In for breakers to IEC 60898 and IEC 60947-2, so it follows from In <= Iz.- NEC motor branch circuit
In <= 2.5 x motor full-load currentNEC 430.52 for an inverse-time breaker; the overload relay protects the motor against overload.- NEC continuous load
In >= 1.25 x IbNEC 210.19(A) and 215.2 for a load running three hours or more.- Maximum earth-fault loop impedance
Zs <= Uo / (upper magnetic multiple x In)Uo is the voltage to earth. Type B uses 5, type C 10 and type D 20.- Breaking capacity
Icu >= prospective fault current at the point of installationUnless a manufacturer-tested cascading combination is used.
The rule that matters most, and why
Of the three coordination checks, In <= Iz is the one that causes fires. If the device rating exceeds the cable's derated capacity, the cable can carry a current above its safe limit indefinitely and the breaker will sit there quite happily, because from its point of view nothing is wrong.
It happens most often when someone sizes the breaker from the load and the cable from a table, without applying the derating factors to the cable. A 32 A breaker on a 6 mm² cable is fine in free air and dangerous when the cable is bunched with five others in a hot ceiling void - the cable's Iz has dropped to around 23 A while the breaker still thinks 32 A is normal.
Curves, inrush and the loop impedance trade-off
A miniature circuit-breaker has two elements: a thermal bimetal for overload and an electromagnet for short circuit. The curve letter describes only the magnetic one. Type B trips between 3 and 5 times rated current, type C between 5 and 10, type D between 10 and 20.
Choosing a higher curve to stop nuisance tripping on inrush has a direct cost: the magnetic element now needs more current to operate, so the earth-fault loop impedance must be proportionally lower. Moving from type B to type D at the same rating makes the loop impedance limit four times tighter, and on a long circuit that can be impossible to achieve. The usual answers are an RCD for shock protection, a larger protective conductor, or a shorter circuit.
Motors are a special case in both codes
A motor draws six or more times its full-load current while starting, for several seconds. A breaker sized at the motor's full-load current would trip every time. Both codes therefore separate the two jobs: the breaker or fuse provides short-circuit protection and is allowed to be well oversized, while an overload relay set from the motor's actual full-load current provides overload protection.
NEC 430.52 gives the multipliers explicitly - up to 250% of full-load current for an inverse-time breaker on a standard motor, more in specific cases. IEC practice reaches the same place through type 1 and type 2 coordination with a manufacturer's published combination of breaker, contactor and overload relay.
Limitations: what the result does not prove
- It applies the ratio rules. It does not read a real device's time/current curve, so it cannot confirm that a particular breaker clears a particular fault in a particular time.
- Discrimination with upstream devices is not checked. Selectivity between breakers depends on their published curves and, for current-limiting devices, on tested combinations.
- Motor circuit protection depends on manufacturer-tested coordination combinations, which no calculator can substitute for.
- Residual current protection is a separate requirement driven by the earthing system and the type of circuit, and is not considered here.
- The loop-impedance ceiling is derived from the curve band, not from a specific device's published maximum Zs values, which are what a certificate records.
- Nothing here is a compliance certificate. A qualified engineer must verify the design against the applicable standard and the local regulations.
Privacy: where your data goes
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Standards and sources
- IEC 60947-2 - Low-voltage switchgear: circuit-breakers - checked 19 Sep 2026
- IEC 60364-4-43 - Protection against overcurrent - checked 19 Sep 2026
- NFPA 70 - National Electrical Code - checked 19 Sep 2026
- IEC 60898-1 - Circuit-breakers for overcurrent protection for household installations
Frequently asked questions
What size circuit breaker do I need for a 24 A load?
The next standard rating at or above the design current, provided it does not exceed the cable's derated capacity. For 24.06 A that is a 25 A device in the IEC series or a 30 A device in the NEC series - but only if the cable can carry it, which is why the cable capacity is a required input rather than an optional one.
What does Ib <= In <= Iz mean?
Design current, device rating, cable capacity, in that order. The device must be big enough not to trip on the normal load and small enough to protect the cable. If no standard rating fits between the two, the cable has to grow - the answer is never to pick a device outside the range.
What is the difference between a type B, C and D breaker?
The current at which the magnetic element trips instantaneously: 3 to 5 times rated current for type B, 5 to 10 for type C and 10 to 20 for type D. Type B suits resistive loads and long runs, type C mixed loads with modest inrush, and type D transformers and direct-on-line motors. All three have the same thermal overload characteristic.
Why can a motor breaker be 250% of the motor current?
Because on a motor circuit the breaker is not the overload device. It exists to clear a short circuit, and it has to be large enough to let six or more times full-load current pass for several seconds during starting without tripping. The overload relay, set from the motor's actual full-load current, protects the motor - which is why a motor starter always has both.
Does the 1.45 rule ever fail for a circuit breaker?
Not in practice. I2 for breakers to IEC 60898 and IEC 60947-2 is 1.45 x In, so the condition I2 <= 1.45 x Iz reduces to In <= Iz, which you have already checked. It is included here because it does not follow automatically for fuses, whose I2 is 1.6 x In - a real difference that surprises people moving between the two.
How do I know what breaking capacity to specify?
It must be at least the prospective fault current at the point where the device is installed, which the short-circuit calculator estimates. Common values are 6, 10, 15, 25 and 36 kA. A lower-rated device can be used only in a cascading arrangement that the manufacturer has tested and published; it can never be inferred.
Can I fit a larger breaker if the cable keeps tripping it?
Only if the cable's derated capacity allows it, which is exactly what the In <= Iz check tests. If a circuit trips repeatedly, the honest answers are that the load has grown beyond the cable, that the curve is wrong for the inrush, or that there is a fault. Fitting a larger device on the same cable removes the protection rather than the problem.
Does the calculator check discrimination with the main breaker?
No. Selectivity between two breakers depends on their published time/current curves and, at high fault levels, on tested combinations rather than on ratings. A rule of thumb of 2:1 or 3:1 between ratings is a starting point for overload discrimination, but it says nothing about behaviour during a short circuit.
Last reviewed by the A2Z.Tools team against the sources listed above.
