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MCB Size Calculator

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Calculate a suitable preliminary MCB rating from load current, kW, kVA or voltage, with power factor, derating, cable ampacity, MCB curve, poles and breaking-capacity checks.

Inputs

1

System and load

Select the supply system and enter the known load quantity.

Use the actual design/load current if already known.

Enter real electrical input power. For motors, use actual electrical input where available.

Use this when the load is specified in kVA.

Single phase: line-to-neutral voltage. Three phase: line-to-line voltage.

Use the actual measured or equipment nameplate PF where available.

2

Design factors

Apply continuous-load, demand and installation correction factors.

Optional design multiplier for continuous or sustained loads. Enter 100% for no additional multiplier.

Use 100% for a known maximum load. Lower values should only be used where the design basis supports diversity.

Enter 100% if no manufacturer derating is required. Use the breaker manufacturer's correction data for elevated ambient temperatures.

Use the actual correction factor from the installation method and number of loaded circuits.

Enter the cable's corrected current-carrying capacity after installation derating. Set 0 only if cable ampacity is not being checked.

3

MCB selection

Choose the desired MCB curve, pole arrangement and fault-duty information.

Select according to load inrush and manufacturer/application requirements.

Use the calculated or measured prospective fault current at the MCB installation point.

Minimum breaking capacity you expect the selected breaker to have.

How to use this calculator: MCB Size Calculator

Calculate a suitable preliminary MCB rating from load current, kW, kVA or voltage, with power factor, derating, cable ampacity, MCB curve, poles and breaking-capacity checks. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the MCB Size Calculator matches the quantity or design check you need.
  2. 2Enter Load current, Real power, and Apparent power using the units printed beside each field.
  3. 3Select the applicable Electrical system, Known load value, and MCB curve options; these choices change the calculation method or factors.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Electrical systemSingle phase ACSelect the option that matches the real installation or scenario.
Known load valueLoad current (A)Select the option that matches the real installation or scenario.
Load current20 AUse the actual design/load current if already known.
Real power4 kWEnter real electrical input power. For motors, use actual electrical input where available.
Apparent power5 kVAUse this when the load is specified in kVA.
System voltage230 VSingle phase: line-to-neutral voltage. Three phase: line-to-line voltage.
Power factor0.9Use the actual measured or equipment nameplate PF where available.
Continuous-load factor100 %Optional design multiplier for continuous or sustained loads. Enter 100% for no additional multiplier.
Demand / diversity factor100 %Use 100% for a known maximum load. Lower values should only be used where the design basis supports diversity.
Ambient temperature factor100 %Enter 100% if no manufacturer derating is required. Use the breaker manufacturer's correction data for elevated ambient temperatures.
Grouping factor100 %Use the actual correction factor from the installation method and number of loaded circuits.
Cable current capacity25 AEnter the cable's corrected current-carrying capacity after installation derating. Set 0 only if cable ampacity is not being checked.
MCB curveC CurveSelect according to load inrush and manufacturer/application requirements.
Number of poles1PSelect the option that matches the real installation or scenario.
Prospective short-circuit current3 kAUse the calculated or measured prospective fault current at the MCB installation point.
Requested breaking capacity6 kAMinimum breaking capacity you expect the selected breaker to have.

Formula, derivation and worked example

This MCB Size Calculator provides a preliminary circuit-breaker selection based on load current, real power, apparent power, voltage and power factor. It applies user-entered continuous-load, demand, ambient and grouping factors, compares the resulting design current with a corrected cable capacity, selects the next standard MCB rating, and checks the requested breaking capacity against prospective short-circuit current. It also provides B, C and D curve guidance and pole selection.

Single phase current: I = P × 1000 / (V × PF)
Three phase current: I = P × 1000 / (√3 × VLL × PF)
kVA current: I = S × 1000 / (√3 × VLL) for three phase
Design current: Ib = I × continuous factor × demand factor
Corrected cable capacity: Iz = cable ampacity × ambient factor × grouping factor
Basic selection principle: Ib ≤ In ≤ Iz
Required breaking capacity: Icn/Icu ≥ prospective short-circuit current

Substitution steps

  1. 1. Operating current
    I = entered load current
    = 20 A
  2. 2. Continuous-load adjustment
    I × continuous factor
    = 20 A
  3. 3. Design current
    I × continuous × demand
    = 20 A
  4. 4. Corrected cable capacity
    Iz × ambient factor × grouping factor
    = 25 A
  5. 5. Standard MCB selection
    next standard rating ≥ design current
    = 20 A
  6. 6. Cable coordination
    Ib ≤ In ≤ Iz
    = PASS
  7. 7. Breaking capacity
    Icn/Icu ≥ prospective short-circuit current
    = 6 kA

Computed example results

Recommended MCB rating
20 A
1P — single-pole, C Curve
Calculated operating current
20 A
Design current
20 A
after continuous and demand factors
MCB utilization
100%
0 A nominal headroom
Corrected cable capacity
25 A
5 A above selected MCB
Cable / MCB coordination
✅ Ib ≤ In ≤ Iz
Recommended breaking capacity
6 kA
fault level entered: 3 kA
Breaking-capacity check
✅ Adequate
MCB curve
C Curve
Approx. 5–10 × In instantaneous magnetic operation
Curve application
General commercial/industrial loads and moderate motor or transformer inrush.
Pole arrangement
1P — single-pole
Load apparent power
4.6 kVA
Load real power
4.14 kW
Overall preliminary status
✅ Preliminary selection passes

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using MCB Size Calculator

  • Do not mix units for Load current (A), Real power (kW), Apparent power (kVA). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not leave Electrical system on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
  • Do not replace the displayed Single phase current: I = P × 1000 / (V × PF) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Recommended MCB rating = 20 A from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the MCB Size Calculator is useful

MCB Size Calculator is designed for cases where Electrical system, Known load value, Load current, Real power are known and you need Recommended MCB rating, Calculated operating current, Design current. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Single phase current: I = P × 1000 / (V × PF). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Electrical system and Known load value: what changes the answer

The worked example uses Electrical system = Single phase AC, Known load value = Load current (A), Load current = 20 A, Real power = 4 kW. With those values, Recommended MCB rating is 20 A. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Electrical system: Select the option that matches the real installation or scenario. Available choices include Single phase AC, Three phase AC. Known load value: Select the option that matches the real installation or scenario. Available choices include Load current (A), Real power (kW), Apparent power (kVA). Load current (A): Use the actual design/load current if already known. Real power (kW): Enter real electrical input power. For motors, use actual electrical input where available.

How to sanity-check a MCB Size Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Recommended MCB rating, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Cable Sizing Calculator

Useful next check because both tools use System voltage and Power factor, while Cable Sizing Calculator answers a different part of the same workflow.

Open Cable Sizing Calculator

Formula

  • Single phase current: I = P × 1000 / (V × PF)
  • Three phase current: I = P × 1000 / (√3 × VLL × PF)
  • kVA current: I = S × 1000 / (√3 × VLL) for three phase
  • Design current: Ib = I × continuous factor × demand factor
  • Corrected cable capacity: Iz = cable ampacity × ambient factor × grouping factor
  • Basic selection principle: Ib ≤ In ≤ Iz
  • Required breaking capacity: Icn/Icu ≥ prospective short-circuit current

This MCB Size Calculator provides a preliminary circuit-breaker selection based on load current, real power, apparent power, voltage and power factor. It applies user-entered continuous-load, demand, ambient and grouping factors, compares the resulting design current with a corrected cable capacity, selects the next standard MCB rating, and checks the requested breaking capacity against prospective short-circuit current. It also provides B, C and D curve guidance and pole selection.

Formulas explained

I = P × 1000 / (V × PF)

For a single-phase AC load, real power depends on RMS voltage, RMS current and power factor. Rearranging the power equation gives the operating current.

I
Load current in amperes
P
Real power in kW
V
Single-phase RMS voltage
PF
Power factor

I = P × 1000 / (√3 × VLL × PF)

For a balanced three-phase system, the total real power is √3 × VLL × I × PF. The equation is rearranged to obtain line current.

I
Three-phase line current in A
VLL
Line-to-line RMS voltage
PF
Power factor

Ib = I × continuous factor × demand factor

The design current can be increased to account for continuous operation and the selected design/demand basis.

Ib
Design current
I
Calculated operating current

Iz = cable ampacity × ambient factor × grouping factor

Cable ampacity can be reduced by installation conditions such as elevated ambient temperature and grouping of loaded circuits.

Iz
Corrected cable current capacity
Iz base
Cable current capacity before correction factors

Ib ≤ In ≤ Iz

A protective device should be selected so that its rated current is not below the design load current while remaining within the conductor's corrected current-carrying capacity.

Ib
Design current
In
MCB rated current
Iz
Corrected cable capacity

Breaking capacity ≥ prospective short-circuit current

The breaker must be capable of safely interrupting the maximum prospective fault current at its installation point.

Icn/Icu
Breaker short-circuit breaking capacity
Isc
Prospective short-circuit current

What is MCB size?

MCB size normally refers to the breaker rated current, such as 6 A, 10 A, 16 A, 20 A, 25 A, 32 A or 63 A. The selected rating must suit both the design load and the protected conductor.

MCB selection principle

A common design relationship is Ib ≤ In ≤ Iz, where Ib is design current, In is the protective-device rating and Iz is the corrected cable current-carrying capacity. Actual installation requirements and applicable standards must also be checked.

Why not simply choose the next MCB above the load current?

Because the cable must also be protected. A breaker selected only from load current can be too large for the conductor after installation derating. Cable ampacity, voltage drop, fault current and coordination all matter.

What does B, C and D curve mean?

The curve describes the magnetic instantaneous operating characteristic of the MCB. B curve generally suits lower-inrush loads, C curve is widely used for general loads, and D curve is intended for higher-inrush applications. Always verify the manufacturer's actual time-current characteristic.

What is MCB breaking capacity?

Breaking capacity is the maximum prospective short-circuit current the breaker is designed to interrupt under its specified test conditions. It must be adequate for the fault level at the installation point.

Why does ambient temperature matter?

MCB thermal performance can be affected by ambient temperature and enclosure conditions. Manufacturer correction factors should be used when the actual operating temperature differs from the reference condition.

Why does grouping matter?

Grouping multiple loaded circuits together can increase conductor temperature and reduce cable current-carrying capacity. The actual correction factor depends on installation method and applicable wiring rules.

MCB versus MCCB

MCBs are commonly used for smaller final and distribution circuits, while MCCBs cover larger current ranges and applications with higher fault levels and adjustable protection requirements. The appropriate device depends on the installation and equipment ratings.

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1-Phase / 3-Phase Power Calculator

Calculate kW, kVA, current and power factor for single-phase and three-phase systems.

Worked example

  1. 1Single-phase example: 4 kW load at 230 V and 0.90 PF.
  2. 2Load current = 4,000 / (230 × 0.90) = 19.32 A.
  3. 3Assuming 100% continuous factor and 100% demand factor, design current = 19.32 A.
  4. 4The next standard MCB rating above the design current is 20 A.
  5. 5If the corrected cable capacity is at least 20 A, the basic Ib ≤ In ≤ Iz check passes.
  6. 6If the corrected cable capacity is only 18 A, a 20 A MCB would not be acceptable for that cable and the cable/design would need to be reconsidered.
  7. 7The final breaker also needs an adequate breaking capacity for the prospective short-circuit current at the installation point.

Assumptions

  • AC RMS voltage is used.
  • Three-phase calculations assume a balanced three-phase load.
  • Three-phase voltage is line-to-line voltage.
  • Power factor is entered as a decimal from 0.10 to 1.00.
  • Continuous and demand factors are user-defined design inputs and are not universal mandatory values.
  • Ambient and grouping factors are user-entered correction factors based on the installation design.
  • Cable ampacity should preferably be the corrected current-carrying capacity after all relevant installation factors.
  • The calculator selects a standard nominal MCB rating but does not replace a complete protection-coordination study.
  • Motor starting current, transformer inrush, harmonic current and selective coordination are not fully determined by this calculator.

Tips

  • Use actual measured current where available.
  • For motors, use nameplate current as an important reference and separately assess starting current.
  • For three-phase systems, use line-to-line voltage such as 400 V or 415 V in the √3 equation.
  • Use the actual cable corrected ampacity rather than an uncorrected catalogue value.
  • Select breaking capacity from the calculated fault level, not simply from the load current.
  • C curve is a common general-purpose choice, but the correct curve depends on the load inrush and manufacturer/application requirements.
  • For long cable runs, check voltage drop separately even when the MCB/cable current relationship passes.

Warnings

  • Do not select an MCB from load current alone. Cable ampacity, installation method, ambient temperature, grouping, voltage drop and fault level must also be checked.
  • The calculated MCB rating is a preliminary engineering selection, not a final protection-coordination decision.
  • For motors and transformers, starting/inrush current can affect instantaneous tripping. Check the manufacturer's time-current curves.
  • The MCB breaking capacity must be greater than or equal to the prospective short-circuit current at the installation point.
  • For high fault levels or industrial installations, an MCCB or other protective device may be more appropriate than an MCB.
  • Do not increase the breaker rating simply because the breaker trips. Investigate overload, cable sizing, voltage drop, motor starting and equipment faults.

Standards & references

  • IEC 60898-1 — Circuit-breakers for overcurrent protection for household and similar installations
  • IEC 60947-2 — Low-voltage switchgear and controlgear: circuit-breakers
  • IEC 60364 — Low-voltage electrical installations

Frequently asked questions

How do I calculate MCB size from amps?

First determine the design current. Then select a standard MCB rating equal to or above the design current while ensuring the selected rating does not exceed the cable's corrected current-carrying capacity. The basic relationship is Ib ≤ In ≤ Iz.

How do I calculate MCB size from kW?

For single phase, calculate I = P × 1000 / (V × PF). For balanced three phase, calculate I = P × 1000 / (√3 × VLL × PF). Then select the appropriate standard MCB rating after checking cable capacity and other design conditions.

What MCB size is suitable for a 4 kW load at 230 V?

At 0.90 PF, the calculated running current is approximately 19.3 A. A 20 A MCB may be a preliminary choice if the protected cable, installation method, voltage drop, fault level and load characteristics all support that selection.

What MCB size is suitable for a 5 kW three-phase load?

The answer depends on voltage and power factor. At 415 V and 0.90 PF, the running current is approximately 7.7 A. The final MCB rating still depends on cable capacity, load characteristics and protection requirements.

What is the formula for single-phase MCB current?

For a resistive/AC real-power calculation with known PF, I = P × 1000 / (V × PF). For a purely resistive load at PF ≈ 1, this becomes I = P × 1000 / V.

What is the formula for three-phase MCB current?

For a balanced three-phase load, I = P × 1000 / (√3 × VLL × PF), where VLL is line-to-line voltage.

What is the difference between B curve and C curve MCB?

B curve has a lower instantaneous magnetic operating range and is generally suited to lower-inrush loads. C curve permits higher inrush and is widely used for general commercial and industrial loads. Always verify the actual manufacturer's characteristic.

When is a D curve MCB used?

D curve is intended for loads with substantially higher inrush, such as certain motors and transformers. It must only be selected when the circuit's fault current and disconnection requirements allow the required magnetic operation.

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