Electrical Sizing Tool

Overcurrent Protection Calculator

Size devices using load, demand, and derating inputs. Compare breaker curves, fault duty, and margins. Review recommended ratings before final code-based engineering approval checks.

Use it to review breaker or fuse sizing, conductor ampacity, trip behavior, and minimum interrupting duty before final engineering sign-off.

Calculated Result

The result panel appears here after you submit the form.

Enter your design values, then press Calculate Protection.

Calculator Inputs

Choose whether current comes from power or is entered directly.
This choice changes the current formula and power factor usage.
Used for trip-characteristic wording and design notes.
Enter measured or design current when you already know it.
Used when current is calculated from the connected load.
Line voltage for AC systems or bus voltage for DC systems.
Applies to single-phase and three-phase AC calculations.
Use equipment efficiency when converting input power to current.
Reduce or keep current based on expected simultaneous loading.
Commonly 125% for continuous service, where required.
Adds room for moderate growth or conservative selection.
Represents current-carrying reduction from temperature effects.
Use this for bundled conductors, trays, or crowded raceways.
A higher value points toward slower trip characteristics.
Enter the prospective short-circuit duty at the device location.
Applies a safety margin before interrupting-duty recommendation.
Select the device ampere series closest to your project standards.

Formula Used

Single-phase AC current: I = P × 1000 ÷ (V × PF × η)

Three-phase AC current: I = P × 1000 ÷ (√3 × V × PF × η)

DC current: I = P × 1000 ÷ (V × η)

Adjusted design current: Iadj = I × demand factor × continuous factor × spare factor

Minimum conductor ampacity: Icond = Iadj ÷ (ambient derating × grouping derating)

Recommended device rating: next standard size at or above Iadj

Required interrupting capacity: available fault current × fault margin

All percentage inputs are converted into decimal factors during the calculation.

How to Use This Calculator

  1. Select whether you want to derive current from power or enter current directly.
  2. Choose the system type and the protective device family.
  3. Fill in power, voltage, power factor, and efficiency if you are using the power method.
  4. Enter demand, continuous loading, spare margin, and conductor derating factors.
  5. Enter inrush multiplier to guide trip-characteristic selection.
  6. Provide available fault current and a fault margin to review interrupting duty.
  7. Press Calculate Protection to show the result above the form.
  8. Export the result as CSV or PDF after reviewing the design notes.

Example Data Table

This sample shows one typical three-phase motor feeder review.

Illustrative example
Input or Output Value Notes
System Type Three-phase AC Motor feeder example
Load Power 18.5 kW Derived current method
Voltage / PF / Efficiency 415 V / 0.88 / 92% Input assumptions
Demand / Continuous / Spare 100% / 125% / 10% Planning factors
Ambient / Grouping Derating 90% / 85% Combined derating 0.765
Inrush / Fault Current 6× / 18 kA Trip and interrupting review
Calculated Operating Current 31.79 A From three-phase power formula
Adjusted Design Current 43.71 A After demand, continuous, and spare factors
Recommended OCPD / Curve 50 A / C curve Next listed rating above design current
Recommended Conductor / Interrupting 57.14 A / 25 kA Derating and fault margin considered

FAQs

What does this overcurrent protection calculator estimate?

It estimates operating current, adjusted design current, conductor ampacity, a suggested protective rating, trip behavior, and an interrupting-capacity target from your entered load and derating assumptions.

Can I use it for motors and feeders?

Yes. It supports direct-current entry or power-based current estimation. Motor loads benefit from the inrush input, while feeder checks benefit from demand, spare, and derating factors.

Does it replace electrical code compliance?

No. It is a planning and review tool. Final device selection must still follow your adopted code, coordination study, conductor rules, and manufacturer data.

How is the recommended rating selected?

The tool calculates an adjusted design current and then chooses the next higher listed device size from the selected regional standard series.

Why is conductor ampacity sometimes higher than device rating?

Ambient and grouping derating reduce usable conductor capacity. When derating is strong, the conductor may need a larger ampacity even if the breaker size seems modest.

What is interrupting capacity?

Interrupting capacity is the maximum fault current a protective device can safely clear. The tool applies your fault margin, then selects the next standard available rating.

Why does the trip curve recommendation change?

Higher inrush multipliers need slower magnetic response to avoid nuisance trips. That is why heavier-starting equipment tends to need curves such as C, D, or K.

Can I export the results?

Yes. After calculation, use the CSV export for spreadsheets or the PDF button for a printable summary of the displayed result table.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.