Breaker rating comparison calculator

Field use cases

Use this page when a protective-device record already has a proposed rating, a calculated load current and a prospective fault-current value. It is useful for final-circuit records, switchboard schedule reviews, maintenance findings and estimating notes where the current and breaking-capacity comparisons need to be visible on one row.

The page does not choose a protective device. It does not read device curves or select settings. It keeps two arithmetic comparisons visible: device rating against load current, and entered breaking capacity against prospective fault current.

Input basis

Device reference should identify the circuit breaker, fuse, RCBO, switchboard row or protective-device schedule. The same current values are less useful when the device cannot be found later.

Load current should come from a load-current worksheet, motor calculation, maximum-demand result or project schedule. Prospective fault current should come from a short-circuit current calculation, DNSP data, study or project record. Breaking capacity should come from the equipment data that applies to the device and installation condition being reviewed.

Review workflow

1. Identify the protective device and circuit or switchboard row.
2. Enter the device rating in amperes.
3. Enter calculated load current from the relevant source worksheet.
4. Enter prospective fault current in kiloamperes for the same point.
5. Enter breaking capacity from the applicable equipment record.
6. Review load-current margin and fault-duty margin separately.
7. Keep the source records and reviewer basis with the export.

Do not force the row into one combined answer. A device can have load-current margin while the fault-duty comparison needs review. The reverse can also happen. The page keeps the two comparisons separate for that reason.

Worked Australian examples

A final-circuit row compares a 32 A device with 24 A load current. The load-current margin is 8 A. The same row compares 6 kA prospective fault current with 10 kA entered breaking capacity, giving a 4 kA fault-duty margin.

A switchboard note compares a 40 A device with 30 A load current, but 12 kA prospective fault current against a 10 kA entered breaking capacity. The load comparison has margin, while the fault-duty comparison shows a -2 kA margin and needs review.

A load-current review compares a 20 A device with 28 A calculated load current. The load-current margin is -8 A. Even if the breaking-capacity comparison has margin, the load-current row should remain open for project review.

Boundary with related calculators

Use the load current calculator when the missing input is load current from kW, kVA, voltage and power factor. Use the short-circuit current calculator when the missing input is prospective current. Use the I2t cable withstand calculator when conductor thermal withstand and clearing time become the next question.

Australian standards boundary

Australian protective-device decisions sit inside current wiring rules, manufacturer data, switchboard ratings, installation conditions, DNSP conditions where relevant and competent-person review. This page names AS/NZS 3000 as context, but it does not reproduce device tables, curves, breaking-capacity rules or selection requirements.

The entered values are placeholders for source records, not hidden authority data. A strong export names the device, the current source, the fault-current source and the breaking-capacity source. State or territory requirements, local authority expectations, product data and project documentation can override the worksheet basis.

Stop points

• The device reference cannot be matched to the schedule or switchboard row.
• Load current and fault current are from different calculation points.
• Breaking capacity is copied without the equipment source.
• Either margin is negative or very small.
• Device curves, let-through energy, selectivity or settings are required.
• The comparison is being used as device selection rather than a review row.