MYTH: A Higher kA-Rated Switchboard Is Always Safer
Specifying 65kA when actual fault current is 25kA costs 40-60% more with zero safety benefit. Match the rating to the actual prospective fault current.
February 26, 2026
The Myth
"Specify 65kA fault rating on all switchboards. You can never have too much fault rating."
This is like specifying a bomb shelter for a residential house. The engineering is sound; the economics are not. And the money wasted on over-rated switchboards could fund actual safety improvements elsewhere.
The Cost Premium
Switchboard fault rating directly drives construction cost through heavier busbars, stronger bracing, higher-rated circuit breakers, and more rigorous type testing:
| Fault Rating | Approximate Main Switchboard Cost | Premium vs 25kA |
|---|---|---|
| 25kA | ~$45,000 | Baseline |
| 35kA | ~$52,000 | +16% |
| 50kA | ~$63,000 | +40% |
| 65kA | ~$72,000 | +60% |
For a commercial building where the actual prospective fault current at the main switchboard is 25kA, specifying 65kA adds $27,000 in cost with zero additional safety. The overcurrent protective devices still need to clear the fault; the switchboard just needs to withstand it long enough for them to do so. Beyond the actual fault level, additional withstand capability provides no benefit.
Fault Current Attenuates
A critical concept many engineers overlook: fault current decreases through cable impedance. The full prospective fault current exists only at the transformer secondary terminals. By the time it reaches a sub-distribution board through 80m of 95mm² cable, it's significantly reduced.
Example cascade:
- Transformer secondary: 50kA
- Main switchboard (2m bus): 48kA (specify 50kA)
- Sub-DB 1, 30m via 150mm²: 28kA (specify 35kA)
- Sub-DB 2, 80m via 95mm²: 15kA (specify 20kA)
- Final DB, 120m via 50mm²: 8kA (specify 10kA)
Specifying 50kA for every board in this installation wastes money at every level below the main switchboard.
When Higher Ratings Are Justified
- Unknown or increasing supply — if the utility is upgrading the local transformer, future fault levels may increase
- Parallel supplies — standby generators or multiple transformer feeds can increase total fault current
- Future expansion — if additional transformer capacity is planned, design for the ultimate fault level
- Standard stock — sometimes the 35kA and 50kA boards are the same price because the manufacturer only stocks certain configurations
The Right Approach
- Calculate the actual prospective fault current at each switchboard location per IEC 60909 or the simplified methods in IEC 61439
- Add margin for supply uncertainty (typically 10-20%, or request the prospective fault current from the utility)
- Select the next standard fault rating above the calculated value
- Document the calculation — this demonstrates due diligence far better than a blanket high specification
Per IEC 61439-1 Clause 10.11, the rated short-time withstand current must equal or exceed the prospective short-circuit current at the point of installation. Meeting this exactly is compliance. Exceeding it is a commercial decision, not a safety one.
Calculate fault levels: Determine the actual prospective fault current at each point with the Short Circuit Calculator.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
Related Articles
- Short Circuit Calculator - Interactive calculator with standards compliance
Try It Yourself
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