MYTH: You Can Always Use the Next Size Up Cable If Your Calculation Is Borderline

The Myth

"The calc says 35mm² is just barely enough, so I'll go 50mm² to be safe."

This seems like conservative engineering. In many cases, it works fine. But in some cases, it breaks the protection coordination in ways that create genuine safety hazards.

What Changes When You Upsize

When you change a cable from 35mm² to 50mm², multiple parameters shift simultaneously:

1. Lower impedance = Higher fault current A 50mm² cable has lower resistance per metre than a 35mm² cable. The earth fault loop impedance decreases, which means the prospective fault current at the end of the circuit increases. On a 50m run:

35mm²: R1 = 0.0266Ω, Zs contribution = 0.0266 + 0.0154 (1.5mm² CPC) = 2.10Ω per km
50mm²: R1 = 0.0187Ω, Zs contribution = 0.0187 + 0.0154 = 1.71Ω per km

Lower Zs means higher fault current. Higher fault current means the upstream protection device operates in a different region of its time-current curve.

2. The adiabatic check shifts The adiabatic equation per BS 7671 Regulation 434.5.2:

k²S² ≥ I²t

For 35mm² with k=115 (Cu/PVC): k²S² = 115² × 35² = 16.2 × 10⁶ A²s For 50mm²: k²S² = 115² × 50² = 33.1 × 10⁶ A²s

The cable withstand doubles — good. But the higher fault current means the protective device's I²t let-through also changes, potentially significantly.

3. Upstream protection coordination A bigger cable with lower impedance presents a larger fault current to the upstream switchboard. If the upstream protective device was coordinated with specific downstream fault levels, the higher fault current may:

Push the upstream device into a faster operating zone, causing sympathetic tripping (nuisance trips on healthy circuits)
Exceed the making capacity of the upstream bus bar or switch
Change the discrimination characteristics between series devices

A Real Example

Original design: 35mm² cable, 100A MCB, 40m to distribution board. Maximum fault current at DB: 3.2kA. Upstream 250A MCCB coordinates with 100A MCB at this fault level — the 250A trips in 0.8s, the 100A trips in 0.02s. Discrimination achieved.

After upsizing to 50mm²: Maximum fault current at DB rises to 4.1kA. At this current, the 250A MCCB now trips in 0.15s (entering its instantaneous region), while the 100A MCB trips in 0.015s. Discrimination margin shrinks from 0.78s to 0.135s. One more cable change in the installation could eliminate discrimination entirely.

The Right Approach

Upsizing a cable is fine, provided you:

Re-run the fault current calculation at the cable's far end
Re-check the adiabatic equation with the new fault current
Re-verify protection discrimination between the circuit's device and the upstream device
Update the protection study documentation

Never treat cable sizing in isolation from protection coordination. They are coupled calculations.

Verify the full picture: Use the Protection Coordination Calculator and Short Circuit Calculator together.

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.

How do I apply derating factors correctly?

Derating factors must be applied multiplicatively: Final Rating = Base Rating × k₁ (ambient) × k₂ (grouping) × k₃ (thermal insulation) × k₄ (ground temp). Each factor comes from specific tables in the relevant standard.

Protection Coordination - Interactive calculator with standards compliance
Short Circuit Calculator - Interactive calculator with standards compliance

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