Myth: Bigger Cable Always Means Safer Installation

“When in doubt, go one size up. Bigger cable is always safer.”

This is one of the most widely held beliefs in electrical installation work. It sounds logical — more copper means more capacity, lower temperature rise, longer cable life. What could go wrong?

Quite a lot, as it turns out.

The safety of a circuit depends not just on the cable surviving normal load current, but on the protection device clearing faults fast enough to prevent fire, shock, and arc flash. Fault clearing time is determined by the fault current magnitude and the protection device’s time-current characteristic.

Here is the critical relationship most engineers overlook: a larger cable has lower impedance per metre, which means higher fault current at the cable end. Wait — doesn’t that mean faster protection? Not necessarily. The issue is more nuanced.

Consider a 63A Type C MCB protecting a 30m circuit at 230V, with 6kA prospective fault current at the origin:

In this scenario, the bigger cable actually helps — more fault current, faster trip. So where does the myth break down?

It breaks down when the protection device is not properly coordinated with the oversized cable, which happens frequently when cables are upsized without re-evaluating the protection. The real danger emerges in two specific scenarios:

1. Termination and Connection Failures

Oversized cables are physically harder to terminate properly. A 35mm² conductor crammed into a terminal designed for 10mm² creates:

• Insufficient contact pressure — the terminal screw cannot compress the conductor adequately
• Strand damage — excess strands cut or folded back during termination
• Mechanical stress — the cable’s stiffness puts bending strain on the terminal

High-resistance connections are the leading cause of electrical fires. Per BS 7671, Regulation 526.2, connections must be suitable for the conductor size. An oversized cable in an undersized terminal is a code violation, not a safety improvement.

2. Let-Through Energy (I²t)

Per BS 7671, Regulation 434.5.2, the energy let-through of the protection device during a fault must not exceed the cable’s withstand capacity (k²S²). An oversized cable has a higher k²S² value, which means it can tolerate more fault energy. This sounds safer, but it can mask a coordination problem upstream.

If an engineer selects a larger upstream fuse or MCB “because the cable can handle it,” the clearing time at lower fault currents increases. The cable survives, but the fault arc burns longer. For a person standing in front of the switchboard, the cable’s survival is cold comfort.

3. The Real-World Example

A facilities manager at an Indonesian mine site replaced a failed 16mm² submain with 50mm² “to be safe.” The existing 80A fuse was not changed. Six months later, a fault at a junction box drew 180A — well below the fuse’s fast-blow threshold. The fuse operated in its time-delay region at approximately 45 seconds. The 50mm² cable was fine. The junction box was not. The resulting fire damaged three adjacent circuits and shut down ventilation to a section of the processing plant for 14 hours.

The correct 16mm² cable at 180A fault would have drawn the same fault current (the cable impedance difference was minimal at this short distance). The real fix was the junction box termination that failed — which was never addressed because “bigger cable” was treated as the solution.

The correct approach is not “biggest available” but “correctly sized and coordinated”:

• Size the cable for the load with proper derating factors applied.
• Verify the protection device coordinates with the cable — both for overload (In ≤ Iz) and for fault (I²t ≤ k²S²).
• Verify fault current at the far end is sufficient for the protection device to operate within the required time (BS 7671, Table 41.1: 0.4s for final circuits, 5s for distribution circuits).
• Verify the cable fits the terminals. Check the terminal manufacturer’s conductor range — it is in the datasheet.

One size up is acceptable when the calculation is marginal and the terminals can accommodate it. Two or three sizes up without re-evaluating protection coordination is not “conservative engineering” — it is uncoordinated engineering.