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Earthing Conductor Size Calculator

There are two rules and the larger of them wins. The table rule scales the protective conductor from the phase conductor; the adiabatic equation asks whether it can survive the fault energy before the protective device clears. On a stiff supply with a slow device, the second demands more.

Earthing conductor size

Two rules, and the larger wins. The table rule scales the protective conductor from the phase conductor; the adiabatic equation S = √(I²t)/k asks whether it can survive the fault energy before the device clears. On a stiff supply with a slow device, the second demands more.

Table rule
Adiabatic
Use
  • Up to 16 mm² the protective conductor is the same size as the phase conductor.
  • No fault current given, so only the table rule was applied. On a large service the adiabatic equation S = √(I²t)/k can demand more, and it is the one that actually reflects the energy the conductor has to survive.
Table rule from IEC 60364-5-54 / IS 3043; k values are for PVC-insulated conductors. The earth electrode is a separate design problem — this sizes the conductor, not the pit.

The formula

S = √(I² t) / k ; table rule: S ≤ 16 → S, 16–35 → 16, above 35 → S/2

I
prospective earth fault current in amperes
t
disconnection time of the protective device, in seconds
k
143 for PVC-insulated copper, 95 for aluminium
S
conductor cross-section in mm²

Worked example

16 mm² copper phase conductor, 6 kA prospective earth fault, cleared in 0.4 s.

  1. Table rule: 16 mm² is at the boundary, so the protective conductor is also 16 mm²
  2. Adiabatic: S = 6000 × √0.4 / 143
  3. S = 6000 × 0.632 / 143 = 26.54 mm²
  4. The adiabatic value is larger, so it governs

35 mm² — the next standard size above 26.54 mm².

Where you'll use it

Sizing the protective conductor in any installation, and showing why a large fault current can demand more than the table suggests. This sizes the conductor, not the earth electrode or the pit — those are a separate design under IS 3043.

The laws behind it

Parts this applies to

Resistor (fixed)Varistor / MOVTVS / Transient SuppressorIGBTSCR (Thyristor)Linear Regulator (78xx / LDO)Switches (toggle/push/slide/rotary/DIP)Solid-State Relay (SSR)

Related calculators

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