Max Z_s calculator

How it works

Z_s(max) = (U₀ × C_min) ÷ I_a

• U₀ = 230 V, the nominal voltage to earth.
• C_min = 0.95, the minimum voltage factor. IET Wiring Matters (issue 105, May 2025) gives it as 0.95 for a low voltage supply provided under the ESQCR.
• I_a comes from the device data.

The circuit figure is Z_s = Z_e + (R₁ + R₂). The maximum applies with the conductors at operating temperature, but test readings are taken cold, so the calculator checks a cold reading two ways, as in the Inspection and Testing Calculations workbook:

1. Rule of thumb: Z_s from room-temperature readings ≤ 0.8 × the maximum Z_s. It is quick and cautious because it also scales Z_e, which does not warm up with the circuit.
2. Corrected to operating temperature: hot (R₁ + R₂) = (R₁ + R₂) × 1.20, then Z_s = Z_e + hot (R₁ + R₂), compared with the full maximum. The 1.20 multiplier takes (R₁ + R₂) from about 20 °C to 70 °C. The IET gives 70 °C as the assumed conductor operating temperature for the tabulated maximum Z_s values (IET Inspection and Testing FAQs), and 70 °C is the rating of thermoplastic (PVC) insulation (IET Wiring Matters 93).

A reading that fails the rule of thumb is not automatically a failure: check it properly with the corrected method. A reading that is already above the full maximum while cold does not comply. The 0.8 factor and the 1.20 multiplier are the workbook’s practice values; which approach to use, and the factor, depend on the guidance you work to.

Rounding. Maximum figures are rounded down and circuit figures rounded up, to 4 decimal places, and the check uses the figures as shown, so rounding can never turn a fail into a pass.

See it worked by hand: How to calculate maximum Zs: worked examples.