Lastsocket

Cable size calculator — Iz and voltage drop, both gates

Give the design current, the breaker, the installation method and the run; Lastsocket walks the IEC 60228 sizes upward until a cross-section passes both gates: Iz after correction factors ≥ the breaker rating, and voltage drop within the limit in your pack. It shows the coordination line Ib ≤ In ≤ Iz with ticks, and each smaller size with the gate that rejected it.

Limits and citations come from the app's pack files. The calculator below is checked against the app's engine on every build · engine 1.

30 A on a 32 A breaker, method C, 20 m: 4 mm² with Iz 36 A and a 2.9 % drop — PASS on both gates.

The same job in the app

30 A on a 32 A breaker, method C, 20 m: 4 mm² with Iz 36 A and a 2.9 % drop — PASS on both gates.

How it's calculated

Gate 1: Iz = Itable × Kambient × Kgrouping × Kinsulation, where the table value depends on installation method, insulation, conductor material and the number of loaded conductors — two for single-phase and DC, three for three-phase, and the three-phase column is lower. The app bundles methods A1, B2, C and E, PVC copper, to 50 mm² today.

Gate 2 is the voltage-drop calculation, at operating temperature. The breaker defaults to the next standard rating above the design current, and the cable must carry the larger of the two.

Worked example

Single-phase 30 A on a 32 A breaker, method C, PVC copper, 30 °C, one circuit, 20 m: 1.5 mm² (19.5 A) and 2.5 mm² (27 A) are rejected on Iz; 4 mm² gives Iz 36 A ≥ 32 A and a 2.9 % drop against 5 % — PASS on both gates, exactly as the screenshot shows.

What this does not check. Short-circuit withstand, earth-fault loop impedance, disconnection time, conduit fill. Lastsocket is a design aid. It does not replace the applicable installation standard, manufacturer data, or a qualified person’s judgement. Sources and limits →
Free or Pro. The two-gate result is Pro, because the app looks the Iz table up for you. Your inputs stay live either way. This screen has no typed Iz — a walk across sizes needs a value per size — so "Type Iz instead" takes you to the ampacity check, where your own value is always free.

More from this screen

Cable size for a free user: the inputs stay live, only the number is locked.
Cable size for a free user: the inputs stay live, only the number is locked.

Questions

What if Iz equals the breaker exactly?

The gate is Iz ≥ In after factors, so a tie passes; a hot ambient or a second grouped circuit is what pushes it under.

Which installation method is mine?

Tap "Which method?" in the app for a description of each.

What about aluminium?

From 16 mm² only, as in the standards.

Related calculators