Sparky Sums

Cable size, volt drop and Zs for electricians, worked out on your iPhone.

Pick the cable, how it's installed and the protective device, then put in the load and the length. Sparky Sums gives the smallest cable that passes and shows its working: the rating after correction factors, the volt drop, and the earth fault loop impedance against the limit for your device. It works with no signal and has no account.

Contact

Questions, problems, or a figure you think is wrong: use the contact form and mention Sparky Sums. We read every message, and we treat a wrong figure as the most urgent kind of bug.

Common questions

Which rules does it follow?

It uses BS 7671:2018+A4:2026 and the On-Site Guide's method. Ratings and volt drop cover copper cables with 70 °C thermoplastic insulation: twin and earth (6242Y), multicore and single cores.

What does it not check?

In the UK: cables in the ground, armoured cables, 90 °C ratings, aluminium, fuses in the Zs check (it covers type B, C and D breakers), and the device's breaking capacity. It is a design aid for a competent person. It is not a substitute for BS 7671, a measured Zs, or your own judgement.

Why does it pick a bigger cable than I expected?

Look at the working under the answer. It shows which check the smaller size fails: the current rating after grouping, ambient and insulation factors, or the volt drop.

How is Zs worked out?

Zs = Ze + (R1 + R2), with R1 + R2 from the conductor resistances at 20 °C, multiplied by 1.20 for a conductor at 70 °C. The limit is 230 V × 0.95 divided by the current that trips the breaker in 0.4 s: 5, 10 or 20 times its rating for type B, C or D. For a cold reading with a tester, the app also gives 80% of that limit. A ring final uses a quarter of the end-to-end R1 + R2.

Is it a subscription?

No. You pay once. No ads, no account, no in-app purchases.