What protects the circuit, whether it disconnects fast enough, and how the installation gets to earth.
A cable sized correctly for load and voltage drop can still be unsafe. Protection is the part of the design that answers what happens when something goes wrong — and how quickly.
This is the least well served stage on the site today, and the one being built out next. It is also the densest part of the job: fault current, loop impedance, disconnection time and conductor sizing are all one interlocking calculation rather than four separate ones.
The equipment grounding conductor to NEC 250.122, from the upstream device rating.
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Motor circuits get their own rules — NEC 430 allows a breaker far above the running current because of starting inrush.
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Zs = Ze + (R1 + R2) against the maximum for the device — the calculation that decides whether the circuit disconnects in time.
What the protective device has to be able to break — from an impedance, a transformer nameplate or your meter readings.
BS 7671 Table 54.7 and the adiabatic route for the CPC; 544.1 for main protective bonding. Not yet built.
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The published figures these calculators are built from, with every value sourced.
Zs = Ze + (R1 + R2) against the maximum permitted for the protective device, to BS 7671 Chapter 41 — with the maximum derived from U0 × Cmin ÷ Ia rather than looked up, and the cold-measurement rule applied separately.
IpfThe highest current a fault could produce, from a loop impedance, a transformer nameplate or your meter readings — checked against the breaking capacity of the device that has to interrupt it.
2 calculators in this category. Every one is free to use on ElectroCalculators, with no account required, and every result cites the standard it came from.