Free electrical calculators for UK electricians.
BS 7671 calculators that show their working: maximum Zs, R1+R2, ring final test, voltage drop, prospective fault current, the adiabatic equation, maximum demand, kW to amps and the EICR codes. Open to everyone, with nothing to sign up to, and every answer is a link you can send.
Testing and inspection
Max Zs calculator: maximum Zs for MCBs, RCBOs and fuses
Pick the device and its rating and read the limit, worked out from Regulation 411.4.4 rather than thumbed from a table. Type in what you measured and it says pass or fail.
R1+R2 calculator: R1+R2 values and expected Zs
Line and protective conductor sizes, the length of the run, and the Ze at the origin. Out comes the R1+R2 you should read, the Zs the circuit will present, and how long it can go before it reaches the device's limit.
Ring final circuit test calculator: r1, rn, r2 and (r1+r2)/4
Type in the three end-to-end readings and it tells you whether they make sense together, what steps 2 and 3 should read at the sockets, and roughly how long the ring is.
Prospective fault current calculator: PEFC, PSCC and Ipf
Ohm's law at the origin: 230 V through the loop you measured. It gives PEFC, PSCC and the Ipf to record, and says whether the board's devices can break it.
EICR codes explained: C1, C2, C3 and FI, with a code checker
Type what you found: "no RCD on sockets", "cracked socket", "borrowed neutral". The usual code, the regulation it breaks and the notes on when it changes come up as you type.
Design and sizing
Voltage drop calculator to BS 7671
Cable size, current, length and what the circuit feeds. It works the drop out from the conductor's own resistance, judges it against Appendix 12, and says how far the cable could go, or what size would.
Adiabatic equation calculator: minimum CPC size
S = √(I²t) ÷ k, Regulation 543.1.3. Give it the fault current and how long the device takes to clear it, pick the insulation, and it gives the minimum copper and the standard size to use.
Maximum demand calculator with diversity
List the circuits and what they draw. The usual household allowances are applied, every one of them shown and changeable, and the total is set against the supply fuse.
kW to amps calculator, single and three phase (UK)
The shower is 9.5 kW: what does it draw, and what breaker does that need? Type the one you know and read the other, at any voltage and power factor.
Every figure is derived from a method BS 7671 publishes or from the conductor standard, and each page shows the working and the regulation it follows. A calculator is not a substitute for the standard, the guidance or the judgement of the person signing the certificate.
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