BS 7671 circuit check
All four checks, in the right order. Sizing on current alone and never checking volt drop or the protective conductor is how cables end up undersized.
Four numbers come from your copy of BS 7671 and are asked for below: the tabulated capacity, the grouping factor, the volt drop rate and the maximum Zs for your device. They are not reproduced here, and the reason is worth reading.
Current-carrying capacity
Voltage drop
Earth fault loop impedance
Protective conductor size
Why the tables are not in here
BS 7671 is copyrighted by the IET and sold as a book. The large lookup tables in it are original engineering work rather than arithmetic: a tabulated current-carrying capacity comes from heating real cable in a laboratory and a committee setting a limit, so there is nothing to derive it from.
There is no legitimate free source. The copies circulating online disagree with one another, and even UK electrical software companies publish only indicative figures and tell you to read the real value from the standard. A mistranscribed rating does not produce a wrong answer, it produces an undersized cable.
So four numbers are asked of you instead, each one a five second look-up, and everything after them is done here. That is also the part that goes wrong: the order of the correction chain, and stopping at the first check instead of running all four.
What the four checks are for
- 01 Current-carrying capacity
- The corrected capacity of the cable must be at least the rating of the protective device, and the device must be at least the design current. Iz ≥ In ≥ Ib.
- 02 Voltage drop
- A cable can be big enough to carry the current and still drop too much voltage over a long run. Passing check 01 says nothing about this.
- 03 Earth fault loop impedance
- The loop has to be low enough for the device to disconnect in time. Measured Ze plus the resistance of the circuit conductors.
- 04 Protective conductor size
- The adiabatic check. The CPC must survive the fault current for as long as the device takes to clear it. The check almost nobody runs.
Questions about circuit design
Why do I have to type in the cable rating myself?
Because the tabulated current-carrying capacities in BS 7671 Appendix 4 are copyrighted by the IET and there is no legitimate free source for them. They are not a formula: each figure comes from heating real cable in a laboratory and a committee setting a limit, so there is nothing to calculate them from.
The copies circulating online disagree with one another, and even UK electrical software companies publish only indicative figures with an instruction to read the real value from the standard. A mistranscribed rating does not give you a wrong answer, it gives you an undersized cable. Asking you for the one number you can read off a page in five seconds is the honest option.
Which four numbers come from the book?
The tabulated capacity It for your cable and installation method, the grouping factor Cg from Table 4C1, the volt drop rate in mV/A/m, and the maximum Zs for your protective device from Tables 41.2 to 41.4.
Everything else is computed here: the ambient temperature factor, the rewireable fuse factor, the thermal insulation factor, the whole correction chain, the volt drop, the loop impedance sum and the adiabatic check.
What actually goes wrong when people size cables?
Not the table look-up. People get that right because they are reading it off a page.
What goes wrong is applying the correction factors in the wrong order or forgetting one, and above all stopping after the first check. A cable can be comfortably large enough to carry the current and still fail on voltage drop over a long run, or leave a protective conductor too small to survive the fault. All four checks have to pass, and this tool runs all four.
Why is voltage drop a separate check?
Because current capacity and voltage drop are unrelated limits. Capacity is about the cable overheating; voltage drop is about how much voltage is lost along the run.
On a long circuit voltage drop is usually the binding constraint, and it is the one that gets skipped. The limits are 3% for lighting and 5% for everything else, measured from the origin of the installation.
What is the 0.8 factor on maximum Zs?
The tabulated maximum values assume the conductors are at 10 °C. In service they are warmer, so their resistance is higher and the real loop impedance is worse than the cold measurement suggests.
Multiplying the tabulated maximum by 0.8 is the common rule-of-thumb allowance for that. BS 7671 also permits a full temperature correction instead, which is more precise. The toggle here lets you do either.
What is the adiabatic check for?
It answers whether the circuit protective conductor can survive the fault current for as long as the device takes to clear it. The equation is S = √(I²t) ÷ k, where I is the fault current, t is the disconnection time and k depends on the insulation.
It is the check almost nobody runs, and the failure mode is invisible until there is a fault: the CPC is damaged before the breaker opens.
Does this replace BS 7671?
No, and it cannot. It applies four of its rules to numbers you supply. It knows nothing about your installation method, whether the cable type suits the environment, RCD requirements, selectivity, diversity, or any of the hundreds of other regulations.
It is an arithmetic check on a design you have already made, not a design tool, and certainly not a substitute for the standard or for competence.
Does it work for other wiring codes?
Largely, yes, because the method is common. The correction-factor chain, the voltage drop calculation and the adiabatic equation work the same way under IEC 60364, AS/NZS 3000 and the Canadian code; only the tabulated values differ, and you are supplying those.
The volt drop percentages and the 0.8 Zs convention are BS 7671 specific, so check those against your own code.
Where these figures come from
Ambient temperature factors are Table 4B1, the rewireable fuse factor is Regulation 433.1.101, the thermal insulation case is Regulation 523.9, the adiabatic k values are Table 54.3, and the voltage drop limits are Regulation 525 with Appendix 12. Each was cross-checked against independent sources that agree.
No BS 7671 table is reproduced here. The tabulated capacities, grouping factors, volt drop rates and maximum Zs values are asked of you instead, for the reason set out above.
This is an arithmetic check, not design advice, and not a substitute for the standard or for a competent person. If a figure here disagrees with your copy of BS 7671, the standard is right and I would like to know.