Calculation methodology
Every engine on this site is built the same way, and the order matters: find the clause, implement the method it prescribes, check the implementation against a value somebody published, then print the derivation next to the result. A calculator that produces a number without saying where the method came from is not an engineering tool, it is a guess with a user interface.
1. The clause comes first
Development starts in the standard, not in the code. Before a tool computes anything, the applicable document and clause are identified — ampacity from IEC 60287 or from the Annex B tables of IEC 60364-5-52 are different methods for different purposes, and picking the wrong one produces a defensible-looking number for the wrong question.
The clause then travels with the result. When a tool prints an initial symmetrical short-circuit current it also prints that this is Formula (33) of IEC 60909-0:2016, clause 7.2.1 — so the number can be checked, argued with, and defended in a design review.
2. Tabulated values are read from the standard, never invented
Where a calculation needs a value out of a table — a current-carrying capacity, a correction factor, an efficiency limit, a voltage factor — that value is extracted from the standard's own table and stored in a shared reference database, with the standard, the clause and the table recorded alongside it. Every tool reads the same rows, so one correction reaches all of them.
What is never done is filling a gap with a plausible number. If a table is not in the database yet, the tool says the value is missing rather than interpolating something that looks reasonable. An invented coefficient is indistinguishable from a real one on screen and completely different on site.
3. Manufacturer data is asked for, not assumed
Standards do not publish breaking capacities, cable zero-sequence ratios, transformer test-report losses or locked-rotor ratios — manufacturers do. Where an engine needs one of those, it asks for it and states that it is asking for datasheet data. It does not carry a hidden default that quietly decides the answer.
Where the standard does allow an assumption in the absence of data — for example RQ = 0,1 XQ for a network feeder in IEC 60909-0 clause 6.2, or RM/XM = 0,42 for a low-voltage motor group in clause 6.10 — that assumption is offered as the default, labelled with the clause that permits it, and remains editable.
4. Every field stays editable
A library value is a starting point, not a verdict. When a cable is picked from the equipment library its resistance and reactance fill in, and both remain editable — because the cable on your project is the cable on your project, and its test certificate outranks any catalogue. A row edited by hand says so, so the provenance of a number is never ambiguous.
5. The derivation is printed with the result
Each tool shows the formula, the substituted values and the result, then the standard reference. The report exports the same content, so a reviewer reads the working rather than trusting the tool. This is also what makes a disagreement productive: when two engineers get different numbers, the difference is visible in one of the lines rather than hidden in a black box.
6. Scope is stated, and enforced
A method has a range of validity and the tool carries it in its own name: short-circuit currents 100 V to 1 000 V in one calculator and 1 kV to 33 kV in another, because IEC 60909 treats the two differently — the low-voltage tool needs the peak-current cap of clause 8.1.2 and minimum currents at the conductor's final temperature, the medium-voltage tool needs parallel sources and earthing arrangements.
Where a chain crosses the boundary, the tools hand over to each other rather than each pretending to cover both ranges: the medium-voltage calculator stops at 1 kV and passes the equivalent source at that busbar to the low-voltage calculator, which continues the same network and prints the joining current so the two can be compared in one line.
7. The free tier performs the whole calculation
A free calculator that stops before the answer is a demo, not a tool. On this site the free tier runs the complete calculation, states the method and shows the derivation. What PRO adds is the work around the calculation: the report, saved projects, equipment libraries, several scenarios at once, batch runs and exports. Nothing in the engineering is held back to force an upgrade.
8. Then the engine is checked against somebody else's numbers
The last step is the one that decides whether any of the above worked: run the engine on a case that was published with its answer — a worked example, an annex table, a reference network — and compare. That comparison ships with the tool as a panel you can run yourself, and the summary across all tools is on the validation page.