Feeder
250 MVA · X/R 10
One radial network,
both duties.
A 250 MVA utility feeder, a 1000 kVA Dyn11 transformer, a main switchboard and a distribution board — the ordinary Australian LV arrangement, taken node by node through maximum and minimum duty. Every figure below is the engine's, and every standards value is a declaration you can see.
Transformer
1000 kVA Dyn11
Rated voltages
11 kV / 433 V
Nominal system
11 kV / 400 V
uk · X/R
6 % · 10
MSB feed
2 × 240 mm² · 50 m
DB feed
95 mm² · 80 m
Earthing
Solid · Z₀/Z₁ 0.9
The transformer is rated 433 V and the system is nominal 400 V — two different numbers doing two different jobs, and the study is computed at 400. The 433 / 415 / 400 V note is the house's explainer of exactly this distinction.
The declaration comes first. Before the engine computes anything, the study declares its voltage factors — c_max 1.25 and c_min 0.75 here, per band — the minimum-case conductor temperature θ_e = 160 °C, and the copper temperature coefficient with its base. Those are deliberately not the values IEC 60909-0's Table 1 gives: the instrument holds no licensed constant, so the worked example declares implausible-on-purpose factors to show the mechanism, and your study declares the real ones from your own copy of the standard, cited to the clause. The declaration prints in §2.0 of the report with the clause, edition, declarer and date beside every value.
Maximum duty
What the equipment must survive
c_max · conductor resistance as declared at θ_ref
| Node | I″k3 (kA) | I″k2 (kA) | I″k1 (kA) | i_p (kA) | κ | X/R of Z₁ |
|---|---|---|---|---|---|---|
| 0 · Transformer LV | 24.06 | 20.83 | 25.38 | 59.40 | 1.746 | 10.00 |
| 1 · MSB | 20.04 | 17.36 | 18.51 | 41.62 | 1.469 | 3.84 |
| 2 · DB-1 | 9.27 | 8.02 | 5.58 | 13.79 | 1.052 | 0.88 |
The earth fault governs at the transformer. With a solidly-earthed Dyn secondary at Z₀/Z₁ = 0.9, the line-to-earth fault reaches 25.38 kA — above the 24.06 kA three-phase figure. On this ordinary arrangement the breaking-capacity duty is the earth fault, which is why the adequacy check grades against the maximum over the fault types actually computed, and why a node where the earth fault was refused carries that refusal on its badge.
Two cables later the duty has fallen by more than half: 9.27 kA at the DB, with the peak factor κ collapsing from 1.746 at the transformer terminals to 1.052 once cable resistance dominates the loop. The MSB carries declared ratings on this study — Icu 36 kA, Ipk 75 kA — and both checks read PASS with their numeric margins printed beside the badge; the unrated nodes read not assessed, never a silent pass.
Minimum duty
What the protection must still see
c_min · R(θ_e) — base-explicit temperature correction on the declared cables
| Node | I″k3 (kA) | I″k2 (kA) | I″k1 (kA) |
|---|---|---|---|
| 0 · Transformer LV | 14.43 | 12.50 | 15.23 |
| 1 · MSB | 11.91 | 10.31 | 10.87 |
| 2 · DB-1 | 4.96 | 4.29 | 2.87 |
The minimum is the compliance quantity: the number that shows a protective device still sees the fault at the far end of the run. At the DB the minimum three-phase current is 4.96 kA and the minimum earth fault just 2.87 kA — the figures a grading study checks a pickup against. The correction is visible arithmetic, not a hidden constant: at θ_e = 160 °C the declared copper α lifts each cable's resistance by a factor of 1.219 over its 90 °C catalogue figure, and the report prints that multiplier per element, alongside its own line stating that transformer and feeder resistances are used as declared, uncorrected.
Rated is not nominal, and the difference is 8.25 %. The equivalent voltage source is c·Un/√3 with Un the nominal system voltage — 400 V here. Type the transformer's 433 V rated secondary into that slot instead and every LV current above rises by 433/400 = 8.25 %, on the first line of a signed document. The calculator therefore takes both voltages, prints both, and never lets one stand in for the other — the 433 / 415 / 400 V note explains why the two numbers exist at all.
§9.0 of the report
The hand-off, with the loop named
every current symmetrical RMS, no decrement applied · each X/R labelled by its own loop
To N° 17 · grading
25.38 / 2.87 kA max · min
To N° 01 · breaking
9.27 kA at the DB
To N° 04 · earth loop X/R
2.74 of Z₁+Z₂+Z₀, not Z₁
The report's closing section prints each computed current against the issuing document number, with the X/R beside it labelled by the loop it came from — 3.84 for the positive-sequence loop at the MSB, 2.74 for the earth-fault loop at the same node. They are different numbers, and an earthing-grid decrement factor computed from the wrong one is wrong by construction. The shipped calculators each carry a fault source document field, so the study that produced a number is named on the document that consumes it.
Every figure on this page was produced by the Ampacities Fault Current engine for the network stated, with c_max 1.25 / c_min 0.75, θ_e 160 °C and α 0.004 /K on a 20 °C base declared as illustrative values — deliberately not the standard's, because the engine holds no licensed constant and your study declares its own, cited to your copy of IEC 60909-0:2016. Enter the same network to reproduce them and export the signed report. Figures are engineering estimates for the stated inputs and declarations.