Cable Sizing: Short-Circuit Rating, Voltage Drop & Economics

Picking a cable by ampacity alone is how feeders survive the load and fail the fault. Here's the full sizing story — I²t withstand, voltage-drop limits and the economic current density that saves you money every year.

Industrial Feeder Sizing & Verification

Here's the uncomfortable truth about cable sizing: ampacity — the table everyone uses — answers only one question ("will it overheat at rated load?"), and it says nothing about the other three questions that actually decide whether a feeder survives its working life. Will it survive the fault? Will the voltage stay within limits at the far end? And is the size economic, or are you paying for years of avoidable losses?

I've lost count of the projects where a feeder sized "correctly" by the ampacity table failed its short-circuit check by a factor of ten, or ran a motor so far from the substation that the motor saw brown-out voltage at full load. The thing is, the check takes five minutes once you know the method — it's just never on the shortcut that procurement hands you.

This guide walks through all four sizing criteria with the equations and worked examples, in the order a competent engineer actually applies them.

1. The Four Criteria, in Order

Every feeder must pass all four. Ampacity is necessary but never sufficient.

#CriterionIt answersPrimary reference
1Ampacity (continuous load)Will it overheat at rated load?IEC 60364-5-52 / IEC 60287
2Short-circuit thermal withstandWill it survive the fault?Adiabatic equation (I²t), IEC 60949
3Voltage dropWill the far end stay in limits?IEC 60364-5-52, BS 7671
4Economic current densityIs this the cheapest size over life?IEC 60287-3-2
Criteria 2–4 routinely change the size selected. Ampacity gives you a floor, not an answer.

2. Short-Circuit Thermal Withstand: The I²t Check

For faults up to about 5 seconds, treat the heating as adiabatic — all the fault energy goes into raising the conductor temperature, and none escapes.

Under a short circuit the conductor must absorb the fault's thermal energy without its temperature rising beyond the insulation's limit. The adiabatic equation puts it in one formula:

I²t = k² × S²  →  Smin = I × √t / k

where I is the fault current (A), t the fault duration (s), S the conductor cross-section (mm²), and k a constant set by the conductor material and insulation. The check: the cable's withstand (k²S²) must exceed the energy the protective device lets through (I²t).

InsulationCopper (Cu) kAluminium (Al) kCopper thermal window
PVC1157670°C → 160°C (>300 mm²: 140°C)
XLPE1439490°C → 250°C
k values per IEC 60949 short-circuit calculation practice. The k itself derives from material properties: k = 226·√(ln(1+(θf−θi)/(234.5+θi))) for copper.

Worked example

A 4 mm² PVC/Cu control feeder, prospective fault 6,000 A, protective device clearing in 0.08 s:

  • Fault energy let-through: I²t = 6000² × 0.08 = 2,880,000 A²s
  • Cable withstand: k²S² = 115² × 4² = 211,600 A²s
  • Verdict: fails by a factor of ~13.6. Minimum size: S ≥ 6000 × √0.08 / 115 ≈ 14.8 mm² → 16 mm².
That's the whole point of the check: a cable that carried the load comfortably was destroyed by the fault. Ampacity would have picked 4 mm²; the adiabatic equation demands 16 mm². The two checks can disagree by four sizes — which is exactly why both must be done.
Note for the field: this is the fast, conservative adiabatic method, valid to ~5 s. For longer faults, non-adiabatic methods (IEC 60949) are more accurate. If a joint is soldered, cap the conductor temperature at 160°C regardless of insulation rating.

3. Voltage Drop: The Far End Decides

A feeder can be thermally perfect and still starve the load at the far end of a long run.

Voltage drop is the easiest criterion to neglect and the hardest to excuse, because it's pure geometry. For a three-phase feeder:

Vd = √3 × I × L × (R·cosφ + X·sinφ)

Design limits vary by national code and client specification — many installations use a 3–5% range (e.g. 4% for feeders, 3% for lighting in some codes), and the governing limit is the applicable national installation code and client specification. Motors are even more demanding — most tolerate 5% at their terminals, but every volt lost at the feeder is a volt the motor doesn't get.

Worked example: 400 V three-phase, 35 mm² copper, 100 A, 200 m run, cosφ 0.8. Working-temperature resistance ≈ 0.6 mΩ/m, reactance ≈ 0.08 mΩ/m:

Vd = 1.732 × 100 × 200 × (0.6×0.8 + 0.08×0.6) / 1000 ≈ 18.3 V ≈ 4.6% → exceeds 4%

Step up to 50 mm² (≈0.44 mΩ/m working): Vd ≈ 13.9 V ≈ 3.5% — within limit. Same load, same duty; the 200 m run forces a size the ampacity table never would have.

4. Economic Current Density: The Criterion That Pays

Once the cable is safe, the economic question takes over: which size is cheapest over its life, including energy losses?

Ampacity and voltage drop pick a minimum. Economics picks the optimum — the size where initial cost plus the present value of I²R losses is lowest. IEC 60287-3-2 formalises this, and it routinely lands one or more sizes above the minimum, because losses are paid in operating hours, year after year.

As a fast engineering rule of thumb, copper feeders are economic around 1.5–3 A/mm² (the classic German/VDE practice band). A feeder drawing 100 A sits economically around 35–70 mm² — while the ampacity minimum might have been 25 mm². The larger size costs a little more today and saves the difference in energy, every year, for the cable's whole life. (Note: for intermittent loads or emergency feeders, economic current density models may not apply — ampacity and voltage drop govern.)

4%Feeder voltage-drop limit
k=143XLPE/Cu short-circuit k
1.5–3 A/mm²Economic range, Cu feeders
~5 sAdiabatic assumption valid to
Cheap wins on paper, expensive wins on the P&L: the economic size is rarely the minimum safe size. In continuous-process plants the energy line alone usually justifies going one or two sizes up. Run the numbers with your own load, hours and energy price before you settle.

5. A 5-Step Sizing Workflow for Engineers

  1. Ampacity: size for continuous load per IEC 60364-5-52, with derating for ambient, grouping, depth and harmonics.
  2. Short circuit: Smin = I·√t/k against the protective device's let-through energy; round up.
  3. Voltage drop: verify the far-end voltage at full load against the 4% (or motor/lamp-specific) limit; increase size if needed.
  4. Economic check: compare the life-cycle cost of the size above; step up while the energy saving exceeds the added capital.
  5. Confirm with the datasheet: the conductor resistance, reactance and k constant you used should match the manufacturer's technical data — ask for it.

6. Engineering Data: Commodity vs. SORIVO Technical Support

Every sizing calculation is only as good as the data it uses.

CharacteristicMarket commoditySORIVO grade
Conductor resistance dataGeneric tablePer-lot measured values on request
Short-circuit data (k, ratings)Not suppliedAdiabatic withstand figures in technical data
Voltage-drop / reactance dataNot suppliedR and X values per size and construction
CertificationSelf-declaredThird-party tested (IEC / UL / TÜV where applicable)
Engineering supportDatasheet downloadApplication engineers for sizing verification
Sorivo publishes the engineering data engineers actually need and will confirm sizing calculations with our application team.

Cable Sizing FAQ

Why isn't ampacity enough to size a feeder?
Ampacity only answers "will it overheat at rated load?" A feeder must also survive the fault (short-circuit I²t withstand), deliver acceptable voltage at the far end, and ideally be economic over life. These checks routinely demand a size one to four steps larger than the ampacity minimum.
How do I use the adiabatic equation correctly?
Compute the fault let-through energy I²t from the protective device and compare it with k²S². Rearranged, the minimum size is S = I·√t/k. Use the k for your insulation and conductor (PVC/Cu 115, XLPE/Cu 143, etc.). The method is valid to about 5 seconds; for longer faults use IEC 60949.
What's a reasonable voltage-drop limit?
Design limits vary — many national codes and client specifications use a 3–5% range (e.g. 4% for feeders, 3% for lighting in some installations). Motor feeders should keep terminal voltage within the motor's tolerance — typically 5% at the motor terminals. Long runs are usually governed by voltage drop, not ampacity.
What is economic current density and when does it matter?
It's the conductor loading (A/mm²) that minimises initial cost plus the present value of energy losses over the cable's life, per IEC 60287-3-2. It matters whenever a cable is loaded for many hours a year — the economic size is typically one or two steps above the safe minimum, and the larger size pays for itself in saved losses.
Where can I get reliable k values and resistance data for the calculation?
k values are given in IEC 60364-5-54 / BS 7671 (Appendix tables) and IEC 60949. Conductor resistances follow IEC 60228. Sorivo publishes per-construction technical data — resistance, reactance and short-circuit figures — and will confirm a sizing calculation with our application engineers before you order.

Need technical data you can calculate with?

Request the Sorivo technical datasheets — resistance, reactance, short-circuit data — and our engineers will help verify your sizing.

Email SORIVO Sales +86 192 8290 5529

Senior cable application engineer at Sorivo
Reviewed by Luo Qiang — Senior Cable Application Engineer, Sorivo
15+ years in industrial and renewable energy cable specification. Experienced in cable specification aligned with IEC standards. Previously contributed to cable selection for 500MW+ solar PV and BESS projects across Asia, Europe, and the Middle East.

Sources & method: Short-circuit thermal withstand calculated via the adiabatic equation per IEC 60949 (Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects), with k values per IEC 60949 and the tables of IEC 60364-5-54 (Clause 543.1) / BS 7671 Reg. 434.5.2 / 543.1.3. k derivation: k = 226·√(ln(1+(θf−θi)/(234.5+θi))) for copper. Voltage-drop limits per the applicable national installation code (e.g. IEC 60364-5-52 / BS 7671), which vary by jurisdiction; economic sizing per IEC 60287-3-2, with the 1.5–3 A/mm² band as established engineering practice for copper feeders. Worked examples use stated assumptions; verify against your protective-device data and plant conditions.