Cable Ampacity Calculator (IEC 60287): Underground, Duct Bank and Free-Air Ratings with Minimum Conductor Size
Cable Ampacity Calculator (IEC 60287): Underground, Duct Bank and Free-Air Ratings with Minimum Conductor Size
Calculate the continuous current rating of a single-core power cable from its data sheet — direct buried, in ducts, in a concrete duct bank or in free air — or enter the current you need and find the minimum conductor size and overall cable diameter. Every layer is drawn to scale, every input has a guide, and the full IEC 60287 calculation is shown step by step.
Cable Ampacity Calculator
Pick an example from Load example to see a complete case, then change one value at a time. Press the ? beside any input for its meaning, typical values and where to find it on the data sheet. Switch the mode to Find minimum size when you have a required current instead of a cable. Print / PDF report gives you the inputs, drawings and every calculation step.
Reading the Cable Data Sheet into the Calculator
The form follows a typical data sheet from the conductor outwards, then moves to the installation. The cross-section drawing updates as you type, so you can check each diameter against the data sheet line by line.
Which conductor data do I need?
- Material and construction: copper or aluminium; round compacted or Milliken (segmental). The construction sets the skin and proximity coefficients
ksandkpfrom IEC 60287-1-1 Table 2. - Area and diameter: nominal mm² and the “diameter of conductor” line. Quick-fill buttons give typical compacted diameters for every IEC 60228 size.
- Max. DC resistance at 20 °C: always on the data sheet. Leave it blank to use the IEC 60228 maximum; the results flag a data-sheet value above that maximum.
- U₀ and frequency: used for the dielectric loss, which only matters from about 60 kV up. A 50 Hz data sheet is slightly optimistic for a 60 Hz system.
How do the insulation and screen layers map to the drawing?
Layers are numbered ① conductor to ⑨ outer sheath, and the same numbers appear on the inputs, the drawing and the build-up table. Everything between the conductor and the metallic screen — conductor screen, insulation, insulation screen and any water-swelling tape — forms the insulation thermal resistance T1. Everything between the screen and the surface forms T3.
What installation and layout data are needed?
- Ground temperature at cable depth in the hottest month — not the air temperature — and the soil thermal resistivity for the driest season.
- Ducts: type, outside and inside diameter, wall resistivity, and whether the duct is dry, flooded or filled with bentonite.
- Duct bank: leave Auto-fit bank to ducts on and give the concrete cover, or untick it to enter an existing bank. Ducts outside the bank are drawn in red and the calculation stops.
- Layout: trefoil or flat, touching or spaced, depth to the top row, and up to 6 × 6 equally loaded circuits.
- Screen bonding: both-ends, single-point or cross-bonded. For large cables this one setting can halve the rating — see the worked example.
The IEC 60287 Method Behind the Numbers
IEC 60287 treats the cable and its surroundings as a thermal circuit: losses generated in the conductor, insulation and screen flow outwards through thermal resistances in series, and the rating is the current that brings the conductor to its maximum temperature.
What is the rating equation?
Variables
Δθ: conductor temperature rise above ambient (K)R: AC resistance at maximum temperature (Ω/m);Wd: dielectric loss (W/m)λ1: screen loss as a fraction of conductor lossT1,T3,T4: insulation, outer sheath and external thermal resistances (K·m/W)
For a group of cables, the calculator replaces T4 with the cable’s own external resistance plus the mutual-heating terms from every other cable, each weighted by that cable’s own losses. It evaluates every cable and the hottest one sets the rating.
How are the conductor, dielectric and screen losses found?
Single-point and cross-bonded screens carry no circulating current, so λ1′ = 0; eddy-current loss λ1″ is then calculated for tubular sheaths. IEC practice neglects eddy loss in wire screens; a toggle lets you include it as a check.
How are the thermal resistances calculated?
Variables
L: depth to cable centre;De: cable (or duct) outside diameterd_pk: distance to cable k;d'_pk: distance to the image of cable k above ground
Worked Example: 72.5 kV 1200 mm² Cable in a Duct Bank
This is the first entry in Load example. Load it and you’ll get the same numbers.
| Cable | 38/66 (72.5) kV, 1 × 1200 mm² Cu Milliken, XLPE 11.5 mm, 77 × 1.5 mm Cu wire screen, PE sheath, De = 88 mm |
| Installation | Concrete duct bank 720 × 480 mm, centre 1.0 m deep; PVC ducts 160/147 mm |
| Layout | 2 circuits stacked, flat, 240 mm spacing; top row at 0.91 m, second at 1.15 m |
| Thermal | Soil 1.2 K·m/W, concrete 1.0 K·m/W, ground 30 °C, conductor 90 °C |
| Bonding | Cross-bonded, 50 Hz |
What does the calculator return?
| Quantity | Value |
|---|---|
| AC resistance at 90 °C, R | 0.02011 Ω/km (ys = 0.0406, yp = 0.0036) |
| Dielectric loss, Wd | 0.154 W/m |
| Screen loss factor, λ1 | 0 (cross-bonded, wire screen) |
| T1 / T3 | 0.3077 / 0.1130 K·m/W |
| T4 own (air gap 0.2948 + duct wall 0.0809 + external 0.5344 + bank correction 0.0591) | 0.9692 K·m/W |
| Mutual heating from the other five cables | 1.9351 K·m/W |
| Continuous rating | 943 A |
| Conductor temperatures at 943 A | Upper row 85.0 / 88.1 / 85.0 °C; lower row 86.9 / 90.0 / 86.9 °C |
What happens if the screens are bonded at both ends instead?
With 136 mm² of copper screen and 240 mm spacing, the circulating current is large: the solidly-bonded λ1′ is 2.34, meaning the screens would generate more than twice the conductor loss.
What if I need 900 A — what’s the smallest conductor?
Switch to Find minimum size and enter 900 A. The calculator keeps the insulation, screen, sheath, duct bank and layout, scales the conductor diameter from the one entered, and tries every IEC 60228 size.
| Size | Conductor Ø | Overall Ø | Rating |
|---|---|---|---|
| 800 mm² | 34.5 mm | 80.3 mm | 750 A |
| 1000 mm² | 38.5 mm | 84.3 mm | 874 A — too small |
| 1200 mm² | 42.2 mm | 88.0 mm | 943 A — minimum size |
| 1400 mm² | 45.6 mm | 91.4 mm | 1,018 A |
Below 1000 mm² the calculator treats Milliken conductors as compact round, because segmental conductors aren’t made that small. The real outer sheath usually grows a little on larger sizes, so confirm the final diameter with the supplier.
What Moves the Rating Most?
The base case is a typical distribution cable — 8.7/15 kV, 1 × 240 mm² Cu XLPE with a 25 mm² wire screen, buried in trefoil at 1.0 m, 60 Hz, screens bonded both ends — rated at 453 A with 1.2 K·m/W soil and 30 °C ground. Each row changes one input from that base case.
How sensitive is the rating to soil, temperature and depth?
| Change | Rating | vs base |
|---|---|---|
| Soil ρ 0.7 K·m/W (very moist) | 565 A | +25 % |
| Soil ρ 1.0 K·m/W | 489 A | +8 % |
| Soil ρ 1.5 K·m/W | 412 A | −9 % |
| Soil ρ 2.0 K·m/W | 362 A | −20 % |
| Soil ρ 2.5 K·m/W (dry sand) | 327 A | −28 % |
| Ground 25 °C / 35 °C | 472 A / 434 A | ±4 % |
| Depth 0.8 m / 1.5 m | 464 A / 436 A | +2 % / −4 % |
| Second circuit alongside, 600 mm apart | 404 A | −11 % |
| Single-point bonded instead of both ends | 464 A | +2 % |
| In free air, 35 °C, shaded / in full sun | 598 A / 457 A | — |
Why Your Result May Differ from the Supplier’s Data Sheet
A data-sheet rating is only valid for the conditions it was calculated under, which are often printed in small type or not at all. When your number and the supplier’s disagree, check these first.
Which assumptions usually explain the difference?
- Soil resistivity and ground temperature: 1.0 K·m/W at 20 °C gives a much higher rating than 1.2 K·m/W at 30 °C.
- Bonding: single-point or cross-bonded versus both-ends bonded — the largest effect on big cables.
- Load factor: ratings calculated with a daily load factor below 100 % are higher than the steady-state IEC 60287 value.
- Frequency: a 50 Hz rating is slightly optimistic on a 60 Hz system.
- Grouping and spacing: a single isolated circuit versus your actual number of circuits.
- Coefficients: Milliken
ks, eddy loss in wire screens, and layer resistivities differ between software and IEC editions — usually only a few percent.
Frequently Asked Questions
Is IEC 60287 the same as Neher–McGrath?
They share the same physics: IEC 60287 grew out of the 1957 Neher–McGrath paper. Both use a thermal circuit, the image method for soil and mutual heating, and the same duct air-gap formula in different units. The main difference is load factor: Neher–McGrath builds daily load cycling into the earth term, while IEC 60287 rates at 100 % load and handles cycling separately in IEC 60853. The NEC’s engineering-supervision calculation uses Neher–McGrath, with RHO 90 (0.9 K·m/W) and 20 °C as the table basis.
What happens when the cable or duct is flooded?
The rating goes up, because water conducts heat far better than air. Choose Water (flooded duct) under duct filling: the duct example above goes from 943 A dry to 967 A flooded. The calculator treats the water as still and the cable as centred, which is conservative. Only use it for ducts that are permanently under water; a duct that dries out in summer must be rated dry.
Can soil around a loaded cable dry out?
Yes. Heat from the cable drives moisture away, and dried soil can reach 2–3 K·m/W, which can lead to thermal runaway. IEC 60287-2-1 covers this with a two-zone soil model that this calculator doesn’t include, so use the soil resistivity for the driest season, or a thermally stable backfill around the cables.
Does the calculator check short-circuit ratings too?
Yes, adiabatically per IEC 60949: I = K·S·√[ln((θf + β)/(θi + β))/t]. For the 1200 mm² copper conductor, 90 → 250 °C in 1 s gives 171.7 kA. The screen check takes a non-adiabatic factor ε if you want to reproduce a supplier’s figure.
