XLPE Insulated Power Cable Selection Guide: Current Rating, Voltage Drop and Cross-Section
Choosing the right cross-section of XLPE insulated power cable is a four-variable problem: load current, installation method, ambient conditions and voltage drop. Get the order wrong and you either overspend on copper or run the cable hot. This guide walks through the calculation sequence used on our own export projects.
Step 1 — Establish the Design Current
Start from the connected load in kVA, apply the diversity factor of the installation and convert to amperes:
Ib = S × 1000 / (√3 × Un) for a three-phase load, where S is kVA and Un is 400 V line-to-line.
A 12 MW industrial park that draws 8 MVA at peak, for instance, needs roughly 11,550 A per phase — which immediately tells you the design will be built from several parallel feeders rather than one cable.
Step 2 — Read the Base Current Rating
The base ampacity of 0.6/1 kV copper XLPE cable, 90 °C conductor, 30 °C air and soil resistivity 2.5 K·m/W, is shown below. Copper and aluminium values differ by roughly one size step.
| Cross-section | In air (single core) | In soil (single core) | In air (3-core) | In soil (3-core) |
|---|---|---|---|---|
| 10 mm² | 77 A | 94 A | 60 A | 77 A |
| 35 mm² | 160 A | 185 A | 125 A | 155 A |
| 70 mm² | 245 A | 270 A | 200 A | 225 A |
| 150 mm² | 400 A | 410 A | 325 A | 345 A |
| 300 mm² | 635 A | 610 A | 505 A | 515 A |
Step 3 — Apply Derating Factors
Manufacturer tables assume an ideal world. Real installations multiply four correction factors together:
| Condition | Range | Typical factor |
|---|---|---|
| Ambient air temperature | 30 °C reference; 40 °C typical | 0.87 – 0.91 |
| Soil temperature | 20 °C reference; 30 °C tropical | 0.87 – 0.93 |
| Soil thermal resistivity | 2.5 K·m/W reference; dry sand 3.5 | 0.80 – 0.85 |
| Grouping, three circuits touching in air | 3 cables per tray | 0.73 |
| Burial depth | 1.0 m reference | 1.00 – 0.95 |
Corrected ampacity = base rating × ktemp × ksoil × kgroup × kdepth. A 150 mm² three-core cable rated 325 A in air becomes 325 × 0.89 × 0.75 = 217 A in a tropical plant room with two parallel circuits — a 33% cut that surprises most first-time buyers.
Step 4 — Check Voltage Drop
Voltage drop for a three-phase circuit is calculated as:
ΔU = √3 × I × L × (R cosφ + X sinφ) / 1000 in volts, with R in Ω/km at operating temperature and L in km.
Using 150 mm² copper XLPE (R = 0.124 Ω/km, X = 0.08 Ω/km), 200 m run, 300 A, cosφ = 0.85: ΔU = 1.732 × 300 × 0.2 × (0.105 + 0.042) = 15.3 V, or 3.8% of 400 V. Within the usual 5% limit, so the 150 mm² selection stands.
Step 5 — Verify Short-Circuit Withstand
XLPE allows a conductor temperature of 250 °C for a fault of up to five seconds. Use the adiabatic equation S = I√t / k with k = 143 for copper XLPE. A 25 kA fault cleared in 0.2 s needs S = 25,000 × 0.447 / 143 = 78 mm², so a 95 mm² cable is chosen on fault duty even though 70 mm² would carry the load.
Quick Selection Checklist
- Design current calculated from kVA, not from installed kW.
- Installation method identified: tray, duct, direct burial, in air.
- All four derating factors applied and documented.
- Voltage drop below 5%, or below the stricter local code.
- Short-circuit withstand checked against breaker clearing time.
- Bending radius and pulling tension compatible with the route.
For the complete construction, type and standards overview, see our XLPE insulated power cable guide.
