HV Cable Sizing Calculation: A Worked Example
For medium and high voltage cables the load current is often small and the fault current is large, so the short-circuit check frequently decides the size. Here is the full method with a worked example.
The example and its assumptions
Illustrative example, not a project. A 2,500 kVA transformer is fed at 11 kV, three phase, by a 300 m copper XLPE cable. The prospective fault level at the source is 25 kA for 1 second. Total derating factor for the installation is assumed to be 0.80, and k for copper XLPE is taken as 143. Take real values from the cable datasheet and the governing standard.
Step 1: Design current
I = kVA ÷ (√3 × V) = 2,500 ÷ (1.732 × 11) = 131 A.
Step 2: Required tabulated rating
Required tabulated current rating = design current ÷ derating factor = 131 ÷ 0.80 = 164 A. The chosen cable's tabulated rating for the installation method must be at least this. Ground temperature, depth of burial, soil thermal resistivity and grouping all feed the derating factor.
Step 3: Short-circuit withstand
S = I × √t ÷ k = 25,000 × √1 ÷ 143 = 175 mm², so the next standard size is 185 mm².
The load current alone could be carried by a much smaller cable. The fault level makes 185 mm² the minimum, which is why the short-circuit check cannot be skipped.
Step 4: Voltage drop
With R = 22.5 ÷ 185 = 0.122 Ω/km, an assumed X = 0.10 Ω/km and cos φ = 0.9 (sin φ = 0.436), the drop is √3 × 131 × 0.3 × (0.122 × 0.9 + 0.10 × 0.436) = about 10 V, which is about 0.1% of 11 kV. Voltage drop is not an issue at this length. See voltage drop formula and examples.
Result
| Check | Requirement |
|---|---|
| Current rating | At least 164 A tabulated, which a small cable meets |
| Short-circuit withstand | At least 175 mm², so 185 mm² (governs) |
| Voltage drop | About 0.1%, so it passes |
Also check the cable screen and its earthing arrangement for the fault current, and the cable's rated voltage and insulation level for the system. The HV and LV cable sizing calculatorruns these checks for your inputs. Related: short circuit basicsand cable sizing basics.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
How do you size an HV cable?
Find the design current, apply derating factors for installation conditions to get the required tabulated current rating, check voltage drop, and check that the conductor can withstand the prospective short-circuit current for the fault clearing time. The largest size required by these checks governs.
Why does short-circuit withstand often decide HV cable size?
Because HV systems have high fault levels while load currents are modest. A conductor sized for load current may overheat during a fault, so the minimum area from the adiabatic equation can be larger.
What is the adiabatic equation?
S = I × √t ÷ k, where S is the minimum conductor area in mm², I the fault current in amperes, t the fault duration in seconds and k a factor depending on the conductor material and insulation temperature limits.
Where do the ampacity and impedance values come from?
From the cable manufacturer's datasheet and the applicable standard for the installation method. The values in this example are illustrative.
Want to learn this properly?
This topic is covered in depth in our Electrical Design – Data Center Specialist program.
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