Engineer's Notes: sizing HV cables
Heat is the real limit
An XLPE cable can run its conductor at 90 °C continuously. Everything that slows heat leaving the cable lowers its rating: hot ground, dry soil, deeper laying, ducts, neighbouring circuits and screen losses. That is why a cable that is fine in Auckland can overheat in Dubai at the same current. Always rate with the utility design conditions.
Three separate checks
A cable size has to pass three tests: continuous current rating, short-circuit withstand of the conductor and the screen, and voltage drop. On short HV feeders in buildings the short-circuit or screen check often decides the size, not the load.
Screens and bonding
The copper screen carries earth fault current and capacitive charging current. How you bond it changes the rating: solid bonding is simple but adds circulating current losses on large single-core cables. Single-point and cross bonding remove those losses but need standing voltage control, sheath voltage limiters and link boxes.
Hold the cables together
Fault currents throw single-core cables apart. Trefoil cleats rated to IEC 61914 and spaced correctly keep them in place. The forces climb with the square of the peak current, so a 50 kA peak can produce more than 10 kN per metre.
Frequently asked questions
Why is the same HV cable rated lower in the GCC than in New Zealand?
Current rating depends on how fast heat leaves the cable. GCC ground temperatures of around 35 degrees C at cable depth and dry sand with high thermal resistivity leave less temperature headroom and slow heat flow, so the same 240 mm2 cable can carry 20 to 30 percent less than in cool, moist New Zealand soil.
How is the copper screen of an HV cable sized?
The screen must carry the network earth fault current for the protection clearing time without overheating its insulation. With the adiabatic method of IEC 60949, the required area is the fault current times the square root of the time, divided by a factor k for copper between the operating and maximum screen temperatures.
What is the difference between solid, single-point and cross bonding?
Solid bonding earths the screens at both ends: simple and safe, but circulating currents heat the screens and reduce the rating of large single-core cables. Single-point bonding earths one end only, removing circulating currents but leaving a standing voltage at the open end. Cross bonding transposes the screens at link boxes so induced voltages cancel over each major section.
What is the standing voltage limit on single-point bonded cables?
It depends on the utility and standard. Values of around 50 to 65 V under full load are common limits for touchable screen ends, with sheath voltage limiters fitted to handle faults. Longer routes use cross bonding instead.
Why do single-core HV cables in trefoil need cleats?
During a short circuit, the three cables repel each other with large electromagnetic forces. IEC 61914 estimates the force per metre as 0.17 times the square of the peak fault current in kA divided by the centre spacing in metres. Cleats must be rated for that force and spaced close enough to hold the cables.
Topics: IEC 60287 current rating · soil thermal resistivity · ground temperature · grouping factors · ducts · cables in air · screen short circuit · IEC 60949 · voltage drop · solid bonding · single-point bonding · cross bonding · standing voltage · cleat force IEC 61914 · cable losses
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