HOW MUCH UNDERGROUND CABLE AMPACITY?
Publication 287 of the International Electrotechnical Commission (IEC 287) is the basis for most underground ampacity tables. The calculations can be complex.
- Published in Southwire's Power Cable Update newsletter in May/June 1998
- Reprint permission granted
Going underground with a medium-voltage or high-voltage installation? What size conductor do you need?
"For many underground applications, you can find ampacities in cable manuals, says Axel Schlumberger, technical marketing manager for Forte Power Systems™. "For specialized installations, or for high-voltage, you'll need a calculated answer."
The basis for most high-voltage underground ampacity calculations worldwide is an international standard, IEC 287. IEC 287 covers medium-voltage and high-voltage cables, many different constructions, and many installation types. Just as the Neher-McGrath method to calculate ampacities is the predominant method used in the U.S., IEC 287 is the predominant method used internationally. Both methods are employed through the application of thermal equivalents of Ohm's and Kirchoff's Laws to a simple thermal circuit.
Thermal Resistances Determine Ampacity
IEC 287 uses a thermal resistivity circuit model of the cable installation.
12R losses in the conductor, dielectric losses in the insulation and eddy currents in the sheath all generate heat. Cable materials and soil represent a series circuit of thermal resistances. The thermal resistances control heat dissipation from the conductor. If you dissipate more heat, you can carry more current.
The key element in the series heat circuit is the external thermal resistance outside the cable. Here are some factors.
Soil thermal resistivity. How well does the soil carry away the heat? Soil resistivity is a specialized measurement. If you don't have actual numbers for your site, many handbooks — such as the EPRI Underground Transmission Systems Reference Book — give design assumptions.
Ambient soil temperature. Warm soil absorbs less heat.
Installation depth. It is assumed that the heat finally goes to the ground surface. Deeper cable sees more total soil resistance.
Installation method A direct-buried cable generally dissipates heat more readily than a cable in a duct.
Nearby Cables Complicate Heat Flow
Adjacent cables contribute heat and may induce additional losses in the cable itself. Closer cables have more effect.
Sheath grounding makes a difference. Multipoint grounding increases circulating currents in the sheath. Single-point grounding eliminates sheath currents, but away from the ground point, induced voltage on the sheath has to remain within certain limits.
"If you have an underground installation that can't be sized with standard ampacity tables, check with your cable vendor," says Schlumberger. "With high-voltage in particular, you may need assistance with the calculations."
Key Variables In Underground Cable Ampacity
Variable Value Effect On Ampacity
Installation depth deeper less ampacity
Cable separation smaller less ampacity
Soil temperature higher less ampacity
Soil thermal resistivity higher less ampacity
Installation method duct, rather than direct burial less ampacity
Shield grounding multipoint, rather than single-point less ampacity |
