Warm soil absorbs less heat. It is assumed that the heat finally goes to the ground surface. Deeper cable sees more total soil resistance. 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 circulat-ing 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 installa-tion that can t be sized with standard ampacity tables, check with your cable vendor, says Schlumberger. Section Six of Southwire s Power Cable Manual also provides information on under- ground cable ampacity calculations.