IEEE S-135-1 1962 Edition, January 1, 1962
POWER CABLE AMPACITIES VOLUME I - COPPER CONDUCTORS INSULATIONS:
IMPREGNATED PAPER VARNISHED CLOTH RUBBER AND THERMOPLASTIC
ASBESTOS-VARNISHED CLOTH INSULATIONS: IN UNDERGROUND DUCTS BURIED DIRECTLY
IN EARTH IN FREE AIR IN CONDUIT (IPCEA PUB.NO. P-46-426)