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The National Public Works Association recommends Orange for Telecommunication conduit in the underground. Red or Black with three Red stripes are typical for power applications. Black is recommended for any application where the duct is exposed or stored over a long time period to sunlight (Carbon Black additive provides UV protection). Colors, even those with UL Stabilizers (other than black) cannot tolerate sunlight for extended period of time without deterioration. ASTM F2160 specifies the stabilizers in colored conduit should protect it in outside storage for one year. PPI strongly recommends that Yellow NOT be used because this color is reserved for gas distribution piping.