(1) Property shall be included in the property factor if it is actually used or is available for or capable of being used during the tax period in the regular course of the trade or business of the taxpayer. Property held as reserves or standby facilities or property held as a reserve source of materials shall be included in the factor. For example, a plant temporarily idle or raw material reserves not currently being processed are includable in the factor.
(2) Property or equipment under construction during the tax period (except inventoriable goods in process) shall be excluded from the factor until such property is actually used in the regular course of the trade or business of the taxpayer. If the property is partially used in the regular course of the trade or business of the taxpayer while under construction, the value of the property to the extent used shall be included in the property factor.
(3) Property used in the regular course of the trade or business of the taxpayer shall remain in the property factor until its permanent withdrawal is established by an identifiable event such as its conversion to the production of nonapportionable income, its sale, or the lapse of an extended period of time (normally five years) during which the property is held for sale.