The tax authorized to be levied under § 1-230 shall be levied at the time this article shall take effect and shall apply to all persons, firms, associations or corporations engaged in the business of furnishing power, light, heat, electricity, water or both natural and liquefied petroleum gas in the city; it shall not apply to any person, firm, association or corporation operating under a valid franchise from the city and said exception shall be so stated in said valid franchise.