Mo. Rev. Stat. § 64.190
Said commission or such members thereof as are designated under the regulations adopted by the county commission shall be authorized to examine all applicants for a license to engage in electrical wiring or installation work and shall have authority under said regulations to revoke or suspend any license issued for refusal or failure to comply with the regulations adopted, and any person, firm or corporation licensed under the provisions of sections 64.170 to 64.200 shall be authorized to make electrical installations in any municipality in the county and be subject to the inspection requirements contained in the regulations adopted hereunder and shall not be licensed by any city, town or village in said county; provided, that the provisions of sections 64.170 to 64.200 shall not apply to any city having or that may hereafter have a population of more than one hundred and twenty thousand inhabitants.
§ 64.190, RSMo