Cal. Pub. Util. Code § 762
Whenever the commission, after a hearing, finds that additions, extensions, repairs, or improvements to, or changes in, the existing plant, equipment, apparatus, facilities, or other physical property of any public utility or of any two or more public utilities ought reasonably to be made, or that new structures should be erected, to promote the security or convenience of its employees or the public, or in any other way to secure adequate service or facilities, the commission shall make and serve an order directing that such additions, extensions, repairs, improvements, or changes be made or such structures be erected in the manner and within the time specified in the order. If the commission orders the erection of a new structure, it may also fix the site thereof. If the order requires joint action by two or more public utilities, the commission shall so notify them and shall fix a reasonable time within which they may agree upon the portion or division of the cost which each shall bear. If at the expiration of such time the public utilities fail to file with the commission a statement that an agreement has been made for a division or apportionment of the cost, the commission may, after further hearing, make an order fixing the proportion of such cost to be borne by each public utility and the manner in which payment shall be made or secured.
Ca. Pub. Util. Code § 762