Cal. Public Utilities Code § 8028

No person shall run, place, erect, or maintain, above ground, within four feet from any wire or cable conducting less than 600 volts of electricity, any wire or cable which conducts at any one time more than 600 volts of electricity, or run, place, erect, or maintain within four feet from any wire or cable which conducts at any one time more than 600 volts of electricity any wire or cable conducting or carrying less than 600 volts of electricity. This section does not apply to wires or cables attached to a transformer, arc or incandescent lamp within four feet (measured along the line of the wire or cable) from the point where such wire or cable is attached to such transformer, arc or incandescent lamp, nor to wires or cables within buildings or other structures, nor to wires or cables where they are placed vertically on poles, or to any lead wires or cables between the points where they are made to leave any pole for the purpose of entering any building or other structure and the point at which they are made to enter such building or structure. As between any two wires or cables, or any wire or any cable, run, placed, erected, or maintained in violation of this section, only the wire or cable last in point of time run, placed, or erected shall be held to be run, placed, erected, or maintained thus in violation of this section. Where no more than one crossarm is maintained on a pole, all the wires or cables conducting at any one time more than 600 volts of electricity shall be placed on the crossarm on one side of the pole, and all the wires or cables conducting less than 600 volts of electricity shall be placed on the crossarm on the other side of the pole. The space between any wire or cable conducting at any one time more than 600 volts of electricity and any wire or cable conducting less than that voltage shall be at least 36 inches clear measurement in a horizontal line. Where the provisions of this section cannot be complied with, the commission may grant permission for the following form of construction; where two or more systems for the distribution of electric light or power occupy the same poles with wires or cables, all wires or cables conducting at any one time more than 600 volts of electricity may be placed on the crossarms on one side of the pole, and all wires or cables conducting less than that voltage, shall in such case, be placed on the crossarms on the other side of the pole; the space between any wire or cable conducting at any one time more than 600 volts of electricity and any wire or cable conducting or carrying less than that voltage shall be at least 36 inches in measurement in a horizontal line; and in such construction all crossarms shall be at least 36 inches apart in a vertical line.