California Code, Public Utilities Code - PUC § 8029

No person shall run, place, erect, or maintain any wire or cable which conducts at any one time more than 600 volts of electricity, without causing each crossarm, or such other appliance as is used in lieu thereof, to which such wire or cable is attached to be kept at all times painted a bright yellow color, or, on such crossarm, or other appliance used in lieu thereof, shall be placed signs, providing, in white letters on a green background, not less than three (3) inches in height the words “high voltage” on the face and back of each crossarm.  The provisions of this section do not apply to crossarms to which are attached wires or cables conducting more than 10,000 volts of electricity, and which are situated outside the corporate limits of a city.