California Code, Food and Agricultural Code (Formerly Agricultural Code) - FAC § 17121

A lawful fence is any fence which is good, strong, substantial, and sufficient to prevent the ingress and egress of livestock.  No wire fence is a good and substantial fence within the meaning of this article unless it has three tightly stretched barbed wires securely fastened to posts of reasonable strength, firmly set in the ground not more than one rod apart, one of which wires shall be at least four feet above the surface of the ground.  Any kind of wire or other fence of height, strength and capacity equal to or greater than the wire fence herein described is a good and substantial fence within the meaning of this article.  The term “lawful fence” includes cattle guards of such width, depth, rail spacing, and construction as will effectively turn livestock.