California Code, Vehicle Code - VEH § 36600

(a) The limitations as to width as set forth in Chapter 2 (commencing with Section 35100) of Division 15 do not apply to implements of husbandry incidentally operated, transported, towed, or otherwise moved over a highway.

(b) Notwithstanding subdivision (a), when an implement of husbandry is transported or moved over a highway which is a part of the National System of Interstate and Defense Highways (as referred to in Section 108 of the Federal-aid Highway Act of 1956)  1 as a load on another vehicle, if the load exceeds 102 inches in width, the vehicle and load shall not be operated for a distance in excess of 25 miles from the point of origin of the trip.  The operator of the transporting vehicle shall be a farmer or a person regularly employed by a farmer or farm corporation, and the operator transporting the load shall have in his or her immediate possession a writing signed by the farmer or farm corporation agent which states the origin and destination of the trip.

(c) Notwithstanding subdivision (a), when an implement of husbandry is transported or moved over any other highway as a load on another vehicle, if the load exceeds 120 inches in width, the vehicle and load shall not be operated for a distance in excess of 25 miles from the point of origin of the trip.  The operator of the transporting vehicle shall be a farmer or a person employed by a farmer or farm corporation, and the operator transporting the load shall have in his or her immediate possession a writing signed by the farmer or farm corporation agent which states the origin and destination of the trip.