A good rule of thumb to determine required Texas trailer marker lights is this: the longer and wider the trailer, the more lights and reflectors mandated by the Texas Transportation Code. The most common types of trailers are general purpose, wide, long, eighteen wheeler, pole, boat, and farm.
A general-purpose trailer is less than 80 inches wide and less than 30 feet long. There are no lighting requirements for the front or sides. The rear of the trailer needs at least one white license plate light at all times, one red reflector on each side, one red stop lamp on each side, electric turn signal lamps and tail lamps.
A wide trailer is one at least 80 inches wide and less than 30 feet long. It requires more illumination than shorter, thinner trailers. In addition to the lighting requirements for general-purpose trailers, a wide trailer needs two amber reflectors, one on each side near the front; two red reflectors near the rear, one on each side; hazard lamps; front and rear clearance lamps; and front and rear side marker lamps.
A trailer at least 30 feet long should have most of the lighting required of a wide trailer, with some exceptions. It does not need two amber reflectors near the front. According to the Texas Department of Public Safety, it does need one amber reflector on each side, "centrally located on body of trailer." It also does not need the front and rear clearance lamps and side marker lamps required for the wide trailer.
A trailer wider than 80 inches and longer than 30 feet is generally identified as an eighteen-wheeler trailer. This trailer has the most lighting and reflector requirements. In addition to all the rear lighting requirements for a trailer more than 80 inches wide, the front should have two amber clearance lamps, and the rear should have two red clearance lamps. Lighting across the top must include a red side marker lamp and amber side marker lamps in the middle and rear. The lower side edge of the trailer requires a red reflector on the front and amber reflectors along the bottom, middle and rear.
A pole trailer requires special lighting, since it is a specialized, thin trailer. Each side needs an amber side marker lamp at the top front and an amber reflector at the bottom front. One combination lamp should be placed on each side of the trailer, positioned roughly at the same height as the bottom front reflector, above the rear axle and between the wheels. The combination lamp should emit amber light to the front, red to the side and red to the rear. The back of the trailer must have one white license plate lamp, along with red tail lights, stop and turn lamps on each side, and two red reflectors, one on each side.
Boat trailer requirements include the same 80 inches and 30 feet breakpoints as general-purpose trailers but additional gross weight qualifiers apply. A boat trailer less than 80 inches wide and shorter than 30 feet with a gross weight of 3,000 pounds or less operated during daylight needs two red reflectors on the rear, one on each side. If the trailer is longer than 30 feet, add one amber reflector on each side, centrally located on the trailer body.
Farm and fertilizer trailers less than 80 inches wide have the same requirements as boat trailers during daylight. Farm and fertilizer trailers wider than 80 inches and less than 30 feet long need two amber reflectors, one on each side near the front; two red reflectors, one on each side near the rear; and two red reflectors on the rear, one on each side. If the trailer is wider than 80 inches and longer than 30 feet, statutes require one additional amber reflector on each side, centrally located on the trailer body.