top of page

Federal Register for 49 CFR Part 393
Title 49 - Transportation

"Volume: 5Date: 2011-10-01Original Date: 2011-10-01Title: PART 393 - PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATIONContext: Title 49 - Transportation. Subtitle B - Other Regulations Relating to Transportation (Continued). CHAPTER III - FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION. SUBCHAPTER B - FEDERAL MOTOR CARRIER SAFETY REGULATIONS."

§ 393.28 Wiring systems. Electrical wiring shall be installed and maintained to conform to SAE J1292—Automobile, Truck, Truck-Tractor, Trailer, and Motor Coach Wiring, October 1981, except the jumper cable plug and receptacle need not conform to SAE J560. The reference to SAE J1292 shall not be construed to require circuit protection on trailers. (See §393.7(b) for information on the incorporation by reference and availability of this document.) [70 FR 48047, Aug. 15, 2005]

Part 393

Below are the available interpretations for the given section. To return to the list of parts, use the Parts link above. The menu to the left provides a full list of sections that have interpretations. To view interpretations for a different section, click on the menu item.  The regulations text of the section can be found on the eCFR website. To view the regulations text, use the link below. For assistance, please send an email to


(Added ruling question)
Guidance for § 393.28: Wiring systems.

Question 1: Does a frame channel of a Commercial Motor Vehicle (CMV) constitute a protective "sheath or tube" as specified in §393.28?

Guidance: No. To be acceptable, a sheath or tube must enclose the wires throughout their circumference. In the absence of a sheath or tube, the group of wires must be protected by nonconductive tape, braid, or other covering capable of withstanding severe abrasion.

3.3.1 Select cable insulation in accordance with the vehicle's working environ-

ment. Consideration is given to physical and environmental factors such as flex-

ing, heat, cold, bend, oil and fuel contact, dielectric, abrasion, short circuit, and

pinch resistance among others. 

3.4.5 Circuit Grounding — Ground terminal lugs shall be solder dipped, cad-

mium, tin, or zinc plated. Ground terminals shall be accessible for service: A ser-

rated paint cutting terminal may be utilized to make proper contact on painted

surfaces. Ground terminal devices shall be eadmium,-.tin^ or zinc plated. In spe-

cial cases, plating may not be required for lugs and/or attaching devices. 

3.8.2 Wiring harness covering shall be adequate to protect the harness in the

vehicle routing environment and shall furnish protection during all phase's of Vehi-

cle assembly and operation.

3.9.2 In general, wire routing shall be such that maximum protection is pro-

vided by the vehicle sheet metal arid structural components. Smooth /protective

channels especially designed for wiring and built into the veliicl& bbdy structure

should be used when practicable. Avoid areas of excessive heat, vibration, arid


3.9.4 If significant vibration levels exist, the edges of all metal members

through which cables and harnesses pass shall be deburred, flanged, rolled, or

bushed with suitable grommets. Suitable tubing or conduit over cables may be

substituted for grommets if properly secured. Clips for retaining cables and har-

nesses shall be securely attached to body or frame member and cable or harness.

Clips also assist in locating and routing at assembly. 

3.9.8 Electrical apparatus with integral wiring shall be supplied with grommets

or other suitable mechanical fasteners for strain relief.

SAE J1292: Automobile, Truck,
Truck-Tractor, Trailer, and Motor Coach
Wiring Sections which apply

Compilation of Existing State Truck Size and Weight Limit Laws Report to Congress

May 2015
Exhibit 4: Common Vehicle Configurations in the U.S. Commercial Truck Fleet.

Single-Unit (SU) Trucks – Also referred to as straight trucks, this class of vehicles includes all trucks on a single frame with two or more axles and dual rear wheels. These vehicles are allowed in all States, but the weight allowed depends on axle limits and axle spacing. All States must allow 20,000 per axle and 34,000 lbs. on a tandem axle on the Interstate Highway System. Due to grandfather provisions, 16 States allow higher axle limits on the Interstate; several more allow higher axle limits on non-Interstate highways through State statute. Many States provide exemptions for public vehicles like fire trucks, snow plows, and others.


Combination Trucks – These are commercial vehicles made up of two or more units, most commonly a tractor and a semitrailer. Double trailer combinations consisting of a tractor, a semitrailer, and a trailer are also common. The so-called STAA Double consists of a tractor and two trailers neither of which exceeds 28 feet in length. All of the vehicles in this group are allowed to travel the Interstate Highway System nationwide up to 80,000 lbs. The number of States that allow higher axle loads on Interstate and non-Interstate highways apply to combination vehicles as well.


Longer Combination Vehicles – A subgroup of combination trucks, LCVs are double and triple trailer combinations that can exceed 80,000 lbs. on Interstate Highways in 23 States. All vehicles have seven or more axles consisting of three or more units, one of which is a tractor or a straight truck power unit. Three common LCVs are in operation across the U.S:

  • Rocky Mountain Doubles – a tractor with two trailers, a long front trailer (usually 48 ft.) followed by a shorter second trailer

  • Turnpike Doubles – a tractor and two long trailers

Triples – a tractor and three short trailers

bottom of page