Trailer Components

VIN Deciphering

The Vehicle Identification Number (VIN) is a unique number given to each trailer by the manufacturer that not only identifies the trailer itself, but identifies certain features such as model year or manufactured location. The VIN can be found on the Manufacturers Certificate of Origin (MCO) or on the VIN label on the trailer itself. The VIN label should be placed on the trailer on the forward half, left side (road side) and be easily visible.

Understanding the Vehicle Identification Number (VIN):

A trailer’s VIN contains helpful identifying information about the unit and its manufacturer. VIN number configuration is governed by federal regulation and must conform to the following;

VIN Deciphering Schemes and World Manufacturer Identifier Lookup

To more easily understand the contents of your specific VIN you can use the manufacturers VIN deciphering scheme. These schemes are registered, approved, and hosted on NHTSA’s website at https://vpic.nhtsa.dot.gov/mid/. Follow these steps to view your trailers VIN deciphering scheme:

Under Organization -> Name: enter the name of the trailer manufacturer
Select the following checkboxes: Part 565, Part 566, & World Manufacturer Identifier (WMI).
Click the blue search box in the lower left hand corner of the screen.
Sort the results by Letter Date by clicking the column twice. This will place the most recent document at the top.
Select the hyperlink in the Org Name column to view the most recent VIN deciphering scheme.

If you do not know the manufacturer or do not have success with the above process you may try altering your search terms such as only using one word of the manufacturer name as opposed to the entire phrase or by searching their home state. You may also search by the World Manufacturer Identifier (WMI). To do so, follow these steps:

Under World Manufacturer Identifier (WMI) -> WMI: enter the WMI from your VIN label. This will be the first three characters from the VIN number.
Select the following checkboxes: Part 565, Part 566, & World Manufacturer Identifier(WMI).
Sort the results by Letter Date by clicking the column twice. This will place the most recent document at the top.
Select the hyperlink in the Org Name column to view the most recent VIN deciphering scheme.

If Your Trailer is Missing a VIN Label

If your VIN label is missing, your MCO/MSO should include this information. Without documentation, you may be able to locate a hidden VIN somewhere on the trailer. The easiest way to do so is to contact the manufacturer to find if there is a hidden VIN and if so, where it is located. If you do not know the manufacturer you may be able to locate one yourself though this may prove difficult as they are usually located in out of the way places such as the underside of the frame.

If the manufacturer does not apply a hidden VIN, you are unable to locate one and cannot obtain one from the manufacturer, or you cannot determine who the manufacturer is, you will then need to contact your state’s Department of Motor Vehicles or relevant office for direction on how you may be able to proceed. Each state has its own regulations governing how to claim lost, stolen, or otherwise unidentifiable trailers.

Brakes

Whether or not your trailer requires brakes is a matter of both federal and state law. Federal law, in short, requires brakes on all axles of the trailer when it is over 3,000 lbs. GVWR and is used for commercial purposes. State law dictates brake requirements beyond what is outlined by federal law, including non-commercial use.

Federal Brake Law:

Federal Law requires brakes be placed on axles of trailers used for a commercial purpose (with notable exceptions found below). However, understanding what a commercial use is, can be confusing.

Per 49 C.F.R. § 390.5, a commercial motor vehicle means, as pertinent to trailers, “any …towed motor vehicle used on a highway in interstate commerce to transport … property when the vehicle -

Has a gross vehicle weight rating or gross combination weight rating, or gross vehicle weight or gross combination weight, of 4,536 kg (10,001 pounds) or more, whichever is greater; or
Is used in transporting material found by the Secretary of Transportation to be hazardous under 49 U.S.C. 5103 and transported in a quantity requiring placarding under regulations prescribed by the Secretary under 49 CFR, subtitle B, chapter I, subchapter C.”

Of particular relevance for trailers in the definition is the requirement that the GVWR or GCWR be 10,001 lbs. or more. This means that in order to come within the definition of a commercial motor vehicle subject to the regulations of FMCSA, either the trailer by itself or the combined GVWR of the trailer and tow vehicle combination must be 10,001 lbs. or more.

Examples of Commercial Purposes:

FMCSA does not provide an express definition of “commercial” or commercial use; however, examples of trailers that could be considered for commercial use are those used for compensation or earning profits, including, but not limited to, uses such as transporting freight, operating landscaping and construction businesses, and populating city and county trailer fleets. Another common, but often overlooked potentially commercial use is transporting show horses or livestock when prize money, a form of compensation, is at stake.

Any commercial trailer meeting the above definition must comply with 49 C.F.R. § 393.42, requiring brakes on all wheels as specified below.

Language and Exceptions:

49 C.F.R. § 393.42 states:

A. “Every commercial motor vehicle shall be equipped with brakes acting on all wheels. This requirement also applies to certain motor vehicles being towed in a driveaway-towaway operation, as follows:

Any motor vehicle towed by means of a tow-bar when another motor vehicle is full-mounted on the towed vehicle; and
Any saddlemount configuration with a fullmount.”

B. “Exception.

Trucks or truck tractors having three or more axles and manufactured before July 25, 1980, are not required to have brakes on the front wheels. However, these vehicles must meet the requirements of § 393.52.
Motor vehicles being towed in a driveaway-towaway operation (including the last truck of triple saddle-mount combinations (see § 393.71(a)(3)) are not required to have operative brakes provided the combination of vehicles meets the requirements of § 393.52.
Any semitrailer or pole trailer (laden or unladen) with a gross weight of 1,361 kg (3,000 pounds) or less which is subject to this part is not required to be equipped with brakes if the axle weight of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.
Any full trailer or four-wheel pole trailer (laden or unladen) with a gross weight of 1,361 kg (3,000 pounds) or less which is subject to this part is not required to be equipped with brakes if the sum of the axle weights of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.
Brakes are not required on the steering axle of a three-axle dolly which is steered by a co-driver.
Loaded housemoving dollies, specialized trailers and dollies used to transport industrial furnaces, reactors, and similar motor vehicles are not required to be equipped with brakes, provided the speed at which the combination of vehicles will be operated does not exceed 32 km/hour (20 mph) and brakes on the combination of vehicles are capable of stopping the combination within 12.2 meters (40 feet) from the speed at which the vehicle is being operated or 32 km/hour (20 mph), whichever is less.”

If 49 C.F.R. § 393.42 requires brakes on commercial-use trailers, then 49 C.F.R. § 393.40 also requires the brakes to be “adequate to stop and hold the vehicle or combination of motor vehicles” and the vehicle to “meet the applicable service, parking, and emergency brake system requirements provided in [§ 393.40].”

State Brake Laws:

Brake laws vary greatly by state. To find an up-to-date list of laws by state visit https://drivinglaws.aaa.com/tag/trailer-brakes/. It is important to note that if you purchase a trailer from another state in which the trailer will be registered and/or primarily used, you will want to ensure the trailer meets the appropriate laws of the state in which it will be registered or primarily used.

Brake Types:

Trailer brakes can be hydraulic (surge), electric, or both. All trailer brake systems must incorporate a trailer breakaway brake system that activates when the brakes detach. Just as your brakes require regular maintenance, so too does your breakaway system.

Electric Brakes:

Electric brake systems require the use of an electric brake controller in the cab of the tow vehicle. In short, when the tow vehicle brake pedal is engaged, the electric brake controller engages the trailer brakes.

It is important to note that electric brake controllers provide a modulation function that varies the current to the electric brakes in proportion to the deceleration of the tow vehicle. This is important because the braking of the tow vehicle and the trailer needs to be synchronized to ensure proper distribution. Synchronization is accomplished by adjusting the “gain” on the tow vehicle controller. With the tow vehicle hooked up to the trailer, make several hard stops on dry pavement. If the trailer’s brakes skid or are too sensitive, decrease the gain on the brake controller. If they do not skid, slightly increase the gain setting until they have reached a desired braking point. This adjustment will need to be made with each varied trailer load. A heavier trailer will require more gain than a lighter trailer. A lighter trailer with brakes synchronized to a fully loaded trailer, will probably lock-up easily with only minimally commanded deceleration.

Electric brakes are very efficient, reliable and can be controlled by the operator from inside the tow vehicle. The draw back is the tow vehicle must have a “brake controller” installed. If the tow vehicle is not equipped with a controller unit, the vehicle cannot actuate the brakes and should not be used to tow a trailer with electric brakes.

Hydraulic Brakes:

Hydraulic or surge brakes function through the use of a special coupler on the trailer that compresses a piston in a hydraulic cylinder when the tow vehicle decelerates and the trailer surges forward. When this occurs, brake fluid is then forced to the brake assembly on each wheel. Because hydraulic brakes are entirely self-contained, any tow vehicle can use a trailer with a hydraulic brake assembly.

It is important to note that some states and the Federal Motor Carriers Safety Administration have placed limitations on the use of surge brakes for commercial vehicles. These limitations surround the GVWR of the trailer as it compares to the GVWR of the tow vehicle.

Electric-Over-Hydraulic Brakes:

Electric-over-hydraulic brakes are a hybrid system that actuates when the brakes of the tow vehicle are engaged and the trailer surges forward. The key difference is the actuator in this combination is electric, providing little to no lag time due to an instant application. This system is used most often on trailers like large horse trailers and boat trailers. It is important to note that the system cannot be submerged (though they are built to withstand rain) so users will need to be careful if using these for hauling boats.

Electrical Breakaway Brake System Batteries

Per Part 393.43 (d) of the Federal Motor Carrier Safety Regulations: Breakaway braking requirements for trailers states: “Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle. The brakes must remain in the applied position for at least 15 minutes.” The last statement is found in most state’s trailering statutes. Therefore, regardless of the application (i.e. commercial or personal use), the breakaway battery should be able to keep the electric brakes applied for 15 minutes.

Trailer Tire Information

Tires are vital to the proper performance of the trailer. Tires that have inadequate load capacity, are under-inflated or over-loaded, can cause loss of vehicle stability and/or loss of control. Additional causes for reduced trailer sway stability are overloading the trailer tires and/or inadequate air pressure. Before towing your trailer, make sure the tires are properly inflated to the max sidewall inflation pressure, and the gross weight (trailer & cargo) is within the trailer tires capacity.

Tires used on trailers are also subjected to additional adverse conditions because many trailers sit out of service for long periods of time. It is actually better for the tire to be rolling down the road than to be idle. During use, the tire releases lubricants that are beneficial to tire life. Using the trailer tires often also helps prevent flat spots from developing.

Further, when trailers are in service, they are often used at their maximum loads. These conditions require diligent tire inspection, inflation checks and maintenance. Each trailer will clearly display the appropriate tire size, wheel size and inflation pressure on the VIN/Certification label, which is mounted on the left front of the trailer body or tongue. Your trailer tires should be the same type, size and construction. A similar label is on the driver’s door pillar of the tow vehicle. The most important factors in tire care are maintaining proper inflation pressure, avoiding excessive loading, avoiding roadway hazards, driving at the appropriate speeds and routine inspection.

It is also very important that lug nut torque be routinely checked. This is especially true when you first pick up a new trailer, or after a tire change. Proper torquing is vital for a correct wheel functioning. Check your owner’s manual for the required torque specifications or contact the manufacturer.

Tire Inflation

The main cause of tire failure is improper inflation. Check the cold tire inflation pressures at least once a week for proper inflation levels. “Cold” means that the tires are at the same temperature as the surrounding air, such as when the vehicle has been parked overnight. Wheel and tire manufacturers recommend adjusting the air pressure to the trailer manufacturer’s recommended cold inflation pressure, in pounds per square inch (PSI) stated on the vehicle’s Federal Certification Label or Tire Placard when the trailer is loaded to its gross vehicle weight rating (GVWR). Refer to the owner’s manual or talk to your dealer or vehicle manufacturer if you have any questions regarding proper inflation practices.

If the tires are inflated to less than the recommended inflation level the load carrying capacity of the tire could be dramatically affected. If the tires are inflated more than the recommended inflation level, handling characteristics of the tow vehicle/trailer combination could be affected.

Tires can lose air over a period of time. In fact, tires can lose 1 to 3 PSI per month. This is because molecules of air, under pressure, weave their way from the inside of the tire, through the rubber, to the outside. A drop in tire pressure could cause the tire to become overloaded, leading to excessive heat build up. If a trailer tire is under-inflated, even for a short period of time, the tire could suffer internal damage.

High speed towing in hot conditions degrades trailer tires significantly. As heat builds up during driving, the tire’s internal structure starts to breakdown, compromising the strength of the tire. It is recommended to drive at moderate speeds.

Statistics indicate the average life of a trailer tire is about five years under normal use and maintenance conditions. After three years, replacing the trailer tires with new ones should be considered, even if the tires have adequate tread depth. Some experts claim that after five years, trailer tires are considered worn out and should be replaced, even if they have had minimal or no use. This is such a general statement that it may not apply in all cases. It is best to have your tires inspected by a tire supplier to determine if your tires need to be replaced.

If you are storing your trailer for an extended period, make sure the tires are fully inflated to the maximum rated pressure and that you store them in a cool, dry place, such as a garage. Use tire covers to protect the trailer tires from the harsh effects of the sun.

Speed Rating of ST Tires

Special trailer tires have a maximum speed rating, just like passenger car and truck tires. Understanding all the writing on the sidewall of a tire can be somewhat overwhelming at first glance. It helps to know what characters you are searching for in order to obtain the tire information you desire. For example, if the tire code was 225/45R17 75L, then 75 would signify the load index and L would signify the speed rating of the tire.

Speed Rating

Load Index

The load index tells you how many pounds a tire can safely carry. As you can see on the load index chart below, the 75 in the example above has been assigned a load carrying capacity of 853 lbs per tire. Multiply the load carrying capacity by the number of the tires on the trailer. As long as that sum is greater than or equal to the assigned GVWR of the trailer, then the tire’s load index is sufficient for its application.

Speed ratings are based on laboratory tests and were established to match the speed capability of tires. In the same tire code example stated previously (225/45R17 75L), L would signify the tire speed rating. According to the attached chart, the speed rating for L is up to 75 mph. It has been proven under testing conditions, that tire can operate for a certain period of time up to 75 mph.

Motor vehicles should always be operated according to the speed limit of the roads they travel on. Never exceed the speed rating of the tow vehicle tires or the trailer tires; drive according to whichever rating is lower. Trailer tires commonly have tire speed ratings in the 65 – 75 mph range.

Unless otherwise noted, the speed rating for trailer tires is 65 mph.

For more information, visit https://www.etrailer.com/faq-trailer-tire-frequently-asked-questions.aspx.

Types and Sizes

Some tires are specially designed to be used on trailers. These tires include the letters "ST" in the size specification that is listed on the sidewall. The "ST" stands for "special trailer". This kind of tire has a stronger sidewall than an automobile or truck tire so it can handle higher air pressures and heavier loads.

ST tires can be found in various sizes. Generally, the size of the tire size is directed correlated to its capacity - i.e. a 16” tire will have greater carrying capacity than a 12” tire. Larger tires generally provide more sway stability. However, remember that trailer fenders are designed for particular tire sizes. So, before you increase the tire size, ensure you have adequate space under your fenders.

There are primarily two types of tires used for trailers: radial and bias. The main difference between a radial ply and a bias ply tire is how the tire is constructed. The cords inside a bias ply tire run at a 32 degree angle to the direction of travel, and the cords on a radial tire run at a 90 degree angle to the direction of travel, or across the tire from wheel lip to wheel lip.

Radial tires, known for their higher cornering coefficients, tend to reduce trailer sway. A radial tire flexes more than a bias tire, giving it better ground contact, traction, stability, and tread wear. A radial tire will normally run cooler than a bias ply tire, especially when the tire is under a load. A tire that runs cooler will last longer.

A bias tire has a stiffer sidewall than a radial, a feature that might be useful for off road applications like farm use. Bias tires have lower cornering coefficients, which tends to decrease sway stability of the trailer while towing at highway speeds. However, many other factors can play a role in how stable the trailer tows. Besides stiffer sidewalls, tire size and capacity affect performance.

For more information on the difference between radial and bias tires, click here.

NOTE: You should always use tires of the same size, load range, and construction on a trailer. If you use tires that don't match, you could experience problems of overloading and overheating, which can lead to tire failure.

Jack Stands

Jacks are primarily used to lift and stabilize the trailer tongue, or front corners; for lifting the trailer to connect and disconnect from the tow vehicle. They are also used to keep the trailer level while not attached to the tow vehicle as well as stabilizing the rear of the trailer when loading and unloading. There are many types and styles of jack stands with as many different names or terms. They also come in a wide range of sizes and lifting capabilities.

Capacity and Length

Jacks can be found with lifting capacities as low as 500 to 1,000 pounds. The most common sizes range from under 1,000 pounds, to 2,000, 5,000, 7,000 and 10,000 pound lifting capacities. While the necessary capacity of the jack stand has been argued differently by different resources, the bottom line is the jack stand needs be of sufficient size and capacity to lift and stabilize the trailer.

As a general rule of thumb, “an adequate size for standard duty trailers is a jack capacity between 25% and 50% of the trailer’s GVWR. This capacity should provide a sufficient safety margin since a fully loaded bumper pull trailer should typically have no more than 10% to 15% of the gross trailer weight on the tongue, or jack.” It is important to note that there are many variables in load size and location on the trailer, so this rule of thumb may not apply to your application.

You should also note that jacks come in different lengths. The length of the jack in both its retracted and extended positions is important. It will need to retract enough to adequately clear the ground when traveling - the more it retracts the better - and high enough to clear the coupler. As a general rule you want to have about 4 inches of clearance about the coupler.

Type of Jacks

Tongue Jacks: Tongue jacks are used to stabilize the trailer tongue or front corners of a trailer both while lifting the trailer during hitching or unhitching, but also to stabilize the trailer during its non-use.

A-frame Trailer Jacks: A-frame jacks are mounted on the front of the trailer as far forward as practicable, placing it as close as possible to the center of the width of the trailer. They either bolt on or are welded onto an A-Frame coupler, or sit in a brace behind the coupler. By centering the jack on the width of the trailer it adds stability to the trailer when it is detached from the tow vehicle. The draw-back to this style jack is the low ground clearance when it is retracted.

Swivel Jacks: Swivel or side mount trailer jacks also mount on the tongue of the trailer. They are usually attached on the driver’s side of a pole-tongue or A-Frame trailer, again as far forward as practical. When not in use, a pin is pulled and the jack swivels or swings up and snaps into place for storage parallel with the tongue. This type of jack has a bracket which mounts to the trailer and the jack mounts to the bracket. It is important to note that over time the pin or ring around with the jack pivots can loosen or wear over time, making the jack, and thus trailer, less table.

Jockey Wheels: A jockey wheel can be mounted on some tongue jacks and are used to help move the trailer after it has been detached from the tow vehicle. Jacks equipped with wheels generally have light duty capacities, under 2,000 lbs. Most tongue jacks without wheels normally have capacities up to 5,000 pounds. For heavier capacities drop-leg or landing-leg jacks are more appropriate.

Drop-leg Jacks: Drop-leg jacks are used with high-use or heavy duty applications. This style jack generally starts at 5,000 pounds and include 7,000, 10,000 and 12,000 pound lifting capacities. It is a rigid jack normally welded to the front bed of the trailer or a bracket in the tongue of the trailer. Unlike other jacks, the drop-leg jack feature allows for fast leveling. This jack features an inner tube, or drop-leg, that slides through a larger outer tube. Several holes are drilled through both tubes and held by a pin to position the leg in the raised or lowered position. Simply drop the inner tube to the ground and place the pin. The handle is cranked to engage the lifting or retracting action of the jack.

Note: The American Society of Agricultural Engineers published, “Implement mounted, Screw-type Jacks”, as ASAE S485, provides performance standards for tongue jacks. This certification should help customer’s confidently select an appropriated rated jack.

Stabilizer Jacks: Stabilizer jacks are found mostly on trailers that load equipment or vehicles. They are usually positioned on the rear corners of the trailer to keep the trailer from dropping too much while raising the front of the trailer. When heavy equipment is loaded on the rear of a trailer, it tends to lift the front resulting in the tow vehicles rear-end being unweighted, possibly even enough to lift the rear tires off the ground. If this occurs when parked on a sufficient downgrade, then both the truck and trailer may move forward, causing an all-round unpleasant situation or possibly even an accident.

There are a number of trailer stabilizing jacks available. More information on the options for your application can be found at https://www.etrailer.com/faq-trailer-jacks.aspx.

Safety Chains

The purpose of safety chains is to keep the towed vehicle attached to the towing vehicle in case of detachment. Safety chains are the first line of defense in this instance. State law varies with regard to the number of required safety chains or attachments. However, their use is recommended. (More information on state safety chain laws can be found at https://drivinglaws.aaa.com/tag/trailer-hitch-signals/.) Each safety chain on its own must be adequate to sustain the fully loaded weight of the trailer, it’s GVWR.The hooks on the end of the chains and the attachment of the chains to the trailer must also be rated equal to or greater than the GVWR of the trailer.

It is not recommended that safety chains be welded to the trailer. Chains, especially high strength chains, are not designed to be welded. The welding process could cause a molecular change in the chain material and possibly make the chain brittle and not as strong. Safety chains can be attached with a bolt or a loop of heavier rolled steel bar. Hot or cold rolled steel bar can be looped through the chain then welded to the trailer. This rolled steel bar with a good weld can match the strength of the chain. If you bolt-on the chains, ensure the strength of the nut and bolt are rated adequately for the chains. When bolting on safety chains you may need to reinforce the trailer tongue with washers or plate steel so the fasteners do not pull through.

The safety chains need to be long enough so it can be attached to a frame or hitch member of the towing vehicle and back to itself, should allow for turning, but not so long that it touches the ground. The chains should be crossed under the coupler and hitch ball to prevent the tongue dropping to the ground if the trailer comes loose.

Fenders

When discussing fenders and tires it is important to know the legal maximum width allowed on the roads you will be traveling. All federal highways and interstates allow a maximum vehicle width of 102” without having to obtain special “over-width” permits. While many State, County, and local road restrictions follow the federal limits, many do not. The most common limit on non-federal roads is 96” maximum width.

Once you know the legal requirement for the roads you are traveling then you need to verify your trailers width. The widest point determines your trailers maximum width, which may be the fenders or the tires. If your tires extend beyond your fenders then you must measure to the tires widest point (excluding the bottom bulge were the tire is compressed to the road from the trailer weight).

Trailer Electrical System

The electrical system on your trailer primarily consists of lights, wiring and connectors (including plugs). Beyond the lights, there may be wiring to connect electric brake assemblies to the tow vehicle, as well as possible auxiliary wiring for various 12-volt accessories. Twelve-volt accessories run off the tow vehicles battery system. Examples of 12-volt accessories may include inside camper lights, electric winches, electric jacks or the electric motor to a hydraulic hoist for dump trailers. At a minimum, if the trailer has electric brakes, an additional 12-volt wire is needed to charge the safety break-away brake battery for the electric brakes.

Lights

The purpose of lights and reflectors on trailers is the same as a tow vehicle, to notify other roadway uses of the trailer’s presence and intentions, i.e. braking and turning. Trailer lights consist of stop, turn and taillights that may be in one housing unit or installed separately. Other trailer lights include clearance lights and side marker lights, both of which may be yellow or red, depending on their location on the trailer. The Department of Transportation has specific lighting requirements manufacturers must follow.

To see the federal lighting requirement, go to their website at: www.nhtsa.dot.gov/cars/rules/standards/conspicuity/Trlrpstr.html

The cost of adding the correct lighting after the purchase of a trailer is significantly more than if the trailer had the correct lighting in the first place. You can find out how the National Association of Trailer Manufacturers is helping consumers more confidently make a purchase here.

Wiring

Lights must be grounded. Most lights are grounded through the housing mounting hardware to the trailer frame. If not, there will be a ground wire to connect to the trailer frame. Clearance and side markers are on the same circuit as the taillights. Wire colors for trailer lights are standard in the industry.

Brown Tail, marker and clearance

Yellow Left Turn

Green Right Turn

White Ground

Other wires in the electrical system include brakes and various accessories. These wire colors are not as standardized, but in general are:

Blue Electric brake system, interior lights

Red 12-volt auxiliary, breakaway safety system

Black 12-volt auxiliary, occasionally ground

Wire size is directly related to the proper function of the electrical system. Generally, for stop, tail and turn lights, a 16 or 18 gauge, 12-volt electrical wire is adequate. For electric brakes, which draw much more current, 12 to 14 gauge is more appropriate. For the ground wire, 14 to 18 gauge is fine. The auxiliary wires should meet the standard for the accessory they are powering. See your owner’s manual for specifications. Ensure the wire you use is not too small, as it may overheat, short out and cause a fire. On the other hand, using wire which is too large is unnecessary and more costly.

Wiring should be protected as much as feasible. Many trailers may even use conduit similar to that in a house. Pay special attention to the wiring joints and connection. Where necessary, use electrical tape to wrap these connections for protection against the elements.

Plugs

The connector plug for the trailer wiring system is generally located where the tow vehicle and trailer are hitched. The socket (vehicle end) is normally attached to the tow vehicle rear-end or bumper, while the plug (trailer end) hangs loose with enough slack wire to reach the tow vehicle socket. Plugs range in size from 4-way plug and socket to a 7-way system or even 12-way system.

4-wire flat style plug is common for trailers without brakes.
5-wire systems provide all the color coded functions of the 4-pole systems plus an additional blue wire, which may be used for brakes or auxiliary lights.
6-wire systems provide all the color-coded functions of the 4-pole systems plus two additional poles, one for 12-volt supply (red wire) and the other (blue wire) for electric brake systems. The 6-way connecter has posed problems related to the wiring of the electric brake s the way in which the trailer and tow vehicle are wired do not always match. Be sure to check how both the trailer and tow vehicle are wired for the electric brakes before your tow the trailer. Not only could this damage the controller in the tow vehicle, but it could also cause problems with other components of the trailer’s electrical system. In some instances you would know if the connector is not wired correctly because the trailer brakes may lock-up. However, there may not be any indicator until you use the brakes and they do not engage. For this reason, it is always a good idea to synchronize the trailer and tow vehicle brakes each time you use the trailer.
7-wire systems provide all the color-coded functions of the 6-pole systems plus an additional pole for interior/exterior lighting.