How do you add power to a cargo trailer?

In today’s blog post, we will answer the following question: ​​How do you add power to a cargo trailer?  We will explain how to connect your cargo trailer to power source and give you advice on mantaining the elctrical system of your trailer once it is installed. 

How do you add power to a cargo trailer?

To add power to a cargo trailer you must connect it to shore power.  Cargo trailers are often used for camping, portable businesses, horse transportation, race car transport and crafts. Operation of AC appliances or tools requires the installation of outlets, wiring and a circuit breaker box. 

Wire to be stretchable and flexible so that you won’t have to rework later to meet various circumstances. You can add a generator later, and the need will arise to plug it into camping “shore power”.

Here’s how you add power to a cargo trailer:

  1. Hole-saw a place for the lanyard hatch on the driver’s side of the trailer. Entry points are customary upper rear or lower front. Install the hatch using the gasket supplied with the hatch, or use trailer exterior sealant.
  1. Install the circuit breaker box near the entry point. For the left and right side walls near the ceiling, screw-in plastic junction boxes using the drill with a 1/4 inch socket – one window at each corner, between one on each side. On range boxes, break the circle with a conduit. On the other boxes, knock out a circle on each side.
  1. Measure and cut pieces of flexible conduit to fit between the junction boxes and between the breaker box and the two inlet boxes, so the ends protrude 1/4 inch through the end holes. Label the parts for the location. Each trailer site run will have its own breaker, so the passenger side run will extend from the roof to the breaker box.
  1. Cut Romex electrical cables for each length of cable, making each piece 12 inches longer than its conduit. Run the pieces of cable through their conduits, leaving six inches hanging out at each end. wire cable and conduit through the junction box holes, letting the wire hang down the front of each box. Screw conduit straps down onto the conduit every 12 inches and 6 inches from each junction box.
  1. Cut 8-inch cable braids for all but the final boxes. Divide and cut 3 inches of outer jacket on each end of the cable, including the mats. Strip 3/4 inch of insulation from all wire boards. Wire-nut incoming, outgoing and pigtail black wire jacket together. Repeat the operation with white or grey threads. Repeat the operation with bare or green threads. End boxes do not have braids or protruding wires. Pull lightly on the wires to test the connection strength.
  1. Loop and tighten the end of the white or gray wire under the neutral side or output terminal screw at each box. Black Threads Screw Down Under Silver Screw Hot Side. The bare or green wire attaches under the green screw that feeds the round earth electrode. Secure the socket assembly to the box and secure the cover plate.
  1. Insert the 30-amp trailer cord through the exterior hatch, and into the breaker box. Screw any bare or green wires down under the Fieldbus in the breaker box, along with the green wire from the cord. Wire-nut all white or gray wires together.
  1.  Connect each black wire to one side of a circuit breaker. Wire-nut the black wire into the black wire cord of two 8-inch pieces of Romex. Connect each black Romex wire to an empty terminal breaker. Close the circuit breaker box.
  1.  Use an adapter to plug the 30 amp cord into a standard outlet.

Note: If you have no experience whatsoever in wiring a trailer, please DO NOT do it by yourself. It is always best to ask a professional to do it. 

Adding power to a cargo trailer: the wiring

Even without being a professional, it is not very complicated to tackle the hookup of a trailer. For 12 Volt 7 and 13 pin sockets, simply follow the wiring as shown below:

Pin numberCodeWire colourCircuit
1YellowTurn Signal
254G BlueFog lights
331 White Ground
4GreenRight turn signal
558R BrownRight side marker light
654 Red Stop lights L and R
758L Black Left side marker light
8Reversing light
9Battery (+ pole)
10Charging line + trailer battery

Connecting your cargo trailer to a socket

You should be extremely careful when connecting a cargo trailer socket. To do this, it is necessary to start by bringing the harness into the trunk of the car. This operation requires dismantling the bumper to reach the cable gland. Subsequently, this same harness must be connected with the coupling socket. 

The connection must be made in accordance with the manufacturer’s instructions. The instructions should therefore be read carefully before proceeding with the installation. This is because the connection method differs depending on whether it is a 7-pin or a 13-pin socket. After this phase, the plug must be fixed to the support with screws.

Subsequently, it is necessary to go through the connection of the electrical harness with the rear lights for old cars. If it is a new vehicle model, the connection is made using a multiplex box. 

To be sure which method to follow, you should also consult the vehicle’s maintenance booklet. At this point, the trailer is normally ready for travel. In the event of a problem, it is necessary to check all the connections. Professional help is suggested in case of difficulty.

Can you connect a cargo trailer generator to shore power?

Yes, you can connect a cargo trailer generator to shore power. A generator provides a backup source of power in your trailer. You may connect your trailer’s generator to your home in the event of a power outage. Trailer generators typically range between 2,500 and 15,000 watts.

Here’s how to connect a trailer generator to a shore power:

  1. Turn off switch boxes in both the trailer and the external source.
  1. Check the generator for capacity in amps and wattage, the amperage should match the items you are driving. Home and trailer  generators include 30 amps – or 110 volts – power.
  1. Extend the trailer plug at the end of your generator cord.
  1. Plug in an generator cord adapter, a trailer plug is different from outlets found in the typical home. The most common adapters are the “dog bone” and “hockey puck” adapters. Both are available in RV stores.
  1. Plug the generator into the shore power. Use a heavy-duty extension cord if connecting the generator to an indoor outlet.

Conclusions and final advice

The electrical system of your cargo trailer should be checked at regular intervals. Ideally, the check is done annually if you are driving a high mileage.

Keeping electrical equipment in good condition and having appliances repaired if necessary is a matter of safety. Damaged or worn electrical cables and cords should be replaced immediately.

It is well to remember not to abuse electrical devices by forcing them to work beyond their capacities … Unplug all internal devices from the caravan before connecting or disconnecting from the site power supply.

Another important tip: check the operation of the differential circuit breaker using the test button each time the caravan is connected to the power supply of a terminal. If a fuse or an earth leakage circuit breaker trips, turn off the power. Then check the cause before replacing the fuse.

And again, if you have no experience in wiring a cargo trailer, it is always best to take your trailer to a professional than to experiment on your own!

FAQ on How do you add power to a cargo trailer?

How to install an RV power supply?

The safest and simplest way to install a home connection for a recreational vehicle or RV is to use a power pedestal manufactured for this purpose. Power pedestals are similar to the hookups in most campsites, usually made with a built-in 50 amp outlet and a 30 amp outlet and one or two 20 amp outlets. 

How to connect a 7 pin harness?

The connection is made to the wires supplying the rear lights of the vehicle. The wires of the harness must be transplanted to those of the vehicle corresponding to the desired function. The wires are never cut, just use the supplied quick connectors.

How to connect the wires of a trailer?

To connect the hitch plug, first pass the harness through the hole provided next to the hitch ball. Then, always refer to your trailer’s manual to find out how to connect the wires.

Can you run an RV generator while plugged into shore power?

Yes, you can run an RV generator while plugged into shore power. We get how this may cause concern as it seems counterintuitive, but remember that RV generators are built with a transfer switch, thus all the power will come from the shore power while the generator will remain charged and risk-free. 


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