A new current

The inverter itself has two cooling fans.

Many truckers purchase small appliances designed for a sleeper cab that run on the 12-volt DC (direct current) that’s in truck batteries. But having 110-volt, AC (alternating current) in the sleeper opens up a world of possibilities. You can then purchase standard, small household appliances that are available everywhere and much less expensive. Also, you can plug in your laptop computer and printer.

Appliances, especially the motor-driven ones, operate a lot more efficiently when using the higher voltage AC, taking less current and using smaller cords. This means less strain on the truck batteries and lower fuel consumption.

We visited K.L. Harring Transportation in Bethel, Pa., where Paul Mitchell, applications engineer of mobile equipment at Xantrex, installed a Xantrex 1800-watt inverter into a new Kenworth W900L. We received the help of Tommy Vajdic, the shop manager, and Robert Reed, technician.

Mitchell first laid out the inverter and wiring, which comes in a premium kit called Cab Power Plus put together by Phillips and Temro. The DC wiring, which carries up to 200 amps, is 2/0 wiring designed for use in welding that has more strands and therefore can carry all that juice with much less resistance and heat than regular battery cable. This gives better voltage to the inverter.

The inverter itself has two cooling fans (A). A cooler inverter works better because its electrical resistance increases with heat. The AC wiring is loomed with a protective, orange plastic cover. The AC wiring connectors have male and female ends and snap together.

The inverter has an internal circuit breaker and ground fault interrupter. Inverters can burn up, says Mitchell, when wiring problems or overload exist, and should carry an Underwriter’s Laboratories safety rating (UL 458) as the Xantrex unit does.

The DC wiring has a 200-amp fuse, critical because a ground in the DC circuit can easily cause a serious fire.

The unit comes with a separate line for shore power. When the driver plugs in, a solid-state internal transfer switch disconnects the batteries and ensures all power is drawn from outside. Although this unit did not have it, an optional feature allows the shore power to feed back into the batteries and charge them up. This unit has a low-voltage cutoff. It operates at 10.5 volts and also has an alarm that warns you voltage is getting low.

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The first step for any do-it-yourselfer is to review the safety instructions in the installation manual. Handling the DC battery output is where the greatest risk lies, as it’s always there and grounding a wire could easily start a fire because of the high amps. AC has enough voltage to give you a serious electrical shock.

One way to reduce the risk is to work backward all the way from the receptacles, through the inverter, and then from the inverter toward the batteries. Wait to connect to the positive battery terminal until the rest of the wiring throughout the vehicle is secure, and the fuse is installed and mounted.

Mitchell first laid out the wiring to visualize how it would have to be routed through the truck. Refer to B for the layout. The rear of the inverter is where the DC from the batteries inputs. The front end is where the AC is output.

B also shows that the inverter’s control panel can be popped out and remote mounted with the signals carried through a standard telephone line included in the kit.

An ideal spot for the unit was under the single bunk in back. There was plenty of room to run the DC cables through the floor and then forward to the battery box on the driver’s side of the truck. Mitchell decided to run the cables along the inside of the frame rail, where there was existing wiring. That way, he could use existing wire carriers and also tie the inverter DC wiring to what was already there.

You may need somebody to help you with a few of the operations – to hold things out of the way or hold wire in place as you work.

Mitchell chose to install the receptacles near access panels, one below and one above, because he could work through the panels to route the wiring and then plug in the receptacles.

Doing the job is relatively straightforward, but the devil is in the details. Planning where and how to route the wire and checking to make sure you’re in the right spot before drilling are critical. It took a good eight hours to perform the work because Mitchell had never installed one of the units in a Studio Sleeper before.

  1. The Xantrex unit was first located under the bunk in the sleeper. After taking careful measurements to make sure the holes would not come out inside the tool compartment, Mitchell then drilled two holes through the floor of the sleeper for the DC wires (C). He used progressive drills of several sizes throughout the day to increase the size of the hole by just drilling deeper. The holes were cut to just the right diameter to fit the threaded area below the ring on each grommet.
  2. He then unscrewed the silver grommet fastening nuts (D) and pulled them off both positive and negative wire grommets. He next positioned the positive and negative cables with the grommets on them through the holes in the sleeper floor. He inserted the lower threaded areas of the grommets through the holes so they would pass down through the floor. He next passed the attaching nuts over the open ends of the cables outside the sleeper. A helper held the grommets so they would sit tight against the floor while he installed and tightened the attaching nuts from underneath (E).
  3. He connected both DC battery cables to the inverter, red to + and black to -, installed the washers and nuts, and tightened the nuts with a wrench (F). The wires were forced down through the grommets until there was just enough so the inverter was in the right position under the bunk, and then the upper grommet nuts were tightened to hold the wires in position in the grommets.
  4. Next came the job of routing the AC wires through the sleeper. The Y-connector was plugged in at the inverter on the left side at the front. Then, Mitchell removed the two panels for access and drilled large holes for the receptacle wiring connectors (G). He drilled smaller holes for the mounting plate screws, using each plate held in position as a template (H). He then mounted the plates with the screws.
  5. He maneuvered the AC cables that would connect the receptacles to the Y-cable through the open panels, and fed the ends that connect to the receptacles out through the large holes in the receptacle mounting plates from inside. He next connected the receptacle connectors to these cables, and then fed the cables and connectors back through the holes, reaching in through the open panels for access. Once this was done, the receptacles were snapped into place onto the mounting plates.
  6. Next, the AC cables were routed down through the walls behind the panels and back toward the bunk. Mitchell now replaced the control panel and speaker panel he had removed. The wires were run backward along the sleeper wall, concealed as well as possible (I), and brought out onto the floor under the bunk.
  7. Then they were connected to the Y output cable (J). The connector for shore power was connected to the right side connector on the inverter and passed through an opening under the bunk and into the tool compartment. It would not normally be used by this fleet, but if this changes, it can easily be mounted in the wall of the tool compartment so it can be connected to a shore power hookup from outside.
  8. At this point, the wiring to and from the inverter was all connected, so it was pushed back under the bunk (K). The unit has feet with slots in them, and Mitchell would normally have drilled small holes into the floor and then installed mounting screws (L). However, the Kenworth’s bunk was mounted on tracks so it could serve as both a sofa and bunk, and was not easily removed, leaving insufficient room to drill holes and start the mounting screws. The inverter will stay in a relatively stable position because of the heavy DC wires running through the floor and held in place with the grommets.

Connecting the DC cables

  1. The DC positive and negative cables were now routed forward toward the battery box. Mitchell used existing wiring carriers for both cables. He used plastic ties to tie both cables to another wiring harness that was already thoroughly fastened all along its length (M).
  2. Mitchell decided to install the fuse block inside the battery box for protection against corrosion. The fuse block has a hole in one end and a slot in the other. After removing the box cover, it was held against the inside wall of the box and the mounting holes were drilled (N). With a slight modification to lengthen the slot, it was successfully put into position and the attaching bolt, washer and permanent locking nut installed on either end to mount it.
  3. The cables were too short to reach all the way from the back of the large sleeper up to the battery box. The shop had on hand some of the proper gauge battery cable. Mitchell first took careful measurements and then cut the cable to the correct length.
  4. He then used a knife to cut and peel the insulation at all the open ends, installed a plastic shrink tube over each cable, and connected the two ends for positive and negative cables with a butt connector. This was crimped at either end with a special tool (O). The shrink tube was then centered over the butt connector and heated carefully with a very small torch to make it shrink and seal. The electrical connectors were then installed on the open ends with the same crimping tool and shrink-tube.
  5. The cables were then routed inside the battery box using the holes the regular battery cables pass through. The positive DC cable connectors were installed on either side, the fuse was installed over the two connector screws, and then the washer and attaching nut was installed and tightened on both sides (P). The rubber cover was then closed tightly.
  6. Once Mitchell was sure all of the wiring was secure, the final step was connection of the connectors on the ends of the DC red and black wires to the red (+) and black (-) connectors on the batteries. This was done using the existing attaching nuts for the truck’s battery cable connectors. We tested the inverter by plugging in Mitchell’s 110-volt, AC test lamp inside the sleeper (Q) and found that it shone brightly.

Mitchell says any driver who is “handy” can install an inverter. Just remember to measure carefully and get extra cable, connectors and shrink-type insulation tubing, as well as the tools necessary to use them, if you need to lengthen the DC wires. Once the unit is installed, you can enjoy running all the household appliances you’ll ever want in the comfort of your sleeper.

For More Information:
(408) 987-6030

Phillips and Temro Industries
(952) 941-9700

Cobra Electronics
(773) 889-8870

(800) 362-5397

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