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Auxiliary Power UniT
When running the air conditioner from an APU, the genset must be large enough to start and run the air conditioning
unit as well as any other electrical appliances you may wish to run at the same time. This can be complicated, since
gensets from different manufacturers may vary considerably in their output characteristics. If you measure the output
of two gensets rated by their manufacturers at 2.5 Kilowatts, you will observe them to be very different.
It’s also important to allow for the surge capacity needed to start the air conditioning compressor. Each time the
compressor cycles on, there is a short electrical surge that is equal to about four times the normal running amps
for several hundred milliseconds. Most good-quality gensets are designed to support these short surges without
tripping off the line.
Shorepower
Although there are relatively few shorepower hookups available at truck stops, rest stops and terminals, these will
become more readily available in the future.
TMC recommends a minimum 20-amp circuit for shorepower connections. You should use any heavy-duty power
cord with a standard three-prong plug. Remember that there will be a voltage drop across the cord, so you should
use the minimum length necessary to reach your outside power source.
SPECIFYING THE SYSTEM
About Insulation.
Before we go any further, let’s talk briefly about insulation. Most truck sleepers today are insulated with a value of
approximately R 1 to R 1.25. Compare this to the insulation of R 30 that is required in most new housing contruc-
tion. Even the “cold-weather packages” offered with some new trucks only have insulation values of R 2. TMC
recommended practices call for R 4.2 as a standard insulation package and greater than R 4.6 in a premium package.
This standard, once adopted by the industry, will go a long way toward improving the performance of auxiliary air
conditioning and heating systems.
If you are buying a new truck, you should order it with additional insulation. There are also a number of measures
you can take to reduce the heat load in your truck. If you have skylights, cover them. Insulate the floors with foam
under the carpet. Add insulation to the doors. Get heavy-duty curtains to separate the sleeper from the cab.
Cover the windshield and windows with reflective shades when parked. Park in the shade whenever possible.
Switch off heat-producing appliances in the sleeper.
Air Conditioning Units
Your first step will be determining the type of air conditioning units (split or self-contained) to meet your requirements
and space constraints, then sizing the units to meet the expected heat load.
Sizing the system can be a complicated affair, since it depends on a number of factors, such as the size and shape of
the sleeper, the amount of insulation (see above), the size and location of windows, the color of the truck (darker
colors absorb more heat) and the ambient temperature outside.The following guidelines are based on TMC recom-
mendations under “average” conditions. If you are operating in very hot regions, you may need to consider extra
capacity.
7,000 BTU Day cab or small sleeper up to 48 in. with flat roof
10,000 BTU Mid-size 60-in. sleeper with low or mid-height roof
14,000 BTU Full-size 60-72-in. sleeper with high-rise roof
Capacity
BTU
Genset Minimum Output
(Continuous KW)
Genset Minimum Surge
Amps
7,000 2.0 38
10,000 2.5 60
14,000 3.0 70
12 gauge Up to 25 ft.
10 gauge Up to 50 ft.