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Coach Wiring Primer
By - LD Tullis
Tullis Electric

     I will state a few facts for those of you who are interested in and in need of this knowledge, I have received a lot of help on this board and am returning the favor. Concern yourself with power consumption, if your appliance or device does not state the power consumption, I will give you some basic formulas to compute any part of the equation.

     The following symbols are the standard forms used in the industry and will be followed by the various simple formulas that will enable you to calculate any parameter of your electrical needs. Bear in mind that any power generation or power handling system has internal losses but if properly designed it should be negligible for an RV. The following will treat the system as having no loss..

I or A
E or V
P or W
Current or Amperage
Power or Wattage.

I[squared]r =Power [a normal calculation to measure power loss in the conductor].
E[squared]/r =Power
IE =Power
P/I =E
P/E =I
E/R =I
E/I =R
3.42 BTU = one Watt
742 W =1 HP
KVA =watts (thousands)
Amp/hr =1 amp draw for 1 hour.


bulletBatteries in series gain voltage capacity, the current capacity remains the same.
bulletBatteries in parallel gain current capacity, the voltage capacity remains the same..

(NOTE:) Don't attempt to measure resistance on a/c circuits and apply it here as there are other factors that enter in that we won't go into.

horizontal rule

The following examples are not an attempt to recommend a particular configuration, just show you the feasibility of the different configurations.

1 - You have a 2500 watt 12v inverter. At full load this unit will draw 2500/12 or 208 amps. Your battery bank will suffer a 208 Amp drain after one hour. (208 amp/hr) To hold the banks charge with 120v generator at this rate of consumption would require 2500w/120v or 20.8 amp. Or a 2500W (2.5kw) generator.

2 - You have a 2500 watt 24v inverter. At full load this unit will draw 2500/24 or 104 amps. To hold the banks charge with 120v generator at this rate of consumption would require 2500w/120v or 20.8 amps. or a 2500W (2.5kw) generator.

3 - You have a rooftop cooler that draws 20a. As we have seen, your 2500 watt inverter could conceivably run this unit (and only this unit) by itself.
Take care that your particular inverter's power quality does not deteriorate as you approach peak capacity.

4 - You have (10) 50w lights. At 12v or 24v or 120v these units will collectively draw 500w (assuming the lights are connected to the voltage they were designed for). Your inverter is a 2500w unit and these lights will utilize only 1/5th of it's capacity at 120v and the inverter will be drawing 500w/12v or 41.6a from your battery bank. If you connect the lights directly to the battery bank (assuming the lights are now 12v units), they will draw 41.6a from the bank so the net gain of 12v over 120v is nil, it's a matter of preference but I submit to you, that 120v lighting is much more common and considerably less expensive unless you are reluctant to put the load on your inverter.

5 - Configuration:

Voltage Load Watts
120v Central air @ 20a 2500
120v Lighting 500
120v Entertainment Center 300
12v Furnace Motor 250
12v Reefer 1000
(I am guessing at these RV units so plug in your actual figures)

Total 120v load = 3300 Watts (3.3KW)
Total 12v load = 1250 Watts.
Generator required for this load = 4550 watts (4.5kw)

12v load connected directly to batteries 1250 watts or 104 amps and the inverter becomes 3300 watts [still a lot of current for batteries].
Inverter required for this load = 4550 watts (@ 120v).
Battery bank required (your supplier will be very happy) 4550/12v =379 amps [not a feasible design].

Well, I hope this helps some of you, as you can see, much of this selection is just personal preference.

LD Tullis
Tullis Electric


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