lt_eggbeater wrote:
How many amps is the ac, and whats the total load of the other devices?
It will vary depending on the make and model of equipment used. One of the best devices to own for determining actual loads is a
Kill A Watt meter. They are typically found for about $35.00, sometimes as low as $20.00. Having one allows you to know exactly what an appliance draws.
Using a Kill A Watt to determine A/C draw is sort of tricky since most A/C units do not have a removeable plug. Sooo..... using a 20 to 30 amp adapter plug your entire RV into a Kill A Watt (you may need adapters on both ends since the Kill A Watt only comes with standard NEMA 5-20 North American Lugs). Turn off all breakers in the RV except the main and A/C. Check Kill A Watt to be sure there is no load (if so, write it down to subtract from your A/C reading later). Turn on the A/C and take either the amps or watts reading from the meter for your current draw. Remember, the LRC (locked current rotor) current for an A/C will be at least 3x more (maybe as much as 6x than the running current so extra capacity is needd during that first 1/15th of a second (4 AC cycles) to get the compressor moving. That is where surge, peak or reserve power from a genny is important. While I do not make a whole hearted endorsement of this "trick", you might want to try holding the genny engine governor open to increase engine speed then turn on the A/C to get a hard start unit running. Be sure to release the governor speed control after the A/C starts so the engine can drop back to 3,600 RPM. This does not necessarily give more voltage or amperage out but it does keep the genny from bogging down too far and loosing the needed power to give the kick to a stuborn A/C. If all of this is needed to get a RV A/C started remember that once the thermostat cuts the compressor off when desired room temperature is reached we start all over again when it cuts back on.
Mathamatically, line voltage times appliance amperage is equal to wattage (power). Inversley, appliance rated wattage divided by line voltage is equal to current in amps. All appliances should have a label showing voltage and wattage -or- amperage (also frequency). If line voltage is low, like 110 VAC, an appliance will pull a higher amperage to deliver a stated power. If the output is going to be 2,000 watts - period- then the amperage for 110 VAC will be 18.18. But, let's say the line voltage is 128 VAC, the same 2,000 watts of power will translate to 15.625 amps. Frequency also has a factor in getting things started. For example, 52 Hz will NOT initially push as hard as 65 Hz. (think of it like 52 people trying to push your RV vs. 65. A higher frequency will give more initial kick to a device like a RV A/C. In any event, keep your frequency from the genny no lower than 55 hz and no higher than 65 hz - but safe operation can go as low as 50 Hz or as high as 70 Hz. Output voltage should ideally be on the higher side - like 125 VAC, than the low side - like 110 VAC. Don't let it drop below 105 VAC or go over 132 VAC for any period of time (10 minutes or so).
One more addition - keep power cords short and of a large gauge. A #10 RV power cord is good for up to 30 amps, but ONLY for about 35 feet. After 50 feet you should be using a #8 or even a #6 AWG cord if you want to stay under a 2% voltage drop. Again, the Kill A Watt meter can tell you how much voltage drop you have in a power cord. Just test both ends and subtract the difference to discover power lost to heating the cable.
Microwave ovens are often misunderstood appliances. A 1000 watt microwave oven draws at least 1,200 watts or (1,200/120 = 10 amps), maybe more depending on efficincy and accessories. Just remember the advertised power/wattage of a microwave is output power, not input power.
Devices without electric motors will be relatively flat on current draw - meaning start up will be close to running load. Devices with induction motors will need much more current to get them started than to keep them running. The only induction motor in your RV will most likely be an A/C compressor - unless you are running a circular saw, additional portable refrigerator, air compressor or water pump requiring 120 VAC.
An "old trick" with a RV A/C is to start the fan motor on low first and let it run a minute or so. Then, turn up the temperature to allow the compressor motor to start. This breaks the start load into two stages. Some RV A/C units do this automatically (compressor delay built in).
Don't forget your LPG fridge also draws wattage when on auto or electric mode. The heater in the fridge can easily pull 500 watts on some models (3+ amps at 120 VAC). Your converter that powers 12 VDC devices and charges your batery can suck up another 400 to 600 watts (5 amps at 120 VAC). Thus, if you have trouble starting your RV A/C, turn off all breakers except main and the A/C to get it going, then you might be able to turn the others back on again. Unless you have a big genny it is most always best to run fridge of LP (turn off auto mode) and turn off AC to DC converter and allow DC power to come from just the batteries.
Don't overlook the possibility of accidentally leaving on an electric water heater element. Many newer RVs now have the option of heating water via LP, electric or both. That is another 900 watts (7.5 amps) than can be eliminated.
Long answer for a short question? Quite possibly more than you askied for or wanted to know? I get on a roll sometimes and rarely give short answers - unles it is simply yes or no. Sorry.... maybe some other reader will find this helpful.
Professor Randy T. Agee & Nancy Agee. Also Oscar, the totally ruined Dachshund.
2009 Cedar Creek 5th Wheel - 2004 Volvo VNL670 class 8 MotorHome conversion as toter.
Turbocharged, 12L, 465 HP and 1,800 ft. Lbs. of torque.
