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Old 07-03-2009, 08:37 PM   #1
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Re: FLORA - 1987 Ford Carpenter

1a. Yes, you can run a dorm-sized refrigerator off of an inverter. But it may not be the best choice. If you are picking it to conserve space, go for it. If you are thinking small to conserve electricity, investigate further.

The power draw over time is not dependent on size, but rather heat leakage and mechanical efficiency. I think I heard that dorm refrigerators have poor insulation, so they run a lot more often. You want to figure your usage of watt-hours, because you are going to have to size your battery bank to provide for it. A 3-amp refrigerator that runs 20 minutes per hour draws 120 watt-hours each hour. (3 amps x 120 volts = 360 watts x 20/60 hour = 120 watt-hours.) A 6-amp refrigerator that runs only 6 minutes per hour uses only 72 watt-hours. (6 x 120 x 6/60 = 72). Though the instantaneous current draw is twice when running, this example actually uses 40% less electricity over time.

If buying new, look at the Energy Star estimates, and pick an efficient model. If you already have an old fridge, get a Kill-A-Watt or Power Angel watt-hour meter to measure the actual accumulated daily load.

If space is not a limitation, pick a size that is just a little bigger than all the cold food you want to carry. If there is extra space inside, fill old jugs with water and store them. Once they chill, they will hold in the cold, and take up space that would otherwise fill up with hot air when you open the door, making the motor run again. If you are plugged in, pre-chill the box before you set out and start draining the batteries to keep it running.

1b. Add up your peak loads, and pick an inverter based on that. Inverters usually have a continuous & peak rating, and you want to plan on staying down in the continuous rating level all the time, especially if you are protecting food. The 6-amp load in the example above draws 6 x 120, or 720 watts when running. A 500 watt continuous/800 watt peak unit would work but be stressed, picking a 1000/1200 unit would be fine.

2. Yes, the ground and neutral on the bus should be separate when on a shoreline, because you are on a sub-feed of the source, and the source provides the ground. You can tie them together if you are disconnected and using power from an inverter or generator, but must 'untie' them when plugging in. RV model inverters with internal automatic transfer have an extra contact compared to the off-grid cabin style, so they can switch the neutral as well as the hot(s).

Its possible that the bus shell could charge up with static electricity from wind-borne particles, but any discharge would be more of a surprise than a danger to people. The most common damage is to antenna pre-amps in sensitive radio receivers. I have gotten static shocks from sliding on a car seat, but never a vehicle shell. Ground crews working with helicopters discharge the accumulated static on hoist loads before grabbing hold and guiding them to the ground, but a lot more air is being blown past that. Besides, the carbon in the rubber bus tires is a conductor at high voltages, and should drain off static over time.

The real danger would be a mis-wired power source for the shoreline, where a neutral or hot wire is connected to the bus ground. Then the bus shell would be hot in reference to earth ground. You should always check your source before plugging in, either with a meter or using an RV tester with status lights. You could drive in ground rods wherever you park and ground the frame to them, but I don't know anyone who does, and it may not make you popular with the property owners.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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Old 07-08-2009, 11:25 PM   #2
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Re: FLORA - 1987 Ford Carpenter

I can easily run two dorm-sized sin fridges plus a TV, small home theater system, and PS2 off just one of my 750 watt inverters. When you figure out how much battery bank you need go ahead and triple that number. You'll understand later.
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