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Old 06-19-2020, 09:09 PM   #1
Mini-Skoolie
 
Join Date: Dec 2018
Location: Tucson, AZ
Posts: 52
Year: 1991
Chassis: Wayne Lifeguard
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Alternating Current Switching

Hi all,

Earlier in my planning I was putting off on having a generator and I figured when I did get one I could just plug its output directly into the shore power input. Well… I changed my mind. It seemed crude to have a pigtail hanging off the side of the bus while driving (in order to run the air conditioning off the generator) so I changed my mind.

The problem then became: how to control where AC (alternating current) power is coming from… I first thought I'd just put a bunch of outlets in my electrical cabinet and move extension cords around but that felt cheesy and impossible for novices (ie, friends who borrow the bus) to figure out so I got a bit more fancy.

Here's the final plan. Using two dual pole dual throw (dpdt) relays I'm able to preferentially provide AC power to my household outlets in this order:
1. Shore power
2. Generator power (which is never running when on shore power, of course)
3. Inverter power.

My house-battery charger and air conditioning will only run on shore or generator power. My outlets will run on shore or generator power, if available, otherwise they'll be on the inverter.

I've attached my schematic in case anyone is interested in how I'm doing it. I'm using two NTE R04−11A30−120 dpdt relays to accomplish this setup. I ordered them from Arrow.com for about $25 each.

The schematic only shows 2 outlets but there are more in the real world.
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Old 06-19-2020, 10:52 PM   #2
Mini-Skoolie
 
Join Date: Apr 2020
Location: St. Louis
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Year: 2003
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I presume that if you're on shore power, you'll want to run everything on shorepower. I mean, why would you run a genset or cycle your batteries while on shorepower?

If that's the case, just put EVERYTHING on the same breaker box (except the house-battery charger), then run your power lines in from shore/genset/inverter into a 3-position changeover switch like this: 3-position changeover switch

If you're running on battery, just don't turn on the heavy draw stuff. This gives you the option of running those heavy-draw items on battery if absolutely necessary (maybe it's 120 degrees out, and you just need to run the A/C for a few minutes), simplifies your overall wiring, and gives you a single panel for all your circuit breakers.
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Old 06-20-2020, 06:46 AM   #3
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When it comes to AC power, I am always nervous about DIY projects. There are a number of automatic transfer switches available (like this one). The automatic part makes for a nice foolproof (??) system. They are also fast enough that I don't normally lose power to appliances (don't have to reset the clocks).
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Old 06-20-2020, 07:35 AM   #4
Bus Crazy
 
Join Date: May 2014
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It seems like what you're trying to do is invent your own version of a transfer switch. Normally I'd be all for making your own stuff and doing it your own way, but those transfer switches typically work really well, and as JD said, the automatic ones are typically fast enough to not notice the flip.
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Old 06-20-2020, 09:26 AM   #5
Mini-Skoolie
 
Join Date: Dec 2018
Location: Tucson, AZ
Posts: 52
Year: 1991
Chassis: Wayne Lifeguard
Engine: 7.3L IDI
Rated Cap: 23,600 lb
Thanks guys. I understand your concerns but this is just basic circuit design using dpdt relays as transfer switches. I'm using parts made for power switching to do power switching.

Had I been able to find a transfer switch that accomplished these two goals and was affordable I would have bought it. Unfortunately, no such thing exists as far as I can tell. My goals were:

1. Run the battery charger and air conditioning ONLY off of shore/generator power.
2. Run the interior outlets preferentially on shore/gen power, then inverter power if need be.

The big thing is that I don't EVER want to run my air conditioning off of the house-battery. I live and travel in the desert southwest. Unless you're investing thousands in a big battery bank and solar then there's just no good way to run AC here off of battery power.

A manual transfer switch was considered but I didn't want users to have to mess about with knowing the correct configuration to put the switches. I also didn't want someone switching to battery power then running the AC and completely draining the house battery in 15 in minutes. The less expertise needed to operate the better. Zero-configuration use is important to me.
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