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Old 12-05-2016, 11:08 AM   #1
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how do i add shore power?

hey everyone!
i've decided that i want to add a 50 amp shore line power to my bus and i've been contemplating how to do it.

my bus was previously a bookmobile so i have an awesome 12kw generator for my main source of power, but it would be nice to plug in, when i'm stopped at somewhere that has shore power available.



the generator output is directly wired to my AC distribution panel. i think the right way to add shore power is to disconnect the generator, and wire the distribution panel to a transfer switch. then wire a shore input to one side of the switch and the generator to the other side.

shore........... transfer switch...............gen
............. distribution panel

am i on the right track? where would i put an inverter? any recommendations on a transfer switch? should i hard wire the shore cord to the switch or use a receptacle? are there any issues that i may cause with ground from the generator?

any sparkies have some answers? i'm all ears
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Old 12-05-2016, 11:18 AM   #2
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50' extension cord and 50A-rated power strip?
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Old 12-05-2016, 02:52 PM   #3
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That power strip comment might have come across as slightly sarcastic... but it's really not far off the mark.

First, though, there are a variety of packaged transfer switches. I found a bunch of nice pictures on eBay, but I avoid posting links to there because everything is so temporal. Do a web search for "double throw safety switch" and you'll find lots of options from eBay to Home Depot to online catalog listings from the big names like Eaton, Schneider, GE, Siemens. A single switch could gate between generator and shore. If inverter is to be added to the mix, it'll take a second switch cascaded so that, for example, switch A selects between generator or shore and switch B selects between inverter or whatever's chosen through switch A. A's output feeds into B; B's output feeds into the distribution panel.

Automatic transfer switches are an option too. Look at these only if pricing on the manual switches didn't make you cringe.

If you want to build it yourself you can look for multi-pole contactors. That's the main component inside a packaged automatic transfer switch, and when bought as a single component, costs a lot less than the packaged solution. But you'll have to gather up a few odds and ends, like a housing, on your own.

Ground is a good thing to think about. It is (should be) connected to neutral in your rig now since you have the on-board generator. When shore powered, that connection should be interrupted. The most oops-resistant method is to use a 3-pole transfer/safety switch with the third pole used to make the ground-neutral connection. Set it up so that the two are connected when generator or inverter is the source, but they're separate when shore power is the source.

Now, coming back around to the extension cord/power strip comment.. You could wire an inlet connector to the distribution panel and a cord-end receptacle to the generator, shore power extension cord, and inverter. The "transfer switch" would simply be unplug one source and plug in another. At the moment I'm having a little trouble choosing an appropriate 50 amp connector for that job. If it were 30 amp I'd suggest the L14-30 twist-lock style. Surely something similar exists with a 50 amp rating. Compared to the cost and size of safety/transfer switches, this really isn't such an outlandish suggestion after all.
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Old 12-05-2016, 09:37 PM   #4
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Something like this?

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1



That's an option I considered for a while.... get a generator with a 50 amp outlet, then my "transfer switch" would be to plug my 50 amp cord into whichever source I wanted to use, shore power outlet or generator output. I've since abandoned that since 50 amp generators are over $1000 and are noisy as hell. Not what you want in a campground. Boondocking with no one else around it would be ok but not if you have neighbors nearby.
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Old 12-05-2016, 11:29 PM   #5
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Yeah, that connector could feed to the distribution panel. The mate could go on the generator and inverter outputs, and one of each gender on the ends of a length of SOOW cable as a shore power cord.
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Old 12-05-2016, 11:48 PM   #6
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If you've got an inverter big enough to supply a full 50 amp service then you'd almost have to have a separate trailer for the battery bank, that's a LOT of amp hours and a LOT of weight.

That connector I posted before was for the shore power cable connection into the rig. If you're looking for the outlet you would connect to a generator or generator head, inverter, etc, to plug the cable into then this is what you would need:



And this cable to plug into it, with the other end in the receptacle connected to the distro panel/load center:

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Old 12-05-2016, 11:58 PM   #7
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i'm a bit confused by the ground and electrical nomenclature.

my generator supplies 50a split phase service. thats 4 wires, 2 hots, a neutral and a ground. so do i need a 3 pole? or 4 pole? transfer switch if i want to switch ground from the generator to shore.

the rv transfer switches i see seem to be 3 pole and share ground but switch 2 hots and the neutral. i feel like if i get one of these, that shared ground will fault and trip the shore line. am i missing a way to wire the 3 pole switch?

am i looking for something that i don't need?

dazed and confused

i did find this switch, but dang$$$$$

Blue Sea Systems 8369 AC Rotary Panel 120 + 120/240VAC/6
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Old 12-06-2016, 12:48 AM   #8
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If it's the neutral to ground bond you're concerned about, don't be.

The bond for shore power is done at the power pedestal.

For a generator, the manual should tell you how to do the bond if it's not already connected.

Larger inverters usually have it connected, with a means to disconnect it if necessary.

I've seen automatic transfer switches for 50 amp service for less than 100 bucks.
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Old 12-06-2016, 10:03 AM   #9
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Mike Holt is an industry expert who makes his business helping people understand the National Electric Code. Among the free resources available on his web site is his Generators and Standby Power Systems based on 2011 NEC summary. There's one here for 2014 NEC also, but the 2011 write-up has more/better illustrations for this discussion.

On page 4, Figure 445-5 illustrates what's needed. This figure shows a generator with a built-in ground-neutral connection. The shore power (aka Service) also has a ground-neutral connection. There shouldn't be two such connections in a system, so the transfer switch is shown disconnecting the generator's neutral line from the panelboard. The generator's (or inverter's) ground-neutral connection has no effect when using shore power because its neutral is interrupted by the transfer switch.

Regarding the NEMA 14-50 connector AlleyCat67 suggested: of course it'll work. I prefer to avoid those NEMA connectors because their insertion/removal force is so much higher as compared to the twist-lock variety. That's why I suggest if using plugs and sockets instead of a hard-wired transfer switch to use the other type. They're more expensive but SOOO much easier to plug/un-plug. Campground pedestals are going to have that 14-50 socket though so ultimately you'll have to have a way of getting into that to use their service.
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Old 12-06-2016, 10:14 AM   #10
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Quote:
Originally Posted by family wagon View Post
. The generator's (or inverter's) ground-neutral connection has no effect when using shore power because its neutral is interrupted by the transfer switch.

that is the answer i was looking for. thanks for explaining it to me.
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