Wiring 100 Amp RV/tiny house panel from two 240v 50A feeds

Question

I'm in the process of building a tiny house. An electrician helped me install my service panel and the connections throughout the house. We either never concluded, or I've forgotten, what the plan was for feeding power into the panel.

The tiny house is intended to be used on a primarily permanent basis, but will have a few moves before it's set somewhere.

I've balanced the load in my 100 Amp (240V) breaker box to not only draw mostly evenly, but also to power necessities on one bus, and extras on another (A/C, washer/dryer, etc.).

Assuming I have a 240v power source with two independent buses, would it be possible to wire two 50A 240v RV plugs (2 at the source panel, 2 plugs on the exterior of the tiny house, 6/3 wire from the plugs to the tiny house panel), and connect them together on each pole of the breaker to form two 100A 120V connections (aka one 240v 120v connection), provided I recombine like phase to like?

Put another way, if the 6/3 cabling has one run from the plug to the tiny house panel, and one run from the other plug to the tiny house panel, both black cables would be tied in at the ground screw to the 100A breaker, and both red cables would be tied in at the other ground screw to the 100A breaker.

If the electrical logic makes sense but it's unsafe to tie them together on a ground screw, would it make more sense to use two 2-pole 50A breakers; or a sub-panel to tie the feeds together on independent buses, then use higher-gauge wire to feed the tiny house panel as a 100A source?

Any input is helpful (though don't worry about code beyond its application to safety please. To my knowledge there will not be any mandated inspections)

Answer 1

You can't do that. It is called paralleling. It is only allowed on really big feeders, not typical residential 50A - 400A. There are a few different concerns:

• If you connect both sets of wire together at both ends (not what you are proposing), then if a wire goes out (e.g., loose connection) then the corresponding parallel wire gets the full load instead of half the load, overheats because the breaker won't catch it (breaker on either end for full load) and your house burns down.
• There is no way to guarantee which set of wires will get the most use. If everything is nearly perfect (same type of wire, same length, good clean connections) then the current should be roughly split. But if any wire comes out then you will end up with an overload on the corresponding parallel wire. If that wire is a hot wire then it should trip a feed breaker. If that wire is a neutral wire then it will not trip a breaker and your house will burn down.
• There are some dangerous scenarios that could occur if one of the 50A feed breakers trip but not the other one, where there would be live power present at a tripped breaker. Everything is fine if you unplug the cords, but if not, somebody could get zapped working on the panel.

The proper way to do this is to live within your means. If you can only get 50A on a single temporary (or even semi-permanent) hookup then that is what you have to work with. The truth is that 50A (which should be planned limited to 40A continuous) is actually quite a bit to work with for a tiny house. Plenty of regular houses rarely go over 50A usage, so you really should be OK, with the following caveats:

• No electric tankless water heating. Period. Don't even think about it.
• No electric resistance heat. A plug-in 1500W heater is OK on an as-needed basis, with the catch that you make sure not to run too much other stuff at the same time. But baseboards, etc.? No way.
• Clothes dryer can be an issue. Typical 30A x 240V connection with usage close to the 5,760W continuous limit. Can be used but might need to limit use to when you are not using too much else (air conditioning, oven). If you have not already bought the dryer, you may want to consider a condensing dryer - costs more up-front but uses a lot less power.
• Air conditioning - First of all, consider a heat pump that can also provide heat. Second, efficiency is absolutely key, as that will significantly affect the power usage.
• Oven/Cooktop - This is the big one. You can easily use 240V x 30A or more. In an ordinary house this is rarely an issue, but at 50A maximum it can be critical. So pick your appliances carefully and consider how you will use them.

With all of that considered, 50A is actually quite plausible - as long as you don't try to cook the Thanksgiving turkey on a really cold day with heat running and dry clothes at the same time, etc.

Answer 2

Assuming I have a 240v power source with two independent buses, would it be possible to wire two 50A 240v RV plugs (2 at the source panel, 2 plugs on the exterior of the tiny house, 6/3 wire from the plugs to the tiny house panel), and connect them together on each pole of the breaker to form two 100A 120V connections (aka one 240v 120v connection), provided I recombine like phase to like?

NO NO NO NO NO. Sorry, best I can do. StackExchange doesn't support huge fonts, red text or emoticons :)

There is absolutely no way on earth to merge power from two separate sources like that and do it in any kind of thing resembling safety. Not possible, not within 1000 miles of possible.

Aside from the 50/50 chance of one outlet being wired backwards from the other and you creating a dead short, see everything manassehkatz says.

And by the way, when the panel says "100A breaker box", that's just a never-exceed redline. Like where your car tires say "130 mph max" or whatever.

Here's how I would do that.

I would install two panels (breaker boxes). One would be the Primary Panel and the other would be the Secondary Panel. The Secondary Panel would get as few loads as possible to avoid the risk of mixing up neutrals, so big ones. It is vital not to cross neutrals among these panels. And especially not hots.

Each panel is fed from a different inlet (or very short cord with a 14-50 plug on it).

Right next to those is a 14-50 outlet (wall socket). Say what?? That outlet is fed by a 50A breaker in the primary panel.

Yes, you guessed it. When I have access to only one RV supply, I plug the primary panel into the grid, and plug the secondary panel into the primary panel via that convenient outlet.

When I have access to two RV supplies, the outlet goes unused and I plug both panels in separately to separate sources.

Of course it's a Code violation, but it's a hell of a lot safer than the other thing.