One Tiny House, Two Sub Panels
Hey there. Question that might be a little weird: so we went with electric oven (.04 cents per kWh and deal on oven made it too good to pass up), which with all burners and the oven at full blast, with everything running needs 40a breaker and draws 11,700 watts at 240 according to Bosch. We have to do rv service to avoid wrath of the county. Rv service is 50a that allows 120/240 and 12k watts, so I’m going to install one subpanel just for our oven and then one panel for everything else. Then we’ll just have basically two independent sub panels via two different 50a rv plugs coming out of the 200a main service panel. A little spendy but should be totally safe. Does this all make sense? Also, for the subpanel with everything else, It shouldn’t be a problem to run the Mitsubishi minisplit on 240 while the rest of the panel uses only 120? Thanks so much!
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No, not totally safe under all conditions. You actually open yourself up to a lot of potential problems here by doing this, specifically the ability to bring in two seperate supplies of power with different ground references. This can be extremely dangerous.
The math on that Bosch unit doesn't work out. A 40A 240V circuit is capable of delivering a maximum, under perfect conditions, of 9,600 watts. Period. That's it. It doesn't matter if Bosch wants to claim anything else. What you're likely seeing is a maximum of all the burners added together, but the unit isn't actually capable of doing that, and can't actually draw more than 9.6kw, if even that.
Are you sure you have to do "RV service", or do you just need a "temporary" type of power service, something using a cable with connectorized ends that can be "unplugged"? If the second will suffice, and I think it likely will, you could use 100A pin and sleeve connectors, which would work, but are expensive. An example is here:
https://www.elecdirect.com/pin-sleeve-devices/100-amp-pin-and-sleeve-devices/ip67-iec309-pin-sleeve-plug-100a-125-250vac-3-pole-4-wire-watertight
You would need 2 gauge, 4 conductor cable of type SO or heavier (possibly type W, I don't work with this stuff often and haven't looked it up). This essentially makes a very heavy extension cord, but will probably satisfy the county. My guess is they don't want you permanently wiring power to a mobile structure, so any disconnectable means using a listed assembly (fancy way to say "a cord with proper connectors made for this purpose") will probably keep them happy.
Bill
Why would they have different ground references? Also these would be separate systems within the house no? How is this different than having multiple sub panels in a house?
It is the *potential* for the problem. If the two connections were to seperately derived sources (which has a specific meaning), you can have problems. You can partially get around this by tying the ground busbars in the two panels together, but that then introduces additional code issues in other cases related ground-neutral bonds and their relative positions in the system. Everything within ONE STRUCTURE (not system) is required to have the same common ground reference. In commercial buildings, this is done by attaching a ground "bond" to the building's steel frame anywhere such a bonding connection is needed. The building still will have only ONE electrical service entrance though in almost all cases, which means the neutral side isn't typically an issue.
In your case, there is the possibility for both grounding AND bonding problems. You avoid this completely with a single power connection, such as by using that 100A pin and sleeve connector I linked to. Note that it's a "3 pole, 4 wire" connector too. That's important: since the panel in your tiny house would be a subpanel connected to the external power source, the ground and neutrals are required to be kept seperate in your tiny house's panel. The ground-neutral bonding (connection) point is required to be at the first service disconnect, which is usually the main breaker in the case of a house. Code also requires that there be ONE, and ONLY ONE, ground-neutral bonding point. That second part gets complicated if you go with multiple panels, and is impossible to accomplish with three wire cord sets.
Do you have no way to use a single panel with a larger service? That would be the simplest and most elegant way to go. The other option would be to have one power connection for your panel, and another for ONLY the electric oven, which leaves the oven as an "appliance" connection, and the plug/cord is just a normal branch circuit. This avoids some of the complexities that come into play if you have two subpanels.
Bill
I think what Bill's getting at is that while, in the situation exactly as proposed / described, it's similar to one main panel feeding two subpanels, the fact that you are providing two connection points on the exterior means that those two connection points could possibly be connected to two different sources, and that's the issue. The proposed structure is allegedly mobile / portable, and thus should be constructed in a way that is "portable" to different sites with different conditions / connections.
Yes, that's exactly what I'm getting at. The possible situation I can think of where this could occur is in an RV park where you plug into two seperate power connections. There is no guarantee those two power connections won't be powered from completely seperate sources, and then you're in a potential safety situation. Depending on the specifics of such a situation, the problem the "two panel tiny house" causes may be limited to only the tiny house, or could potentially cause a problem for the entire RV park, and it would probably be a hidden problem that doesn't show up until something happens that lets out magic smoke somewhere.
Bill
Oh very true I was not thinking about it moving, good point.
The question I would have is, what code applies to an RV? Because that's what the OP is building. And I have no idea. There's nothing wrong with having an electrical service with two 50A receptacles side by side, and that's where the jurisdiction of the NEC ends.
The RV is more like an appliance that's plugged into the receptacles. Does it have to be UL listed? Is there some other code? I don't know.
DC, you get into portable/temporary power setups, and the NEC does cover these. If the structure is subject to other building codes, it would be subject to the electric code too. The NEC has sections for things like theatrical lighting, and other somewhat oddball things, so it doesn't necassarily end at the receptacle. The general rule that if power is being distributed to other things, the NEC likely applies. If the cord is connected to an appliance, then it's usually covered by UL/CSA and NEMA.
Bill
DC,
Tiny houses on wheels still inhabit a grey zone. Practically, they are site built RVs, but avoid the regulation and standards manufactured ones must meet. At the same time they aren't subject to building codes - and I've never seen a one that met, or could meet, our code.
The most common impediments are:
- Sleeping lofts accessible by ladders
- Ceiling heights
- Room, hallway, and door sizes
- Non-CSA rated appliances
- Required insulation
- Structural sizing
What I'm getting at is that the whole point of building a tiny house is to sidestep regulation. Usually zoning is the primary concern and other codes fall by the wayside.
Some municipalities have gotten smart about it and started to regulate them as RV's. The next step is to pass a bylaw prohibiting camping or limiting it to a certain number of consecutive days.
DC,
Yes. Tiny house on wheels, (as opposed to small ADUs), create more problems than they solve.
Well I went down a rabbit hole and am still there, still a bit unclear as to any answers but for those that are curious NEC 551 and Nfpa 1192 covers recreational vehicle. I feel like if you are building one you should become familiar with these sections.
Edit- I don’t think you can run a 100 amp supply or two 50 amp
“551.44 Power-Supply Assembly
Each recreational vehicle shall have only one of the main power-supply assemblies covered in 551.44(A) through (D).
(A) Fifteen-Ampere Main Power-Supply Assembly
Recreational vehicles wired in accordance with 551.42(A) shall use a listed 15-ampere or larger main power-supply assembly.
(B) Twenty-Ampere Main Power-Supply Assembly
Recreational vehicles wired in accordance with 551.42(B) shall use a listed 20-ampere or larger main power-supply assembly.
(C) Thirty-Ampere Main Power-Supply Assembly
Recreational vehicles wired in accordance with 551.42(C) shall use a listed 30-ampere or larger main power-supply assembly
(D) Fifty-Ampere Power-Supply Assembly
Recreational vehicles wired in accordance with 551.42(D) shall use a listed 50-ampere, 120/208—240-volt main power-supply assembly.”
(Response to #20)
Basically if you think codes are a good thing it's hard to get behind tiny houses. And judging from the questions we get here it seems that a lot of the problems wouldn't have existed in the first place if they'd been built to code.
Personally, I believe overall in the codes. There may be a few nonsensical things but for the most part they make sense, especially since the IRC gained steam and we don't have to deal with local flavors so much any more. I like to think if I was building a house for myself some place where there wasn't code enforcement I'd still build it to code.
Zoning on the other hand. Well, don't get me started.
DC,
I agree. If you follow our current code you end up with a really well constructed house. You have "built the worst house you are allowed to", as is often said here on GBA, but our code makes sure that "worst house" is a pretty good one.
I’ll look into the 100a connector for the service. This seems to be the smartest way to go!
That said, this structure will never ever move the main 200a panel and meter is about 20 ft away. Was going to wire two rv outlets in about 5ft from where I need to plug in on the trailer.
I wish I could just run a standard subpanel, but you’re correct, I just need to be able to unplug the house if the county comes.
Thanks for all of your thoughts!
EDIT: I also find it funny that Bosch's math is weird. For most of their appliances, it's a real pain to try and find the power consumption.
Tbbg,
If the building is never going to move, it should be built to code like any small residential structure. Using the fact it is on wheels, and potentially portable, to get around those requirements helps no one. It's either an ADU or a site built RV.
It is built to code but the county isn't concerned with it because it is on wheels. Hence, why I'm asking how I should do this correctly because I'm not allowed to wire a permanent subpanel due to code...
I understand the electrical theory, and there's many ways to skin a cat, so I'm just hoping to find the best way to skin said cat.
And there is a chance the house moves down the road to a different part of the property once we have the money for foundation and regular stick-built house, thus it has to be mobile.
Tbbg,
I should have been clearer: If at some point it is going to be a permanent structure, the electrical service should reflect that.
With the 100A receptacle, conversion to a “proper” electrical service would be easy: remove the receptacle/plug/cordset, wire in the permanent power, bond the ground and neutral busbars together in whichever is the main panel now — probably the tiny house? — (do NOT forget this step), done. Everything from the panel on should be built to regular code already.
Bill
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How often do you expect to be running every single burner on the stove and the oven full blast while doing a bunch of other stuff in your tiny house?
Pretty much never. Every other appliance is pretty efficient and water is our only propane-powered appliance.
On the "main sub panel" We have: 15A breaker for Mitsubishi H2i 12k but mini split. Hard to find info, but I'm guessing 1500 watts while at full blast?. 15A for Refrigerator, call it 800 watts. 15A dishwasher (700 watts for heated cycle), 15A for washing machine, 700 watts. two 20A for kitchen circuits (blender and rice cooker, mainly), 20a circuit for main lighting, 20a circuit for living space (computer charger, phone charger), 20a circuit for heat tape for water line under the house (50 watts for 10 feet of heat tape), 20a for roof heat tape (750 watts for 150ft) and maybe led Christmas lights and tools on the roof if I ever have to work up there.
The second sub panel would have only the range.
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