Wicked Tiny House Electrical System Overview - Part 1
August 14, 2018
What's the difference between a tiny house and pioneers house built in the 1800's? Not much if you think about it. They're both small, hard to fit a family of eight into, and many have a very rustic feel. But I bet you've never seen an authentic version of the Little House on the Prairie with outdoor lighting, a 32" flat screen TV, and solar panel hookups! So, for my second installment of "Yes, I built it myself!", let's do an overview of the Wicked Tiny House's electrical system.
As with the plumbing system, the whole process started with clarifying what my electrical needs were. I knew I wanted to have all the comforts that the modern world offers - lighting, a microwave, TV, computer, and all that good stuff. I also knew that I wanted to be able to have the option to install a solar setup and go off-grid at some point in the future. From there, I calculated what my maximum power usage would look like, with the goal of keeping it below 20 amps, which what typical outdoor outlets are rated for. Why 20 amps? I wanted to be able to park pretty much anywhere, run an extension cord to an electrical outlet, plug in, and be good to go. Requiring a 30-amp or 50-amp hookup would've greatly reduced my options for future living situations, and my goal was to keep my options as wide open as possible. This all seems pretty simple, until you start to consider that the most commonly used electrical fixtures (lights, fans, electrical breaker boxes) aren't designed with a tiny house in mind. It required some "creative" use of some materials, but with a little planning, the electrical system is safe, up to code in the important places, and hasn't let me down in the two years that I've been living in the Wicked Tiny House. This first portion of the electrical system overview takes a look at the "rough" electrical system, including planning, wiring, and distribution. See part two for more information about electrical finishes such as lighting, smart outlets, fans, and more.
Electrical Calculations - The first step in this process is to figure out what the maximum current draw of your house will be. If you plan on hooking into a 20-amp receptacle like I did, then you want to make sure your house won't ever draw more than 20-amps at one time. If you exceed this amount, you'll trip the breaker and it will need to be reset. Although this technically won't hurt anything, it is a annoyance and means that you're stressing your electrical system.
In order to calculate your maximum current draw, think about what electrical items you'll have in your house and which ones you'll be running at the same time. Item's that have a large draw include microwaves, power tools, and anything that produces a lot of heat like a blow drier or iron. So, if you think you might be running the microwave, while you blow dry your hair and watch TV with the lights on, then the that would be your max draw. Lets say you have a 1200 watt microwave, 900 watt blow drier, 120 watt TV, and your lights draw 80 watts, your total draw will be 2,300 watts. We convert that to amps by dividing by the voltage which is 120 volts in this case, and find that our total draw is 19.2 amps. Right at the limit of our 20 amp goal, and for me, too close for comfort. In this case I would recommend toning down the multitasking a bit.
You can see from the example above that it doesn't take long to reach that 20 amp number. This is why many tiny houses will use a propane (or natural gas) water heater, oven, and heat. Using electricity for these appliances is a HUGE draw that will often require you to have a minimum 30-amp electrical service. By choosing to use propane for these high power appliances, you are keeping your options open down the road.
Electrical Planning - After figuring out that a 20-amp service would work for my power needs, my next step was drawing a very basic electrical diagram. You most likely have a breaker box in your house/apartment/yurt that distributes power from the outside world to you house. Typically power at the street will be 7,200 volts (or more) and is stepped down by a transformer at the street to the typical 240 volts alternating current (AC, more on that later) which enters the home and is then distributed through a breaker box (aka load center). Almost all household appliances run on 120 volts, except for electric ovens, driers, water heater, and a few other notable high load exceptions which run on 240 volts. The breaker box will have separate breakers for any 240 volt appliances, and will then essentially "split" into two separate 120 volt currents. This is what the rest of the breakers in your breaker box are. Each breaker is its own power "circuit" which is typically 15 amps, or in the case of bathrooms, kitchens, outdoor outlets, and other potentially high load areas, 20 amps. A typical household breaker box will have room for 20 or more breakers/circuits. However, if you remember, I planned for my house to run on a maximum of 20 amps, so I would only need one circuit for my entire house, right?
Technically, yes. Practically though it doesn't make sense. I decided on a six breaker "sub-panel" that I would use for the main breaker box in my house as it was the smallest panel I could find at a reasonable price. This allowed me to break my house into separate circuits. I planned two circuits for lighting (front and back of the house), one for the kitchen/bathroom/outdoor outlets, one for the living space, and one for the loft. Is this overkill? Yep. Am I glad I did it. Yep! It made troubleshooting a breeze because I could easily isolate circuits to track down trouble spots. As for the lighting on their own circuits? I did that because all of my lighting is LED lighting that can be run directly off direct current (DC, rather than AC), so in the future, when I install a solar setup, the DC LED's can be run directly off the battery instead of converting from DC, to AC, and then back to DC for the LEDs. The difference between AC and DC current is beyond the scope of this post, but it is readily explained in a plethora of places online.
Below are the actual items that I installed in my house. Installation was very simple, and after two years of use I haven't had any issues. Note that this breaker box is a shell, and that you need to buy breakers and a bus bar separately. All of the goodies you need are linked below.
Planning for Future Solar - If you have no interest in powering your house with a generator/solar/wind/etc, then feel free to skip this section. But I highly recommend that even if you aren't considering alternative energy in the future, $120 and an hour of extra work now will leave your options open down the line. It's much easier to install this now than having to do a retrofit job down a few years down the road.
In order to use both traditional grid power (IE, plugging into an existing electrical system or house) and alternative energy such as solar, you need to have a way to switch between power sources. This typically comes in the form of a transfer switch. If you have a generator that you use when the power goes out, its the same idea. Go out into the garage, throw the transfer switch from grid power to generator power, fire up the generator, and you're good to go. In some fancy systems, the transfer switch will actually recognize when the power goes out and automatically switch so that the user will hardly notice that the power went out. That kind of system is a bit rich for my blood, so I chose a manual transfer switch.
I mounted the transfer switch near where power enters my house. "Grid power" comes into the house, runs through the transfer switch, and then on to the breaker box. If I also had a battery bank (or generator), a second line would run into the transfer switch from that power source. Switching between grid power and battery power is as simple as.... you guessed it..... flipping the switch. Because I don't have my solar setup installed yet, I just have an unused 8-gauge wire coming out of the transfer switch. When I'm ready for solar, all I have to do is hook my battery bank into the transfer switch, and I'm good to go. Now hopefully you can see why I advocate for installing the transfer switch now. Having to do this work down the road would be much more expensive and time consuming.
Electrical Rough In - Even if you don't feel confident doing all the electrical wiring yourself, I believe most people are cable of doing the "roughing in" of the electrical system, which basically entails running the wires through the walls to where you want your outlets, lights, and appliances to be hooked up. It's not rocket science. You can probably cut the cost of having your electrical work professionally done in half by running the wires yourself, as it can be time consuming to route wires through walls. I'm not going to do a full tutorial on roughing in electrical wires, but essentially if you are planning 5 circuits like I did, you'll have five wires leaving from the breaker box to your outlets/lights/appliances. So if you have all your outlets on one circuit, one of those five wires will need to travel to each outlet, essentially daisy chaining them together.
Romex (trade name) is the most commonly used type of wire you'll find. It's basically 3 (or 4) wires wrapped into one bundle. Back in the old days, you would need to run the three wires separately, but now you can get them conveniently bundled together. Romex is typically named with the gauge and number of wires. A 14-2 Romex will have a black(hot) and a white(neutral) wire as well as an unsheathed copper wire for ground. A 14-3 Romex will have a black(hot), red(hot), white(neutral), and bare copper. The red on a 14-3 is used for three-way light, if you choose to do have that functionality. 14-gauge Romex is used for 15-amp circuits and 12-gauge Romex is used for 20-amp circuit (kitchen, bathroom, outdoor outlets). Keep in mind that 14-gauge is significantly easier to pull around corners, so only use 12-gauge where you really need it and plan out your runs in advance.
As for tools for working with Romex, you'll want a decent quality set of wire cutters/strippers and a Romex sheathing ripper which makes your life SOOOO much easier when removing the Romex sheathing. I used the tools below, which were cost effective and worked well. There are more expensive tools that work incrementally better, but for a project the size of a tiny house, these worked really well. Also, note the cost difference between the 14-2 and 12-2 Romex. Only use the 12-gauge where you really need it!
Exterior Electrical Hookup - After finishing the interior rough in, and installing the transfer switch and breaker box, it was time to actually bring power into the house. Now, because this isn't a typical home and I'm going to bring power into the house similar to an RV, I decided to install a 30-amp "twist lock" type connection that is typically found on RV's. Why a 30-amp connection when I'm only planning on hooking into 20-amp power? Just like with solar, for future convenience. This gives me the option to use 30-amp power in the future if needed with no modifications required.
Another advantage is the "twist lock". As the name implies, when I plug into this receptacle, the extension cord twists and locks into place, so there isn't an issue with the extension cord ever coming loose. In order to make this work, I actually built my own extension cord (see next section) with a 20-amp (typical 3-prong plug) end that plugs into grid power at the house and a 30-amp (3 prong but different shape) plug that plugs into the tiny house. This is a very simple modification that costs about $15 for a new extension cord end and about 10 minutes work of time. You can also do this by purchasing a regular extension cord, hacking off the end you don't need, and adding your desired plug type.
Extension Cord Selection for Hooking into the Grid - This is an often overlooked part of the electrical system. A lot of people don't realize that voltage will actually drop over a long run. If you plan to connect to an existing electrical system with an extension cord, you should think about what gauge extension cord you need. A 200 foot run of 18-gauge (very cheap) extension cord pulling 15-amps will result in a 32% drop in voltage! That means that the 120 volts you are getting at the start of the run is only 82% when it gets to your tiny house. Voltage this low can both damage appliances and result in an overloaded extension cord, potentially resulting in a fire! A 10-gauge extension cord will only see at 5% drop over that same run, which is acceptable.
If you really don't want to spend the extra money for the thicker extension cord, you can build you own! You can buy whatever gauge cord you need in bulk from a hardware or electrical supply store and add you own ends for less! Just make sure you get cord that is rated for outdoor use.
Look for Part 2 coming soon, which will include the electrical finishes including LED lighting, smart electrical outlets, fans, and more!