Disclaimer: I’m not a certified electrician, just a stubborn DIY’er.
tl;dr: Scroll to the bottom for the schematic.
Electricity is one of the most important aspects of building a house. If you get it wrong, your house can burn down. Or you’ll constantly be flipping breakers and frying devices. Definitely hire a professional electrician if you just want a system that works and is safe. But if you’re a stubborn DIY’er-at-all-costs like me, read on.
Since the tiny hacker house is designed to operate off grid, the electrical wiring closely resembles what you’d find in a typical RV. I have to say, information on wiring an RV is extremely hard to come across on the web. The main hangup is accommodating for both 12V and 120V supplies. 120V wiring is straightforward enough – just follow your local building code’s guidelines. But throw in 12V appliances, a battery bank, inverter, “shore power” and you have a little more thinking to do.
Picking an Inverter
Needless to say, inverters are extremely important. The good ones seem to do it all safely and with flying colors, and the really bad ones can hiss, crackle, spark, and smoke if you even get close to their rated capacity. There are a few key metrics you need to keep in mind when picking one. Here’s what I learned:
There are two types of inverters: pure sine and modified sine wave. The modified sine wave inverters “mimic” 120V AC with a step wave so while fine for most devices, it can cause problems for sensitive devices like fluorescent lights, battery chargers, and some medical equipment. Pure sine wave inverters, however, generate an AC supply identical to what you’d receive from the city power grid. Presumably, it makes more sense to go with a pure sine model, but they can be nearly double the price for the same capacity. Personally, I’m making the tradeoff for a pure sine model. I don’t want any surprises on the road.
Assuming you’ve already determined your power requirements and settled on a battery bank configuration, you need to pick an inverter that matches your bank voltage. Most inverters are designed for 12V, but if you need more than 4000W or so they typically require 24V or even 48V DC sources instead. A 24V or 48V system is fine, but then you’ll need a step-down converter to power any 12V appliances you may have (yes, “technically” you can just tap your 48V battery bank at a 12V point to avoid the converter, but this will drain the bank unevenly and could reduce its lifetime). If you’re using solar as well, this will have to match your solar MPPT charger’s output. Also, you should pick an inverter with about 120% of the power capacity you plan to use in an ongoing basis. Most aren’t designed to run at 100% capacity for long periods of time.
Charging / Pass-through Capability
If you plan to ever hook up to a city power grid or shore power at an RV park (or even some generators), a charging inverter may suit you better. These smart devices are essentially an inverter with an AC input to automatically charge your batteries when you connect to an AC source. The switching happens on the fly – when you disconnect (or the generator runs out of gas), the inverter will automatically and instantly switch from charging mode to inverting mode with no loss of power to your house in between. Some of the beefier inverter/chargers can even pass split-phase 240V AC input through to power your devices. So if your tinyhouse uses a 50A shore power inlet, you’ll be able to use both 120V hot legs of the supply instead of just one, basically doubling your available power. More on that later.
According to everything I’ve read so far, the Magnum Energy ones are some of the best. They have a 48V 4000W+ inverter that supplies split-phase 240V so it could power electric ranges, large ACs, and electric dryers! We won’t be needing any of that for the tiny hacker house, but it is nice to know I am future-proofed in case my electrical needs grow.
30A or 50A Shore Power: What’s the Difference?
When you pull up to your spot at an RV park or campground, you’ll typically see an electrical panel like this:
Called the power pedestal, these are pretty much the “main panel” for your electrical system – they’re where the neutral is bonded to ground. Note: that means you DO NOT bond your neutral to ground in your tinyhouse’s electrical panel!
As you may know, 120V systems in the US utilize three wires: hot, neutral, and ground. Power is applied to your devices via the difference in potential between the hot and neutral wires. The ground is just a safety measure – if the hot leg accidentally shorts to the casing of the device it’s powering, electricity will safely short to ground, tripping the associated breaker instead of shorting through you if you happen to touch the afflicted device (think: old rusty washing machine). This is why old homes and some devices are in “two-prong” configuration – the ground can sometimes be omitted.
Now, you may notice in that picture the 50A outlet on the left utilizes four wires. Three of them are the same as any ordinary outlet, but the fourth is actually another hot wire. This means there are two hots, one neutral (shared between the hots), and one ground. Without getting too technical, this is to provide 240V AC to certain high-power devices. The additional hot is 120V as well, but shifted 180° out of phase with the other one, so the average potential between them sums to 240V. Pretty neat huh?
The corresponding breaker (the two handles on the left) is a double-pole breaker. That simply means it is actually two 50A breakers with a mechanism in place such that if one breaker trips, it forces the other 50A one to trip as well. So with a standard four-wire 50A outlet we actually get 50A + 50A = 100A at 120V, or 12,000W power. That’s over three times what the 30A outlet can provide and five times what a standard 20A outlet can provide.
So what does this mean for a tinyhouse? If you think you’ll ever need more power than what the three-wire 30A outlet can provide (namely 3,600W), you need to make sure to use a 50A inlet and wire your house accordingly. That means picking an inverter that can pass through split-phase AC input, and having at least two separate 50A electrical circuits for your tiny house: one for each hot leg of the 50A power.
Finally, here is what all that adds up to:
In my design, I actually have three panels in order to electrically isolate the inverter without having to rewire anything. Hence the “main panel” and transfer panel. You could just as easily omit these and feed directly into the inverter and then into your subpanel.
The subpanel consists of four 120V circuits: two 20V runs and two 15V runs. I’d have to upgrade the subpanel to make room for a double-pole breaker if I wanted to feasibly run a 240V appliance on this system.
Also note the current ratings written on the panels in the diagram are a bit misleading. Those are the total current ratings for each leg of 120V, so divide by two in the context of split-phase 240V.
Good luck and email me or comment with any questions!Read all posts like this:
- I'm Building a Tiny Hacker House
- Tiny Hacker House Design Part I: Overview
- Tiny Hacker House Design Part II: Power
- Tiny Hacker House Design Part III: Bathroom
- Tiny Hacker House Design Part IV: Loft
- Tiny Hacker House Design Part V: Supply Plumbing / Heating
- Tiny Hacker House Design Part VI: Wiring
- Tiny Hacker House Build Part I: Steel Framing
- Tiny Hacker House Build Part II: Plumbing & Electricity
- Tiny Hacker House Build Part III: Sheathing and Insulation