As I puzzle through wiring this building I find myself humming along with Thomas Dolby.
Sandra and I have spent a lot of time over the last couple of weeks thinking about all things electrical.
I mentioned a few weeks ago that I was trying to come up with a sketch of our electrical layout, but what I discovered in the process was that I did not have enough information (more importantly knowledge) to even start a sketch.
Let me see if I can explain …
Standard Residential Wiring
A typical house in North America is wired for 120/220 volts AC (alternating current). Given the simple reality that virtually all residential wiring is done this way the process is well understood and very well documented. Some excellent references for residential wiring that deal specifically with the Canadian Electrical Code are Electrical Code Simplified: Residential Wiring and Electrical Code Simplified: Commercial & Industrial Wiring both by P.S. Knight.
Our project deviates from standard residential wiring.
The most fundamental difference for our wiring is that we plan to generate our own electricity via solar, wind, wood or a combination of all three. We would also like sell this power back to our power utility with the aim of being energy neutral, and possibly some storage capacity (batteries). In other words we hope to generate as much electricity as we consume on an annual basis.
This means that our electrical layout will include generation, storage, and transmission (back to the grid) of power. I am ignoring all of these issues for now and concentrating on what I thought was the simpler problem of lighting and appliance branch circuits.
Efficient Lighting
We have evaluated a number of lighting options including incandescent, compact fluorescent, and LED.
Incandescent – There was no real evaluation here. Incandescent lighting is terribly inefficient, with the vast amount of electricity consumed being wasted as heat. Ironically, in older houses that are refitted with compact fluorescent lights as part of an efficiency upgrade there is some debate to the value of the upgrade. In some cases the heat lost from the incandescent lights in the winter has to be replaced by more expensive heating options!
Compact Fluorescent – This is the current efficiency choice and does have some merit. I dislike compact fluorescent bulbs due to the required ballast (it is environmentally bad), and the fact that all the ones we have ever installed have not lived up to their longevity claims.
LED – This is probably the newest efficiency option and is comparable to compact fluorescent technology in energy saved. It is a DC (direct current) technology and if it is used in an AC branch circuit a transformer must also be included to convert the power. Home generated power is stored in batteries (also DC technology) so there is a definite fit between LED lighting and home power. LED lighting does not need to be inverted (changed from DC to AC voltage) and consequently you avoid conversion power losses of approximately ten percent. LED lighting is not as effective at diffuse lighting, and right now is used more in task lighting.
We plan to use LED lighting for both our room and task lights.
We have not done significant research on efficient appliances but are fairly convinced that a DC fridge makes sense in our situation.
The problem lies in laying out the branch circuits for these DC appliances and lighting.
Branch Circuits
A branch circuit is simply electrical wiring that supplies power to some of the electrical loads in a building and terminates at the electrical service or panel with breaker or fused protection. In the case of 120/220 volt AC residential branch circuit wiring the rules for lighting and receptacles are well established and wiring is straight forward.
DC and AC appliances cannot share the same wiring so the DC branch circuits must be laid out separately. Further, the rules for AC branch circuit wiring do not apply as DC appliances operate at lower voltages and higher currents. In fact, low voltage DC wiring merits its own section in our electrical code and is referred to as a Class II circuit. Practically, this means that an equivalent DC circuit usually requires heavier gauge wire.
Class II branch circuits are not common in residential wiring and consequently there is more thinking (and learning) required to figure them out.
Some Questions that I have been thinking about, and the (hopefully correct) answers I have come up with are:
- Are there any issues with using standard NMD-90 wire for DC circuits? No. As with AC wiring the wire needs to be suitable for its intended location, and be sized correctly for the ampacity of the circuit in question.
- How effective is LED room lighting and how do we evaluate (style, wattage and quantity in a room)? I am assuming an LED room lighting solution equivalent to a 60 watt incandescent bulb will be 13 watts. I am using this assumption to lay out my branch circuits.
- What gauge wire should I use for my DC branch circuits? How many lights and receptacles can I hang off each branch? Do I simply need to design each one? (i.e. no established rules) Yes, I will be designing each one. The best reference I have found to help with this job is a book I tracked down in the library, Photovoltaics: Design and Installation Manual by Solar Energy International.
Our House
I think I have enough information to rough in the branch circuit wiring for our house. This means I can FINALLY get on with decking the roof. I was starting to believe we would never place the first board!
I leave you with …
Good morning,
My father and I got a chance to visit you last year while you where still filling tires on the U’s. I am very interested in your electrical system. I pulled my code book out and in the CEC it doesn’t matter if it is AC or DC the wire must be sized for the load. Wire size decides breaker size. I do suggest larger appliances that have known loads be on there own circuit as it makes servicing much easier and safer.
If you have any questions I can help with please send them my way as I would love to start picking peoples brains at work. Do post your prints or sketches when you get a chance I would love to see them.
Good luck
Jeremiah
it doesn’t matter if it is AC or DC the wire must be sized for the load
Yes … this is my understanding as well.
Wire size decides breaker size
I have not read enough on Class II circuits but I believe you are allowed much lighter breakers for comparable wire sizes.
I will try to post my work on the electrical as I proceed.
Any comments are welcome!
A great resource for planning out you DC electrical system is to look at the boat industry. There are tons of resources available out there.
DC wiring principles are well established in boats, RVs and yachts. Fire (including electrical fire) is one of the leading causes of loss of life and property at sea so the industry takes safety and efficiency quite seriously. Check out this primer from a major boat equipment supplier:
http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorView?langId=-1&storeId=11151&catalogId=10001&page=Marine-Wire
For folks just running a couple of LED lighting circuits, that’s probably enough to get you in business.
For folks planning on trying to go totally off the grid or just doing as much DC as possible, I highly recommend Charlie Wing’s book “Boatowner’s Illustrated Electrical Handbook” and Nigel Calder’s book “Boatowner’s Mechanical and Electrical Manual.
DC wiring is much more sensitive to voltage loss than AC wiring. That’s whole reason AC was invented and adopted. This voltage loss is also sensitive to temperature, so while rules about the maximum number of conductors in a given size of conduit are designed to keep temperatures down low enough not to start an electrical fire, we’re also really interested in not squandering our electricity by generating electrical resistance heat in the walls.
For wiring a whole house in DC it really pays to get a good understanding of the unique characteristics of DC and designing to your needs in addition to the code–it would take a whole lot of extra solar panels (with a lot of embodied energy in them) to make up for an ill-designed DC distribution system.
Also, since some of the biggest energy loads in most homes is refrigeration, the section in Calder’s book on designing and building your own fridge/freezer is invaluable.
As a side benefit, since it draws on bountiful experience in the boat building world, it may be easier to gain buy-in from building officials when your design falls too far outside the conventional AC-wired home.
A terrific bonus of maximizing the use of DC and just having a few AC plugs for convenience items is that you can bypass expensive frequency matching DC-AC inverters for grid tie. If you’re going to grid-tie, connect from the grid to a battery charger then to your battery bank. For household AC loads, run an basic inverter off your battery bank. Your DC generation equipment ties into the batteries on the load terminals. Since the batteries separate the grid from your “power station” there’s no need for special planning permission from your utility and no fancy equipment. The drawback is that you are not set up to sell back excess power to the power company so you’ll need a dummy load like an electric hot water heater tank for when your batteries are full and supply>demand. You also won’t get to benefit from net-metering. It would take a huge battery bank and inverter set-up to handle large 220v appliances like an electric clothes drier or electric cooking range, so you could run those directly to your AC panel. It might not be for everyone, but it goes to show there are more ways to go than the standard PV to frequency-matching-inverter-charger net-metering.
Sorry for the wall of text. I must have some engineer blood in me–I can geek out on systems design a bit.
Great info…I will make sure Chris sees this!