Wiring Guidelines for Off-Grid Homes

ricielectric

عضو جديد
إنضم
13 نوفمبر 2009
المشاركات
684
مجموع الإعجابات
34
النقاط
0
Wiring Guidelines for Off-Grid Homes


The first efficient and reliable inverters for off-grid homes came out in 1988. Prior to then, early off grid pioneers wired their homes for DC lights, switches, and motors. Everything was custom-made. Early power systems were often 12V DC for the simple reason that appliances were borrowed from the RV industry.
Starting with the advent of good inverters, home systems have steadily moved away from specialized DC wiring to conventional AC wiring, with most or all loads run off of inverter power. With the shift toward AC systems and conventional wiring, systems have also grown in size and capability. Homeowners have wanted the same comforts and capabilities as their utility-powered friends. Twelve volts is now obsolete for all but the smallest systems; modern 24- and 48-volt systems are more efficient and more capable of growing with future needs.
Thankfully, the days of car fuses, cigarette lighter sockets and 12-volt TVs are gone. Nowadays, for the most part, the wiring in an off-grid home is just like any other house. This article will point out the key things to keep in mind as you plan the wiring for your off grid home. This article is written for conventional electricians as well as homeowners.
First of all, don’t be afraid to ask us for advice. Most of what you are encountering for the first time we see on a routine basis. We've seen what works. The best time to work out problems at the design stage, as it's always cheaper to make changes on paper than in copper.
We are licensed electrical contractors, and we pull our own permits on our installations. We generally only build the power system, and don’t do conventional building wiring. We routinely work alongside wiring contractors; generally, the breaker panel is the point of connection between our work and theirs. If we are working with you to develop a solar electric power system, we can provide you with general answers on the scope of our work and theirs, Code issues to be aware of, special circuits, and answers to specific questions you may face. We also offer consultant services on an hourly or contract basis, whether or not you are one of our clients.
No Shared Neutrals
In conventional home wiring, two adjacent circuit conductors can share the same neutral (white) conductor. This is called a “multiwire branch circuit” or “shared-neutral home run”. This is perfectly safe because the two hot (red and black) conductors are out of phase with each other. The neutral conductor only carries the difference between what is carried on each hot conductor.
The device that makes AC power is called an inverter. Nearly all modern inverters make 120V AC power in sufficient quantity for most home needs. The 120V AC inverter output is usually jumpered to both hot lugs in the load center. When this is done, the two hot (red and black) conductors are now in phase with each other, and the neutral conductor carries the sum of the loads carried on each hot conductor. As such, multiwire branch circuits can cause the neutral to be overloaded and are unacceptable. You must run a separate home run for each circuit.
There appears to be a growing trend to offer inverters capable of split-phase 120/240V AC power, which will render this a moot issue.
240V AC Loads
On all but the largest home power systems, 240V AC loads are to be avoided. To get 240V power, you either need two inverters or a step-up autotransformer. Either choice drives up the system cost and complexity. Most 240V loads are large bulk consumers of electricity, and are thus inappropriate to run off of a PV system. Propane or natural gas hot water heaters, ranges, and dryers are obvious alternatives.
The one common exception is a submersible deep well pump. Up through about one horsepower, we use a step-up autotransformer to run a deep well pump, with the control switchgear on the primary side of the autotransformer. That is usually within our scope of work. If you are running wire for a 240V pump, call us--we'll work out a plan with you.
We will often specify a split-phase 120/240 inverter if any two of the following three conditions are present in a particular application:
• The backup generator has already been selected, and is only capable of full rated output at 240V AC;
• The home has an existing 240V AC well pump, or must use one.
• The home has multiwire branch circuits.
 
التعديل الأخير:

مواضيع مماثلة

ricielectric

عضو جديد
إنضم
13 نوفمبر 2009
المشاركات
684
مجموع الإعجابات
34
النقاط
0
Subpanels
Some clients with utility power want a PV power system to function as backup power in the event of a utility outage, like a big UPS. They may want certain circuits to run off of the inverter power, but other circuits, like an electric oven or dryer, will be unnecessary during an outage. In this case, a dual panel arrangement is best. A main load center is fed by utility power, and is usually set up for 240V AC. A second "critical loads" load center is fed by inverter power. If a common gutter can serve both panels, home runs can be switched later between panels if the client wishes.
Bonding
With conventional utility power, power only flows from source to load and there is one AC neutral-to-ground bond point in a building. Our PV systems have multiple sources, including one or more inverters, a generator, and sometimes utility power as well. We have pretty much worked out Code-compliant methods to ensure proper single-point bonding. The bond point is generally not in the AC load center.
Phantom Loads
Phantom loads are things that are always on, drawing power, even when turned off. Microwave ovens, VCRs, TVs, computers, and most stereo equipment are a few of the typical household phantom loads. Each device may draw only a few watts, but for several reasons they can use up a surprisingly large portion of a power system's output. The simple solution is to add dedicated wall switches to control outlets wherever such specific devices will be located. This allows an easy way to eliminate such waste.
Transformers
All hardwired transformers are phantom loads. If installed in a heating system, for example, low voltage DC power is inverted to 120, and then transformed back down into low voltage power, with a huge efficiency loss along the way. It's possible to run an entire hydronic heating system directly from the battery's low voltage power—see our web page about the SETH SystemTM.
All wall cubes for small electronic devices are phantom loads. Likewise, low voltage halogen lighting with a transformer on each luminaire is very inefficient. And if you absolutely must install a transformer for a doorbell, install a switch or disconnect between the load center and the transformer.
Fixture Choice and Control
First, efficiency pays. Wherever possible, choose lighting fixtures that can fit compact fluorescent bulbs. They don't flicker, the color is good, and they are readily available in many sizes and wattages, including 3-way and dimmable units. Even better are LEDs, although these remain an emerging technology and are still relatively expensive.
Second, allow for more lighting control. It takes less energy to focus one light where it's needed than to light a whole room to achieve the same result. For instance, instead of putting a dozen lights in the kitchen on one switch circuit with "all-or-nothing" control, break it up. Allow the client to turn on just the lights over the counter, the island or the sink. Plan three-way switching, so clients can easily turn off the lights when they leave a room or hall. A second bedroom or living room wall switch controlling switched outlets allows a desk or reading lamp to be turned on when entering a room.
DC-Appropriate Loads
There are a few instances where DC power can still be the best approach, especially in smaller systems. DC power is usually run on dedicated circuits, for specific loads that are better served by direct battery power. Water pumping (especially when a pressure pump is installed), hydronic heat, ceiling fans and halogen task lighting are a few examples. Some off-grid homes will want one or two DC receptacles to charge cell phones. We can help you decide when to use DC over AC, calculate the proper wire size, and give advice on special wiring methods.
 
التعديل الأخير:
أعلى