The following resources are currently available as resources to electricians:
--Bidding a Solar Electric Home: A Guide for Electricians
--Solar Photovoltaic Systems Inspector Checklist
Bidding a Solar Electric Home
A Guide for Electricians
Your client has asked you to prepare a bid to wire a solar electric (photovoltaic, or PV) powered home. Your client is looking to you for answers, and you have only questions. First, relax. For the most part, the wiring in an off-grid home is just like any other house. Thankfully, the days of car fuses, cigarette lighter sockets and 12-volt TVs are gone. This handout is designed to point out the key things to keep in mind to help you bid your work accurately.
FIRST OF ALL, ASK!
When in doubt, ask us for advice. Most of what you are encountering for the first time we see on a daily basis, and we've seen what works. If we haven't seen your particular problem before we'll track down the answer. Remember that it's always cheaper to make changes on paper than in copper.
We are licensed electrical contractors as well, and we generally pull our own permits on our installations. If we are working with a client to develop a solar electric power system, we can provide you with general answers on the scope of our work and yours, code issues to be aware of, special circuits, and answers to specific questions you may face. We offer consultant services on an hourly or contract basis, whether or not you are installing for one of our clients.
NO SHARED NEUTRALS
The device that makes AC power is called an inverter. Nearly all inverters make 120V AC power in sufficient quantity for most home needs. The inverter output is usually jumpered to both hot lugs in the load center. As such, multiwire branch circuits will cause the neutral to be overloaded and are unacceptable. You must run a separate home run for each circuit.
240 VOLT 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.
SUBPANELS
Some clients want their 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. While it won't affect your bid, the bond point may not be in the AC load center.
CUT OFF 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 wall switches to control outlets wherever such specific devices will be located. This allows the client an easy way to eliminate such waste.
ELIMINATE 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--ask us how. 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.
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 be the best approach. It 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 or to power cordless phones and answering machines. Square D QO-series breakers and load centers are DC-rated for use in 12V and 24V systems. We can help you decide when to use DC over AC, calculate the proper wire size, and give advice on special wiring methods.
We have available free copies of "Photovoltaic Power and the National Electrical Code: Suggested Practices" that will resolve further questions. As we have said, just ask us!
Solar Photovoltaic Systems Inspector Checklist
The following checklist is an outline of the general requirements found in the 1999 National Electrical Code (NEC) -- Article 690 for Photovoltaic (PV) Power Systems installations. This list should be used in conjunction with Article 690 and other applicable articles of the NEC. It includes inspection requirements for both stand-alone PV systems (with and without batteries) and utility-interactive PV systems. References in brackets [ ] are to the 1996 and 1999 NEC and other relevant documents. Where Article 690 differs from other articles of the NEC, Article 690 takes precedence [690-3].
The local authority having jurisdiction (AHJ) or inspector has the final say on what is or is not acceptable. Local codes may modify the requirements of the NEC.
This checklist is provided by Positive Energy as a service to our customers. For a more comprehensive guide, request a free copy of
Photovoltaic Power Systems and the National Electrical Code: Suggested Practices,
Publication number SAND2001-0674, from Sandia National Laboratories,
Telephone 505 844-4383.
For questions, call
John Wiles,
New Mexico State University,
505-646-6105.
Last revised 11/22/99.
CHECKLIST FOR PHOTOVOLTAIC POWER SYSTEM INSTALLATIONS
PV Arrays
___Listed PV modules are available from numerous manufacturers [110-3].
Conductors
___Conductor type--USE-2, UF(not a good choice), or SE if exposed [690-31(b)]; RHW-2, THWN-2, or XHHW-2 in conduit [310-15]. 90°C, wet-rated conductors are necessary.
___Conductor insulation rated at 90°C [UL-1703] to allow for operation at 70°C+ near modules and in conduit exposed to sunlight.
___Temperature-derated ampacity calculations should be based on 156% of short-circuit current (Isc), and the derated ampacity must also be greater than rating of overcurrent device (156% Isc -see below) [690-8,9].
___Suggest temperature derating factors of 60-65°C in cooler areas, 70°C in hotter areas, and 75°C in desert areas be used for ampacity calculations.
___Portable cords are allowed only on moving tracker connections [690-31(c), 400-3].
___Strain reliefs/cable clamps or conduit should be used on all cables and cords [300-4, 400-10].
Overcurrent Protection
___DC-rated and listed fuses and circuit breakers are available from several sources. If device is not marked dc, then verify dc listing with manufacturer.
___Rated at 1.25 x 1.25 = 1.56 times short-circuit current from modules [UL-1703, 690-8, module instructions]. Both of these 125% factors are now in the 1999 NEC.
___Supplementary devices allowed, but branch-circuit rated devices preferred [690-9(c)].
___Each module or series string of modules must have an overcurrent device protecting the module [UL-1703/NEC 110-3(b)]. Frequently this requirement, marked on the back of modules, is ignored by installers.
___Located near the charge controller or battery [690-9(a) FPN].
___Must protect smallest conductor used to wire modules. Sources of overcurrent are parallel-connected modules, batteries, and ac backfeed through inverters [690-9(a)].
Charge Controllers
___Listed devices are available separately and inside listed PV power centers [110-3].
___There should be no exposed terminals that are readily acessible.
Disconnects
___Listed, dc-rated devices are available: Square D QO breakers for 12-volt dc systems, Square D Heavy Duty Fused Safety Switches and others for up to 600 volts dc.
___Listed PV Load Centers by Pulse Energy, Trace, and others for 12, 24, and 48-volt systems contain charge controllers, disconnects, and overcurrent protection for entire dc system with possible exception of module protective fuses.
___Must provide disconnects for all current-carrying conductors of PV source [690-13].
___Must provide disconnects for equipment [690-17].
___Grounded conductors are not fused or switched, but may have bolted disconnects.
Inverters (Stand-alone Systems)
___Listed stand-alone inverters are available from several manufacturers [110-3].
___DC input currents must be calculated for cable and fuse requirements: Input current = rated ac output in watts divided by lowest battery voltage divided by inverter efficiency [690-8(b)(4)].
___Cables to batteries must handle 125% of inverter input currents [690-8(a)].
___Overcurrent devices should be located within 4-5 feet of batteries.
___Overcurrent/Disconnects mounted near batteries and external to PV load centers are suggested if cables are longer than 5-6 feet to batteries or inverter.
___Listed, dc-rated fuses and circuit breakers are available. AIC should be at least 20,000 amps. Littelfuse and Bussmann mark dc rating, others sometimes do not [690-71(c), 110-9]. Verify listed, dc-rating with manufacturer, if unmarked.
___120-volt inverters connected to 120/240 load centers with multiwire branch circuits have the potential for neutral overloading in the branch circuit [100Branch Circuit, Multiwire].
Batteries
___None are listed.
___Cables should be building-wire type cables [Chapter 3]. Welding cables and auto battery cables don't meet NEC. Flexible USE, RHW, or THW cables are available. Article 400 cables are OK for cell connections, but not in conduit or through walls [690-74, 400-8]. See stand-alone inverters for ampacity calculations.
___Access should be limited [690-71(b)]. Install in well-vented areas (garages, basements, out-buildings, not living areas). Manifolds or power venting are not required and should be avoided.
___Cables to inverters, dc load centers, and/or charge controllers should be in conduit [300-4].
Inverters (Utility-interactive Systems)
___Listed units are available from several manufacturers and should be used for safety of utility personnel by eliminating the possibility of energizing unenergized utility lines.
___Must be on dedicated branch circuit with back-fed overcurrent protection [690-64].
___Must have external dc and ac disconnects and overcurrent protection [690-15,17].
___Total rating of overcurrent devices connected to ac load center (main breaker plus PV breaker) must not exceed load-center rating (120% of rating in residences) [690-64(b)(2)].
Grounding
___Only one grounding electrode conductor connection to dc circuits (grounded conductor) and one connection to ac circuits (neutral) should be used for system grounding [250].
___AC and dc grounding electrode conductors may be connected to the same grounding electrode system (ground rod) [690-41,47].
___Equipment grounds are required even on ungrounded, low-voltage systems [690-43].
___On 12-volt, ungrounded systems [690-41], disconnects and overcurrent devices are required in both of the ungrounded conductors in each circuit [240-20(a)].
___Equipment grounding conductors for dc circuits from PV array may be run apart from other conductors [250-57 (b) Ex 2] and this routing is suggested to minimize damage in areas where lightning surges are common.
Conductors (General)
___Standard building-wire cables and wiring methods should be used [300-1(a)].
___Wet-rated conductors should be used in conduits in exposed locations [100 Definition of Location, Wet].
___DC color codes are the same as ac color codes--grounded conductors are white and equipment-grounding conductors are green or bare [200-6(a), Ex 5].
