When used for specific loads, they are often the sole power source. But when used for general power, they usually supplement another power source. Each individual wind turbine must be power rated 100 kW and less [694.1].  Keep in mind that while Article 694 does not limit the number wind turbines that can be installed at one location, a single wind turbine is the typical installation covered by this article.

What must be considered is wind availability. With a sufficiently large enough battery system (covered by Article 480), it’s possible to overcome this limitation also. But only in areas with short-term low wind conditions.

Though SWS generally aren’t a good option for the sole power source to a building, they quite often are an excellent option for the sole source to specific loads.

For example, a farm might use an SWS to provide power for well water pumps. The pumps fill a storage tank while there’s sufficient power to run them. The tank has enough capacity to ride out the low wind period.

Similarly, an industrial facility might use an SWS to run sump pumps. Because rain and wind arrive together, this is an ideal application.

Another great application is powering a pond aerator. The aerator doesn’t need to run constantly, so if the wind dies down for a day or so the outage isn’t a problem. The SWS also eliminates the need to run power from the main building out to the pond.

Today’s typical SWS installation uses an engineered kit. These kits come in designated kW output packages. For example, you can buy a 10 kW wind turbine kit. The mechanical engineering that goes into these kits removes much of the complexity from assembly and installation, while also boosting power efficiency.

The basic electrical system of an SWS consists of three components: a generator (or alternator), inverter, and controller [694.1]. The system can have an ac or dc output. It can have electrical storage (e.g., batteries) or not. The turbine assembly might be mounted on a mast, tower, flange, or roof. If it’s mounted on a guyed tower, then the installation has another layer of complexity added to it. So the total installation effort can be considerable and involve engineering challenges across several disciplines—even if you’re using a predesigned kit.

Sole power source systems
If you’re using the SWS as the sole power source (stand alone power source), the premises wiring must be adequate to meet the NEC requirements for a similar installation connected to a service [694.18].

The NEC doesn’t require energy storage or backup for SWS, but your operating requirements might. The ac inverter of a stand-alone SWS can provide power to the structure disconnect, but only if the current levels are less than the calculated load to that disconnect. Use an inverter rated large enough to supply the largest single utilization equipment connected to the system. The inverter output rating can't be less than the load of the largest single utilization equipment connected to the system. You can’t use the calculated general lighting loads as a single load [694.18(A)].

Size the circuit conductors between the inverter output and the structure disconnect based on the inverter output rating. Provide overcurrent protection per Article 240, and locate overcurrent protection devices (OCPDs) at the output of the inverter [694.18(B)].

You can supply 120V circuits from the inverter output to single-phase, 3-wire, 120/240V service equipment, but only if there are no 240V outlets or multiwire circuits [694.18(C)]. This equipment must be marked with the following words or equivalent:

WARNING
SINGLE 120V SUPPLY
DO NOT CONNECT
MULTIWIRE BRANCH CIRCUITS!

In addition, a plaque or directory must be installed on the exterior of a building or structure at a readily visible location, indicating the location of the stand-alone system disconnect. The plaque also must state that the structure contains a stand-alone electrical power system [694.54(A)].

If the SWS has energy storage (e.g., batteries), mark the storage system with the maximum operating voltage, any equalization voltage, and the polarity of the grounded circuit conductor [694.52].

Supplementing other power sources
While powering small loads is one way that SWS are useful, they are also useful as a means of supplementing the main source. That source isn’t always the electric utility grid. In a remote location, for example, the main source might be a gas turbine.

When an SWS supplements the utility power source, we say that the system is interactive. It’ll have an, inverter to change  the power coming in from the wind turbine to  power being supplied by the utility power source. Ideally, it will also have a bypass switch though the NEC does not require that.

Nor does the NEC require you to provide an operational tutorial to homeowners when you’ve installed an interactive residential system. However, doing so is a good business practice for a variety of reasons including potential liability.

You need to provide some marking and signage for interactive systems, above what’s required for non-interactive systems. Mark all points of interconnection (with other sources) at an accessible location at the disconnecting means [694.50]. The marking must show the rated ac output current and the nominal ac voltage.

Install a permanent plaque or directory to indicate the location of the service disconnecting means and of the SWS disconnecting means [694.54(B)]. It is this plaque that will inform firefighters how to de-energize the premises. The NEC doesn't specify the location of this plaque but the exterior of the building or structure at a readily visible location should meet the intent of this requirement.

Connecting interactive systems
Use only inverters listed and identified as interactive [694.60]. If you’re connecting this SWS to a utility source, comply with Article 705 [694.62]. If connecting to a gas turbine source or other non-utility source, this isn’t necessary. However, regardless of what the other source is, all points of connection must comply with 705.12.

An SMS connected to dedicated branch or feeder circuits can exceed normal voltage operating ranges on these circuits. But only if the voltage at any other distribution equipment supplying loads remains within normal ranges [694.66].

Overcurrent protection
Protect turbine output circuits, inverter output circuits, and storage battery conductors and equipment with OCPDs sized per Article 240. As these are continuous loads, size the OCPDs to carry at least 125% of the calculated maximum circuit current [694.12].

What’s the calculated maximum circuit current? Actually, there are three:

1. Turbine output. Circuit current of the wind turbine operating at maximum output power [694.12(A)(1).
2. Inverter output. Continuous output rating of the inverter [694.12(A)(1)].
3. Inverter input (stand-alone). Stand-alone continuous inverter input current rating of the inverter producing rated power at the lowest input voltage [694.12(A)(3).

Disconnects
An SWS that uses the turbine output circuit for regulating turbine speed doesn’t require a means of disconnecting the turbine output circuit from the premises wiring [694.20 Ex.]. For all other types of SWS, you must supply a readily accessible means of disconnecting the SWS current-carrying conductors from all other conductors in the structure [694.20].

Don’t install a switch, breaker, or other device in a grounded conductor if its operation leaves the grounded conductor energized.

Permanently mark each SWS disconnect to identify it as such. Install a plaque per 705.10 [694.24(C)(2)].

Wiring methods
You can use any applicable Chapter 3 wiring method. You can also use any wiring systems and fittings specifically intended for use on SWS. If an accessible circuit operates at over 30V, run it in raceway [694.30(A)].

Flexible cords used to connect the moving parts of the SWS must comply with Article 400. They must be:

• Suitable for extra-hard usage.
• Listed for outdoor use.
• Water-resistant.

If exposed to sunlight, they must also be sunlight-resistant [694.30(B)].

If installed indoors, turbine output circuits that are dc must be enclosed in metal raceway and enclosures [694.30(C)].

Grounding
Except for the tower and guy lines, bond all non-current-carrying metal parts and equipment to the equipment grounding conductor [694.40(A)]. This puts the SWS in compliance with Article 250, Part V and prevents dangerous differences of potential.

For lightning protection purposes, ground the tower to at least one auxiliary electrode. Auxiliary electrode(s) can be installed in accordance with 250.54. Concrete-encased electrodes installed in accordance with 250.52(A)(3) that are part of the tower foundation can be used as the auxiliary electrode [694.40(C)].

Run an equipment grounding conductor between the turbine (turbine/tower assembly) and the premises grounding system, per 250.110, [694.40(C)(2)]. Failure to do this could result in a large difference of potential between premises wiring and the turbine.

Small wind system issues
With standard kits, the design issues of the turbine/generator system itself are already solved for you. But a successful installation depends on many other factors. Those include intended use, turbine siting, controls location, and user sophistication level. You may also have to consider minimum, maximum, mean, and average wind conditions. You need some training and experience in dealing with those issues, if you are responsible for the whole project.

While it doesn’t address these other issues, the NEC does address the electrical safety requirements of these installations. For the most part, the electrical work is like that of any normal installation. For example, you size your circuits the same way you’d size any continuous load circuits.

But Article 694 adds signage and marking requirements among a few other SWS-specific requirements. If your company plans to do many SWS installations, develop a checklist of these additional requirements and use that in the field.

Taken from Mike Holt's 2011 Understanding the NEC® Volume 2 Textbook.
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