Alternative Energy Systems, Part 2 - Based on the 2011 NEC

By Mike Holt for EC&M

The 2011 NEC extensively revised the rules for Photovoltaic (PV) system wiring methods. For example, the 2011 revision:

• Provides a new Informational Note to 690.31, clarifying how to perform raceway fill calculations when using cables. This Note correlates with Note 5 in Chapter 9.
• Allows you to use Type MC cable as a wiring method for PV system source or output conductors run inside a building [690.31(E)].
• Adds requirements for separating conductors from the undersides of roofs. Like the identification requirement [690.4(F)], this helps prevent firefighters from cutting through energized conductors while ventilating the roof [690.31(E)(1)].
• Adds marking and labeling requirements, to reduce the likelihood of improper connection to conductors [690.31(E)(4)].

Equipment bonding

Unfortunately, Article 690 retains the use of "grounding" where it means "bonding" (see Article 100 definitions). Wherever you see "equipment grounding" such as in 690.43 (or elsewhere in the NEC), the intent is equipment bonding.

However, the 2011 NEC provided much better formatting of the equipment "grounding" provisions of Article 690, Part V. When multiple requirements or provisions are in a single section, a "list" format is often easier to read and understand. Section 690.43 now reflects that fact.

This revision also includes some technical changes. For example:

• Subsection (C) now requires that metal mounting racks be identified as equipment grounding conductors (EGC) or have a bonding jumper(s) or devices installed between the separate metallic sections. In addition, the metallic racks must be connected to the grounding system which can be done via an equipment grounding conductor.
• Subsection (D) (added with the 2011 revision) requires that devices for securing PV modules be identified for equipment grounding if they're used as an EGC.

Grounding electrode system

In contrast to "equipment grounding conductor," the NEC really does mean "grounding" when it talks about the grounding electrode system. The key changes to requirements for PV grounding electrode systems [690.47] are:

• Section 690.47(B) now is clear that you can use a common grounding electrode conductor (GEC) to ground multiple inverters. This concept isn't new to the NEC; similar provisions are in 250.30 for separately derived systems.
• Section 690.47(C) underwent extensive revision, with the intention of incorporating the concepts of the 2005 and 2008 editions into clear, easily understandable text.
• Section 690.47(D) wasn't revised. It was deleted. It required that ground and pole-mounted PV arrays have a grounding electrode. The intention in the 2008 revision was for this to be optional, but the language used made it mandatory. Deleting Section 690.47(D) makes the rule optional again.

All of 690.47(C) is worth reading closely, but let's look at its last paragraph. Subsection 690.47(C)(3) allows you to use a single conductor as the DC grounding electrode conductor (GEC) as well as for the AC equipment grounding conductor. This GEC must be unspliced (or irreversibly spliced). Run it from the marked direct-current grounding electrode connection point along with the alternating-current circuit conductors to the grounding busbar in the associated alternating-current equipment.

You must size this GEC to the larger of 250.122 or 250.166, and install it per 250.64(E).

To prevent inductive choking of grounding electrode conductors, ferrous raceways and enclosures containing grounding electrode conductors must have each end of the raceway or enclosure bonded to the GEC per 250.92(B) [250.64(E)]. Nonferrous raceways don't need to meet this requirement. To save a lot of time and effort, install the grounding electrode conductor in PVC conduit suitable for the application [352.10(F)].

Article 692 provides the requirements for fuel cells. This technology isn't yet ready for wide-scale adoption, due to such issues as cost-effectively obtaining the hydrogen. So a greatly increased demand is still off in the future.

Small Wind Electric Systems

The 2011 NEC added Article 694, which applies to small wind (turbine) electric systems that consist of one or more wind electric generators having a rated power up to and including 100 kW. Typical system components are a generator (or alternator), inverter, and controller [694.1].

Getting physical

One way in which wind differs from solar is physical access. The typical roof-mounting of residential and commercial solar means requires minimal climbing for the installers. But access to a wind system nacelle (where the generator or alternator is) may require climbing 75 ft up a tower.

On larger towers, the ladders are inside the tower body. The towers of the smallest residential wind turbines (under 10KW) are really pole-mounted systems and you reach the nacelle from the outside of the structure.

You need climbing safety training and climbing equipment to work on any but the very smallest of wind systems. You need to be adept at elevated work, also. So you carefully plan each task. And you follow procedures that prevent you from dropping your parts bucket onto the customer's new pickup truck 75 ft below.

Height is also a factor in the NEC requirements. Obviously, a disconnect in a nacelle 75 ft off the ground isn't "readily accessible" as required by 694.22(C)(1).The language of 694.22(C)(1) specifically permits locating the disconnect at a readily accessible location either on or adjacent to the turbine tower, on the outside of a building or structure or inside, at the point of entrance of the wind system conductors.

Circuit requirements

Part II of Article 694 applies to systems up to (and including) 600V. That's the voltage limit for systems connected to one- and two-family residences. Systems over 600V must comply with Part IX, instead [694.10(A)].

Two of the circuit sizing requirements are familiar from solar installations:

• Inverter Output Circuit Current. It's equal to the continuous output current marked on the inverter nameplate [694.12(A)(2)].
• Stand-Alone Inverter Input Circuit Current. It's the stand-alone continuous inverter rating when the inverter is producing power at the lowest rated input voltage [694.12(A)(3)].

The NEC also requires determining the turbine output circuit currents. It says to base these on the circuit current of the wind turbine operating at maximum power 694.12(A)(1)]. But what does that mean?

If you need to calculate the number yourself then it's a matter of converting the turbine kW rating to amps using the turbine output voltage and not the inverter voltage. For a residential system, the inverter output voltage is typically 120V or 240V. But the turbine output voltage may be considerably higher. Typically, the manufacturer has already done this because these systems are usually sold as packages and the manufacturer provides a wiring kit along with everything else.

Overcurrent protection

Size the overcurrent protection devices per Article 240. Also, consider the loads to be continuous [694.12(B)(1)]. That means the circuit conductors and overcurrent devices must be sized to carry not less than 125% of the maximum current calculated in 694.12(A) [694.12(B)(2)].

Stand-alone systems

You won't find "stand-alone" defined in 694.2, but it is defined in 690.2 for solar PV systems, and in terms of fuel cell systems in 692.2. Since this term is now being used in two or more articles, its definition should find its way into Article 100.

Stand-alone systems must meet the installation requirements of the Code for a similar installation connected to a service. The inverter output can supply power to the structure disconnecting means, but only at current levels less than the calculated load connected to that disconnect [694.18(A)].

The inverter must be rated at least large enough to supply the largest single utilization equipment connected to the system. Don't consider calculated general lighting loads as a single load [694.18(A)].

Grounding

Bond all non-current-carrying metal parts of towers and equipment to the equipment grounding (bonding) conductor [694.40(A)]. That is, follow the bonding requirements of Article 250, Part V. But ground the tower itself by connecting it to one or more auxiliary electrodes for lightning protection purposes [694.40(C)].

A bright future

Advances in solar technology are making it ever more practical and affordable. And wind systems are now available in predesigned kits that result in a better installation at lower cost.

As the demand for alternative energy systems increases, you now have a good foundation for correctly applying the NEC to that work.

Where the wind blows

Conversion of wind power to electricity is not new. This concept was used in rural farming areas prior to the Rural Electrification Act of 1936 which brought rural electrical cooperatives into existence. Installing and maintaining a battery system remains one of the barriers to wide-spread use of wind power. As a result, the stand-alone small wind electric system is not seen as commonly as interconnected systems to supplement the utility power supply.

Most of the wind generation installation work is for large-scale wind farms. Wind farms consist of wind turbines too large to be covered by Article 694. The types of installations that fall under Article 694 are typically for a specific building or facility. They can serve as the sole supply for a structure, but more commonly serve as a supply in addition to other sources [694.7(A)]. The scope of Article 694 limits it to one or more wind electric generators with individual generators having a rated power up to and including 100 kW [694.1].

Taken from Mike Holt’s 2011 Understanding the NEC Volume 2 Textbook. To order your copy, please click here, or call 888-632-2633

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