Application Bulletin NEC Reference Guide for SolarBridge-Enabled AC Module Installers

Transcription

1 9229 Waterford Centre Dr. Bldg C, Suite 110 Austin, Tx Application Bulletin NEC Reference Guide for SolarBridge-Enabled AC Module Installers Current Version: 13 AC Modules are a new product in the Renewable Energy marketplace. Questions have come up from installers, PV System Inspectors, and First Responders as these modules are being specified for new installations on homes and businesses across the United States. First, it is important to distinguish between discrete microinverters installed in the field and the AC module as 1 defined in the NFPA 70 National Electrical Code (NEC), Section (see below). A discrete microinverter (with AC connectors rated for disconnect under load) is packaged separately and will be ready for connection to a DC GEC/EGC, a DC PV Module(s) and an AC dedicated branch circuit in the field per all applicable requirements in Article 690 of the NEC (including those for PV DC circuits such as grounding, disconnects and GFDI). The AC module is a manufactured product tested and listed as a complete unit by a Nationally Recognized Test Laboratory (NRTL). The manufactured product includes the PV module, the microinverter, cables and connectors. The product is packaged and shipped as a single listed unit from the factory & 2011 edition of the National Electric Code (NEC) Section 690.2: Definitions -2 Inverter: The NEC defines an inverter as "a device that changes DC input to an AC output". This definition is accurate whether the inverter is rated at hundreds of kw's or more and contained in a large, actively ventilated cabinet (typical of central or "string" inverters) or it is rated as low as a 100 Watts and contained in a small, sealed enclosure (typical of microinverters). AC Module: The NEC defines an AC module as a "complete, environmentally protected unit consisting of solar cells, optics, inverter and other components, exclusive of tracker, designed to generate AC power when exposed 3 to sunlight". The microinverter used with an AC module assembly is evaluated to the same Underwriters Laboratory (UL ) 1741 requirements as a central inverter with the exception that the environmental protection of the microinverter enclosure and PV module is evaluated to UL1703 and the AC Module section of UL1741 (instead of using the UL50 type ratings such as 3R). The PV module is evaluated to UL

2 Section 690.6: Alternating Current (AC) Modules This section of the NEC defines the electrical output of an AC module and provides guidance on how AC modules will be installed in the field (A): "Requirements of Article 690 pertaining to PV source circuits shall not apply to AC modules" (such as: multiplying short circuit DC currents by 125 percent, DC disconnects, DC grounding, DC ground fault detect and interrupt, protection by a listed PV type (DC) arc-fault circuit interrupter, and application of DC voltage correction factors). When installing AC modules, there is no need for protection or installation of DC wiring in the field (B): "The output of an AC module shall be considered an inverter output circuit". However, there are differences between how an AC module is interconnected to the utility service on site versus centralized (i.e. string) inverters. AC modules are always considered "utility interactive" devices and will stop producing power when disconnected from the external source of AC power. A single overcurrent protector is only required on the utility side of the dedicated feed to an AC module branch circuit. NOTE: In addition to IEEE utility interactive requirements, each SolarBridge microinverter provides electronic overcurrent regulation / protection. The instructions and marking for the AC module contains information about the dedicated branch circuit it can be connected to, including maximum overcurrent protector rating on the branch circuit to the AC modules and maximum number of AC modules that can be attached to that branch circuit so the combined rated current does not exceed 80% of the branch circuit protector rating. AC modules are provided with interconnecting cables and connectors per 690.6(C) that are listed with the assembly for the application. These interconnecting cables allow the modules to be combined together up to the maximum allowed and allow for connection to the dedicated branch circuit wiring (per NEC and local codes) (C): "A single disconnecting means in accordance with and , shall be permitted for the 6 combined AC output of one of more AC modules". An excellent example of this would be a properly rated, readily accessible, externally operable AC circuit breaker with Open/Closed indicator or any other wiring device listed as a disconnect. "Additionally, each AC module in a multiple AC module system shall be provided with a 7 connector, bolted or terminal type disconnecting means". SolarBridge AC plugs and receptacles, included with each AC module are rated for disconnect under load by the NRTL and are "dead front" type connectors, ensuring that operators are not exposed to live parts when separating or inserting the plugs and receptacles (D): "AC module systems shall be permitted to use a single ground fault detection device to detect only 8 AC ground faults and to disable the array by removing AC power to the AC module(s)". A properly rated and listed AC branch circuit breaker will fulfill this requirement. 2

3 690.6 (E): "The output circuits of AC modules shall be permitted to have overcurrent protection and conductor 9 sizing in accordance with 240.5(B)(2)". This section is for fixture wiring and allows for sizing wires tapped off branch circuit conductors based on length and branch circuit protector rating. The SolarBridge interconnecting cabling and connectors are listed and tested with the microinverter. Sizing of conductors and connectors are based on end product testing by the NRTL, 240.5(B)(2) and Section The dedicated branch circuit wiring to the group of AC modules will need to be performed to Chapter 3 of the NEC and per local codes. Arc Fault Detection SolarBridge-enabled AC modules operate with internal DC voltages less than 80 VDC and therefore are not subject to any of the Arc Fault Protection requirements detailed in Article 690 of the 2011 edition of the National Electrical Code. SolarBridge-Enabled AC Module Cables A SolarBridge-enabled AC module is tested and listed with a microinverter cable that includes an AC module interconnecting (i.e. "trunk") cable as part of the complete unit. The AC module interconnecting cables are rated as Tray Cable Extended Run (TC-ER) outdoor rated. They are rated for -40C to 90C, insulated for direct burial and they are sunlight resistant. The SolarBridge-enabled AC module cables are 4-conductor cables rated for 20 2 amps (AWG #12, 3.31 mm ). The cable contains 2 current-carrying conductors (black and red), a single (white) Neutral conductor and a single (green) Ground conductor (EGC). These cables have plug and receptacle connectors rated for disconnect under load. SolarBridge AC cable receptacles and plugs are locking type and require a tool to separate. They are not rated or designed for use in conduit. Installers will use exterior-rated cable tray (solid or mesh) to manage cables within the array field where the cables cannot be secured to the module support rails. Cable tray is also recommended for managing cables as they leave the array field and are routed to electrical work boxes or interconnection points. It is recommended that the cable tray be fitted with an opaque cover. It is required that the tray be properly secured and supported against environmental pressure such as strong wind or heavy snow accumulation and movement. Metallic tray must be grounded. Nonmetallic tray is acceptable if properly secured and supported against environmental pressures. Grounding AC Modules NEC Chapters 1-4 always apply for grounding PV systems however requirements of Article 690 pertaining to PV source circuit grounding shall not apply to AC modules [see 690.6(A)]. This paper will address three topics related to grounding of AC modules; 1. Equipment safety grounding (i.e. EGC) 2. Establishing a supply reference (i.e. grounding the neutral), and 3

4 3. Lightning / surge protection Equipment Safety Grounding An equipment grounding conductor (EGC) must be provided with the dedicated branch circuit (installed within the same raceway, conduit or cable as the phase conductors) to the AC modules per Article 250 sections VI and VII in order to provide a low impedance path back to the AC module branch circuit breaker. This EGC must be properly connected to the SolarBridge-enabled AC module interconnecting cable system using suitable field wiring devices. It also must be connected to any other metallic devices that could become energized by the AC circuits, such as: wiring boxes and AC module support structures). It should be verified that this EGC has been installed properly all the way back to the site's service entrance system and ground rod. The SolarBridge-enabled AC module includes an equipment grounding conductor within the AC module interconnecting cables and cable receptacles and plugs. The SolarBridge microinverter cable with integral EGC is evaluated as part of the AC module listing with the NRTL. The transition cable's green conductor is connected to the EGC from the utility dedicated branch circuit. All plugs have ground pins that are longer than the circuit pins. This extra length ensures the ground is the first to make contact when connecting modules and the last to break contact when disconnecting modules. The AC ground wire inside the microinverter makes a bolted termination to the chassis and is environmentally sealed. The microinverter chassis is bonded at the factory to the module frame with stainless steel hardware. Establishing a Supply Reference Supply systems (AC or DC) must establish a local reference to ground. This helps keep voltages (with respect to ground) within safe equipment operating levels. The type of supply system and how it attaches to the building supply system will determine what requirements of the NEC apply. AC module systems are utility interactive, do not contain any field wiring DC circuits and are connected into the load side of the site's service entrance. Article 690 of the NEC pertaining to the DC PV circuits does not apply to AC modules since the DC is considered internal to the listed assembly. AC module arrays do not require a grounding electrode conductor (GEC) since there are no DC circuits to reference to the grounding electrode. The AC reference must only be at the site's service entrance. Guidance on the proper installation of the site's AC grounding electrode system (and AC GEC) is provided in Article 250, Section III of the NEC. It should be verified that the site has the grounded conductor (neutral) properly bonded to the site's ground electrode system. Since the site already has the neutral connected to ground, the AC module will always have the neutral isolated from ground like the phase conductors. The AC neutral MUST NOT be connected to ground anywhere else within the AC module system. 4

5 Local permitting agencies and electrical inspectors across the country have become familiar with PV systems that utilize PV modules wired in series and connected to single (or small numbers of) central inverters (i.e. "string" inverters). The 2011 edition of the NEC and NEC Handbook provide excellent guidance on the installation of grounding systems for these types of systems. Below is an excerpt from a John Wiles article in Home Power #103, Code Corner explaining common practices when grounding PV systems with DC PV modules and wall-mounted central inverters. JW: Because most inverters have transformers that isolate the DC-grounded conductors from the AC-grounded conductors, two grounding systems are usually required. Conductors are routed from a single main bonding point in the DC system and a single main bonding point in the AC system to grounding electrodes (the part of the grounding system that is in physical contact with the earth, such as a ground rod). The conductors from these main bonding points to the grounding electrode(s) are called the "grounding-electrode conductors" (GECs). JW: There will usually be a DC grounding-electrode conductor and an AC grounding-electrode conductor in systems with an inverter. In a PV installation where there is an existing AC electrical system, the AC grounding electrode is already installed. The DC grounding-electrode conductor may be connected to the AC grounding electrode or it may be connected to a new DC grounding electrode. This new DC grounding electrode must be bonded to the AC grounding electrode. Grounding-electrode conductors may be either bare or insulated. No color is specified for this conductor (if insulated), but conventional practice suggests that it be black and not white, green, or green with yellow stripes. The smallest allowable size is #8 (8 mm2); however, a grounding electrode conductor this small must be installed in conduit for physical protection. Many inspectors allow a #6 (13 mm2) grounding-electrode conductor to be installed without conduit if it is attached to a building surface. The #8 and #6 conductors are normally allowed when the GEC is connected to a ground rod. Lightning / Surge Protection Lightning or surge protection equipment provides an attractive path to ground for high current events such as direct or nearby lightning strikes. In areas where lightning strikes are possible (e.g. 8 strikes per year per square mile, confirm with local authorities), installers can bond the AC module array together and run a separate, suitably sized wire from the array directly to the home or building's AC grounding electrode system / grounding electrode. It is recommended that PV system designers specify lightning arrestors at the utility service panel to help suppress electrical surges coming in on the utility service conductors. A best practice for all solar module installers, especially those in areas with frequent lightning, is to secure each module to the rack using grounding hardware that has been certified to meet requirements for grounding systems. Connect the entire grounded structure to a grounding electrode system using a dedicated, suitably sized, copper conductor. Safety 5

6 Safety is important for the installer. If the PV system components are safe to handle, the installer can work with confidence and at a high rate of speed. PV module DC conductors are captured within the AC module. This ensures installers are not exposed to DC current and voltage from the PV module. AC modules are OFF during installation. Although the microinverter will detect current and voltage from the PV module when exposed to sunlight, microinverter AC plugs and receptacles will not be energized until connected to a stable utility power source (5 minute re-connect time). Safety is important for the system owner. AC modules are connected to the building's electrical system using standard AC wiring practices. AC module systems do not require field wired, high-voltage DC conductors to be routed through the building (from the roof to the AC breaker box). AC module systems eliminate specialized DC equipment and rely on standard AC wiring components Safety is important for First Responders. Fire Fighters, Police Officers and Security personnel understand or are familiar with typical AC wiring hazards in homes and buildings. When responding to fire or other life-threatening situations on site, electrical hazards can be reduced to a minimum by cutting power to the building at the Utility service entrance. When the Utility service has been disconnected from the building, all appliances in the building, including the AC modules, will be thrown into the OFF position and all associated building wiring will be de-energized. Footnotes: 1 National Electrical Code and NEC are registered trademarks of the National Fire Protection Association, Inc., Quincy MA NFPA 70, National Electrical Code 2011, National Fire Protection Association, Quincy, MA