A recommended reference is the: Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices Sandia National Laboratories John Wiles NTIS Order Number: PB2005-108229 Available in CD, Microfiche, Downloadable, and Color Document.

Available from: U.S. Department of Comerce National Technical Information Service 5285 Port Royal Rd Springfiled, VA 22161 Telephone: (800) 53-6847 Fax: (865) 605-6900 e-mail: orders@ntis.fedworld.gov web: http://www.ntis.gov/search/results.asp?loc=3-0-0

A few short excerpts from this reference follow:

PHOTOVOLTAIC MODULES

Numerous PV module manufacturers offer listed modules. In some cases (building integrated or architectural structures), unlisted PV modules have been installed, but these installations should have been approved by the local authority having jurisdiction (electrical inspector).

MODULE MARKING

Certain electrical information must appear on each module. The information on the factory-installed label shall include the following items [690.51]:

Information Supplied by Manufacturer

• Polarity of output terminals or leads
• Maximum series fuse for module protection
• Rated open-circuit voltage
• Rated operating voltage
• Rated operating current
• Rated short-circuit current
• Rated maximum power
• Maximum permissible system voltage [690.51]
  

Although not required by the NEC, the temperature rating of the module terminals and conductors are given to determine the temperature rating of the insulation of the conductors and how the ampacity of those conductors must be derated for temperature [11O.14(C)]. While module terminals are usually rated for 90°C, most other terminals throughout the PV system will have terminals rated only for 60°C or 75°C. These terminal temperatures may significantly affect conductor ampacity.

Note: Other critical information, such as mechanical installation instructions, grounding requirements, tolerances of indicated values of Isc, Voc and Pmax, and statements on artificially concentrated sunlight are contained in the installation and assembly instructions for the module.

PHOTOVOLTAIC ARRAY DISCONNECTS

Article 690 requires all current-carrying conductors from the PV power source or other power source to have disconnect provisions. This provision includes the grounded conductor, if any [690 III]. Ungrounded conductors must have a switch or circuit breaker disconnect [690.13, IS, 17]. Grounded conductors which normally remain connected at all times, may have a bolted disconnect (terminal or lug) that can be used for service operations and for meeting the NEC requirements. Disconnect switches must not open grounded conductors [690.13]. Grounded conductors of faulted source circuits in roof-mounted dc PV arrays on dwellings are allowed to be automatically interrupted as part of ground-fault protection requirements in 690.5. [690.13]

In an ungrounded 12-volt PV system (as allowed by [690.41]), both positive and negative conductors must be switched, since both are ungrounded. Since all systems must have an equipment-grounding system, costs may be reduced and performance improved by grounding 12-volt systems and using one-pole disconnects on the remaining ungrounded conductor.

Ungrounded systems operating at higher voltages, as will be allowed by the 2005 NEC in 690.35, will also require switched disconnects and overcurrent protection in all of the circuit conductors since both the positive and negative circuit conductors will be ungrounded. See Appendix L for additional discussions of ungrounded PV systems.

INVERTERS

Inverters can have stand-alone, utility-interactive, or combined capabilities.

The ac output wiring is not significantly different from the ac wiring in residential and commercial construction, and the same general requirements of the Code apply. In the case of utility-interactive systems and combined systems, ac power may flow through circuits in both directions. This two-way current flow will normally require overcurrent devices at both ends of the circuit.

The dc input wiring associated with stand-alone or hybrid inverters is the same as the wiring described for batteries. Most of the same rules apply; however, the calculation of the dc input current needs special consideration since the NEC does not take into consideration some of the finer points required to achieve the utmost in reliability. Appendix' F discusses these special requirements in greater detail.

The dc input wiring associated with utility-interactive inverters is similar, in most cases, to the wiring in PV source and output circuits.

Inverters with combined capabilities will have both types of dc wiring: connections to the batteries and connections to the PV modules.

SUGGESTED PRACTICES

APPENDIX E:

Example Systems

The systems described in this appendix and the calculations shown are presented as examples only. The calculations for conductor sizes and the ratings of overcurrent devices are based on the requirements of the National Electrical Code (NEC) and on UL Standard 1703 which provides instructions for the installation of UL-Listed PV modules. Local codes and site-specific variations in irradiance, temperature, and module mounting, as well as other installation particularities, dictate that these examples should not be used without further refinement. Tables 310.16 and 310.17 from the NEC provide the ampacity data and temperature derating factors.

CABLE SIZING AND OVERCURRENT PROTECTION

The procedure presented below for cable sizing and overcurrent protection of that cable is based on NEC requirements in Sections 690.9, 690.8, 110. 14(C), 21O.20(A), 215.2, 215.3, 220.10, 240.3(B), and 240.6(A). See Appendix I for a slightly different method of making ampacity calculations based on the same requirements.

1. Circuit Current. For circuits carrying currents from PV modules, multiply the short-circuit current by 125% and use this value for all further calculations. For PV circuits in the following examples, this is called the CONTINUOUS CURRENT calculation. In the Code, this requirement has been included in Section 690.8, but also remains in UL 1703. This multiplier should not be applied twice. For dc and ac inverter circuits in PV systems, use the rated continuous currents. AC and dc load circuits should follow the requirements of Sections 210, 220, and 215.

2. Overcurrent Device Rating. The overcurrent device must be rated at 125% of the current determined in Step 1. This is to prevent overcurrent devices from being operated at more than 80% of rating. This calculation, in the following examples, is called the 80% OPERATION.

3. Cable Sizing. Cables shall have a 30°C ampacity of 125% of the current

determined in Step 1 to ensure proper operation of connected overcurrent devices. There are no additional deratings applied with this calculation.

4. Cable Derating. Based on the determination of Step 3 and the location of the cable (raceway or free-air), a cable size and insulation temperature rating (60, 75, or 90°C) are selected from the NEC Ampacity Tables 310.16 or 310.17. Use the 75°C cable ampacities to get the size, then use the ampacity from the 90°C column-if needed-for the deratings. This cable is then derated for temperature, conduit fill, and other requirements. The resulting derated ampacity must be greater than the value found in Step 1. If not greater, then a larger cable size or higher insulation temperature must be selected. The current in Step 3 is not used at this point to preclude over sizing the cables.

5. Ampacity vs. Overcurrent Device. The derated ampacity of the cable selected in Step 4, must be equal to or greater than the overcurrent device rating determined in Step 2 [240.4]. If the derated ampacity of the cable is less than the rating of the overcurrent device, then a larger cable must be selected. The next larger standard size overcurrent device may be used if the derated cable ampacity falls between the standard overcurrent device sizes found in NEC Section 240.6.

Note: This step may result in a larger conductor size than that determined in Step 4.

6. Device Terminal Compatibility. Since most overcurrent devices have terminals rated for use with 75°C (or 60°C) cables, compatibility must be verified [110.3(8)]. If a 90°C-insulated cable was selected in the above process, the 30°C ampacity of the same size cable with a 75°C (or 60°C) insulation must be greater than or equal to the current found in Step 1

[110 .14( C)]. This ensures that the cable will operate at temperatures below the temperature rating of the terminals of the overcurrent device. If the overcurrent device is located in an area with ambient temperature higher than 30°C, then the 75°C (or 60°C) ampacity must also be derated [11O.3(B)].

The entire National Electrical Code® can be purchased from the National Fire Protection Association (NFPA) at their web site - www.NFPA.org