12.2 Solar Photovoltaic, Interconnection and Stand-Alone Systems

Article 690 — Solar Photovoltaic Systems

690.1 Scope. Article 690 applies to solar PV systems, other than those covered by Article 691, including the array circuit(s), inverter(s), and controller(s) for such systems.

Circuit Sizing — 690.8

690.8(A) Calculation of Maximum Circuit Current. The maximum current for the specific circuit shall be calculated as follows:

• (A)(1) Photovoltaic Source Circuit Currents. The maximum current shall be calculated by one of the following: (a) the sum of parallel-connected PV module rated short-circuit currents multiplied by 125 percent; or (b) for systems with an associated maximum current calculated by industry-standard methods and provided by a licensed professional electrical engineer, the calculated value.
• (A)(2) Photovoltaic Output Circuit Currents. The sum of parallel source circuit maximum currents as calculated in (A)(1).
• (A)(3) Inverter Output Circuit Current. The inverter continuous output current rating.
• (A)(4) Stand-Alone Inverter Input Circuit Current. The stand-alone continuous inverter input current rating when the inverter is producing rated power at the lowest input voltage.

690.8(B) Conductor Ampacity. PV system currents shall be considered to be continuous. Circuit conductors shall be sized to carry not less than the larger of (B)(1) or (B)(2):

• (B)(1) Before Application of Adjustment and Correction Factors — the maximum currents calculated in 690.8(A) multiplied by 125 percent;
• (B)(2) After Application of Adjustment and Correction Factors — the maximum currents calculated in 690.8(A), with the ampacity of the conductor determined after applying the correction and adjustment factors.

So a PV source circuit is effectively sized at 156 percent of module short-circuit current: 125 percent for the irradiance factor in 690.8(A)(1), then 125 percent again for the continuous-load factor in 690.8(B)(1).

690.9 Overcurrent Protection. PV system DC circuit and inverter output conductors and equipment shall be protected against overcurrent. Overcurrent protective devices shall be sized to carry not less than 125 percent of the maximum currents calculated in 690.8(A).

690.9(C) Photovoltaic Source and Output Circuits. A single overcurrent protective device shall be permitted to protect the PV modules and interconnecting conductors.

Rapid Shutdown — 690.12

690.12 Rapid Shutdown of PV Systems on Buildings. PV system circuits installed on or in buildings shall include a rapid shutdown function to reduce shock hazard for emergency responders.

• (A) Controlled Conductors. Requirements apply to PV system circuits supplied by the PV system.
• (B) Controlled Limits. The use of the terms array boundary in this section is defined as 305 mm (1 ft) from the array in all directions. Controlled conductors shall be limited to the following:
  • (1) Outside the Array Boundary. Controlled conductors located more than 1 foot from the array shall be limited to not more than 30 volts within 30 seconds of rapid shutdown initiation.
  • (2) Inside the Array Boundary. The PV system shall comply with one of the following: (a) the PV array shall be listed or field labeled as a rapid shutdown PV array; (b) controlled conductors located inside the boundary or not more than 1 foot from the point of penetration of the surface of the building shall be limited to not more than 80 volts within 30 seconds of rapid shutdown initiation; (c) PV arrays with no exposed wiring methods and no exposed conductive parts, installed more than 8 feet from exposed grounded conductive parts or ground.
• (C) Initiation Device. The initiation device(s) shall initiate the rapid shutdown function of the PV system. The device off position shall indicate that the rapid shutdown function has been initiated for all PV systems connected to that device.
• (D) Equipment. Equipment that performs the rapid shutdown functions, other than initiation devices such as listed disconnect switches, circuit breakers, or control switches, shall be listed for providing rapid shutdown protection.

690.56(C) Buildings with Rapid Shutdown. Buildings with PV systems shall have permanent labels as described in 690.56(C)(1) through (C)(3), including a label indicating "SOLAR PV SYSTEM IS EQUIPPED WITH RAPID SHUTDOWN" and a plaque or directory located at the rapid shutdown initiation device.

Disconnecting Means and Wiring

690.13 Photovoltaic System Disconnecting Means. Means shall be provided to disconnect the PV system from all wiring systems including power systems, energy storage systems, and utilization equipment and its associated premises wiring. The PV system disconnecting means shall be installed at a readily accessible location, shall be permanently marked to identify it as a PV system disconnect, and shall indicate whether it is in the open or closed position. Where the line and load terminals may be energized in the open position, a warning label shall be mounted on or adjacent to the disconnecting means stating "WARNING — ELECTRIC SHOCK HAZARD — TERMINALS ON THE LINE AND LOAD SIDES MAY BE ENERGIZED IN THE OPEN POSITION."

690.31 Wiring Methods. (A) Wiring Systems. All raceway and cable wiring methods included in the Code, and other wiring systems and fittings specifically listed for use on PV arrays, shall be permitted. Where PV source and output circuits operating at voltages greater than 30 volts are installed in readily accessible locations, circuit conductors shall be installed in a raceway. (C) Single-Conductor Cable. Single-conductor cable Type PV wire or Type distributed generation (DG) cable shall be permitted in PV source circuits within the PV array.

690.41(A) PV System Grounding Configurations. One or more of the following system configurations shall be employed: 2-wire circuits with one functional grounded conductor; bipolar circuits with a functional ground reference; circuits not isolated from the grounded inverter output circuit; ungrounded circuits; solidly grounded circuits as permitted; or other methods that accomplish equivalent system protection.

690.41(B) Ground-Fault Protection. PV systems shall be provided with DC ground-fault protection meeting the requirements of (B)(1) and (B)(2) to reduce fire hazards, with an exception for ground-mounted or pole-mounted PV arrays with not more than two PV source circuits and with all DC source and output circuits isolated from buildings.

690.47 Grounding Electrode System. A building or structure supporting a PV system shall have a grounding electrode system installed in accordance with Part III of Article 250. PV array equipment grounding conductors shall be sized in accordance with 690.45, and where the PV array is on a building, the equipment grounding conductors shall be connected to the grounding electrode system of the building.

Article 705 — Interconnected Electric Power Production Sources

705.12 Load Side Source Connections. The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises, subject to the requirements of the section.

705.12(B)(3) Busbars. One of the following methods shall be used to determine the ratings of busbars:

1. (1) The sum of 125 percent of the power source output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
2. (2) The 120 percent rule. Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power source output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the power source that displays: "WARNING — POWER SOURCE OUTPUT CONNECTION — DO NOT RELOCATE THIS OVERCURRENT DEVICE."
3. (3) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar, with a permanent warning label applied.
4. (4) A connection at either end of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power source output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the ampacity of the busbar.
5. (5) Connections shall be permitted on busbars of panelboards that supply loads and are fed by primary power sources, where the busbar is sized by engineering supervision.

Worked Example — the 120 Percent Rule

A 200-ampere residential panelboard with a 200-ampere busbar and a 200-ampere main breaker. What is the largest PV back-fed breaker permitted at the opposite end of the busbar?

120% of busbar ampacity = 200 x 1.20 = 240 A
Less the main breaker rating .......... - 200 A
Maximum back-fed breaker ..............    40 A

At 240 volts, a 40-ampere breaker supports an inverter output of 40 / 1.25 = 32 amperes, or roughly 7.7 kW. This calculation drives residential PV system sizing more often than the roof area does.

705.13 Power Control Systems. A power control system shall be listed and marked, and shall limit current on conductors and busbars.

705.20 Source Disconnecting Means. Means shall be provided to disconnect power source output circuit conductors of electric power production equipment from conductors of other systems. The disconnecting means shall be one of the specified types, shall be lockable open in accordance with 110.25, and shall be marked with the specified warning where line and load terminals may be energized in the open position.

705.30 covers overcurrent protection, and 705.40 Loss of Primary Source requires the output of an electric power production source to be automatically disconnected from all ungrounded conductors of the primary source upon loss of voltage in that primary source, and to remain disconnected until the primary source voltage is restored. This is the anti-islanding requirement that prevents a PV inverter from back-feeding a de-energized utility line and injuring a lineman.

Article 710 — Stand-Alone Systems

710.6 Equipment Approval. All equipment shall be approved for the intended use.

710.10 Identification of Power Sources. A permanent plaque or directory shall be installed at each service equipment location, or at an approved readily visible location, denoting all electric power sources on or in the premises. Where a stand-alone system is not connected to a service, the plaque shall be located at the disconnecting means for the stand-alone system.

710.15 General. Premises wiring systems supplied by stand-alone systems shall be installed in accordance with (A) through (F):

• (A) Supply Output. Power supply to premises wiring systems fed by stand-alone or isolated microgrid power sources shall be permitted to have less capacity than the calculated load. The capacity of the power source shall be equal to or greater than the load posed by the largest single utilization equipment connected to the system. Calculated general lighting loads shall not be considered as a single load.
• (B) Sizing and Protection. The circuit conductors between the stand-alone source and the first overcurrent device shall be sized based on the sum of the output ratings of the stand-alone source(s).
• (C) Single 120-Volt Supply. Stand-alone systems shall be permitted to supply 120 volts to single-phase, 3-wire, 120/240-volt service equipment or distribution panels where there are no 240-volt outlets and where there are no multiwire branch circuits. In all installations, the sum of the ratings of the power sources shall be less than the rating of the service disconnect or distribution panelboard. The grounded conductor shall be connected to the neutral bus in the service equipment or panelboard.
• (D) Energy Storage or Backup Power System Maximum Voltage. The maximum voltage of a stand-alone system shall comply with 706.30(B).
• (E) Back-Fed Circuit Breakers. Plug-in-type back-fed circuit breakers connected to a stand-alone or isolated microgrid system shall be secured in accordance with 408.36(D).

Subsection (A) is the important one and is unique to stand-alone systems. In a grid-connected building, the service must handle the calculated load. In a stand-alone system, the source need only handle the largest single piece of utilization equipment, on the reasonable assumption that the user manages consumption. That is exactly how a remote Alaska cabin with a small inverter and a 100-ampere panel is legally wired.

Article 712 — Direct Current Microgrids

712.1 Scope. Article 712 covers direct current microgrids, which are a power distribution system consisting of more than one interconnected DC power source, supplying DC-DC converters, DC loads, or AC loads powered by DC-AC inverters.

712.10 Directory. A permanent plaque or directory shall be installed at each DC microgrid, denoting all electric power sources on or in the premises.

712.30 Disconnecting Means and 712.35 Wiring Methods apply the general Code rules with DC-specific modifications, and 712.65 Marking requires DC microgrid systems to be marked with the maximum operating voltage, the short-circuit current, and the maximum operating current.

DC microgrids matter for Alaska because many remote village power systems and telecom sites are exactly this architecture: multiple DC sources — PV, wind, a diesel-driven rectifier, and battery storage — feeding a common DC bus.