11.3 Solar Photovoltaic Systems, Fuel Cells, Wind & Fire Pumps (Articles 690-695)

Solar photovoltaic systems (Article 690)

Maximum system voltage (690.7)

The maximum PV system voltage is the sum of the open-circuit voltages of the series-connected modules, corrected for the lowest expected ambient temperature. Voltage rises as temperature falls. Three methods are allowed:

1. the module manufacturer's temperature coefficients;
2. for crystalline and multicrystalline silicon modules, the correction factors of Table 690.7(A);
3. for systems of 100 kW or more, a documented design by a licensed professional electrical engineer.

In the 2020 NEC, PV DC circuits on or in one- and two-family dwellings may have a maximum system voltage up to 600 V. On other buildings the limit is 1000 V.

Washington (WAC 296-46B-690): use the listed module's temperature coefficients. Otherwise use NEC Table 690.7 with a correction factor of 1.25, unless another factor is justified and approved by the AHJ.

Circuit current and conductor sizing (690.8)

• Maximum PV source circuit current = sum of the parallel modules' short-circuit current (Isc) x 1.25.
• Conductors and overcurrent devices are sized at 125 percent of that maximum current, before adjustment and correction factors. This is why PV math often multiplies Isc by 1.56 (1.25 x 1.25).
• After applying adjustment and correction factors, the conductor ampacity must still be at least the maximum current.

Example: strings of modules with Isc = 10 A.

• Maximum current = 10 x 1.25 = 12.5 A
• Minimum conductor ampacity and fuse rating = 12.5 x 1.25 = 15.6 A, so a 20 A fuse and matching conductors are used.

Rapid shutdown (690.12)

PV systems on or in buildings need rapid shutdown of the conductors they control:

For one- and two-family dwellings, the initiation device is at a readily accessible outdoor location, such as the service disconnect. Buildings with rapid shutdown get the required labels of 690.56(C).

Disconnects and wiring

• PV system disconnect (690.13): readily accessible, able to disconnect the PV system from all other wiring, no more than six disconnects grouped, and marked "PV SYSTEM DISCONNECT." If the line and load terminals may both be energized in the open position, it needs a warning label.
• DC circuits inside a building (690.31(D)): from the point where they enter the building to the first readily accessible disconnect, run them in metal raceways, Type MC cable meeting 250.118(10), or metal enclosures. Label them "PHOTOVOLTAIC POWER SOURCE" or "SOLAR PV DC CIRCUIT" at intervals of no more than 10 ft.
• Grounding: follow 690.41 through 690.47, including equipment grounding of module frames and the grounding electrode requirements.

Washington PV rules (WAC 296-46B-690)

• A PV system design review must be on site from the first inspection until inspection is finished.
• Purely mechanical supports that are not part of a bonding path may be built without an electrical permit.
• Supply-side PV connections must use the service wiring methods of WAC 296-46B-230(7) (Section 13.7).
• Plan review is not required for power production sources of 9,600 W or less (WAC 296-46B-900).

Fuel cells and wind (Articles 692, 694)

Both articles follow the PV pattern: a readily accessible system disconnect, conductor sizing based on the source's output current, and interconnection under Article 705 (Section 12.2). Wind turbines also need a way to shut down the turbine for maintenance. Washington requires a system design review on site for both (WAC 296-46B-692, -694).

Fire pumps (Article 695)

A fire pump is built to keep running during a fire even if it damages itself. The NEC therefore reverses normal motor protection: the pump's circuit must hold through locked-rotor current and may not trip on overload.

Power sources (695.3)

A reliable source is required. The options include:

• a separate service;
• a connection ahead of the service disconnecting means, in its own enclosure or section, not in the same cabinet, enclosure, or vertical section as the service disconnect;
• an on-site power production facility;
• multiple sources, such as a utility service with an on-site standby generator.

A connection ahead of the main keeps the pump running when firefighters open the building's service disconnect.

Conductor sizing (695.6(C))

Conductors supplying fire pump and pressure maintenance (jockey) pump motors, plus their accessory equipment, must be rated at least 125 percent of the sum of the fire pump and pressure maintenance motor full-load currents, plus 100 percent of the accessory load. Use motor full-load currents from Article 430 tables.

Overcurrent protection (695.4(B), 695.5, 695.6)

• Overcurrent devices must carry indefinitely the sum of the locked-rotor current of the fire pump and pressure maintenance motors, plus the full-load current of the accessory equipment (695.4(B)(2)). Where that value is not a standard rating, the next larger standard size is allowed.
• Locked-rotor currents come from Table 430.251(B) or the motor's code letter, not from a rule of thumb. An induction motor's locked-rotor current is typically around 6 times its full-load current.
• Transformers dedicated to a fire pump have primary protection sized to carry the locked-rotor current indefinitely, with no secondary overcurrent protection (695.5).
• No automatic overload protection: power circuits may not have automatic protection against overloads. Except for the transformer primary protection in 695.5, branch-circuit and feeder conductors are protected against short circuit only (695.6(D)). The listed fire pump controller provides locked-rotor protection, not running overload trips.
• Fire pump supply conductors should be routed outside the building. Where they must pass through it, 695.6(A)(2) requires protection such as encasement in at least 2 in. of concrete, a dedicated fire-rated assembly, or a listed electrical circuit protective system.