Solar Photovoltaic Systems, Part 1 - based on the 2011 NEC!

By Mike Holt

Article 690 provides the electrical requirements for photovoltaic systems (PVS), and it consists of 9 Parts. Part I begins with an extensive list of definitions, and includes four diagrams:

1. AC module system.
2. Interactive system.
3. Hybrid system.
4. Stand-alone system.

The diagrams help you identify the system components, circuits, and connections. They don’t show every detail, and are not intended for design advice. They are intended to help you understand which type of system you’re working with, so you can correctly apply the pertinent Article 690 requirements.

Notice, the last one on the list is a “stand-alone system.” A PVS doesn’t always tie into a utility power system or other source. It can supply power to a building or other structure independently of an electrical production and distribution network [690.4(A)].

Other Articles apply [690.3]. All PV installations must conform to specific requirements from Article 90 and Chapters 1, 2, 3, and 4 except where these requirements are supplemented or modified by Article 690. And you may need to apply requirements from Chapters 7 and 8.

Installation
You can install conductors for direct current and alternating current in the same raceways, outlet and junction boxes, or similar fittings with each other. But keep them independent of all other (non-PVS) wiring [690.4(B)]. Figure 690-04B0 01.

Identify PVS conductors by separate color coding, marking tape, tagging, or other approved means. Identification of different systems isn’t required where the identification of the conductors is evident by spacing or arrangement.

The following must be identified at termination, connection, and splice points:

• PV Source Circuits [690.4(B)(1)].
• PV Output and Inverter Circuits [690.4(B)(2)].
• Multiple Systems. Conductor of each system where multiple systems are present [690.4(B)(3)] .

Grouping. Where the conductors of more than one PV system occupy the same junction box (or raceway with removable cover), group the ac and dc conductors of each system with cable ties at least once and at intervals of 6 ft or less. You don’t have to do this if the PV circuit enters from a cable or raceway unique to the circuit (making the grouping obvious) [690.4(B)(4)].

Module Connection. Arrange module connections so the removal of a module doesn’t interrupt the grounded conductor to other PV source circuits [690.4(C)].

Equipment. Inverters, photovoltaic modules, source-circuit combiners, and charge controllers intended for use in photovoltaic power systems must be identified and listed for the application [690.4(D)]. Figure 690-04D0 01
Circuit Routing. Route PV source and output conductors along building structural members (beams, rafters, trusses, and columns), where the location of those structural members can be determined by observation [690.4(E)].

If you have PV source and output conductors imbedded in built-up, laminate, or membrane roofing materials in areas not covered by PV modules and associated equipment, clearly mark their location [690.4(E)].
PV equipment, associated wiring, and interconnections must only be installed by qualified persons [690.4(F)].

Multiple Inverters. Where inverters are remotely located from each other, you must install a permanent plaque (or directory) denoting all electric power sources on the premises. Install such a plaque at each service equipment location and all interconnected electric power production sources [705.10]. A directory isn’t required where all inverters and PV DC disconnecting means are grouped at the main service disconnecting means [690.4(G)].

Circuit requirements
Part II of Article 690 starts off by requiring you to calculate the maximum PV system voltage. This equals the sum of the rated open-circuit voltage of the series-connected PV modules. Apply correction for the lowest-expected ambient temperature, per Table 690.7. For one- and two-family dwellings, the maximum PV system voltage allowed is 600V [690.7(C)]. Figure 690-07C0 01.

Circuit Sizing and Protection
In order to properly size the circuit conductors and protection, you must first, determine your maximum circuit current by making three calculations [690.8(A)]:

1. PV source circuit current. Multiply the module nameplate short-circuit current rating by 125 percent. This multiplier accounts for the fact that PV circuits can deliver output currents higher than their rated short circuit current for more than 3 hours near solar noon. Figure 690-08A1 01
2. PV output circuit current. Sum up the parallel PV source circuit currents calculated in 690.8(A)(1).

Author’s Comment: The PV output circuit is the circuit conductors that run from the combiner to the inverter [690.2].

3. Inverter output current. It equals the continuous output current marked on the inverter nameplate. Figure 690-08A3 01

Author’s Comment: The inverter output circuit is the circuit conductors that run from the alternating-current output terminals of the inverter to the alternating-current disconnect [690.2].

Next, size your overcurrent devices (OCPDs) [690.8(B)]. They must:

• Be sized to carry at least 125 percent of the maximum circuit current as calculated in 690.8(A) [690.8(B)(1)(a)]. Figure 690-08B1a 01
• Comply with the terminal temperature limits in 110.14(C) [690.8(B)(1)(b)],
• Be adjusted per manufacturer’s temperature correction factors for the ampacity rating, if operating above 40°C [690.8(B)(1)(c).

Device ratings may be sized in accordance with 240.4(B), 240.4(C), and 240.4(D) [690.8(B)(1)(d)].

Next, determine conductor ampacity. Size the PV circuit conductors to carry at least 125 percent of the currents as calculated in 690.8(A), before the application of conductor adjustment and correction of 310.15. Size the conductor overcurrent protection to the conductor ampacity after the application of conductor adjustment and correction of 310.15 and apply 240.4(B), 240.4(C), and 240.4(D).

This example problem helps illustrate how to do these calculations. What’s the minimum circuit ampacity for the inverter output circuit conductors, if the maximum continuous nameplate current rating is 24A? The conditions of use include an ambient temperature of 93ºF, two current-carrying conductors in a raceway, and conductors have an insulation rating of 90ºC.

Conductor Ampacity = Inverter Nameplate Current Rating* × 1.25
Conductor Ampacity = 24A* × 1.25
Conductor Ampacity = 30A, with a 30A OCPD
Ampacity Conditions of Use = 10 AWG rated 40A, Table 310.15(B)(16) at 90ºC **SA note to MH, make this change also in 2011 UNEC file
Ampacity Conditions of Use = 40A × 0.96 [310.15(B)(2)(a)]
Ampacity Conditions of Use = 38.40A, permitted to be protected by a 30A OCPD
*[690.8(A)(3)]

Stand-Alone systems
A stand-alone system supplies power independently of the utility [690.2].  When utilizing a stand-alone system, the premises wiring system must meet the requirements of the Code for a similar installation connected to a service. [690.10].

Additionally, wiring on the supply side of the building or structure disconnecting means must comply with the Code except:

• The AC current output from the stand-alone inverters can be less than the calculated load connected to the disconnect, but not less than the largest single utilization equipment connected to the system [690.10(A)].
• The alternating-current circuit conductors must have overcurrent protection at the output of the inverter, sized per Article 240 [690.10(B)].
• The inverter output can supply a 120V to single-phase, 3-wire, 120/240-volt distribution panel, if the panel is marked with a warning not to connect multiwire branch circuits [690.10(C)].
• Energy storage or backup power supplies aren’t required [690.10(D)].
• If a plug-in circuit breaker is backfed from field-installed conductors, secure it by an additional fastener that can’t be released from the panelboard just by pulling on it [690.10(E)].

Author’s Comment: The purpose of the breaker fastener is to prevent the circuit breaker from being accidentally removed from the panelboard while energized, thereby exposing someone to dangerous voltage.

• Don’t backfeed circuit breakers that are marked line and load [690.10(E)].

Disconnecting means
The disconnect must open ungrounded direct-current circuit conductors [690.13]. But you can’t use a switch, circuit breaker, or other device to open the grounded direct-current conductor. Figure 690-13 01
You don’t have to follow this restriction, if all of these conditions are met [690.13 Ex 2]:

• The switch is used only for PV array maintenance.
• The switch is accessible only by qualified persons.
• The switch is rated for the maximum direct-current voltage and current, including ground-fault conditions.

Figure 690-13x2 01
The PV direct-current disconnect must be a manually operable switch (or circuit breaker) located where readily accessible, externally operable without exposing the operator to contact with live parts, and plainly indicating whether in the open or closed position [690.13 and 690.17]. Figure 690-14A 01

Where all terminals of the PV direct-current disconnect may be energized in the open position, place a warning sign on (or adjacent to) a PV direct-current disconnect to indicate the hazard of energized terminals [690.14(A)]. Figure 690-14A 02
You can place a PV source circuit isolating switch on the PV side of the PV disconnecting means, but it’s not required [690.14(B)]. Figure 690-14B 01

Place the DC disconnect at a readily accessible location. Figure 690-14C1 01. If you put the disconnect inside a building or structure, place it nearest the point of entrance of the PVS conductors [690.14(C)(1)]. Figure 690-14C1 02

Exception: If the PVS conductors are in metal raceway, Type MC cable, or metal enclosure inside a building or structure, you can locate the PVS disconnect remote from the structure’s point of entry [690.14(C)(1) Ex]. Figure 690-14C1x 01

The PVS DC disconnect must be identified as such. Figure 690-14C2 01, and it must be suitable for the prevailing environmental conditions [690.14(C)(2)].
Utility-interactive inverters can be mounted on roofs or exterior areas that aren’t readily accessible if:

• The PVS DC disconnect is within sight of or in the inverter [690.14(D)(1)].
• The inverter AC disconnect is within sight of the inverter [690.14(D)(2).
• You install an additional AC disconnect at a readily accessible location [690.14(D)(3)].
• A permanent plaque identifies the location of the service and inverter AC disconnect if not at the same location [690.14(D)(4) and 705.10].

An interactive system is a solar photovoltaic system that operates in parallel with, and may deliver power to an electrical production and distribution network. This is commonly called a grid tied system.  

Artice 690 matches specific PVS components to requirements commonly associated with electrical sources such as services and separately derived systems, but make no mistake, a PVS system doesn't fit either of these definitions. A PVS system can be a dangerous system to those who are not qualified to work on them, because it is a source of electrical power which is often energized in places that a conventional electrical service would not be. Article 690 provides the requirements specific to photovoltaic systems which must be followed carefully. This topic will be continued in Part 2, as we discuss grounding, wiring methods, and related topics for solar photovoltaic systems in Article 690.

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