NEC Requirements for PV Systems, Part 1 based on the 2011 NEC

NEC Requirements for Photovoltaic Systems, Part 1
Based on the 2011 NEC

By Mike Holt

With Article 690, the NEC addresses the electrical aspects of installing Solar Photovoltaic (PV) systems. The general requirements of Chapters 1 through 4 also apply to PV installations, except as specifically modified by Article 690. You may need some vocabulary work, too (see Sidebar).

Conductor installation
You can install ac and dc conductors together in the same raceways and junction boxes. But keep them separate from non-PV system conductors [690.4(B)].

Identify PV system conductors by separate color coding, marking tape, tagging, or other approved means. Do this at termination, connection, and splice points. You can skip this where identity is evident by spacing or arrangement.

Group the conductors as follows:

PV source circuits.
PV output and inverter circuits.
Multiple systems.

Where the conductors of more than one PV system occupy the same junction box (or raceway with removable cover), group the conductors of each system by cable ties (at intervals of 6 ft or less).. You can skip grouping if the PV circuit enters from a cable or raceway unique to the circuit (making the grouping obvious).

Arrange module connections so module removal doesn’t interrupt the grounded conductor to other PV source circuits. Route PV source and output conductors along structural members (beams, rafters, trusses, columns) where the location of those structural members can be determined by observation.
Clearly mark the location of PV source and output conductors imbedded in built-up, laminate, or membrane roofing materials in areas not covered by PV modules and associated equipment.

Where multiple utility-interactive inverters are remote from each other, place a directory at each PV system disconnecting means (ac and dc) and at the main service disconnect. Ensure it shows the location of all PV system disconnecting means in the structure.

PV systems other than ungrounded dc must have ground-fault protection [690.5, 690.35].

Circuit requirements
How do you determine the maximum PV system voltage? Add up the rated open-circuit voltage of the series-connected PV modules, correcting for the lowest-expected ambient temperature per Table 690.7. If the manufacturer provides open-circuit voltage temperature coefficients in the instructions for PV modules, use those instead of Table 690.7.

But the maximum PV system voltage is

600V for or one- and two-family dwellings.
…in bipolar systems, the highest voltage between the conductors of the 2-wire circuit. But only if one conductor of each circuit of a bipolar subarray is solidly grounded and each circuit connects to a separate subarray. GFCI and AFCI devices can interrupt this connection to ground.

Review your PV voltage calculations to ensure the modules can produce sufficient voltage to start the system [110.3(B)].

Circuit sizing and protection
Before you can size conductors and protective devices, you must know the maximum circuit currents of:

PV source
PV output
Inverter output

So, how do you calculate the maximum PV source circuit current? Just multiply the module nameplate short-circuit current rating (Isc) by 125% [690.8(A)(1).

The other two are also easy to determine:

To get the maximum PV output circuit current, add up the parallel PV source circuit currents you just calculated.
The maximum inverter output current is equal to the continuous output current marked on the inverter nameplate or installation manual.

Once you know these current maximums, you can specify the conductor ampacities and overcurrent protection device (OCPD) ratings for each circuit.

Where conductor overcurrent protection is required [690.9], size the OCPD at least 125% of the maximum circuit current.

For example, you need to size the OCPD for a single PV source circuit having a nameplate Isc of 11.80A.

OCPD = (Module Isc × 1.25)* × 1.25
OCPD = (11.80A × 1.25)* × 1.25
OCPD = (14.75A)* × 1.25
OCPD = 18.44A
OCPD = 20A [240.6(A)]
*690.8(A)(1)

Now let’s look at conductor ampacity. PV circuit conductors must be sized to the larger of 690.8(B)(2)(a) or 690.8(B)(2)(b).
(a) To carry at least 125 percent of 690.8(A) current before the application of conductor ampacity correction [310.15(B)(2)(a) and 310.15(B)(3)(c)] and adjustment [310.15(B)(3)(a)].
(b) To carry 100% of 690.8(A) current after the application of conductor ampacity correction [310.15(B)(2)(a) and 310.15(B)(3)(c)] and adjustment [310.15(B)(3)(a)].

It might have been easier to remember if the NEC said “a” is “after” and “b” is “before,” but it’s the other way around. Let’s work out an example of “before.”

What’s the minimum PV source circuit conductor ampacity before applying conductor correction/adjustment for a string having a short-circuit current rating of 11.80A? Assume all terminals are rated 75°C.

Conductor Ampacity = (Module Isc × 1.25)* × 1.25
Conductor Ampacity = (11.80A × 1.25)* × 1.25
Conductor Ampacity = (14.75A)* × 1.25
Conductor Ampacity = 18.44A
Conductor Ampacity = 14 AWG rated 20A at 75ºC [Table 310.15(B)(16)]
*690.8(A)(1)

Conductors on terminals rated 75°C are sized per the ampacities in the 75°C column of Table 310.15(B)(16).

Now let’s work out an example of “after.” What’s the PV source circuit conductor ampacity after temperature correction for two current-carrying size 12 THWN-2 conductors in a circular raceway on the roof, ambient temperature is 90°F with 60°F temperature added per Table 310.15(B)(3)(c), supplying modules having a nameplate Isc rating of 11.80A?

Conductor Ampacity = Table 310.15(B)(16) Ampacity at 90°C Column x Temperature Correction[
Temperature Correction = 0.58, Table 310.15(B)(2)(a) based on 150°F (ambient plus 60°F roof temperature adder)
Conductor Ampacity = 30A x 0.58
Conductor Ampacity = 17.40A, which has sufficient ampacity after correction/adjustment to supply 690.8(A)(1) calculated current of 14.75A (11.80A x 1.25).

Let’s add a twist to it. Same problem, but this time with four current-carrying size 12 THWN-2 conductors.

Conductor Ampacity = Table 310.15(B)(16) Ampacity at 90°C Column x Temperature Correction x Adjustment
Temperature Correction = 0.58, Table 310.15(B)(2)(a) based on 150°F (ambient plus 60°F roof temperature adder)
Adjustment = 0.80, Table 310.15(B)(3)(a), based on four current-carrying conductors in a raceway
Conductor Ampacity = 30A x 0.58 x 0.80
Conductor Ampacity = 13.92A, which does not have sufficient ampacity to supply carry 690.8(A)(2) calculated current of 14.75A [11.80A x 1.25], therefore a 10 AWG THWN-2 conductor would be required.

Overcurrent protection
PV source circuits, PV output circuits, inverter output circuits, and equipment must have overcurrent protection per Article 240 [690.9(A)].
Six key requirements:

For an ungrounded system [690.35], both positive and negative conductors require overcurrent protection.
For grounded systems, only one conductor requires overcurrent protection (typically the positive conductor) [240.15].
Overcurrent protection isn’t required for PV dc circuits where the short-circuit currents (Isc) from all sources can’t exceed the ampacity of the PV circuit conductors or the maximum overcurrent device size specified on the PV module nameplate. This can occur when the PV source or PV output circuits consist of no more than four conductors.
OCPD for PV source circuits don’t have to be readily accessible.
OCPD for PV dc circuits must be listed for dc circuits and the voltage and current for the applied circuit.
You can use a single OCPD to protect PV modules and interconnecting PV source circuit conductors.

Stand-Alone systems
Nothing says a PV system has to connect to the utility. Many, in fact, serve as the sole power supply for a structure. If your installation is stand-alone, the following requirements apply:

The ac current output from a battery-based inverter cannot be less than the largest single utilization equipment connected to the system [690.10(A)].
Inverter ac output circuit conductors must have overcurrent protection per Article 240.
The battery-based inverter output can supply a 120V single-phase, 3-wire, 120/240V distribution panel marked with a warning against connecting multiwire branch circuits to it.
Energy storage or backup power supplies aren’t required.
Plug-in circuit breakers must be secured in place by an additional fastener that requires other than just a pull to release the breaker from the panelboard.
Circuit breakers that are marked line and load must not be backfed.

AFCI protection for direct current
Photovoltaic dc circuit conductors operating at 80V or greater within a building must be protected by a listed PV type dc arc-fault circuit interrupter [690.11]. The AFCI protection device must:

Detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component in the dc PV source and output circuits.
Disable or disconnect inverters (or charge controllers) and any system components within the arcing circuit.
Have an annunciator that provides a visual indication that the circuit interrupter has operated.

Additionally, you can’t have an auto restart in the event that equipment is disabled or disconnected (manual restart required).

Disconnecting means
Provide a disconnecting means that opens all ungrounded dc circuit conductors. What about the grounded dc conductor? It can be switched only under the conditions noted in Exception 1 and Exception 2 of 690.13.

PV system disconnect
You can use a PV source circuit isolating switch on the PV side of the PV system dc disconnect [690.14(C)(5)]. The PV system disconnect must open all ungrounded conductors.

Install each PV system disconnect at a readily accessible location outside the building/structure, or inside nearest the point of dc PV system conductor entry. If such location isn’t practical, install the dc PV conductors in a metal raceway, Type MC cable, or metal enclosure [690.31(E)].

Permanently mark each PV system disconnect to identify it as the PV system disconnect. As with non-PV installations, you have a limit of six disconnects and they must be grouped.
You can install utility-interactive inverters on roofs or other areas that aren’t accessible, if [690.14(D)]:

A disconnect for the inverter dc input circuit is within sight of (or in) the inverter.
A disconnect for the inverter ac output circuit is within sight of (or in) the inverter.
An additional ac disconnect is at a readily accessible location.
A permanent plaque identifying the locations of the service and inverter ac disconnect(s) is at service and inverter ac disconnect(s) locations.

PV equipment disconnect
PV equipment must have a disconnecting means that opens all ungrounded circuit conductors from all sources of power [690.15]. But what if equipment is energized from more than one source?

Install a disconnecting means for each source.
Group dc and ac disconnects.
Permanently mark each disconnect to identify its purpose.

Disconnects for fuses
Provide a means to disconnect [690.16]:

Each PV source circuit fuse, independently of other PV source circuit fuses [690.16].. Combiners containing pull-out fuses and finger-safe fuse holders meet this requirement.
PV output circuit fuses (independence from other PV output circuit fuses not required). This disconnect must be within sight of (or integral with) the PV source circuit fuse holders.

Nonload-break fuse pullouts or holders must be marked “Do not open under load.”

Disconnect requirements
For a disconnect, use a manually operable switch or circuit breaker meeting all of the following [690.17]:

Externally operable without exposing the operator to live parts.
Plainly indicating whether in the open or closed position.
Interrupting rating is sufficient for the circuit voltage and the current available at the line terminals of the equipment.

But you can use connectors in place of switches (just comply with 690.33). The most common application of this exception is for micro-inverters that handle the output of one or two PV modules. These use plug connectors for the dc and ac disconnect.

Except for grounding and batteries, we’ve now reviewed the electrical requirements for PV installations. We’ll start Part 2 by discussing the grounding requirements.

Sidebar: Words Matter
You’ll find two dozen definitions in 690.2. In addition, PV installers must know many definitions from Article 100.

Taken from Mike Holt’s Understanding NEC Requirements for Solar Photovoltaic Systems, based on the 2011 NEC textbook.

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