Alternative Energy Systems, Part 1 - Based on the 2011 NEC

By Mike Holt for EC&M

Solar and wind are two alternative energy technologies that are increasingly providing work for electrical contractors. Both require expertise in several non-electrical areas, which the NEC doesn’t address. But the NEC does provide requirements that seek to reduce the electrical hazards that may arise from installing and operating such systems.

The requirements for solar power installations are in Article 690. It consists of nine Parts, the ninth of which applies only to systems over 600V. The requirements of Chapters 1 through 4 of the NEC also apply to solar power installations, except as specifically modified by Article 690.

What’s in a name?

With the 2012 Code revision, Article 690 replaces the term “photovoltaic” with the industry’s commonly used acronym “PV.” It does that with the term “solar,” also.

Installation

Section 690.4 includes many changes that clarify and expand the requirements for PV systems. For example:

• Where conductors of more than one system occupy the same enclosure (or raceway with a removable cover), you must group the conductors unless grouping is obviously not necessary [690.4(B)(1)(d) and Ex].
• When routing PV-related conductors along structural members, you have to mark their location if it’s not obvious. This helps protect firefighters who ventilate a roof where PV conductors are present [690.49F)].
• When you install multiple inverters remote from each other, you must also install a permanent plaque or directory denoting all electrical power sources on or in the premises [690.4(H)]. Where all inverters and PV dc disconnecting means are grouped at the main service disconnecting means, the plaque or directory is optional [690.4(H) Ex].
• Now only qualified persons can install PV systems. Other (arguably more dangerous) areas of the Code have never required this, as this concern is typically covered by state or local agencies and their applicable licensing regulations. Many people consider this change to be revolutionary. It may set an interesting precedent for future editions of the NEC [690.4(E)].

Maximum PV voltage

The 2011 NEC added an Informational Note to 690.7, referring the Code user to the temperature data in the ASHRAE Handbook – Fundamentals. This addition should result in a more uniform interpretation and enforcement of the requirements for maximum PV voltage calculations.

Circuit sizing and protection

The sizing provisions for conductors and overcurrent devices now align with other Code requirements for conductors and overcurrent devices.

Although Code Chapters 1 through 4 (including the conductor and overcurrent device rules) already apply [90.3], some PV system installers may not understand that. Or they might not be very proficient with applying the rules from Chapters 1 through 4.

• To make it easier for these people, the 690.8 conductor and overcurrent device rules have been “copied and pasted” into this section. Section 690.8 includes rules for determining maximum circuit currents. Those rules are:
• PV Source Circuit Current (Isc). Multiply the module nameplate short-circuit current rating (Isc) by 125 percent [690.8(A)(1)].
• PV Output Circuit Current. Add up the parallel PV Source Circuit Currents [690.8(A)(2)].
• Inverter Output Circuit Current. According to 690.8(A)(3), this is equal to the continuous output current marked on the inverter nameplate.
• Stand-Alone Inverter Input Circuit Current. It’s the stand-alone continuous inverter rating when the inverter is producing power at the lowest rated voltage [690.8(A)(4)].

Sizing overcurrent protection devices (OCPDs)

Size OCPDs to carry not less than 125 percent of the current as calculated in 690.8(A) [690.8(B)(1)(a)]., and:

• Apply the terminal temperature limits of 110.14(C) [690.8(B)(1)(b)].
• If operating above 40˚C, use the manufacturer’s temperature correction factors to adjust the ampacity rating of the OCPD [690.8(B)(1)(c)].
• Use settings or ratings permitted for OCPDs by 240.4(B), (C), and (D) [690.8(B)(1)(d)].

For OCPDs 800A or less, you can use the next higher standard rating of an OCPD listed in 240.6 (above the ampacity of the ungrounded conductors being protected). But only if you satisfy all three conditions listed in 240.4(B).

You must size the circuit conductors to carry the larger of:

• 125 percent of the currents as calculated in 690.8(A) before the application of conductor adjustment and correction of 310.15 [690.8(B)(2)(a)].
• The maximum current calculated in 690.8(A) after the application of conductor adjustment and correction of 310.15 [690.8(B)(2)(b)].

And you must size the conductor overcurrent protection to the conductor ampacity after the application of conductor adjustment and correction of 310.15 per 240.4(B), 240.4(C), and 240.4(D).

Overcurrent protection

A change to 690.9(E) clarifies that the single overcurrent device being discussed can protect both the module and the source conductors.

The exception to 690.9(A) now includes language about the maximum overcurrent device of the PV module. Where the short-circuit current of the PV modules or PV source doesn’t exceed the ampacity of the conductors or equipment, there’s no hazard of overcurrent. Consequently, there’s no need for overcurrent protection.

So you must protect PV source circuits, PV output circuits, inverter output circuits, and equipment per the requirements of Article 240. But you don’t have to provide overcurrent protection for PV direct-current circuits where the short-circuit currents (Isc) from all sources don’t exceed the [690.9(A) Ex]:

• Ampacity of the PV circuit conductors, or
• Maximum overcurrent device size specified on the PV module nameplate.

Stand-alone systems

Provisions for backfed circuit breakers have been added in 690.10(E). Although the requirements of Article 408 already apply to PV installations [90.3], the 2011 Code revision borrows language from 408.36(D) and plunks it into 690. Because these rules were already in Article 408, this change wasn’t a technical one.

Plug-in circuit breakers that are backfed from field-installed conductors must be secured in place by an additional fastener. This fastener must require something other than a pull to release the breaker from the panelboard. Circuit breakers that are marked line and load must not be backfed [690.10(E)].

The purpose of the breaker fastener is to prevent the circuit breaker from being accidentally removed from the panelboard while energized. That situation would expose someone to dangerous voltage.

Disconnecting

Part III of Article 690 provides the requirements for the disconnecting means. Section 690.13 now includes an exception for switching devices that open the direct-current grounded conductor.

The rule in 690.13 requires a disconnecting means that opens ungrounded direct-current circuit conductors. A switch, circuit breaker, or other device isn’t permitted to open the grounded direct-current conductor.

Exception 2 (which is new with the 2011 revision) allows you to install a disconnecting switch that opens the grounded direct-current conductor. But only if the switch is:

• Used only for PV array maintenance,
• Accessible only by qualified persons, and
• Rated for the maximum direct-current voltage and current, including ground-fault conditions.

Why this change? It permits the standard practice of switching all circuit conductors of dc PV systems, including the grounded conductor. Now we have specific requirements that help ensure a safe installation to qualified persons.

Fuses

A disconnecting means is now required for fuses on some PV output circuits. Why this new rule? Replacing a fuse under load is obviously an unsafe work practice. Suppose the inverter has fuses on the input circuit fuses that combine the internal PV source and output circuit (a fairly common arrangement). And suppose they are connected directly to the inverter input terminals. There’s only one way to make this safe. And that is to install an external disconnecting means.

Thus, 690.16 (A) requires you to provide a means to disconnect a fuse from all sources of supply if energized from both directions. The disconnect must be capable of being disconnected independently of fuses in other PV source circuits. One way to satisfy this requirement is to use disconnects with pull-out fuses (to disconnect a fuse from all sources of energy).

Similarly, 690.16 (B) requires you to install a disconnecting means for PV output circuits where fuses that must be serviced can’t be isolated from energized circuits. The disconnect must be:

• Within sight and accessible to the fuse or integral with the fuse holder.
• Externally operable without exposing the operator to contact with live parts.
• Plainly indicating whether in the open or closed position [690.17].

Where the disconnecting means is more than 6 ft from the fuse, then at the fuse location you must install a directory showing the location of the fuse disconnect. Mark any non-load-break rated disconnecting means “Do not open under load.”

Taken from Mike Holt’s 2011 Understanding the NEC Volume 2 Textbook. To order your copy, please click here, or call 888-632-2633