By Marvin Hamon, P.E., principal, Hamon Engineering
The 2017 triennial update of the National Electrical Code (NEC) is complete and has been released for adoption. Though AHJs may take months or years to adopt the latest edition (California won’t adopt the 2017 NEC until 2020), it’s important for solar contractors to know what’s coming. Here’s an overview of the changes that will have the most impact on solar installation.
Rapid shutdown expands
Probably the most argued change to the 2017 NEC is in article 690.12, Rapid Shutdown System (RSS). Compared with the 2014 RSS, the 2017 edition is much more proscriptive and, more importantly, will usually require module-level isolation for compliance.
What’s changed: All PV system conductors are now controlled conductors. The limitation placed on a conductor depends on its location in relation to the PV array boundary or point of entry into a building. The PV array boundary is located 1 ft from the PV array measured in any direction. Conductors inside this area, or less than 3 ft from point of entry into a building, must be reduced to 80 V or less within 30 seconds of RSS initiation. Conductors outside this area, or more than 3 ft from the point of entry into a building, must be reduced to 30 V or less within 30 seconds of RSS initiation.
Impact on installation: There are three options for complying with RSS inside the PV array boundary:
- The PV array can be listed or field-labeled as a RSS PV array.
- Conductors will be limited to 80 V within 30 seconds of initiation of RSS.
- If the PV array has no exposed wiring, no exposed conductive parts, is not closer than 8 ft to any other exposed grounded conductive parts and at least 8 ft from the ground, then compliance with RSS is not required.
Option 1 is not yet available because the standard has not been developed. Option 3 would require raceway for all wiring in the PV array up to the PV module junction box, which few PV modules currently support. But the other requirements could be met with existing equipment. This leaves option 2 as the one most likely to be used to comply. The requirement is two part, first the 80 V limit and second is the 30-second timeout. The 80 V limit will require breaking up higher voltage strings into segments that are 80 V or less. Because almost all modules have a temperature-corrected Voc of more than 40 V, each module must be isolated. This can be accomplished by many means including microinverters, DC-DC converters or simple remote controlled contactors.
The compounding factor is the 30-second timeout. If the array is disconnected from an inverter, the voltage on the conductor can take several minutes to decay below 80 V on its own due to the DC input capacitors in the inverter. Active discharge of the voltage, or a contactor internal to the inverter, is usually required to meet the 30-second requirement, which means that putting a disconnect on the module alone is usually not enough to provide a compliant system. The RSS system must also address the inverter discharge.
While in the 2014 NEC there was no prescriptive requirement for initiation, the requirements for RSS initiation are called out in detail in the 2017 NEC. Basically, there will be three ways to initiate RSS as outlined in 690.11(C):
- Service disconnect, in which removing utility power to the main service panel would initiate RSS.
- PV system disconnect, disconnecting the PV system would initiate RSS.
- A stand-alone RSS initiation switch.
It’s anticipated that simply removing utility power will initiate RSS in most PV systems without any backup power functionality. This is the simplest method to achieve compliance. With the growing use of energy storage or direct PV system backup power without storage, allowing the PV system to provide backup power RSS initiation will require the use of option 3 with a shutdown initiation switch.
System grounding redefined
Significant changes have been made to the wording concerning system grounding, but this will actually have little impact on how systems are installed.
What’s changed: With the advent of non-isolated inverter designs that do not allow for DC system grounding, a new term “functional grounded” has been introduced. This is defined in the NEC as, “A PV system that has an electrical reference to ground that is not solidly grounded.” The term “electrical reference to ground” is not defined but is basically a system designed to detect when there is a failure in the insulation of any ungrounded conductor, raise an alert and shut down the PV system. This is a sea change in system grounding design because much of what was previously defined as solidly grounded in the DC circuits is now functional grounded and is installed in much the same way as an ungrounded system. In the process, 690.5, describing GFDI, has been moved to 690.41(B), and GFDI will now be another form of functional grounding. Section 690.41(A) lists the acceptable forms of PV array system grounding with 690.41(A)(6) being a catch-all to allow for new equipment. Equipment grounding is not affected by any of these changes. The only solidly grounded PV systems will be small systems allowed by the 690.41(B) exception.
Impact on installation: For most PV arrays, functional grounding will be provided by the inverter. DC circuits will be installed as functional grounded circuits with no conductor marked as a grounded conductor, similar to ungrounded systems. Disconnects will open both current carrying conductors, and section 690.9(C) will require an overcurrent protection device (OCPD) only on one polarity of the circuit—the positive or negative—as long as all OCPDs are in the same polarity of the circuit.
Section 690.47 covering the grounding electrode system has been significantly simplified from prior NEC cycles to take advantage of the new functional grounded type of system grounding.
Other NEC changes worth noting
Section 690.7(A)(3) was added to allow a PV system 100 kW or larger to have the maximum PV array voltage calculated under the supervision of an electrical engineer using methods other than the lowest expected ambient temperature.
Section 690.31(C) will now allow the use of USE-2 as well as PV wire as single conductor in PV arrays with any type of system grounding.
Section 690.47(D) requiring an additional auxiliary grounding electrode has moved to 690.47(B) and has been made optional.
Section 705.12(B)(2)(3)(d) provided a method for interconnecting to center-fed panelboards using the 120% busbar rating allowance instead of requiring engineering review. This change will also be made to the 2014 NEC.
Section 705.12(B)(6), the requirement for AFCI protection of microinverter AC circuits, has been deleted. This change will also be made to the 2014 NEC.
Section 310.15(B)(3)(c) requiring temperature correction for conductors in raceway exposed to sunlight has been significantly simplified. It now only applies if the raceway is mounted closer to the roof than 7/8 in. and the correction is 33°C. Because most standoffs are going to be higher than this, temperature correction will rarely be a factor.
Section 312.8(B) was added to provide requirements for the addition of power monitoring equipment such as CTs to switch or OCPD enclosures. Because PV monitoring system CTs are often added to the PV AC fused disconnect, this section will need to be understood to size the enclosure appropriately.
You can visit nfpa.org/codes-and-standards for more information and assistance with NEC questions.