Ecmweb 8323 Nec Photovoltaics Pr
Ecmweb 8323 Nec Photovoltaics Pr
Ecmweb 8323 Nec Photovoltaics Pr
Ecmweb 8323 Nec Photovoltaics Pr
Ecmweb 8323 Nec Photovoltaics Pr

Article 690, Solar Photovoltaic Systems — Part 2

July 18, 2016
Don’t get tripped up over ground fault protection and circuit requirements for solar installations.

You must provide ground fault protection for DC photovoltaic (PV) arrays. But you don’t have to, if they are ground- or pole-mounted PV arrays with not more than two source circuits isolated from buildings. Also, Art. 690 requirements pertaining to DC PV circuits don’t apply to AC PV modules because AC PV modules don’t have a DC source or output circuit [690.6].

The ground-fault protection requirements for DC PV arrays depend upon whether they are grounded or ungrounded. If the latter, the ground fault protection must comply with Sec. 690.35(C). Let’s first look at the grounded requirements.

Fig. 1. It’s important to know where and how to measure maximum system voltage.

If the PV array is grounded, you must provide it with DC ground-fault protection meeting the requirements of Sec. 690.5(A) through (C).

A) Ground-fault detection and interruption (GFDI). The ground-fault protection must:

1) Be capable of detecting a ground fault in the PV array DC current-carrying conductors and components, including any intentionally grounded conductors,

2) Interrupt the flow of fault current,

3) Provide an indication of the fault, and,

4) Be listed for providing PV ground-fault protection.

B) The faulted circuits must be automatically isolated. This can be by disconnecting the conductors (e.g., a fuse opens) or by a change in controller output such that it stops supplying power to output circuits.

C) A warning label that isn’t handwritten and is of sufficient durability to withstand the environment involved must be permanently affixed [110.21(B)] on the utility-interactive inverter at a visible location at PV system batteries.

So what if the PV array isn’t grounded? We see similar requirements in Sec. 690.35(C). A key difference with the ungrounded arrays is the ground fault protection must indicate that a ground fault has occurred [90.35(C)(2)].

Circuit requirements

The maximum PV system voltage for a DC circuit is equal to the rated open-circuit voltage (Voc) of the series-connected PV modules, as corrected for the lowest expected ambient temperature [690.7]. You use this voltage to determine the voltage rating of conductors and equipment for the DC circuits (Fig. 1).

Voc is the voltage when there’s no load on the system. When Voc temperature coefficients are supplied by the manufacturer as part of the installation instructions for listed PV modules, you must use these values to calculate the maximum PV system voltage (instead of using Table 690.7), as required by Sec. 110.3(B).

One source for lowest-expected ambient temperature is the Extreme Annual Mean Minimum Design Dry Bulb Temperature found in the ASHRAE Handbook — Fundamentals. See www.solarabcs.org/permitting/map.

PV module voltage has an inverse relationship with temperature. At lower temperatures, the PV modules’ voltage increases from the manufacturer’s nameplate Voc values. At higher temperatures, the PV modules’ voltage decreases from these values.

PV system voltage

You have several ways to determine the PV system voltage. One way is to base it on the Manufacturer Temperature Coefficient %/°C.

Example: Using the Manufacturer Temperature Coefficient of -0.36%/°C, what’s the maximum PV source circuit voltage for 12 modules each rated Voc 38.30 at a temperature of -7°C?

PV Voc = Rated Voc × {1 + [(Temp. ºC - 25°C) × Module Coefficient %/°C]} × # Modules per Series String

Module Voc = 38.30 Voc × {1+ [(-7°C - 25°C) × -0.36%/°C]}

Module Voc = 38.30 Voc × {1 + [-32°C × -0.36%/°C]}

Module Voc = 38.30 Voc × {1 + 11.52%}

Module Voc = 38.30 Voc × 1.1152

Module Voc = 42.71V

PV Voltage = 42.71V × 12 = 513V

Another way is to base it on Table 690.7 temperature correction

Example: Using Table 690.7, what’s the maximum PV source circuit voltage for twelve modules each rated Voc 38.30, at a temperature of -7°C?

String Voc Table 690.7 = Module Voc × Table 690.7 Correction Factor × # Modules per Series String

Module Voc = 38.30 Voc × 1.14 correction factor

Module Voc = 43.66V

PV Voc = 43.66V × 12 modules = 524V

For one- and two-family dwellings, the maximum PV system DC voltage is limited to 600V, which is equal to the standard voltage insulation of electrical conductors [690.7(C)].

The maximum PV system DC voltage for other than one- and two-family dwelling units can to be up to 1,000V [690.7(C)]. If it’s 1,000V, then the working space, voltage rating of conductor insulation, and equipment (such as disconnects and fuses) must be based on the maximum PV DC system voltage of 1,000V.

Related

Bipolar circuits

For a 2-wire circuit connected to bipolar systems, the maximum system voltage of the circuit is the highest voltage between the conductors of the 2-wire circuit if all of the following conditions apply [690.7(E)]:

1) One conductor of the 2-wire circuit is solidly grounded.

2) Each 2-wire circuit is connected to a separate subarray.

3) The bipolar equipment has a permanently affixed label that isn’t handwritten and is of sufficient durability to withstand the environment involved [110.21(B)].

Circuit current and circuit sizing

Calculate the maximum PV source circuit current by multiplying the module nameplate short circuit current rating (Isc) by 125% [690.8(A)(1)]. The 125% current multiplier exists because of the module’s ability to produce more current than its rated value based on the intensity of the sunlight. That can be affected by altitude, reflection from snow or other buildings, or dryness of the air.

Maximum circuit currents

The maximum PV output circuit current is equal to the sum of parallel PV maximum source circuit currents [690.8(A)(2)] as calculated in Sec. 690.8(A)(1). The PV output circuit consists of circuit conductors between the PV source circuit (DC combiner) and the DC input terminals of the inverter or DC disconnect [690.2 Definition].

Fig. 2. Pull the maximum inverter output current value straight from the unit’s nameplate.

The maximum inverter output current is equal to the continuous output current marked on the inverter nameplate or installation manual (Fig. 2). The inverter output circuit consists of the circuit conductors from the inverter output terminals or AC modules [690.6(B)] to AC premises wiring [690.2 Definition]. Use the instruction manual values because the inverter output current can change based on the input voltage. Regardless of the modules, conductors and overcurrent devices are based on the output current whether there’s one module or 1 million modules.

The maximum output current for a DC-to-DC converter is the converter continuous output current rating [690.8(A)(5)].

Conductor sizing

PV circuit conductors must be sized to the larger of Sec. 690.8(B)(1) or (2) [690.8(B)].

690.8(B)(1). Before Ampacity Correction or Adjustment.
PV circuit conductors must have an ampacity of at least 125% of Sec. 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)].

Conductors terminating on terminals rated 75°C are sized per the ampacities listed in the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(3)], if the conductor insulation temperature rating is 75°C or 90°C.

Example: What’s the minimum PV source circuit conductor ampacity before the application of conductor correction or adjustment for the PV source circuit (string) conductors having a short circuit current rating of 8.90A, assuming all terminals are rated 75°C?

Conductor Ampacity = (Module Isc × 1.25)* × 1.25

Conductor Ampacity = (8.90A × 1.25)* × 1.25

Conductor Ampacity = (11.13A)* × 1.25

Conductor Ampacity = 13.91A

Conductor Ampacity = 14 AWG rated 20A at 75°C [Table 310.15(B)(16)]

*690.8(A)(1)

690.8(B)(2) After Ampacity Correction or Adjustment.
Circuit conductors must have an ampacity to carry 100% of Sec. 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)].

Fig. 3. Make sure you use the right NEC table when referencing conductor ampacity values in your calculations.

When performing conductor ampacity correction and adjustment calculations, use the conductor ampacity listed

in the 90°C column of Table 310.15(B)(16) for RHH/RHW-2/USE-2 [310.15(B)] and PV wire at 90°C [110.14(C)(1)(b)(2)] (Fig. 3).

Example: What’s the conductor ampacity after temperature correction for two current-carrying size 10 RHH/RHW-2/USE-2 or PV wires rated 90°C installed at a location where the ambient temperature is 90°F, supplying a 24A inverter output circuit?

Conductor Ampacity = Table 310.15(B)(16) Ampacity at 90°C Column × Temperature Correction

Temperature Correction = 0.96, Table 310.15(B)(2)(a) based on 90°F ambient temperature

Conductor Ampacity = 40A × 0.96 [Table 310.15(B)(2)(a)]

Conductor Ampacity = 38.40A, which has sufficient ampacity after correction to supply the inverter AC output circuit current of 24A [690.8(A)(3)].

Coming up next

This gives you an overview of ground fault protection and some of the circuit requirements for PV systems. We’ll begin Part 3 of this series by looking at overcurrent protection requirements.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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