by Mike Holt, NEC Consultant

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, Article 690 requirements pertaining to dc PV circuits don’t apply to ac PV modules since ac PV modules don’t have a dc source or output circuit [690.6].

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

If the PV array is grounded, you must provide it with dc ground-fault protection meeting the requirements of 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 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.

Open circuit voltage (Voc) is the voltage when there’s no load on the system. When open-circuit voltage 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 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 manufacturers 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 %/ºC.

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

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

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

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

Module Voc = 38.30 Voc × {1 + 11.52%}

Module Voc = 38.30 Voc × 1.1152

Module Voc = 42.71V

PV Voltage = 42.71V x 12

PV Voltage = 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

PV Voc = 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.

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 percent [690.8(A)(1)].

The 125% current multiplier exists because the module’s ability to produce more current than its rated value based on the intensity of the sunlight. And that can be affected by altitude, reflection due to snow or other buildings, or even the 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 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].

The maximum inverter output current is equal to the continuous output current marked on the inverter nameplate or installation manual. 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 one 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 690.8(B)(1) or 690.8(B)(2) [690.8(B)].

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

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 75ºC [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 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)].

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)].

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 x Temperature Correction

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

Conductor Ampacity = 40A x 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.