Residential PV Systems: Common Code Violations: Page 4 of 6

Labeling

Required labels. Article 690 lists numerous requirements for labeling equipment, with more requirements added each Code cycle. For proper signage, installers must refer specifically to Sections 690.17, 690.31(E)—see 2011 NEC Section 690.31(E) sidebar—690.35, 690.53, 690.54 and 690.64 (705.12 in the 2011 NEC). Labeling the equipment is an extremely important, yet commonly overlooked, requirement in PV installations. It is one of the final steps in an installation, but the labeling requirements should be identified before the first module is placed.

“Direct-Current Photovoltaic Power Source,” one of the labels required in NEC 690.53, calls for a permanent label for the DC disconnecting means that specifies the PV maximum system voltage, short-circuit current, and the rated maximum power point voltage and current.

The following example calculations for a typical residential system illustrate how to obtain the values for labels:

• PV array capacity: 5,390 W STC; 22 SolarWorld 245 W modules
• Module specifications: SolarWorld Sunmodule SW245; 37.7 Voc, 30.8 Vmp, 8.25 Isc, 7.96 Imp, -0.124 V/°C Voc temperature coefficient
• Array configuration: 11 modules per series string with two strings paralleled in a roof-mounted combiner
• Record-low site temperature: -10°C
• Inverter: PV Powered PVP4800, 4.8 kW, 500 VDC maximum input, 200–450 VDC MPPT range

The rated maximum power point voltage and current values are rather straightforward—they are determined by the module manufacturer’s specifications at STC. The 2005 Code requires listing the operating current and voltage, and the 2008 Code helps clarify that language.

Imp = Module Imp × strings in parallel

7.96 A × 2 = 15.92 A

Vmp = Module Vmp × number of modules per string
30.8 V × 11 = 338.8 V

Section 690.53(3) in the 2008 Code contains a fine-print note (FPN) that instructs listing the maximum voltage as calculated in Section 690.7. Depending on the Code version you are working with, this value may be the array’s open circuit voltage at STC multiplied by the correction factor listed in 690.7, or it may be calculated from module manufacturer data.

Max Voc = {37.7 V + [(T_min - T_STC) x coeff]} x 11 

{37.7 V + [(-10°C- 25°C) x -0.124 V/°C)]} x 11 = 462.4 V

The fourth piece of information required is the array’s short-circuit current. Again, the 2008 NEC  has added an FPN to clarify this calculation that references Article 690.8(A), which requires multiplying the array’s short-circuit current by 1.25 to determine the value to list on the label.

Isc = Module Isc rating x strings in parallel x 1.25

8.25 A x 2 x 1.25 = 20.63 A

Rounding to the nearest whole numbers, the label applied at this site should read:

DC PV POWER SOURCE

Imp: 16 A

Vmp: 339 V

Max Voc: 462 V

Isc: 21 A