# String Theory

In the past few years, hundreds of thousands of solar photovoltaic (PV) systems have been installed for residential (dwelling) units, commercial buildings, industrial facilities and utility companies. Many have been subsidized by grants from utility companies as well as by local, county, state and federal tax rebates. The main problem with the amount of PV installations across the country is the lack of comprehensive training and understanding of the National Electrical Code (NEC) requirements for these systems.

Anyone working on PV systems must reconfigure their conceptual ideas by developing a better understanding of the direct current (DC) output of the solar cells to the input side of the inverter system, in addition to the connection of the alternating current (AC) output to the utility company power. One of the first concepts that must be realized when comparing a PV system to a normal utility-company-supplied power system is the vast difference between the low available fault current in a PV system and the extremely high available fault current in the utility-company-supplied AC system.

For example, the available fault current in a typical series string of PV modules is usually only a couple of amperes (A) above the normal functional current of the modules. In contrast, the available fault current in a utility-supplied service for a single-family dwelling can be 10,000A and higher, with a utility--supplied commercial or industrial service at a substantially higher fault-current value. Since the fault current in a PV system is so low, overcurrent--protective devices for PV systems will not react with the same time-current curve as those installed for an AC system.

Markings on photovoltaic modules will indicate the maximum power (Pmax), the open-circuit voltage (Voc), the maximum short-circuit current (Isc), the rated voltage (Vpmax), the rated current (Ipmax), maximum system voltage and the maximum fuse size. The voltage rating and open-circuit voltage of PV modules installed in a series must be a conglomerate of all modules in the series or stringer.
Voc is the voltage prior to any current flow through the module. Both open-circuit voltage and rated voltage for a module is dependent on the lowest ambient temperature in which the module will be operated, based on an ambient temperature of 25°C (77°F). For crystalline and multi-crystalline silicon modules, Table 690.7 or the manufacturer’s instructions based on 110.3(B) must be used to correct the open-circuit and the rated voltage output of the module, since the colder the temperature of the module, the higher both the open-circuit and the rated voltage output of the module will be.

While the voltage in a string of modules is additive, the current in the series string is the same for the entire string, based on Ohm’s Law, where amperage in series is always the same for the series circuit. By installing the module strings in parallel, the current of each string is additive to the other parallel strings. The conductor ampacity for the PV output circuit currents (the conductors from the combiner to the inverter) shall be the sum of all of the parallel-source circuit currents (the nameplate short-circuit current output of each string of modules) calculated at 125 percent as located in 690.8(A)(1) and (A)(2).

PV system currents are considered to be continuous, as stated in 690.8(B), so any calculation of current for conductors will always be at 125 percent. In addition, 690.8(B)(2) requires PV circuit conductors, other than those already located on listed modules and properly sized at the factory, to be sized to carry not less than the larger of either 690.8(B)(2)(a) or 690.8(B)(2)(b). Section 690.8(B)(2)(a) requires the conductors to be sized at 125 percent of the maximum currents calculated in 690.8(A) without any additional correction factors for conditions of use. This would require the sizing of the conductors from the combiner to the inverter at 125 percent times the 125 percent required in 690.8(B)(2)(a).

Conditions of use would involve higher ambient temperature, number of current-carrying conductors in a raceway, or being placed in a wet location. Section 690.8(B)(2)(b) requires the conductors to be sized at the maximum currents calculated in 690.8(A) after conditions of use have been applied (125 percent times 125 percent or 156 percent), but ambient temperature, outdoor use and other conditions of use would have to be figured. The higher value of conductor ampacity between the two calculations must be used for sizing the conductors from the combiner box to the inverter.

It’s evident that a thorough knowledge of the definitions is very helpful in determining what conductors and modules the NEC covers.

ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.