For ambient temperatures other than 30°C (86°F), multiply the 30°C ampacities [310.16, 310.17] by the appropriate factor. Source: NEC 2002

For ambient temperatures other than 30°C (86°F), multiply the 30°C ampacities [310.16, 310.17] by the appropriate factor. Source: NEC 2002

for 24 volts rather than the present 48 volts. These breakers protect the #1/0 (53 mm2) conductors, which have a temperature-corrected ampacity of 148 amps—170 amps at 30°C times 0.87 correction factor for 45°C operating temperature (170 x 0.87 = 148).

The two, 75 amp circuit breakers are connected to a single, 175 amp circuit breaker mounted in the same enclosure as the battery disconnect. This 175 amp circuit breaker serves as the main PV disconnect, and is connected to the conductors going to the battery. Conductors sized at #1/0 are used to connect this breaker to the main battery circuits and to the 75 amp breakers.

Equipment-Grounding Conductor Size. For this ground-mounted PV system, NEC 690.45 requires that the PV array equipment-grounding conductor be able to carry a current equal to the continuous current from the modules (each set of ten), which is calculated by multiplying the short circuit current (Isc) of 27.3 amps (5 x 5.46 = 27.3) by an NEC factor of 1.25, which in this case yields 34.1 amps (27.3 x 1.25 = 34.1). This requires a #10 (5 mm2) equipment-grounding conductor.

NEC 250.122(B) requires that this conductor be increased in size if the circuit conductors are increased in size for voltage drop. Circuit conductors were increased from #8 (8.4 mm2) to #1/0 (53.5 mm2), a ratio of 6.4 to 1. Applying this ratio to the #10 (5 mm2) conductor indicates that the equipment-grounding conductor should be increased to about a #2 (33 mm2) conductor. A #2 black, insulated conductor is marked on both ends with green tape and routed in each conduit containing the #1/0 circuit conductors.

Conduit Fill. "Conduit fill" refers to the number of wires of a particular size and type allowed in a particular size of conduit. There are no short-cuts or easy explanations about conduit fill. The code and all electricians handle it with numerous tables (more than 50) that are a function of the exact conductor type, exact conductor size, and the conduit material, type, and size. The NEC tables must be used.

We chose a 2.5 inch (64 mm) conduit to use from the PV array location to the DC power center, and it carries the four, #1/0 (53 mm2) circuit conductors and the #2 (33 mm2) equipment-grounding conductor. There is additional room in this conduit for using larger conductors if additional modules are ever added to the array. The conduits are run underground and beneath the concrete slab of the house from the array to the DC disconnect. The house is built on a pad made of sand so the trenching was easy.

A T will be installed near the combiner boxes at the PV array. The single, #2 (33 mm2) equipment-grounding conductor will be spliced into two (one to each combiner box). Separate 1.5 inch (38 mm) conduits will run from the T to each combiner box.

Battery to Inverter Circuit

Conductors. The inverter has a 24 VDC nominal input and a rated AC output of 4,000 watts at 120 VAC. At the lowest battery voltage of 22 volts, the inverter efficiency is 85 percent. A maximum continuous DC input current for the inverter is calculated using the AC power output divided by the inverter DC-to-AC efficiency to get a DC power input. This DC power input is then divided by the lowest input battery voltage to get a continuous DC input current of 214 amps (4,000 - 0.85 - 22 = 214). An additional factor of 1.25 is used to allow for the 80 percent conductor derating required by the NEC. The resulting ampacity requirement is 267 amps (1.25 x 214 = 267) for the conductors between the inverter and the batteries [690.8(A)(4)].

A 90°C (194°F), 300 kcmil (152 mm2) conductor has an ampacity of 291 amps when used in conduit and corrected for an operating temperature of 40°C (104°F) (320 x 0.91 = 291). We chose to use two, #2/0 (67 mm2) conductors connected in parallel (four in conduit). Each of these #2/0

conductors has an ampacity of 195 amps when used in conduit at 30°C. Using these two conductors instead of the one above required that a conduit fill correction factor of 0.8 and a temperature correction factor of 0.91 be used to calculate their combined ampacity of 284 amps (2 x 195 x 0.8 x 0.91 = 284).

Conductors #1/0 (53 mm2) and larger may be connected in parallel to increase the ampacity if they are exactly the same length and connected at each end in exactly the same manner to the same point. When large conductors are required, paralleling smaller conductors to achieve the required ampacity is common practice. Besides, the Heinemann GJ 250 circuit breaker that we used will accept no conductor larger than 250 kcmil (127 mm2). Nearly all electricians start paralleling conductors above #4/0 (107 mm2) because the ampacity does not go up as fast as the conductor size increases.

Terminal Temperature. Most overcurrent devices have upper limits on the temperature at which their terminals are allowed to operate. If these temperatures are exceeded, the device may be subject to nuisance trips and premature failures.

We must estimate the actual temperature of the 90°C insulated conductor when carrying actual currents to ensure that the conductor temperature is not higher than the terminal to which it is connected. This estimation is made by taking the same size conductor (2 x #2/0 in this case) and finding the temperature derated ampacity when these conductors are insulated with an insulation having the same temperature rating as the terminal (in this case 75°C).

We can look up the 30°C ampacity of the paralleled #2/0 conductors in the 75°C insulation column in Table 310-16, apply the new (75°C insulation/45°C ambient) temperature correction factor and the conduit fill factor (0.8) to get the ampacity of the cable. If the actual currents in the cable are lower than this ampacity, then we can be assured that the cable will operate below 75°C.

The actual maximum continuous current of 214 amps is less than the conduit fill and temperature-corrected ampacity of a pair of 75°C (167°F) insulated #2/0 (67 mm2) conductors (2 x 175 x 0.8 x 0.88 = 246), so the terminals on the batteries and circuit breakers always operate below their temperature rating of 75°C.

Battery Disconnect. The battery disconnect is a 250 amp circuit breaker rated for 100 percent duty in its listed enclosure. This breaker serves as overcurrent protection for the battery cables and as a disconnect for the batteries. This circuit breaker can carry the continuous current of 214 amps and also protects the paralleled #2/0 (67 mm2) conductors between the disconnect and the inverter. A 2 inch (51 mm) conduit is used between the inverter and the battery disconnect and between the disconnect and the first battery enclosure.

Battery String Circuits. The four, 6 volt batteries in each string are connected in series using 1/8 by 1 inch (3 x 25 mm) copper bus bars in free air that have an equivalent area of #2/0 conductors (ampacity is greater than 300 amps). The four strings of batteries (four batteries per string) are connected in parallel using high current terminal blocks with #2/0 conductors running from the common terminal block (one positive, one negative) to the ends of each battery string. The ampacity of each of these conductors at 30°C (86°F) is 265 amps in free air, which is significantly more than the 54 amps (one-fourth of the 214 amps continuous current) that they may be expected to carry.

This oversizing allows for battery aging, where one of the four battery strings may have to carry higher current than the other three strings. In fact, with an ampacity of 265 amps, the conductors in a single string of batteries could carry the entire 214 amps maximum expected continuous current.

DC Circuit Equipment-Grounding Conductors. The battery enclosures are nonconductive, so no equipment-grounding conductors are required between the battery enclosures and the battery disconnect. The battery disconnect is in a metal enclosure and is connected to the inverter with metal conduit, providing the equipment-grounding conductor. A #6 (13 mm2) bare equipment-grounding conductor is also used between the inverter and the disconnect to provide additional insurance of good bonding.

A #6 bare conductor is used as an equipment-grounding conductor between the generator and the enclosure containing the 175 amp battery starting circuit breaker. This enclosure is also bonded to the main battery/inverter disconnect with a #6 bare copper conductor [Table 250.122].

The pump circuit uses a #8 (8 mm2) equipment-grounding conductor—oversized from the #14 (2 mm2) minimum requirement by NEC 250.122. The DC lighting circuit uses a #14 equipment-grounding conductor.

AC Circuits

Generator to Inverter. The rated continuous AC output current of the generator is 54.2 amps at 120 volts (6,500 watts) up to an elevation of 3,000 feet (915 m). At an estimated elevation of 4,500 feet (1,370 m), the output current is reduced to about 51 amps because of lower air pressure. The generator manual gives the correction factor of 0.9475 (54.2 x 0.9475 = 51).

Increasing the output current of 51 amps by a factor of 1.25 to meet code requirements yields a required cable ampacity of 64 amps (1.25 x 51 = 63.75). A #4 (21 mm2), 90°C (194°F) conductor in conduit at 40°C (104°F) has a temperature-corrected ampacity of 86 amps (95 x 0.9 = 85.5).

Checking Terminal Temperatures. The actual generator current of 51 amps is less than the temperature-corrected ampacity of a #4, 75°C insulated conductor, which is calculated, using the NEC tables, to be 75 amps (85 x 0.88 = 75). So the circuit breakers protecting these conductors operate with terminal temperatures of less than their rating of 75°C.

A 70 amp circuit breaker is used at the generator to serve both as overcurrent protection for this circuit and as a disconnect located outside at the generator. The 70 amp overcurrent protection dictates a #8 (8 mm2) equipment-grounding conductor for this circuit [250.122].

A 1 inch conduit is used between the generator and the inverter bypass switch to carry the two, #4 (21 mm2) conductors and the #8 equipment-grounding conductor [Ch. 9, Table C-10].

Inverter Output. The continuous output of the inverter in the inverting mode is about 33 amps. That is calculated by dividing the rated inverter power of 4,000 watts by the AC output voltage of 120 volts (4,000 -f 120 = 33.3). In the battery charging mode, the inverter may draw up to 51 amps from the generator and send it to the house AC load center. The NEC 1.25 factor increases the needed conductor ampacity to 64 amps (51 x 1.25 = 63.75). A #4 (21 mm2) conductor in conduit at 40°C (104°F) has a temperature-corrected ampacity of 86 amps (95 x 0.91 = 86).

A second 70 amp circuit breaker (part of the inverter bypass switch) is mounted near the inverter in the AC output circuit of the inverter and provides overcurrent protection and a disconnect for the AC circuit to the house AC load center. Once again, the 70 amp overcurrent protection requires the use of a #8 (8 mm2) equipment-grounding conductor [250.122].

Calculation Process

This calculation process is lengthy, but it is necessary to achieve a safe, code-compliant, reliable, and durable system. If you have questions about the NEC, or the implementation of PV systems that follow the requirements of the NEC, feel free to call, fax, e-mail, or write. See the SWTDI Web site for technical notes and articles on installing code-compliant PV systems and frequently asked questions (and answers—of course).

Sandia National Laboratories sponsors my activities in this area as a support function to the PV industry. This work was supported by the United States Department of Energy under Contract DE-FC04-00AL66794. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.


John C. Wiles, Southwest Technology Development Institute, New Mexico State University, Box 30,001/MSC 3 SOLAR, Las Cruces, NM 88003 • 505-646-6105 • Fax: 505-646-3841 • [email protected]

Sponsor: Sandia National Laboratories, Ward Bower, Department 6218, MS 0753, Albuquerque, NM 87185 • 505-844-5206 • Fax: 505-844-6541 • [email protected]

The 2002 NEC and the NEC Handbook are available from the National Fire Protection Association (NFPA), 11 Tracy Dr., Avon, MA 02322 • 800-344-3555 or 508-895-8300 • Fax: 800-593-6372 or 508-895-8301 • [email protected] ^

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