Page 113 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM CONFIGURATIONS 89

Various device terminals, such as terminal block overcurrent devices, also must have
the same insulation rating as the cables. In other words, if the device is in a location that
is exposed to higher temperatures than the rating of the feeder cable, the cable must be
further derated to match the terminal connection device. The following example is used
to illustrate these design parameter considerations.
A Wiring Design Example Assuming that the short-circuit current I from a PV
sc
array is determined to be 40 A; the calculation steps should be as follows:

1 PV array current derating: 40 × 1.25 = 50 A.
2 Overcurrent device fuse rating at 75°C: 50 × 1.25 = 62.5 A.
3 Cable derating at 75°C: 50 × 1.25 = 62.5. Using NEC Table 310.16, under the 75°C
columns we find a cable AWG 6 conductor that is rated for 65-A capacity. Because
of UV exposure, XHHW-2 or USE-2 type cable should be chosen, which has a
75-A capacity. Incidentally, the “-2” is used to designate UV exposure protection.
If the conduit carrying the cable is populated or filled with four to six conductors,
it is suggested, as noted previously, by referring to NEC Table 310.15(B)(2)(a), that
the conductors be further derated by 80 percent. At an ambient temperature of
40–45°C, a derating multiplier of 0.87 is applied: 75 A × 0.87 = 52.2 A. Since the
AWG 6 conductor chosen with an ampacity of 60 A is capable of meeting the
demand, it is found to be an appropriate choice.
4 By the same criteria, the closest overcurrent device, as shown in NEC Table 240.6, is
60 A; however, since in step 2 the overcurrent device required is 62.5 A, the AWG 6
cable cannot meet the rating requirement. As such, an AWG 4 conductor must be
used. The chosen AWG 4 conductor under the 75°C column of NEC Table 310.16
shows an ampacity of 95 A. If we choose an AWG 4 conductor and apply conduit fill
and temperature derating, then the resulting ampacity is 95 × 0.8 × 0.87 = 66 A; there-
fore, the required fuse based on NEC Table 240.6 will be 70 A.

Conductors that are suitable for solar exposure are listed as THW-2, USE-2, and
THWN-2 or XHHW-2. All outdoor installed conduits and wireways are considered to be
operating in wet, damp, and UV-exposed conditions. As such, conduits should be capable
of withstanding these environmental conditions and are required to be of a thick-wall type
such as rigid galvanized (RGS), intermediate metal conduit (IMC), thin-wall electrical
metallic (EMT), or Schedule 40 or 80 polyvinyl chloride (PVC) nonmetallic conduits.
For interior wiring, where the cables are not subjected to physical abuse, CNM-,
NMB-, and UF-type cables are permitted. Care must be taken to avoid installation of
underrated cables within interior locations such as attics where the ambient tempera-
ture can exceed the cable rating.
Conductors carrying direct current are required to the use color-coding recommenda-
tions stipulated in Article 690 of the NEC. Red wire or any other color other than green
and white is used for positive conductors, white for negative, green for equipment
grounding, and bare copper wire for grounding. The NEC allows nonwhite grounded
wires, such as USE-2 and UF-2, that are sized No. 6 or above to be identified with a white
tape or marker.