Page 99 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM WIRING 69

such as terminal block overcurrent devices must also have the same insulation rating
as the cables. In other words, if the device is in a location that is exposed to a higher
temperature 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 Isc from a PV
array is determined to be 40 A, the calculation 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 ultraviolet (UV) exposure, XHHW-2 or USE-2 type cable, which has a 75-A
capacity, should be chosen. 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 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 to 45°C a derating multiplier of 0.87 is also to be applied: 75 A × 0.87 = 52.2 A.
Since the AWG #6 conductor chosen with an ampacity of 60 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 Table 310-16
shows an ampacity of 95.

If we choose an AWG #4 conductor and apply conduit fill and temperature derat-
ing, then the resulting ampacity is 95 × 0.8 × 0.87 = 66 A; therefore, the required fuse
per 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 steel (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, special
NEC code approved wires must be used. 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 dc current are required to use color coding recommendations
as stipulated in Article 690 of the NEC. Red wire or any color other than green and
white is used for positive conductors, white for negative, green for equipment ground-
ing, and bare copper wire for grounding. The NEC allows nonwhite grounded wires,