Essentially solar power installations include a hybrid of technologies consisting of basic
ac and dc electric power and electronics—a mix of technologies, each requiring specific
technical expertise. Systems engineering of a solar power system requires an intimate
knowledge of all hardware and equipment performance and application requirements. In
general, major system components such as inverters, batteries, and emergency power
generators, which are available from a wide number of manufacturers, each have a
unique performance specification specially designed for specific applications.
The location of a project, installation space considerations, environmental settings,
choice of specific solar power module and application requirements, and numerous
other parameters usually dictate specific system design criteria that eventually form
the basis for the system design and material and equipment selection.
Issues specific to solar power relate to the fact that all installations are of the outdoor
type, and as a result all system components, including the PV panel, support structures,
wiring, raceways, junction boxes, collector boxes, and inverters must be selected and
designed to withstand harsh atmospheric conditions and must operate under extreme
temperatures, humidity, and wind turbulence and gust conditions. Specifically, the electrical wiring must withstand, in addition to the preceding environmental adversities,
degradation under constant exposure to ultraviolet radiation and heat. Factors to be
taken into consideration when designing solar power wiring include the PV module’s
short-circuit current (Isc) value, which represents the maximum module output when
output leads are shorted. The short-circuit current is significantly higher than the normal or nominal operating current. Because of the reflection of solar rays from snow, a
nearby body of water or sandy terrain can produce unpredicted currents much in excess
of the specified nominal or Isc current. To compensate for this factor, interconnecting
PV module wires are assigned a multiplier of 1.25 (25 percent) above the rated Isc.
PV module wires as per the NEC requirements are allowed to carry a maximum
load or an ampacity of no more than 80 percent; therefore, the value of currentcarrying capacity resulting from the previous calculation is multiplied by 1.25, which
results in a combined multiplier of 1.56.
The resulting current-carrying capacity of the wires if placed in a raceway must be
further derated for specific temperature conditions as specified in NEC wiring tables
(Article 310, Tables 310.16 to 310.18).
All overcurrent devices must also be derated by 80 percent and have an appropriate
temperature rating. Note that the feeder cable temperature rating must be the same as
that for overcurrent devices. In other words, the current rating of the devices should be
25 percent larger than the total sum of the amount of current generated from a solar
array. For overcurrent device sizing NEC Table 240-6 outlines the standard ampere
ratings. If the calculated value of a PV array somewhat exceeds one of the standard
ratings of this table, the next highest rating should be chosen.
All feeder cables rated for a specific temperature should be derated by 80 percent
or the ampacity multiplied by 1.25. Cable ratings for 60, 75, and 90°C are listed in
NEC Tables 310.16 and 310.17. For derating purposes it is recommended that cables
rated for 75°C ampacity should use 90°C column values. Various device terminals,
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:
1PV array current derating = 40 ×1.25=50 A.
2Overcurrent device fuse rating at 75°C = 50 ×1.25=62.5 A.
3Cable 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.
4By 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 derating, 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 temperature 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 grounding, and bare copper wire for grounding. The NEC allows nonwhite grounded wires,
70 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS
such as USE-2 and UF-2, that are sized #6 or above to be identified with a white tape
or marker.
As mentioned earlier, all PV array frames, collector panels, disconnect switches,
inverters, and metallic enclosures should be connected together and grounded at a single service grounding point.
ac and dc electric power and electronics—a mix of technologies, each requiring specific
technical expertise. Systems engineering of a solar power system requires an intimate
knowledge of all hardware and equipment performance and application requirements. In
general, major system components such as inverters, batteries, and emergency power
generators, which are available from a wide number of manufacturers, each have a
unique performance specification specially designed for specific applications.
The location of a project, installation space considerations, environmental settings,
choice of specific solar power module and application requirements, and numerous
other parameters usually dictate specific system design criteria that eventually form
the basis for the system design and material and equipment selection.
Issues specific to solar power relate to the fact that all installations are of the outdoor
type, and as a result all system components, including the PV panel, support structures,
wiring, raceways, junction boxes, collector boxes, and inverters must be selected and
designed to withstand harsh atmospheric conditions and must operate under extreme
temperatures, humidity, and wind turbulence and gust conditions. Specifically, the electrical wiring must withstand, in addition to the preceding environmental adversities,
degradation under constant exposure to ultraviolet radiation and heat. Factors to be
taken into consideration when designing solar power wiring include the PV module’s
short-circuit current (Isc) value, which represents the maximum module output when
output leads are shorted. The short-circuit current is significantly higher than the normal or nominal operating current. Because of the reflection of solar rays from snow, a
nearby body of water or sandy terrain can produce unpredicted currents much in excess
of the specified nominal or Isc current. To compensate for this factor, interconnecting
PV module wires are assigned a multiplier of 1.25 (25 percent) above the rated Isc.
PV module wires as per the NEC requirements are allowed to carry a maximum
load or an ampacity of no more than 80 percent; therefore, the value of currentcarrying capacity resulting from the previous calculation is multiplied by 1.25, which
results in a combined multiplier of 1.56.
The resulting current-carrying capacity of the wires if placed in a raceway must be
further derated for specific temperature conditions as specified in NEC wiring tables
(Article 310, Tables 310.16 to 310.18).
All overcurrent devices must also be derated by 80 percent and have an appropriate
temperature rating. Note that the feeder cable temperature rating must be the same as
that for overcurrent devices. In other words, the current rating of the devices should be
25 percent larger than the total sum of the amount of current generated from a solar
array. For overcurrent device sizing NEC Table 240-6 outlines the standard ampere
ratings. If the calculated value of a PV array somewhat exceeds one of the standard
ratings of this table, the next highest rating should be chosen.
All feeder cables rated for a specific temperature should be derated by 80 percent
or the ampacity multiplied by 1.25. Cable ratings for 60, 75, and 90°C are listed in
NEC Tables 310.16 and 310.17. For derating purposes it is recommended that cables
rated for 75°C ampacity should use 90°C column values. Various device terminals,
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:
1PV array current derating = 40 ×1.25=50 A.
2Overcurrent device fuse rating at 75°C = 50 ×1.25=62.5 A.
3Cable 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.
4By 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 derating, 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 temperature 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 grounding, and bare copper wire for grounding. The NEC allows nonwhite grounded wires,
70 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS
such as USE-2 and UF-2, that are sized #6 or above to be identified with a white tape
or marker.
As mentioned earlier, all PV array frames, collector panels, disconnect switches,
inverters, and metallic enclosures should be connected together and grounded at a single service grounding point.
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