Page 29
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2017 Sensata Technologies
Installation
Find the PV System conductors and OCPD size (continued):
Continued Scenario: The PV output conductors of the combiner (two current-carrying conductors)
run through conduit 2” above the rooftop and connect to a PV disconnect breaker installed in an
MMP enclosure with a PT-100 charge controller. The lowest temperature rating of any terminal
is 75°C.
B) Find the PV Output circuit conductor and OCPD size:
Step 1 -
Calculate the maximum current for the PV output circuits (I
MAX
).
• M
ax current for PV output (PV
OUTPUT
-I
MAX
) = Module I
SC
x 1.25 x number of strings =
6.99 x 1.25 x 3 strings =
26.2
Step 2 -
Determine the conductor size for the continuous currents (I
CONT
).
a)
Calculate the continuous current for the PV output circuits (I
CONT
).
• C
ontinuous current for PV output (PV
OUTPUT
-I
CONT
) = PV
OUTPUT
-I
MAX
x 1.25 = 26.2 x 1.25
=
32.8
b)
Find the conductor sized for continuous current (Lowest terminal temperature = 75°C).
• Co
nductor sized to continuous currents =
10 AWG @ 75°C (35 amps)
.
Per Table 310.15(B)(16), 10 AWG is the smallest cable listed above 26.2 amps @ 75°C.
Step 3 -
Determine the conductor size for the maximum currents for the PV output circuits
derated after adjustment factors (I
DERATE
).
a)
Calculate the ampacity of the conductors after derating by “conditions of use” factors:
• Ma
x PV output circuit current with conditions of use (PV
OUTPUT
-I
DERATE
) = (PV
OUTPUT
-I
MAX
÷ Fill Factor ÷ Temp Factor) = 26.2 ÷ 1 (no Fill Factor derate) ÷ 0.76 =
34.5
1.
Conduit Fill Factor - there is less than three current carrying conductors, so there is no derate.
2. Temperature Correction Factors (ambient tempe rooftop adder) - The ambient
temperature is 26°C and there is a rooftop adder of 28°C [per
Table 310.15(B)(3)(c)]
because the
PV output conductors are installed 2” from the rooftop. The ambient temperature (26°C) + rooftop
adder (28°C) give an effective temperature of 54°C. A temperature derate is required since this
is greater than 30°C. Using Table 310.15(B)(2)(a), a USE-2 (90°C) cable at a temperature of 54°C
uses a Temp Factor derate of 0.76.
b)
Find the conductor size after derating factors have been applied:
[Lowest terminal temperature = 75°C, and using USE-2 (90°C) cable]
• Co
nductor sized after derating factors =
10 AWG @ 75°C (35 amps).
P
er Table 310.15(B)(16), 14 AWG @ 90°C has an ampacity of 25 amps; however, since
the terminal temperature is 75°C, the cable ampacity under the 75°C column is used.
So, 14 AWG is the smallest cable listed under the @ 75°C column and is rated 20 amps.
Step 4 -
Determine the minimum required conductor size:
10 AWG @ 75°C (35 amps)
.
The largest conductor between the continuous current conductor (Step 2b) and the derating factors
conductor (Step 3b) is the minimum conductor size used, in this instance they are the same.
Step 5 -
Size the Overcurrent Protection Device (OCPD) to protect the selected conductor.
The OCPD should be a commonly available size (see NEC Article 240.6), and sized to be:
a) equal to or greater than the Continuous Current (see Step 2a) = 32.8 amps; and
b) less than or equal to the Minimum Required Conductor
ampacity (see Step 4) = 35 amps
A 35 amp OCPD is recommended
, this is the next highest standard value above the
design current of 32.8 amps. This size will meet all requirements for the PV output
conductor ampacity.
Note:
If the OCPD is rated for 100% operation, then it can be sized to the lower I
MAX
current (Step 1).
Note:
Consult manufacturer for OCPD rating if exposed to temperatures greater than 40
°
C.
