This design proposes a small, off-grid solar power system for a cabin in Boone, NC. The system includes 4 260W solar panels, a MidNite Solar charge controller, 4 350Ah batteries, and a 3,200W Sunny Boy inverter. Key components are mounted on a DPW rack and wired to a MidNite Solar combiner box. The total cost is estimated at $4,160.90 for a 1,040W system, or $4 per watt generated.

1. Final Design:
Small PV System for Cabin in Boone, NC
James Koehler
Scanlin

2. 1) Intro
For this project, a small battery-based, grid-tied PV system was designed for a small cabin in the Ap-
palachian Mountains of Boone, NC. It will include a 24V battery-based system with a MPPT controller,
60 cell modules and a connection to the grid.
2) Loads
These loads will all be AC-powered with the exception of heating, which is assumed to be powered by an
alternative energy source (grid or other).
3) Modules
For this system, Canadian Solar polycrystalline 260W, 24V, 60-cell solar modules were used. The
cost of a module is $0.90 per Watt, or $233.

3. 4) Array Sizing
Using the Excel spread sheet, it was found that 2 series strings with 2 modules per string (4
total) would be required to power the loads when using a MPPT controller with a maximum input voltage
of 150 volts.

4. 5) Array Schematic
6) Array Wire Sizing Calculations
Maximum amperage in each series string will be Isc x 1.56 = 8.99 x 1.56 = 14.02 A. Therefore a
90 C, wet-rated, and sunlight resistant USE-2 or PV wire must be selected that has a temperature derated
ampacity greater than 14.02 A. # 14 USE-2 copper wire in free air has a rated ampacity of 35 amps at 86
degrees and should be enough to carry the 14.02 amps. However, since the conductors will be located in
the hot junction boxes the conductors will need to be temperature derated to a temperature of 141 – 158 F.
This requires a 58% derating or 35 x .58 = 20.3 A. Even with this derating, # 14 USE-2 conductors in
free air would be enough to carry the maximum amperage and as a result # 14 USE-2 will be used.
7) Rack
A DPW Power Rail P4 racking system will be used to mount these modules to the roof of the cabin. The
modules each have a width of 3.225 feet; since we have 4 total modules, the racking system will have to
have a minimum length of 13 feet (156”). So, there would need to be two 13-foot racks mounted to the
roof, which would cost roughly $96 ($48 each).
Important Module Specs
*8.99 Isc
*37 Voc
*0.58 temperature derating
for record cold temp of
141-158 F
#14 USE-2 Cop-
per wire: 90C, wet
rated, sunlight
resistant

5. 8) Combiner Box and Overcurrent Protection Devices
Since there are 2 module series string, there will be 2 overcurrent protections in the combiner box of this
system. The fuses or circuit breakers must be greater than the short circuit current of the module multi-
plied by 1.56, and the fuse or circuit breaker size must be less than the series fuse rating of the module.
The Isc of the module is 8.99 amps and the series fuse rating is 15 amps.
Also, we can use circuit breakers only if the open circuit voltage Voc multiplied by the number
of module in the series string and by 1.25 (for record cold temperatures), is less than 150 Volts.
As the result is smaller than 150 V, we can use circuit breakers in the combiner box. Therefore we need 2
– 15 amp circuit breakers with a DC rating of 150 volts, which would cost roughly $22.70.
These 2 circuit breakers will be put in the MidNite Solar MNPV3 combiner box, which costs $99. The
MidNite lightning arrestor shown in schematic on the next page costs $250. Below are the characteristics
of this combiner box:
9) Array Schematic and Combiner Box
Name Amps
DC Voltage
Rating
Cost
MidNite Solar
MNEPV
15 150 $11.35
Max 150
VDC
breakers
Max out-
put cir-
cuits
Max out-
put wire
size
MNPV
combiner
dimensions
Cost
3 1 #1/0 11x5x4 $99

6. 10) Wire Sizing between Combiner Box and BOS Components
The conductors coming out of the combiner box and traveling to the house will have to carry the Isc X
1.56 x 2 strings or 8.99 A x 1.56 x 2 = 28.04 Amps. Since these conductors are in a potentially hot envi-
ronment inside the combiner box, they will have to be temperature derated and will need to have a tem-
perature derated capacity of 28.04 amps. # 8 – 90 C conductors in conduit have an ampacity of 55 amps at
86 F, but since they will be installed inside a potentially hot combiner box we will derate the ampacity by
multiplying by .58. 55 x .58 = 31.9, which is greater than the 28.04 A required. They will be installed in
conduit which will run between the combiner box and the house. We assume that there is 100’ distance
between the combiner box and the BOS components. To avoid a voltage drop greater than 2%, we are
going to size the wire by calculating the Voltage Drop Index. We should use the Imp coming out of the
combiner box and do not have to multiply by 1.56. Since this is not a code violation or safety issue we
just want to model typical performance.
Using the VDI Chart, the copper wire size #4 will carry the Imp with no more than a 2 % voltage drop
and therefore # 4 will be used.
11) DC Junction Box and Charge Controller
The DC junction Box will be an E-panel from MidNite Solar and will include a Positive bus bar
to transition from the larger #4 to small gauge wires that will be easier to work with and a 30 amp breaker
on the incoming side of the charge controller. This breaker needs to be larger than the Isc x 1.56 amps
coming out of the combiner box, but less than the ampacity of the wire used, which will be #12 which can
carry the 30 amps that are coming into the DC junction box. We will have to have 2 of these: 1 before the
charge controller and one after going to the batteries.

7. The charge controller used will be MidNite Solar’s The KID, which costs $315. The specifications are
listed below:
The two breakers used will also cost a combined total of $22.70. Specifications are listed below:
Name Amps
DC Voltage
Rating
Cost
MidNite Solar
MNEPV
30 150 $11.35

8. These breakers will be installed in a MidNite Solar Mini-DC Disconnect Power Center MNDC250, which
costs $184.92. The specifications are listed below:
12) Array, Combiner and DC Junction Schematic
Combiner Box
DC Junction Box

9. 13) Inverter Sizing
The total of AC power in the cabin is 2,912 W. Therefore, the continuous watts of the inverter must be
greater than or equal to 2,912 W. So, the Sunny Boy SMA SB5000TL-US-22 3,200 W inverter was cho-
sen, which costs around $2,000. Specifications are listed below:
14) Breaker Sizing for Inverter Wiring
The minimum breaker size for inverter wiring is the maximum DC wattage divided by the cut-out voltage
with the maximum DC wattage, which is equal to the maximum continuous AC output wattage divided by
the inverter efficiency. Moreover, it has to be less than the ampacity of the wire use (#4/0) 90 C conductor
which is 260 amps.
There is only the DC rated circuit breaker MNEDC 250, which costs $86.18. Specs listed below:
Amps Poles Voltage Rating (DC) Cost
250 1 125 $86.18

10. 15) Battery Sizing
For this design, 4 “L-16” 350 Ah batteries would have to be used. This would create a com-
bined cost of $152.50.
Specifications are listed below:

11. 16) Final Schematic
Total costs listed below:

12. 17) Cost Estimate and Conclusion
Total cost of system: $4,160.90 (not including taxes and wire costs)
Cost of power: $4,160.90/1,040W = $4 per Watt
Batteries $152.50
Rack $96
260 A Breaker $86.18
Sunny Boy 3,200 W Inverter $2,000
DC Junction Box $184.82
4 20-30A Breakers $45.4
Charge Controller $315
Lighting Arrestor $250
4 260W Modules $932
Combiner Box $99