## Example

* – Array Size*: 10, 12-volt, 51-watt modules; Isc= 3.25 amps, Voc= 20.7 volts

*800 amp-hours at 12 volts*

**– Batteries:***5 amps DC and 500-watt inverterwith 90% efficiency.*

**– Loads:**## Description

The PV modules are mounted on the roof. Single-conductor cables are used to connect the modules to a roof-mounted junction box. Potential reverse fault currents indicate that a PV combiner be used with a series fuse for each PV module.

* UF two-conductor sheathed cable* is used from the roof to the control center.

Physical protection (*wood barriers or conduit*) for the UF cable is used where required. The control center, diagrammed in * Figure 1*, contains disconnect and overcurrent devices for the PV array, the batteries, the inverter, and the charge-controller.

## Calculations

* – The moduleshort-circuit current* is 3.25 amps.

*1.25 x 3.25 = 4.06 amps*

**– CONTINUOUS CURRENT:***1.25 x 4.06 = 5.08 amps per module*

**– 80% OPERATION:**The maximum estimated moduleoperating temperature is * 68°C*.

### From NECTable 310.17:

- The derating factor for USE-2 cable is 0.58 at 61-70°C.
- Cable 14 AWG has an ampacity at 68°C of 20.3 amps (0.58 x 35) (
*max fuse is 15 amps*). - Cable 12 AWG has an ampacity at 68°C of 23.2 amps (0.58 x 40) (
*max fuse is 20 amps*). - Cable 10 AWG has an ampacity at 68°C of 31.9 amps (0.58 x 55) (
*max fuse is 30 amps*). - Cable 8 AWG has an ampacityat 68°C of 46.4 amps (0.58 x 80).

**The array is divided into two five-module sub-arrays.**

The modules in each sub-array are wired from module junction box to the PV combiner for that sub-array and then to the array junction box. Cable size 10 AWG USE-2 is selected for this wiring, because it has an ampacityof 31.9 amps under these conditions, and the requirement for each sub-array is * 5 x 4.06 = 20.3 amps*.

*Evaluated with 75°C insulation, a 10 AWG cable has an ampacity of 35 amps at 30°C, which is greater than the actual requirement of 20.3 amps (5 x 4.06).*

In the array junction box on the roof, two 30-amp fuse sin pullout holders are used to provide overcurrent protection for the 10 AWG conductors. These fuses meet the requirement of 25.4 amps (*125% of 20.3*) and have a rating less than the derated cable ampacity.

*A 4 AWG UF cable (*

**The ampacity requirement is 40.6 amps (10 x 4.06).***4-2 w/gnd*) is selected for the run to the control box. It operates in an ambient temperature of

*and has a temperature-corrected ampacity of*

**40°C***. This is a*

**86 amps (95 x 0.91)***cable with 90°C conductors and the final ampacity must be restricted to the 60°C value of 70 amps, which is suitable in this example.*

**60°C**Appropriately derated cables must be used when connecting to fusesthat are rated for use only with 75°C conductors. A 60-amp circuit breaker in the control box serves as the PV disconnect switch and overcurrent protectionfor the UF cable.

**The minimum rating would be 10 x 3.25 x 1.56 = 51 amps.**

The NEC allows the next larger size; * in this case, 60 amps*, which will protect the 70 amp rated cable. Two single-pole, pullout fuse holders are used for the battery disconnect.

**The charge circuit fuse is a 60-amp RK-5 type.**The inverter has a continuous rating of 500 watts at the lowest operating voltage of * 10.75 volts* and an efficiency of

*at this power level. The continuous current calculation for the input circuit is*

**90%***.*

**64.6 amps ((500 / 10.75 / 0.90) x 1.25)**The cables from the battery to the control center must meet the inverter requirements of 64.6 amps plus the DC load requirements of 6.25 amps (1.25 x 5).

A 4 AWG THHN has an ampacityof 85 amps when placed in conduit and evaluated with 75°C insulation. This exceeds the requirements of 71 amps (64.6 + 6.25). **This cable can be used in the custom power center and be run from the batteries to the inverter.**

The * discharge-circuit fuse* must be rated at least

*. An*

**71 amps***should be used, which is less than the cable ampacity.*

**80-amp fuse***ampacityof 30 amps*) and protected with a

*.*

**15-amp circuit breaker**The grounding electrode conductor is 4 AWG and is sized to match the largest conductor in the system, which is the array-to-controlcenter wiring. This size would be appropriate for a concrete-encased grounding electrode. Equipment-grounding conductors for the array and the charge circuit can be 10 AWG based on the 60-amp overcurrent devices.

The equipment ground for the inverter mustbe an * 8 AWG conductor* based on the

*. All components should have at least a DC voltage rating of*

**80-amp overcurrent device***.*

**1.25 x 20.7 = 26 volts****Reference:** Photovoltaic Power Systems And the 2005 National Electrical Code – John Wiles Southwest Technology Development Institute New Mexico State University