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Small Photovoltaic (PV) Residential Stand-Alone System

A Photovoltaic (PV) Residential Stand-Alone System is a self-sufficient solar power setup intended for home use. The following are the main components and characteristics of this system:


  1. PV Modules (Solar Panels):

    1. PV modules (also known as solar panels) convert sunlight into direct current (DC) electricity.

    2. These modules are typically mounted on rooftops or other suitable locations to capture sunlight efficiently.

  2. Charge Controller or Maximum Power Point Tracker (MPPT):

    1. The charge controller regulates the voltage and current from the PV modules to the battery and the load.

    2. It ensures that the battery is charged optimally and prevents overcharging or deep discharging.

  3. Battery Storage:

    1. Stand-alone PV systems often include batteries to store excess energy generated during sunny hours.

    2. Batteries provide power when there is little or no solar input (e.g., during nighttime or cloudy days).

  4. Inverter (Optional):

    1. If the system needs to power AC (alternating current) loads (such as household appliances), an inverter is used.

    2. The inverter converts DC power from the PV modules or batteries into AC power for household use.

  5. Load:

    1. The load represents the electrical devices or appliances that consume power in the residential setting.

    2. Loads can be DC (direct current) or AC, depending on the system configuration.

  6. Grid Interaction (Optional):

    1. Some stand-alone systems can use the utility grid as an input for backup power.

    2. However, true stand-alone systems do not supply power to the grid and operate independently.


Applications:

  • Remote Locations: Stand-alone PV systems are ideal for areas where connecting to the utility grid is impractical (e.g., rural locations, cabins, or isolated properties).

  • Low-Power Requirements: They are commonly used for low-power applications, such as lighting, small appliances, water pumping, or attic fans.

  • Backup Power: In some cases, stand-alone systems can use the grid for backup power during extended periods of low solar input.


It is crucial to adhere to safety guidelines, electrical codes, and proper installation procedures when installing a residential stand-alone photovoltaic (PV) system. Such systems provide a sustainable and dependable option for off-grid living or as a supplement to grid power.


Calculation Example for a Small Photovoltaic (PV) Residential Stand-Alone System

PV modules are installed on the roof, and single-conductor cables connect them to a junction box also located on the roof. The presence of potential reverse fault currents suggests the necessity of a PV combiner equipped with a series fuse for each module.


2kW Solar PV Array on Campus Crossings at Briarcliff (photo by Soenso Energy)

Example:

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

  • Batteries: 800 amp-hours at 12 volts

  • Loads: 5 amps DC and 500-watt inverter with 90% efficiency.

A UF two-conductor sheathed cable runs from the roof to the control center.

Physical protection, such as wooden barriers or conduits, is utilized for the UF cable where necessary. The control centre, depicted in Figure 1, includes disconnect and overcurrent protection devices for the PV array, batteries, inverter, and charge controller.


Figure 1 – Small Residential Stand-Alone System


Calculations

  • The module short-circuit current is 3.25 amps.

  • CONTINUOUS CURRENT: 1.25 x 3.25 = 4.06 amps

  • 80% OPERATION: 1.25 x 4.06 = 5.08 amps per module

  • The estimated maximum operating temperature for the module 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 ampacity at 68°C of 46.4 amps (0.58 x 80)

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

The modules within each sub-array are connected from the module junction box to the PV combiner designated for that sub-array, and subsequently to the array junction box. A 10 AWG USE-2 cable size is chosen for this wiring due to its 31.9 amps ampacity under these conditions, which comfortably meets the 20.3 amps requirement for each sub-array, calculated as 5 times 4.06 amps.


When assessed with 75°C insulation, a 10 AWG cable possesses an ampacity of 35 amps at 30°C. This exceeds the actual requirement of 20.3 amps, which is calculated as 5 times 4.06 amps.


Within the array junction box on the roof, two 30-amp fuses in pull-out holders are utilized to offer overcurrent protection for the 10 AWG conductors. These fuses satisfy the requirement of 25.4 amps (125% of 20.3 amps) and possess a rating below the derated cable ampacity.


In this junction box, the two sub-arrays merge into a single array output. The required ampacity is 40.6 amps (10 x 4.06). For the run to the control box, a 4 AWG UF cable (4-2 w/gnd) has been chosen. This cable operates at an ambient temperature of 40°C and possesses a temperature-adjusted ampacity of 86 amps (95 x 0.91). Being a 60°C cable with 90°C conductors, the final ampacity is limited to the 60°C rating of 70 amps, which is adequate for this scenario.

Cables that are properly derated should be utilized when connecting to fuses rated exclusively for 75°C conductors. In the control box, a 60-amp circuit breaker acts as both the PV system disconnect switch and the overcurrent protection for the UF cable.


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

The National Electrical Code permits the use of the next larger size; in this instance, 60 amps, to protect the cable rated for 70 amps. For the battery disconnect, two single-pole, pullout fuse holders are utilized. The fuse for the charge circuit is of the 60-amp RK-5 variety.


The inverter maintains a continuous power rating of 500 watts at a minimum operating voltage of 10.75 volts, with an efficiency of 90% at this level of power. The calculated continuous current for the input circuit is 64.6 amps, derived from ((500 / 10.75 / 0.90) x 1.25).


The cables connecting the battery to the control center need to comply with the inverter's requirements of 64.6 amps, in addition to the DC load requirements of 6.25 amps, which is 1.25 times 5.


A 4 AWG THHN wire has an ampacity of 85 amps when installed in a conduit with 75°C rated insulation. This surpasses the required 71 amps (64.6 + 6.25). Therefore, this cable is suitable for use in the custom power centre, running from the batteries to the inverter.


The fuse for the discharge circuit must have a rating of at least 71 amps. Therefore, an 80-amp fuse would be appropriate, as it is below the cable's ampacity.


The DC load circuit is connected using a 10 AWG NM cable, which has an ampacity of 30 amps, and is safeguarded by a 15-amp circuit breaker.

The grounding electrode conductor is 4 AWG, matching the size of the largest conductor in the system, which is the wiring from the array to the control centre. This conductor size is suitable for a concrete-encased grounding electrode. The equipment-grounding conductors for both the array and the charge circuit may be 10 AWG, in accordance with the 60-amp overcurrent protection devices.


The grounding conductor for the inverter should be an 8 AWG conductor, considering the 80-amp overcurrent protection. Additionally, all components must have a minimum DC voltage rating of 1.25 times 20.7, which equals 26 volts.



Reference:

Photovoltaic Power Systems And the 2005 National Electrical Code – John Wiles Southwest Technology Development Institute New Mexico State University

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