Solar To Sub Panel (How To With Calculations)

Large roof or ground-mounted solar arrays are often a combination of rows of panels wired in series, and then the separate rows are connected in series. A solar combiner box helps to organize and reduce the risk of errors when wiring the solar array together.

Large solar arrays are often 24V or 48V series strings that need to be joined in parallel to boost the current strength. Preventing current from flowing back to the solar array requires using a sub-panel or combiner box with an appropriately sized fuse. A solar power shut-off switch is mandatory.

It would be best if you considered the following aspects when planning the wiring and component diagram for your system:

  • Combiner box with fuse and shut off switch
  • Calculating the correct size fuse to use
  • Dedicated electrical control box for the solar system
  • How to protect your system components with fuses and shut off switched
  • How to connect the combiner sub panel to the inverter

The more orderly and well-labeled you make your solar power system, the lower the risk of damage or severe injury. Let’s look at how to safeguard yourself, the members of your household, and the expensive components of your solar power system.

Solar Panels Covering The Roof Of A House
Solar Panels Covering The Roof Of A House

The Best Way To Wire Large Solar Panel Arrays

Small 12 V solar power systems are easy to wire, and it is common to wire the solar panels directly to the solar charge controller and the battery bank. When a solar array is a complex mix of solar panel strings in series combined with other strings in parallel, the potential for error and injury is significant.

For larger, more complex solar array installations, it is recommended to use a PV Combiner Box where each string of solar panels is connected via a dedicated circuit breaker.

Let’s look at how a 4kW 48V solar power system should ideally be set up:

Our solar array comprises twenty 195W 12V solar panels that need to be wired into a 4.8kWh 48V Lithium-ion battery bank via a solar charge controller and feed the household AC loads via a 3,500 W 48V to 110V pure sine-wave inverter.

This system can generate 16kWh of power with four hours of sunshine per day and could power small household appliances such as a refrigerator, Cell phones, tablets and laptops, Lights, Television, Fans, Washing, Machine, and Well pump.

To achieve the 3,900 W of solar input at 48 V per hour, we have to wire the solar panels in strings of four 12V 195W solar panels in series to generate 780W at 48V per string. The five strings will be combined in parallel in a PV combiner junction box for a total of 3900 W at 48V.

Each string of four solar panels produces a combined 780 W of power at 48V at 16.25 Ampere (A), sufficient power to warrant a dedicated 10 A circuit breaker. We have five such strings of power to combine in an organized PV breaker box.

Advantages of PV Combined

The advantage of using a PV combined/beaker box is that you have an easy way to connect the solar array that will be suitable for grid-tied and off-grid solar setups.

Photovoltaic Voltage Combiner boxes or sub-panels can be purchased to suit the most commonly found large array configurations. Each series string of solar panels can be connected to a dedicated circuit breaker and built-in overload and over-charge protection.

The PV combiner box also has built-in protection against backflow currents, and the high voltage output of each string can safely be combined in parallel to boost the 24V or 48V to a higher amperage.

What Are The Key Advantages Of Using PV Combiner Boxes?

The main advantage of wiring multiple solar panel strings in series and then combining them in parallel in the combiner box is that the installation is easier, safer, and more professional.

The combiner box or sub panel also protects against lightning, short circuit protection, and grounding protection. The solar panel circuit breaker protects the system installer and users and makes additional solar panels easy to incorporate when expanding generation capacity.

The interface between the PV combiner sub-panel and downstream components such as the solar charge controller and battery bank can be done without power surges.

The master shut-off switch on the panel can shut down the flow of DC from the solar panels entirely if maintenance is required.

What Is The National Electric Code (NEC) “120% Rule”?

The “120% Rule” requires that the sum of the Amp-ratings of the trip switches in power to the busbar must not exceed the rating of the busbar of conductive wire. The busbars’ rating may not exceed 120% of its rated current carrying capacity.

This is a mouthful, so lets me break it down into a simple formula:

(Busbar Rating (A) x 1.2) – Main Breaker Rating (A) = Max (PV + Battery) (A)

The busbar Ampere rating multiplied by 120% (factor 1.2) less the Main Panel Breaker Ampere Rating must be equal to or less than the maximum current from the Solar Panel Array to the Battery bank.

This rule applies to Grid-Ties Solar Systems, where a feed to the Main Electrical Panel (Load Center) and a feed of power from the Solar Panel Array. The Battery Bank can be charged during the day from the solar panels and feed power to the load center as required by the demand on the inverter.

To prevent current from the grid from causing a backflow to the solar panel and vice versa, the 120% rule defines the maximum allowable amperage from the solar panels. Let’s assume we have a 200 Amp breaker on the grid power connected to a 200 Amp rated busbar.

The 120% Rule would dictate:

(200 A x 1.2) for busbar – (200 A) for main breaker = (240A – 200A) = 40A

The dedicated breaker for the Solar Array feed must be 40A to comply with the 120% Rule. The power feed from the solar system cannot exceed a maximum of 40A of input to the main breaker busbar.

Don’t be too flustered by this rule, as you will not be allowed to make such an installation on your own. Qualified technicians must do Grid-tied installations to ensure that the grid and the house are adequately protected.

You can do most of the structural installations yourself but rely on a qualified electrician to do the final connections and testing before switching on the system.


General Solar Panel FAQ

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