Matching PV Combiner Box Current & Voltage Ratings

In a solar photovoltaic (PV) system, the combiner box plays a central role in consolidating power and ensuring safety. It’s a junction box where the output of multiple solar strings are connected in parallel, combining their current before sending it to the inverter. More importantly, it houses the critical overcurrent protection devices (fuses or circuit breakers) that protect the system’s wiring and components from potentially dangerous faults.

Properly sizing your PV combiner box and its internal components is not just a best practice—it’s a fundamental requirement for a safe, reliable, and code-compliant installation. An undersized component can lead to fire hazards, while an oversized one may fail to provide adequate protection.

This guide will walk you through the essential steps for matching your combiner box’s voltage and current ratings to your specific solar array, with a focus on the requirements outlined in the National Electrical Code (NEC). We will cover:

• Core safety principles from NEC Article 690
• Calculating maximum system voltage based on site conditions
• Sizing DC string fuses and main overcurrent protection
• 

The Foundation: Understanding NEC Article 690

For any PV installation in the United States, NEC Article 690 is the ultimate authority. This section of the code is specifically dedicated to solar PV systems, addressing the unique characteristics and potential hazards of DC power generation. It provides the framework for everything from wiring and grounding to disconnecting means and overcurrent protection. Adhering to Article 690 is essential for ensuring your system is safe and will pass inspection.

Part 1: PV System Voltage Matching

The first and most important parameter to determine is the maximum system voltage. All components in a circuit, from the wires to the fuses and the combiner box itself, must have a voltage rating equal to or greater than the maximum voltage they might experience.

In a PV system, the maximum voltage is determined by the number of modules in a series string and their open-circuit voltage (Voc), found on the module’s datasheet. However, voltage is inversely proportional to temperature; a solar panel’s voltage is highest on the coldest possible day. NEC requires you to calculate the maximum voltage for the lowest expected ambient temperature at your site.

How to Calculate Maximum System Voltage:

1. Find the Voc: Look up the Open-Circuit Voltage (Voc) on your solar module’s datasheet.
2. Determine Lowest Temperature: Find the record low temperature for your location. You can often find this from local weather data or ASHRAE design temperature tables.
3. Find the Temperature Correction Factor: The module datasheet will provide a temperature coefficient for Voc, expressed as a percentage or V/°C. Use this to find the correction factor for your lowest temperature relative to the standard test condition (STC) temperature of 25°C. The NEC provides tables in 690.7 for a simplified calculation.
4. Calculate Corrected Voc: Corrected Voc = Voc × [1 + (Lowest Temp °C - 25°C) × (Voc Temp Coefficient %/°C)]
5. Calculate Max System Voltage: Max System Voltage = Corrected Voc × Number of Modules in Series

Example:

• Module Voc: 48.5V
• Lowest Site Temperature: -10°C
• Number of modules per string: 12
• Voc Temperature Coefficient: -0.28%/°C
• Corrected Voc = 48.5V × [1 + (-10 - 25) × (-0.0028)] = 48.5V × [1 + (-35 × -0.0028)] = 48.5V × 1.098 = 53.25V
• Max System Voltage = 53.25V × 12 = 639V

In this case, the system requires a combiner box, fuses, and breakers rated for at least 639V. Therefore, you would select components from a standard 1000V DC class, as 600V components would be insufficient.

Part 2: PV Combiner Box Sizing & DC String Fusing

After determining the voltage rating, the next step is to size the overcurrent protection devices (OCPDs), which are typically fuses. The primary purpose of string fuses is to protect against reverse current. A fault in one string can cause other strings to back-feed into it, creating a dangerous overcurrent condition.

Per NEC 690.9, string-level fusing is required whenever you have three or more strings in parallel. With only two strings, the reverse current potential is not high enough to exceed the module’s fuse rating, so fuses are not required by code (though they are still sometimes used as a disconnecting means).

How to Size String Fuses:

The NEC requires that the fuse be rated to handle continuous duty and potential irradiance spikes. This is accomplished by multiplying the module’s Short-Circuit Current (Isc) by a factor of 1.56.

• 1.25 factor for continuous load (circuits that can operate for 3+ hours).
• 1.25 factor for potential “irradiance enhancement” where sun exposure can exceed the standard 1000 W/m².
• 1.25 × 1.25 = 1.56

Fuse Rating Formula: Minimum Fuse Rating ≥ Module Isc × 1.56

After calculating this minimum, you select the next standard fuse size up.

Crucial Limitation: Maximum Series Fuse Rating\
Every solar module has a “Maximum Series Fuse Rating” listed on its datasheet. This value is an absolute limit. Your calculated fuse size must not exceed this rating. If it does, your system design is flawed, typically meaning your chosen module cannot be used with that many strings in parallel.

Example:

• Module Isc: 10.5A
• Module Maximum Series Fuse Rating: 20A
• Minimum Fuse Rating = 10.5A × 1.56 = 16.38A

Based on this calculation, you would choose the next standard size up, which is a 20A fuse. Since 20A is equal to the module’s maximum series fuse rating, this is a valid selection. If the calculation had resulted in 21A, you would not be able to use a 25A fuse, and the design would need to be re-evaluated.

Part 3: Overcurrent Protection Coordination

Protection coordination ensures that in the event of a fault, the correct OCPD opens first. For a fault within a single string, you want the individual string fuse to blow, isolating only that string without taking the entire array offline. The main OCPD at the combiner box output should only trip if there is a major fault on the main conductors leading to the inverter.

This is achieved by ensuring the downstream device has a lower rating than the upstream device.

Frequently Asked Questions (FAQ)

Do I need fuses for only two parallel strings?\
No. According to NEC 690.9, overcurrent protection is only required when there are three or more strings in parallel. With two strings, the maximum fault current that one string can source to the other is limited to the Isc of one string, which is below the module’s maximum fuse rating.

Can I use AC-rated fuses or breakers for DC circuits?\
No, you must use components specifically rated for DC circuits. DC arcs are much more difficult to extinguish than AC arcs. An AC-rated device will likely fail to safely interrupt a DC fault, leading to a significant fire and safety hazard.

What happens if I install the wrong size fuse?

• Too Small: The fuse will be subject to “nuisance tripping,” blowing under normal operating conditions (e.g., on a cool, very sunny day), causing unnecessary system downtime.
• Too Large: A fuse that is too large (especially one exceeding the module’s Maximum Series Fuse Rating) will not protect the module from damaging reverse currents. This can lead to module failure and creates a fire risk.

Why is the 1.56 multiplier used for sizing?\
It’s a combined safety factor required by the NEC. It consists of two separate 1.25 multipliers: one to account for the fact that solar circuits are considered “continuous loads” (running at max current for more than 3 hours), and a second to account for irradiance levels that can exceed standard test conditions, causing current to rise temporarily above the rated Isc. (1.25 x 1.25 = 1.56).

What is a module’s “Maximum Series Fuse Rating”?\
This is a safety rating determined by the module manufacturer and certified by UL. It specifies the maximum current the module can withstand without being damaged when subjected to a reverse current. You must never install a fuse or OCPD rated higher than this value.

Conclusion

Matching a PV combiner box’s current and voltage ratings is a systematic process guided by engineering principles and the strict safety standards of the NEC. The core takeaways are:

1. Voltage First: Always determine the maximum system voltage using the lowest expected temperature for your site. All components must be rated for this voltage.
2. Current Second: Size string fuses based on the module’s Isc and the 1.56x multiplier, but never exceed the module’s Maximum Series Fuse Rating.
3. Code is Key: When in doubt, defer to NEC Article 690 and the manufacturer’s datasheets for all components.

By following these guidelines, you can ensure your solar array’s combiner box is sized safely, reliably, and professionally, forming a robust foundation for your entire PV system.