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EG4 18KPV: PV Disconnect Sizing Guide for 4 Strings

Sizing a PV disconnect for an EG4 18KPV hybrid inverter with 4 strings? Learn the correct disconnect amperage and voltage for NEC compliance and safety.

Frequently Asked Questions

•What is the minimum voltage rating for the PV disconnect?

The minimum voltage rating for the PV disconnect must be equal to or greater than the maximum calculated PV array voltage, accounting for temperature effects.

•Where should the PV disconnect be located?

The PV disconnect should be located in an easily accessible location as required by NEC 690.13, typically near the inverter or the PV array.

•What is the NEC article related to PV disconnects?

NEC Article 690 covers solar photovoltaic (PV) systems and provides requirements for disconnects, overcurrent protection, and grounding.

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Marcus leads the technical research department at Storage Faults. Together with a team of systems analysts, he specializes in translating complex solar storage error codes into actionable diagnostic steps for the US market.

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Pro Tip:

To quickly estimate the required disconnect amperage, multiply the total PV array Isc by 1.5625 (1.25 for continuous current and 1.25 again for the disconnect rating). This provides a conservative estimate for disconnect sizing. Example: if your Isc is 8A per string and you have 4 strings, the minimum disconnect rating would be 8A * 4 * 1.5625 = 50A.

Solution:

1

Obtain the PV module datasheet and identify the Voc, Isc, and temperature coefficient for Voc.
2

Calculate the maximum system voltage: Voc * (1 + (Temperature Coefficient * (Minimum Expected Temperature - 25°C))). Use the lowest expected ambient temperature for the location.
3

Calculate the maximum PV array current: Isc * 1.25 (NEC 690.8) * Number of Parallel Strings. The 1.25 factor accounts for continuous operation.
4

Select a PV disconnect with a DC voltage rating equal to or greater than the maximum system voltage and a DC current rating equal to or greater than the maximum PV array current. Verify UL listing.
5

Ensure the disconnect is appropriately rated for the environmental conditions (e.g., NEMA rating for outdoor use).
6

Install the disconnect in an accessible location as required by NEC 690.
7

Use a properly sized enclosure.
8

Verify the system with a qualified electrician using a multimeter to measure voltage and a clamp meter to measure current after commissioning.

Possible Causes:

Incorrect calculation of the maximum PV array voltage (Vmp * temperature coefficient).
Incorrect calculation of the maximum PV array current (Isc * 1.25 * number of strings).
Failure to account for temperature correction factors affecting Voc.
Using a disconnect with an inadequate voltage or current rating.
Ignoring NEC requirements for disconnect placement and type.
Improper understanding of series and parallel string configurations.

How It Works:

The EG4 18KPV is a hybrid inverter that combines solar PV input, battery storage, and grid connectivity. The PV input section converts DC power from the solar panels into DC power suitable for the inverter's DC bus. A correctly sized disconnect provides a safe way to isolate the PV array from the inverter for maintenance or emergency situations. Key parameters include maximum PV string voltage (Voc), short-circuit current (Isc), and the number of parallel strings. The National Electrical Code (NEC) Article 690 provides guidelines for PV system disconnect sizing.

Symptoms:

Determining the correct PV disconnect size for an EG4 18KPV hybrid inverter connected to four PV strings is crucial for safety and NEC compliance. Incorrect sizing can lead to inadequate protection during overcurrent events or maintenance.

Maintenance Tips:

Regularly inspect the PV disconnect for corrosion, damage, or loose connections
Check the PV module datasheet for updated specifications
Periodically verify the maximum PV string voltage and current, especially after adding or replacing modules
Consult with a qualified electrician for any modifications to the PV system