String Sizing and Voltage Drop — Wiring a Compliant and Efficient Solar System in NZ

Introduction

String sizing and voltage drop are fundamental aspects of designing a reliable and code-compliant solar photovoltaic (PV) system in New Zealand. Mistakes in string configuration or cable design can result in failed inspections, reduced system efficiency, and safety risks. This article explores the technical considerations and calculations required to properly size PV strings and manage voltage drop according to AS/NZS 5033 and AS/NZS 3000.

1. Understanding PV String Configuration

A PV string is a series-connected group of solar panels. Panels in a string must operate within the inverter’s Maximum Power Point Tracking (MPPT) voltage window and adhere to upper and lower voltage limits defined by both the equipment and relevant standards.

Key Parameters:

• Voc (Open Circuit Voltage): Highest voltage under no load (used for cold temperature calculations)

• Vmp (Voltage at Maximum Power): Operating voltage under load

• Isc (Short Circuit Current): Maximum current under fault conditions

These values are found on the panel datasheet and must be corrected for local temperature conditions.

2. Voltage Correction for NZ Conditions

AS/NZS 5033 requires that Voc be corrected for the lowest recorded ambient temperature at the installation site. This ensures the system does not exceed inverter or equipment limits during cold mornings.

Formula:

Voc (corrected) = Voc (STC) + [temperature coefficient × (Tmin - 25°C)]

Where:

• Tmin = lowest site temperature (e.g., −10°C for Queenstown)

• Temperature coefficient typically = −0.3 to −0.35%/°C

Example:

• Panel Voc at STC = 49 V

• Temp coefficient = -0.32%/°C

• Tmin = -5°C

Corrected Voc = 49 + [(-0.32/100) × (-5 - 25)] = 49 + 9.6 = 58.6 V

If you have 10 panels in series: Total Voc = 10 × 58.6 = 586 V

Ensure this is below the inverter’s and AS/NZS 5033’s max allowable DC voltage (usually 600 V for residential).

3. MPPT Voltage Window Compliance

Inverters have a MPPT voltage window, typically something like 120 V to 550 V. The Vmp of the string must fall within this range under expected operating temperatures (usually 25°C to 45°C).

Example:

• Vmp per panel = 41 V

• 10 panels × 41 V = 410 V

This is within most inverters' MPPT range and will operate efficiently. However, in hot conditions, voltage will drop, so check minimum voltage at Tmax (e.g., 45°C).

Use: Vmp (corrected) = Vmp (STC) + [temp coefficient × (Tmax - 25°C)]

4. Current Considerations

The short-circuit current (Isc) of the string is used to determine cable ratings, breaker sizing, and overcurrent protection. AS/NZS 5033 requires you to apply a safety factor:

Isc (string) = Isc (panel) × 1.25 × number of parallel strings

Use this to ensure the current-carrying capacity (CCC) of your DC cabling is compliant under AS/NZS 3008.1.

5. Voltage Drop Requirements

AS/NZS 5033 and AS/NZS 3000 recommend limiting voltage drop to <3% per segment (DC and AC side). Excessive voltage drop causes efficiency losses and thermal stress.

DC Voltage Drop Formula:

V = I × L × 2 / (A × σ) Where:

• V = voltage drop (volts)

• I = current (amps)

• L = cable run length (metres)

• A = cross-sectional area (mm²)

• σ = conductivity (~56 for copper)

Then: % Voltage Drop = (Vdrop / Vsystem) × 100

Select cable sizes using AS/NZS 3008.1 tables, considering:

• Ambient temperature

• Cable grouping

• Conduit type and installation method

Example:

• Current = 9 A

• Length = 30 m

• Vsystem = 400 V

• Cable size = 4 mm²

Voltage drop = ~1.2%, which is acceptable

6. Parallel Strings and Mismatch Risk

When using parallel strings:

• Ensure identical panel orientation, tilt, and irradiance

• Avoid mixing different brands/models

• Use string fusing if inverter doesn’t have integrated protection

Mismatch between strings can reduce total power output and stress individual modules.

7. Compliance with AS/NZS 5033 and 3000

Key mandates include:

• DC isolators rated appropriately or eliminated if not required

• Cable mechanical protection (e.g., UV-resistant conduit on rooftops)

• Clearly labelled string voltages and shutdown procedures

• Adequate earthing and bonding for frame and array supports

AS/NZS 3000 also governs the AC side:

• Protection devices (RCBOs, MCBs)

• MEN (Multiple Earthed Neutral) integrity

• Cable colour coding and segregation

8. System Design Documentation

Documentation required for compliance:

• String layout diagrams

• Voltage and current calculations (min/max)

• Cable sizing and voltage drop summary

• Isolation and protection scheme

• Inverter datasheets and MPPT range

These are essential for sign-off by inspectors and electricity distributors.

Conclusion

String sizing and voltage drop are non-trivial aspects of PV system design in New Zealand. Installers must rigorously calculate temperature-corrected voltages, stay within MPPT windows, and comply with AS/NZS standards to ensure safe and efficient operation. These calculations form the backbone of a reliable solar installation that performs as expected over its 25+ year lifetime.

Next article: "Battery-Ready Grid-Tie Systems — Planning for Future Storage in Residential Installations"