Photovoltaic (PV) System Design Flashcards

Under NEC 690.7, which temperature is used to calculate the maximum PV system DC voltage?

Answer: Lowest expected ambient temperature

Module Voc rises as temperature falls, so the record or extreme low ambient temperature gives the highest system voltage.

A module has Voc = 40 V and a Voc temperature coefficient of -0.30%/°C. With a design low of -10°C, what is the maximum number of modules per string for a 600 V inverter?

Answer: 13

Corrected Voc = 40 × (1 + 0.003 × 35) = 44.2 V, and 600 ÷ 44.2 = 13.6, which rounds down to 13 modules.

What usually sets the minimum number of modules in a string?

Answer: Vmp at the highest expected cell temperature compared with the inverter's minimum MPPT voltage

Hot cells lower Vmp, so the string has to stay above the inverter's MPPT minimum on the hottest days.

A design with a DC/AC ratio of 1.3 is most likely to experience which effect?

Answer: Inverter clipping during peak irradiance

With more DC capacity than AC rating, the inverter caps output at its AC limit during high-irradiance hours.

Under NEC 690.8, what combined factor is applied to module Isc when sizing PV source circuit conductors for continuous current?

Answer: 1.56

Isc × 1.25 for irradiance gives the maximum circuit current, and a second 1.25 continuous-duty factor gives 1.56.

What is the main purpose of bypass diodes in a crystalline PV module?

Answer: To let current pass around shaded cell groups and prevent hot spots

Bypass diodes route current around shaded or damaged substrings so those cells are not reverse-biased.

Fixed-tilt arrays in the continental US commonly set row spacing so the array is shade-free during which window?

Answer: 9 AM to 3 PM solar time on the winter solstice

The sun is lowest on the winter solstice, so spacing for that day keeps the array unshaded during the main production hours all year.