WiremanWireman · One PV string producing low: troubleshooting for electricians · 2023 NEC ·
One PV string producing low
Compare strings at the same moment to tell shading, a dead module, a bypass diode or a bad connection apart.
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How it works
Strings with the same modules facing the same way should read almost the same at the same moment. Module current rises and falls with sunlight, while voltage depends mostly on temperature (it drops as modules heat up) and only somewhat on light. That’s why you compare strings to each other at the same time instead of comparing to a number on the label: a low voltage points to missing or bypassed modules, while low current points to shading, dirt or damaged cells.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- How do you know the string is low?
- It reads zeroTest: Check the string fuse and voltageExpect: Normal Voc on the array side and a fuse that reads continuity.
- Voc good, fuse blownBlown string fuse
- No voltage on the array sideOpen in the string wiring
- It produces, but less than the othersDoes anything shade that string at any time of day?
- Yes, or it’s visibly dirtyShading or soiling
- No shade, same orientation as the othersTest: Compare open-circuit voltage between stringsExpect: Strings with the same module count within a few percent of each other.
- Low by about one module or a fraction of oneShorted bypass diode or missing module
- Voltage matches the good stringTest: Compare operating current between stringsExpect: Matching strings within about 5–10% of each other in steady sun.
- Low current on that stringDegraded module or high-resistance connection
- Same as the othersString is healthy; check the monitoring
- It reads zeroTest: Check the string fuse and voltageExpect: Normal Voc on the array side and a fuse that reads continuity.