Wiring Siemens solar modules: series, parallel and bypass diodes
The datasheets set the wiring rules model by model: maximum system voltage, 6 V versus 12 V versions and cold-weather Voc. Here they are in one place, with worked strings.
Key takeaways
- The small modules (SM6, SM10, SM20) have a 20 to 25 V system-voltage limit: wire them in parallel only. The SM20 can exceed its own limit when cold.
- Cold weather raises Voc by about 0.34 % per degree: four SM55 in series reach about 101 V at −20 °C.
- The 6 V versions of the SP and SR modules need large bypass diodes with heat sinks if they are used in high-voltage arrays.
- Size cable and fuses for about 1.56 × Isc and fuse each string when three or more are paralleled.
The datasheets contain more wiring information than most owners realise. Three things decide how a module may be connected: the maximum system voltage, the cell count and, for the SP and SR families, the 12 V or 6 V version. This guide puts the rules in one place, with worked examples built from the numbers on the model pages.
Series and parallel in one picture
Energy is the same either way. Series is chosen for lower current, which means thinner cable and less loss on a long run; parallel keeps the voltage at the battery level and lets a weak module drag down only its own contribution.
Maximum system voltage depends on the model
| Model | Maximum system voltage (as published) | Consequence |
|---|---|---|
| SM6, SM10 | 25 V | Two modules in series (about 39 to 40 V open-circuit) exceed the rating: parallel only |
| SM20 | 20 V | The module’s own Voc rises to about 20.6 V at −20 °C, above the rating: use it singly or in parallel, and only where cells stay above about 0 °C |
| SM46, SM50-H, SM55/SM50, SM110/SM100 | 1000 V (EC), 600 V (UL 1703) | Series strings allowed within the controller or inverter limit |
| SP18, SP36, SP75/SP70 | 1000 V (EC), 600 V (UL 1703) | Same; note the footnote on the 6 V version |
| SR50, SR100/SR90 | 1000 V (EC), 600 V (UL 1703) | Same; the source text marks the EC rating slightly differently, so confirm on the original |
| ST5, ST10, ST20, ST40 | 25 V (as published; the ST table layout is garbled) | Treat as parallel only until confirmed on the original datasheet |
Voltage rises when it gets cold
The datasheets give a Voc coefficient of −0.34 % per kelvin. A string’s open-circuit voltage is therefore highest on a cold, sunny morning, and that is the number your controller or inverter has to survive. For the SM55, whose Voc is 21.7 V at 25 °C, the chart shows what one, two and four modules in series reach.
At −20 °C a single SM55 reaches 25.2 V, two reach 50.4 V and four 100.8 V. A 12 V controller with a 25 V input limit is not enough for one module in a cold climate; a 100 V controller is not enough for four.
The 6 V versions and bypass diodes
The SP and SR modules were offered in a 12 V and a 6 V version; the 6 V version halves the voltage and doubles the current (SP75: Vmp 17.0 V at 4.4 A or 8.5 V at 8.8 A). The datasheet footnote is specific: if the designer uses the 6 V version in a high-voltage array configuration, the designer must install custom, large bypass diodes and heat sinks around every 6 V parallel string. The likely reason is that a shaded 6 V block in a high-voltage string has to pass the full string current through its bypass diode.
Worked examples
| System | Wiring | Array result (from datasheet) |
|---|---|---|
| 12 V battery, one SM55 | 1 module | Vmp 17.4 V, Imp 3.15 A, Voc 21.7 V; Voc at −20 °C about 25.2 V |
| 24 V battery, SM55 | 2 in series | Vmp 34.8 V, Imp 3.15 A, Voc 43.4 V; at −20 °C about 50.4 V: check the controller’s input limit |
| 12 V battery, four SM55 | 4 in parallel | Vmp 17.4 V, Imp 12.6 A, Isc 13.8 A; controller current at least 1.25 × 13.8 = 17.3 A |
| 48 V battery, SM110-24 | 2 in series | Vmp 70 V, Imp 3.15 A, Voc 87 V; at −20 °C about 101 V |
Cable and protection
- In the US National Electrical Code (article 690), the maximum circuit current is 1.25 × Isc, and conductors and overcurrent devices are then sized at a further 125 %, about 1.56 × Isc. Confirm the rule in the edition your jurisdiction has adopted.
- A common rule of thumb is to keep voltage drop under about 2 to 3 %. It is a design habit, not a datasheet value; cable runs on remote masts can be long enough for it to matter.
- Fuse each parallel string when three or more are paralleled; a fault in one string can then be fed by the others.
- Do not rely on old connectors. Replace weathered junction-box seals and cable glands even when the modules test well.
Five mistakes that cost people modules
- Putting two SM6 or SM10 modules in series and exceeding the 25 V rating.
- Sizing a controller for the Voc at 25 °C instead of at the coldest cell temperature.
- Mixing 12 V and 6 V versions in one string.
- Paralleling three or more strings without fuses.
- Using long thin cable on a 12 V array and then blaming the modules for low charge current.
Questions readers ask
Can I wire an SM55 to charge a 24 V battery?
Yes, with two in series: Vmp 34.8 V, Imp 3.15 A, Voc 43.4 V (about 50 V at −20 °C). The controller’s input rating must exceed that cold figure.
What happens if I exceed the maximum system voltage?
The datasheet qualification is for the stated voltage; above it the insulation is outside what was tested. Treat the limit as a hard design rule, not a guideline.
Do SP and SR modules have a switch for 6 V and 12 V?
The datasheets list a 12 V and a 6 V version in separate columns and do not describe a user-selectable switch. The two versions use different cell wiring; confirm which one you have before you connect anything.
Is series or parallel better for a long cable run?
Series, because it lowers the current for the same power and so the cable loss. It raises the Voc, so check the controller’s input rating at your coldest temperature.
Do I need bypass diodes on SM and ST modules?
The datasheets discuss bypass diodes only in the footnote on the 6 V SP and SR versions. For other modules, check the junction box of your unit and the original datasheet.
Related guides
Charge controllers for Siemens solar modules
How to choose a charge controller for Siemens modules: PWM or MPPT, why 36 cells suit 12 V batteries, cold-weather Voc limits and how to size the current.
Read more →Replacing or expanding a Siemens solar module
How to replace a failed Siemens solar module or expand an old system: test the existing modules, read the results, match voltage and choose a replacement.
Read more →Batteries for a 12 V Siemens solar system
Which battery to pair with Siemens modules: lead-acid versus LiFePO4, how to size from the daily load, worked examples and what the installation notes recorded.
Read more →Pick a module, set series and parallel counts and see cold-weather Voc.