Size the cable from the current it must carry and the distance it must carry it over, then check voltage drop. Ampacity alone is not enough: a cable that is safe on current can still drop too much voltage on a long motor run, and that is when a pump runs hot and delivers less head.
What the Two Cable Runs Are
The two cable runs in a solar pumping system are the DC string cable from array to inverter, and the AC motor cable from inverter to pump — and they are sized by different rules.
Why Cable Sizing Causes So Many Faults
The existing 380V article notes that more solar pumping faults trace back to cable than to electronics, and that a submersible pump may sit 80 to 150 metres below the inverter.
When to Size the Cable
Size cable at design time, before trenching or dropping a pump; re-check it whenever a pump is set deeper or a run is extended.
Where the Rules Differ
DC runs, motor runs, and the submerged splice section each have their own requirements.
How to Size Cable for a Solar Pump Inverter
- Read the rated current from the motor nameplate and the array current from the module data.
- Measure the full run length, not the straight-line distance.
- Check the cable’s ampacity for that current.
- Check voltage drop over the full run and keep it to a small design target (about 3 percent is a practical planning target, not a product specification).
- If the run is long, go one cable size up from the bare ampacity answer, as the 380V article advises.
- Keep joints out of the splash zone and use waterproof splices for the submerged section.
- Keep DC, AC input and motor output wiring physically separate and labelled.
Run type vs sizing rule:
| Run | Sized by | Common mistake |
|---|---|---|
| DC string cable | Array current, UV-rated solar cable | Using indoor cable outdoors |
| AC motor cable | Motor current plus voltage drop over distance | Sizing on current alone for an 80 to 150 metre run |
| Submerged section | Waterproof splice and joint position | Joints in the splash zone |
When to go one size up:
| Situation | Action |
|---|---|
| Long motor run | One size up from the ampacity answer |
| Pump set deeper than planned | Re-check drop before running at full load |
| Hot enclosure or high ambient temperature | Allow for derating and ventilation, do not just accept trips |
Related reading: the wiring article, the panel sizing article, and the QB100-PV solar pump inverter product page.
Frequently Asked Questions
Is the thickest cable always the best choice?
No: oversizing beyond the drop requirement adds cost and termination difficulty without more water.
Can I use ordinary indoor cable for the DC string?
No: it must be UV-rated solar cable for outdoor exposure.
Why does my pump run hot on a long cable?
Voltage drop starves the motor, so it draws more current and delivers less head.
If the inverter keeps tripping, will a bigger inverter fix an undersized cable?
No: the cable drop remains, and the motor still runs hot; fix the cable first.
Do I need to re-check cable size if I move the pump deeper?
Yes: a longer run means more drop, so re-check before full-load running.
To get a cable check, send motor nameplate and cable length for a check.