Solar Pump for a 100 m Deep Well: Sizing, Voltage & Cable Choices

The short answer: a solar pump for a 100 m deep well is normally a 110 V DC 3-inch or 4-inch submersible rated for 110-135 m of head — not because the hole is 100 m deep, but because you size from the water level, not the bottom of the hole. Measure the static water level, add drawdown, add the lift to your tank and pipe friction, add 10-15% margin, and match that to a rated head. In our range that usually lands on 3DPC3.8-123-110-1100, 3DSC5-112-110-1100, 3DSC5-135-110-1500 or 4DSC7.5-100-110-1500. The two things buyers get wrong at this depth are picking a 48 V pump (cable losses eat it alive) and buying a cable that is too thin.

This is the depth band I get asked about most. Deeper than a homestead shallow well, shallower than the 150-300 m municipal jobs — and it is where most sizing mistakes happen.

Why 100 m is the depth where shallow pumps simply stop working

A surface pump cannot suck water up more than about 6-7 m in practice (the physical limit of atmospheric pressure is 10.3 m of water column, and you never get all of it). Below that, the pump has to go down the hole and push the water up. That is the whole reason deep wells use submersibles.

It also rules out the small cheap pumps people try first:

If someone quotes you a “100 m solar pump” at 24 V and 300 W, it is not a 100 m pump. Check the rated head on the spec sheet, not the marketing line.

The number that matters: head from the water level, not the hole

Here is the single most useful thing in this guide. Drilled depth and required head are two different numbers.

• A 100 m borehole with the static water level at 40 m does not need 100 m of head.
• A 100 m borehole with the static water level at 85 m needs roughly 105-120 m of head.

The formula (full version in how to calculate pump head):

TDH = static water level + drawdown + vertical lift above ground + pipe friction + 10-15% margin

Worked example A — water at 40 m, tank on a 12 m tower, 80 m of pipe

• Static water level: 40 m
• Drawdown while pumping: 6 m
• Lift to tank inlet: 12 m
• Pipe friction (80 m ÷ 10): 8 m
• Subtotal: 66 m → with 12% margin: ~74 m TDH
• Pick: 4DSC7.5-100-110-1500 (100 m head, 7.5 m³/h) or 3DSC5-112-110-1100.

Worked example B — water at 85 m, tank 8 m up, 50 m of pipe

• Static water level: 85 m
• Drawdown: 10 m
• Lift to tank: 8 m
• Pipe friction (50 m ÷ 10): 5 m
• Subtotal: 108 m → with 10% margin: ~119 m TDH
• Pick: 3DSC5-135-110-1500 (135 m head) or 3DPC3.8-123-110-1100 (123 m head).

Same “100 m well”, two completely different pumps. This is why I always ask for the static water level before quoting.

The model ladder for a 100 m borehole

Progression is simple: 2DPC/3DPC (plastic impeller, smaller) → 3DSC/4DSC (stainless impeller, mid-range) → 6DSC/8DSC (AC/DC high-voltage, high flow). If you are still choosing between down-the-hole and surface setups, read submersible vs surface pump first.

That last row is worth reading twice. The 8DSC is the biggest pump we sell, but its rated head is 37 m — it moves enormous volume over a short lift. It is the wrong machine for a deep borehole, and buyers who shop by “biggest model” get burned here.

For depths beyond 100 m — 150-300 m — the whole design changes to high-voltage AC/DC; that is covered in solar pumps for deep wells (150-300 m).

Why 48 V struggles at 100 m (and 110 V does not)

Current is what kills you on a long drop cable. Power is fixed, so lower voltage means higher current, and voltage drop scales directly with current.

Keep total drop under 3% of system voltage. At 100 m depth, that effectively means: 48 V for shallow wells only, 110 V for the 80-130 m band, 380/520 V AC/DC once you are moving 20 m³/h or more.

Cable sizing for a 110 m drop

Using the same formula published in the solar pump cable sizing guide: ΔV = 2 × L × I × 0.0175 ÷ A, where L is one-way length in metres, I is current in amps, A is cross-section in mm², and 0.0175 is copper resistivity.

Voltage drop over a 110 m run (100 m well + 10 m to the controller), by cable size:

Bottom line: 10 mm² for a 1100 W pump, 16 mm² for a 1500 W pump. Anything thinner and you are paying for panels that never reach the motor.

One practical note: most submersible models ship with only about 2 m of cable on the nameplate spec. A 100 m installation needs the full 110 m of submersible drop cable ordered and spliced separately — budget for it, because it is not a small line item at 16 mm².

Panel configuration and the VOC limit

Use the 1.3x rule: panel wattage at least 1.3 times pump wattage.

• 1100 W pump → 1,430 W → 3 × 550 W panels
• 1500 W pump → 1,950 W → 4 × 550 W panels

Then check voltage, not just power. A 550 W panel is about 42 V Vmp / 50 V Voc. For a 110 V controller the open-circuit limit is 220 V:

• 3 panels in series: Voc ≈ 150 V — safe
• 4 panels in series: Voc ≈ 200 V — safe at 25 °C, but Voc rises about 0.3% per degree below 25 °C, so in a cold morning at 0 °C it climbs to roughly 215 V and gets close to the limit

If you are in a cold climate or your panels have a higher Voc, wire 2 strings of 2 in parallel instead of 4 in series. Full wiring detail is in the controller wiring diagram guide.

Installation checklist specific to 100 m

1. Stainless safety rope or cable gripper. Never lower a 100 m pump on the power cable or the pipe alone — the whole weight plus water column sits on it.
2. Waterproof the splice properly. A 100 m deep splice is unreachable. Use a resin-filled submersible splice kit, not electrical tape. Tape fails, and a failed splice means pulling 100 m of pipe.
3. Set the pump above the bottom. Hang it 3-5 m off the hole bottom so it does not ingest settled sand.
4. Confirm dry-run protection is on. The controller should stop the pump when the well draws down, not burn the motor. See solar pumps in sandy wells if your water is gritty.
5. Torque the pipe joints. At this depth a joint that backs off is a lost pump.
6. Prime the controller slowly on first start and watch the current for the first ten minutes — it should settle, not hunt.

Sizing a 100 m borehole? Run your numbers through the sizing tool with your static water level and daily demand, or message me on WhatsApp with your water level, depth and daily volume — I will tell you the exact model, cable size and panel count.