Wire that's too thin for the depth causes voltage drop at the motor — which shows up as overheating, reduced pumping capacity, and shortened motor life. Here's how to size it correctly.
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Every foot of wire adds resistance, and resistance causes voltage drop. A motor that's supposed to receive 230V but is actually getting 210V at the bottom of a deep well will draw more current to compensate, run hotter, and wear out faster — even though everything above ground looks completely normal. This is why wire that's perfectly fine for a 100-foot well can be dangerously undersized for the same pump at 300 feet.
| Pump HP | Wire Gauge (up to 100 ft) | Wire Gauge (100-200 ft) | Wire Gauge (200-400 ft) |
|---|---|---|---|
| 1/2 HP | 14 AWG | 12 AWG | 10 AWG |
| 3/4 HP | 14 AWG | 12 AWG | 10 AWG |
| 1 HP | 12 AWG | 10 AWG | 8 AWG |
| 1.5 HP | 12 AWG | 10 AWG | 8 AWG |
| 2 HP | 10 AWG | 8 AWG | 6 AWG |
These are general guidelines, not a substitute for a proper voltage drop calculation. Actual required gauge depends on your specific voltage, amperage draw, and acceptable drop percentage. Always verify against the pump manufacturer's wire sizing chart or have a licensed electrician calculate it for your exact setup.
Submersible pump cable is a specific product rated for continuous underwater submersion — not standard THHN or NM cable. It's typically sold as 3-wire (plus ground) jacketed cable rated for direct burial and submersion, and should always be replaced with the same submersible-rated product, upsized in gauge if your depth calculation calls for it.
The general voltage drop formula is: VD = (2 x K x I x D) / CM, where K is a constant for copper (approximately 12.9), I is the motor's running amperage, D is the one-way wire distance in feet, and CM is the wire's circular mil area (found on any wire gauge reference chart). Most electricians target no more than a 3% drop for a dedicated motor circuit like a well pump — beyond that, step up a wire gauge. This is a genuinely useful check even if you're having a contractor size the wire, since it lets you verify their recommendation rather than taking it on faith.
Any splice below the waterline needs to be done with a proper submersible splice kit — heat-shrink tubing rated for continuous submersion, or a resin-filled splice kit, never standard electrical tape or wire nuts alone. A failed underwater splice is one of the more common causes of a pump that suddenly stops working with no warning, and it's also a real safety issue since it can energize the well water itself. If your well was serviced by someone who used a non-submersible splice method, it's worth having a licensed electrician redo it properly the next time the pump is out.
Submersible pump cable itself typically runs $1-$3 per foot depending on gauge, so a 200-foot well might need $200-$600 in wire alone for a full rewire. Because rewiring requires pulling the pump — the same labor-intensive step as a pitless adapter or torque arrestor job — total cost including labor typically runs $500-$1,500 for moderate depths, more for very deep wells. If you're already paying for a pump pull for any other reason, upsizing undersized wire at the same time avoids paying for a second pull later.
No — submersible pump cable has a specific jacket rated for continuous water submersion. Standard NM or THHN wire will degrade and fail underwater.
Voltage drop increases, causing the motor to draw more current to compensate, which leads to overheating and premature motor failure.
Yes — undersized wire can reduce actual pumping capacity and flow rate even when the pump appears to run normally.
Underground direct-burial splice and repair kit with heat shrink.
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