Why is the wire larger than 12 AWG?

12 AWG copper is the usual 20 A conductor. Over 150 ft the drop calculation can require 10 or 8 AWG to stay inside 3 percent.

Should the pump be 120 or 240 volts?

This preset is 240 V. A 120 V pump at the same amps and distance has a much higher percent drop. Change the system control if the nameplate is 120 V.

Does PVC in a trench change resistance?

Not in this model. Conduit changes the cost. Temperature and bundling derating are called out as exclusions in the formula panel.

20 Amp Well Pump Wire Size at 150 Feet

A long 150 ft well branch where drop, not the 20 A breaker, picks the wire.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

8 AWG · 8.37 mm²

Single conductor set

Voltage drop

4.69 V

1.95% · end 235.3 V

Power loss

94 W

I²R at the operating current

Required ampacity

20.0 A

Each conductor 50 A

Material estimate

$747.00

3 conductors + PVC

Conductor weight

28.8 lb

13.1 kg · approximate insulated weight

Single-line and conductor gaugeLive scaled blueprint

Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total
copper 8 AWG · 8.37 mm²	150 ft one way	cuts excluded	3 conductors	USD	$604.50
PVC conduit	150 ft	—	150 ft	USD	$142.50

Copper versus aluminum at this run
Material	Minimum size	mm²	Drop	End voltage	Loss
copper	8 AWG · 8.37 mm²	8.4	1.95%	235.3 V	94 W
aluminum	6 AWG · 13.3 mm²	13.3	2.02%	235.2 V	97 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
100 ft	10 AWG · 5.26 mm²	4.97 V	2.07%	235.0 V
150 ft	8 AWG · 8.37 mm²	4.69 V	1.95%	235.3 V
200 ft	8 AWG · 8.37 mm²	6.25 V	2.60%	233.7 V
300 ft	6 AWG · 13.3 mm²	5.90 V	2.46%	234.1 V

Show the deterministic formula

Single phase: Vdrop = 2 × K × I × D / cmil. Three phase: Vdrop = 1.732 × K × I × D / cmil. K is 12.9 for copper and 21.2 for aluminum. D is one-way feet.

Percent drop = Vdrop ÷ source voltage × 100. Green is 3% or less, amber is 3 to 5%, and red is over 5%. Power loss is I²R on the current-carrying conductors.

The smallest conductor that meets both the ampacity (125% when continuous) and the drop limit is selected. Ampacity does not include bundling or temperature correction.

Contractor sign-off

Project: 20 Amp Well Pump Wire Size at 150 Feet

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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.

Engineering notes

Twenty amps at 240 V does not need a large conductor for ampacity. At 150 ft the 3 percent limit often does. The preset is copper in PVC, not continuous, because a well pump cycles. If the manufacturer lists a continuous or high-service-factor load, turn the 125 percent box on. Voltage drop at the pump reduces starting torque, so this is a case where meeting the percentage matters more than matching the breaker with the smallest legal wire.

Code reference

NEC 210.19(A) and 215.2(A) informational notes recommend 3 percent on a branch circuit and 5 percent total with the feeder. Ampacity follows the 75°C column of Table 310.16 plus the small-conductor limits. Confirm the adopted edition.

Field questions

Why is the wire larger than 12 AWG?: 12 AWG copper is the usual 20 A conductor. Over 150 ft the drop calculation can require 10 or 8 AWG to stay inside 3 percent.
Should the pump be 120 or 240 volts?: This preset is 240 V. A 120 V pump at the same amps and distance has a much higher percent drop. Change the system control if the nameplate is 120 V.
Does PVC in a trench change resistance?: Not in this model. Conduit changes the cost. Temperature and bundling derating are called out as exclusions in the formula panel.