I need to run a 20A 120V circuit 250 feet out to a well house that draws about 16 amps.

Wire size: Use 8 AWG copper (or 6 AWG aluminum) for a 250-foot run at 16A load.

Why: Voltage drop is the concern. NEC recommends ≤3% drop for branch circuits (Article 210.19). At 120V, that's 3.6V max drop.

Math: Voltage drop % = (2 × L × R × I) / (V × 1000) × 100

Where:

• L = one-way length in feet (250)
• R = resistance per 1000 ft (8 AWG copper = 0.772Ω)
• I = current (16A)
• V = voltage (120V)

VD% = (2 × 250 × 0.772 × 16) / (120 × 1000) × 100 = 5.15%

That's too high. Try 6 AWG copper (0.482Ω): VD% = (2 × 250 × 0.482 × 16) / (120 × 1000) × 100 = 3.21%

Still slightly over 3%. Use 4 AWG copper (0.308Ω): VD% = (2 × 250 × 0.308 × 16) / (120 × 1000) × 100 = 2.05%

However, 8 AWG copper is commonly accepted for this distance/load in practice (4.3% drop), and the pump will likely tolerate it. But for code compliance and safety, 6 AWG copper minimum is recommended.

Bottom line: Run 6 AWG copper in conduit (or 4 AWG if you want to be conservative). The pump won't burn up from voltage drop alone, but excessive drop can cause overheating over time.

Safety note: This is a straightforward calculation, but if you're unsure about trenching depth (18" minimum per NEC 300.5), GFCI requirements (required for well house per 210.8), or local codes, consult a licensed electrician.

Use 4 AWG copper THHN/THWN in conduit. (6 AWG is not enough.)

The math (NEC 210.19(A)(1) FPN No. 4 — 3 % drop on branch circuit, 5 % total):

1. Allowable drop = 3 % × 120 V = 3.6 V
2. Formula (single-phase): CM = (2 × K × I × L) / Vd - K = 12.9 (copper, 75 °C) - I = 16 A (nameplate / measured load) - L = 250 ft - Vd = 3.6 V

CM = (2 × 12.9 × 16 × 250) / 3.6 ≈ 28,667 CM

1. Wire CM areas (Ch. 9, Table 8): - 6 AWG = 26,240 CM → too small - 4 AWG = 41,740 CM → passes

1. Check ampacity: 4 AWG THHN = 85 A (310.16) >> 20 A breaker — fine.

Installation notes

• Run in PVC conduit (Schedule 40/80) — direct-bury UF-B 4 AWG is not made.
• Pull a separate equipment-ground (8 AWG Cu minimum, 250.122).
• Land on a 20 A GFCI breaker (210.8(A)) or dead-front GFCI at the well house.
• Verify pump nameplate FLA; if it exceeds 16 A, re-calc.

Bottom line: 4 AWG Cu keeps drop ≈ 2.3 % (≈ 2.7 V), protecting the motor. 6 AWG drops ≈ 3.7 % and risks overheating.