Voltage Drop Reference

Wire voltage drop — copper AWG resistance per 1 000 ft, with formula and NEC rules of thumb.

Reference Reference Updated Apr 19, 2026
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Reference

Formula

V_drop (single-phase)
= 2 · I · R_per_length · length
V_drop (three-phase)
= √3 · I · R_per_length · length
NEC recommendation
≤ 3% on feeders, ≤ 3% on branch (5% combined)

Copper resistance

AWG Ω / 1 000 ft Ω / km
4/0 0.049 0.161
2/0 0.078 0.256
1/0 0.098 0.323
2 0.156 0.513
4 0.248 0.815
6 0.395 1.296
8 0.628 2.061
10 0.999 3.277
12 1.588 5.211
14 2.525 8.286
16 4.016 13.17
18 6.385 20.95
20 10.15 33.31
22 16.14 52.96

Quick-pick examples

Load Length (one-way) Min AWG V_drop @ 120 V
15 A @ 120 V 50 ft 14 AWG < 3%
15 A @ 120 V 100 ft 12 AWG < 3%
20 A @ 120 V 100 ft 10 AWG < 3%
30 A @ 240 V 100 ft 10 AWG < 3%
50 A @ 240 V 150 ft 6 AWG < 3%

Notes

  • NEC voltage-drop figures are recommendations (not code requirements in most jurisdictions) but widely followed.
  • Aluminum wire has ~60% the conductivity of copper — size up 2 AWG larger for the same drop.

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