Reference · NEC 2023 Ch. 9 Table 8 · Copper 75°C
How far can wire run before voltage drop fails
Maximum one-way feet at 3% drop (copper, K-factor method — the same math as our calculator). Distances are panel-to-load, one way. Aluminum runs ≈40% shorter per size; motors and steel conduit need the Advanced Table 9 mode.
Branch circuits — max one-way distance at 3%
| Wire (Cu) | 20A · 120V | 30A · 240V | 50A · 240V |
|---|---|---|---|
| 12 AWG | ~46 ft | ~61 ft | ~36 ft |
| 10 AWG | ~72 ft | ~97 ft | ~58 ft |
| 8 AWG | ~115 ft | ~154 ft | ~92 ft |
| 6 AWG | ~183 ft | ~244 ft | ~146 ft |
| 4 AWG | ~291 ft | ~388 ft | ~233 ft |
| 2 AWG | ~463 ft | ~617 ft | ~370 ft |
Feeders & EV chargers — max one-way distance at 3%
| Wire (Cu) | 100A · 240V | 48A EV · 240V |
|---|---|---|
| 3 AWG | ~147 ft | ~306 ft |
| 2 AWG | ~185 ft | ~386 ft |
| 1 AWG | ~234 ft | ~487 ft |
| 1/0 | ~295 ft | ~614 ft |
| 2/0 | ~371 ft | ~773 ft |
Runs that fail (and the fix)
Kitchen 20A · 120V · 100ft · #12
VD = 2×12.9×20×100÷6530 = 7.90V (6.58%) — fails. #10 = 4.14%, still over. #8 = 2.60% ✓. Answer: #8 minimum.
EV 48A · 240V · 150ft · #6
VD = 2×12.9×48×150÷26240 = 7.08V (2.95%) ✓ — just inside. At 175ft it fails; #4 carries to ~243ft.
12V solar 10A · 30ft · #10
VD = 2×12.9×10×30÷10380 = 0.75V — only 0.75V, but 6.2% of 12V. #6 = 2.5% ✓. On 12V every volt is 8.3%.
100A subpanel · 240V · 200ft · #3
3% limit hits at ~147ft on #3. #2 reaches ~185ft — still short. #1/0 reaches ~295ft ✓. Feeders this long want aluminum pricing too.
Rules this chart assumes
One-way feet, always
Panel to load. The ×2 (DC/1φ) and √3 (3φ) multipliers are baked in — entering round-trip length halves your safety margin on paper only.
Basic K-factor, 75°C copper
K = 12.9 Cu / 21.2 Al. Motors (PF 0.8–0.9), steel conduit, or feeders above #4: switch the calculator to Advanced Table 9 — drop reads 10–30% higher.
Ampacity still applies
A wire inside its drop distance can still fail derating in a hot attic or packed conduit. Size for both; the larger wire wins.
3% branch, 5% total
NEC recommends ≤3% per run and ≤5% feeder + branch combined. A 2.9% branch on a 2.5% feeder already violates the combined limit.
Distance questions, answered
How far does #12 go at 20A?
≈46ft one-way at 120V/3%. #10 ≈72ft, #8 ≈115ft. At 240V the same wire goes roughly twice as far in percent terms.
20A at 100ft — what size?
#8 copper minimum for 3% (#12 = 6.6%, #10 = 4.1%, #8 = 2.6%).
50A EV charger distance?
#6 copper holds 3% to ~146ft (48A to ~153ft). Past that: #4, a subpanel closer in, or 5%-total engineering.
Why is 12V so short-range?
1V lost = 8.3% of 12V vs 0.8% of 120V. 10A on #10 only reaches ~14ft at 3% — go #6, shorten, or step up voltage.
One-way or round-trip?
One-way. Round-trip input is the #1 voltage drop mistake and exactly halves the real answer.
Is 3% mandatory?
No — an NEC recommendation most inspectors enforce as practice. Size critical loads (motors, EV, medical) to ≤2–3% regardless.
Check my exact run →Worked examples →K vs Table 9 formula →
Related: Wire size chart ·Ampacity vs voltage drop ·All electrical calculators