Reference · NEC 2023 Table 310.16 · 75°C terminals
Wire size chart — what size for your amps
Minimum copper and aluminum at base conditions (≤3 conductors, 30°C ambient) with 240.4(D) breaker caps already applied. Hot attic, bundled conduit, or a long run? Read the derating rows below, then verify with the calculator.
| Load | Copper min | Aluminum min | Breaker | Typical use | Headroom pick |
|---|---|---|---|---|---|
| 15A lighting | #14 Cu (20A) | #12 Al (20A) | 15A | Lighting branch circuits | #12 Cu |
| 20A receptacle | #12 Cu (25A) | #10 Al (30A) | 20A | General receptacles, kitchen small-appliance | #10 Cu |
| 30A dryer / WH | #10 Cu (35A) | #8 Al (40A) | 30A | Dryer, water heater | #8 Cu |
| 40A range | #8 Cu (50A) | #8 Al (40A) | 40A | Range, large appliance | #6 Cu |
| 50A EV / RV | #6 Cu (65A) | #6 Al (50A) | 50A | EV charger, RV outlet (check 125% if continuous) | #6 Cu · #4 Al |
| 60A subpanel | #6 Cu (65A) | #4 Al (65A) | 60A | Workshop / garage subpanel | #4 Cu |
| 100A feeder | #3 Cu (100A) | #1 Al (100A) | 100A | Subpanel, small service | #2 Cu · #1/0 Al |
| 150A feeder | #1/0 Cu (150A) | #3/0 Al (155A) | 150A | Large subpanel / service | #2/0 Cu |
| 200A service | #3/0 Cu (200A) | #250 Al (205A) | 200A | Residential service entrance | #4/0 Cu · #300 Al |
When the chart minimum isn't enough
Hot attic: #10 → ~29A
#10 THHN at 104°F with 6 conductors: 40 × 0.91 × 0.80 = 29.1A — a 30A load needs #8. Attics hit 122°F where the factor drops to 0.75.
Continuous loads ×125%
EV chargers and heaters running 3+ hours: 50A × 1.25 = 62.5A → #6 Cu minimum. The breaker is sized to the continuous load, not the other way around.
Bundled conduit derates fast
4–6 conductors ×0.80 · 7–9 ×0.70 · 10–20 ×0.50. Four circuits in one pipe turn #12's 30A (90°C start) into 24A before the terminal cap.
Long runs fail drop first
20A at 120V on #12 passes ampacity but exceeds 3% past ~60ft one-way. Ampacity and drop are independent — the larger wire wins.
Wire size questions, answered
20A circuit — what size?
#12 copper (25A, 20A breaker). #14 is capped at 15A per 240.4(D) and cannot serve 20A.
30A dryer — what size?
#10 copper (35A, 30A breaker). Aluminum needs #8.
100A subpanel — what size?
#3 Cu / #1 Al minimum; installers usually pull #2 Cu / #1/0 Al for headroom.
200A service — what size?
#3/0 Cu / 250 kcmil Al minimum; #4/0 Cu / 300 Al is the common default.
EV charger on 50A?
Size at 125%: 62.5A → #6 Cu minimum, then check drop for the run length.
Copper vs aluminum?
Al needs 1–2 sizes larger for equal ampacity. Use AL-rated lugs and antioxidant; avoid Al for 15–20A branches.
Size my exact load →Worked examples →Table 310.16 →
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