Common copper sizes under the NEC: 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A, and 6 AWG for 50–65 A. Enter your breaker size and this wire size calculator returns the smallest conductor that meets NEC Table 310.16, the small-conductor rule in 240.4(D), the 125% continuous-load rule, and — if you give a length — a 3% voltage-drop limit.
This is a planning tool, not professional advice. Your local inspector enforces the code edition your jurisdiction has adopted, and there may be local amendments. Have a licensed electrician design and install anything you aren't qualified to do.
How to use this calculator
- Choose a branch circuit/feeder/subpanel or a whole-dwelling service.
- Enter the breaker or load current in amps.
- Pick copper or aluminum and the temperature rating you can use: 60 °C for NM-B or UF cable, 75 °C for THWN/XHHW with 75 °C terminals, 90 °C for THHN when you need to derate.
- Say whether the load is continuous — expected to run at full current for 3 hours or more.
- Add the one-way length, voltage and phase for a voltage-drop check, plus ambient temperature and the number of current-carrying wires if above 30 °C or more than three.
- Read the minimum wire size, its derated ampacity and the voltage drop.
How it's calculated
- Required ampacity. For a continuous load, the conductor must carry the noncontinuous load plus 125% of the continuous load (NEC 210.19(A) for branch circuits). A continuous load is one whose maximum current is expected to continue for 3 hours or more. For a qualifying dwelling service, 310.12(A) instead allows 83% of the service rating.
- Table ampacity. Each size's allowable amps come from NEC 2023 Table 310.16 for the chosen metal and temperature column.
- Derating. That value is multiplied by the ambient correction factor from Table 310.15(B)(1)(1) and the bundling adjustment from Table 310.15(C)(1).
- Small-conductor cap. 14, 12 and 10 AWG are then capped by 240.4(D).
- Voltage drop. Drop % =
k × L × I × R ÷ 1000 ÷ V × 100, where k is 2 for single-phase or √3 for three-phase, L is the one-way length in feet, I the current, R the resistance in ohms per 1,000 ft from Chapter 9 Table 8, and V the system voltage. - The result is the smallest size that passes every check.
Worked examples
30 A circuit, 120 V, 50 ft, NM-B copper
10 AWG meets the ampacity rule, but its voltage drop at 50 ft is 3.03% — just over 3%. The calculator steps up to 8 AWG. At 150 ft, 8 AWG drops 5.8%, so you would need 4 AWG (3.7% on 6 AWG, still too high).
32 A continuous load, 240 V, copper at 75 °C
32 × 1.25 = 40 A required. 10 AWG (35 A at 75 °C) is too small, so the answer is 8 AWG (50 A).
30 A, THHN in a hot conduit with 6 current-carrying wires
At 45 °C ambient, 10 AWG THHN starts at 40 A (90 °C column), × 0.87 for heat × 0.80 for six conductors = 27.84 A — below 30 A. The calculator picks 8 AWG.
200 A dwelling service, aluminum
200 × 0.83 = 166 A at 75 °C, which needs 4/0 AWG aluminum (180 A) or 2/0 AWG copper.
Common answers
| Breaker | Copper (NM-B, 60 °C) | Copper THHN (75 °C) | Aluminum (75 °C) |
|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 6 AWG | 8 AWG | 6 AWG |
| 60 A | 4 AWG | 6 AWG | 4 AWG |
| 70 A | 4 AWG | 4 AWG | 3 AWG |
| 80 A | 3 AWG | 4 AWG | 2 AWG |
| 100 A | 1 AWG | 3 AWG | 1 AWG |
| 125 A | 1/0 AWG | 1 AWG | 2/0 AWG |
| 150 A | 3/0 AWG | 1/0 AWG | 3/0 AWG |
| 200 A | 250 kcmil | 3/0 AWG | 250 kcmil |
Wire ampacity chart (NEC 2023 Table 310.16)
Allowable amps for up to three current-carrying conductors in a raceway, cable or earth at 30 °C (86 °F) ambient.
| Size | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 60 °C | Al 75 °C | Al 90 °C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | — | — | — |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 AWG | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
Table values for 14, 12 and 10 AWG are higher than the breaker you may actually use: 240.4(D) caps copper at 15 A, 20 A and 30 A respectively, and aluminum at 15 A (12 AWG) and 25 A (10 AWG).
Which temperature column should I use?
- 60 °C — NM-B (Romex) and UF cable, and any circuit 100 A or less unless terminals are marked 75 °C.
- 75 °C — most breakers and lugs today; THWN, XHHW, USE conductors.
- 90 °C — THHN insulation; the 90 °C value is only for derating, the final size must still satisfy the terminal rating.
Wire size for service and subpanels
For a 120/240 V service or feeder rated 100–400 A that carries the entire load of a one-family dwelling (or of one dwelling unit in a two-family or multifamily building), NEC 310.12(A) lets you size conductors at 83% of the rating. It applies only when no ambient or bundling correction is needed:
| Service | Copper | Aluminum |
|---|---|---|
| 100 A | 4 AWG | 2 AWG |
| 125 A | 2 AWG | 1/0 AWG |
| 150 A | 1 AWG | 2/0 AWG |
| 175 A | 1/0 AWG | 3/0 AWG |
| 200 A | 2/0 AWG | 4/0 AWG |
| 225 A | 3/0 AWG | 250 kcmil |
| 250 A | 4/0 AWG | 300 kcmil |
| 300 A | 250 kcmil | 350 kcmil |
| 350 A | 350 kcmil | 500 kcmil |
| 400 A | 400 kcmil | — |
A subpanel feeder does not qualify for the 83% rule unless it carries the entire dwelling load — use the full breaker rating.
Derating for heat and bundling
Table ampacities assume 30 °C (86 °F) and no more than three current-carrying conductors together. Hot attics, rooftops and full conduits reduce what a wire can safely carry:
| Current-carrying conductors | Adjustment |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Derating starts from the conductor's insulation rating (e.g. 90 °C for THHN), but the result can never exceed the terminal rating of the breaker or lug.
Voltage drop
An Informational Note to NEC 210.19 (advisory, not enforceable) suggests no more than 3% drop on a branch circuit and 5% total for feeder plus branch circuit. Single-phase drop is 2 × L × I × R / 1000, where L is one-way length in feet, I the load in amps and R the DC resistance per 1000 ft from NEC Chapter 9 Table 8 (copper or aluminum, at 75 °C).
Common mistakes
- Using the 90 °C column for the final size. THHN is rated 90 °C, but breakers and lugs usually are not. 110.14(C) limits circuits of 100 A or less (or 14–1 AWG) to the 60 °C column unless the equipment is listed and identified for 75 °C.
- Putting 14 AWG on a 20 A breaker. 240.4(D) caps 14 AWG copper at 15 A no matter what the table says.
- Forgetting the 125% rule. A load that runs 3 hours or more needs 25% more ampacity than its running current.
- Using the 83% service rule on a subpanel. It applies only to conductors that carry the entire load of a dwelling unit.
- Sizing aluminum like copper. Aluminum carries less current per size, so it usually needs one or two sizes larger.
Frequently asked questions
What size wire for a 30 amp circuit?
10 AWG copper (or 8 AWG aluminum) for a standard 30 A breaker. Go up a size for long runs where voltage drop exceeds 3%.
What size wire for 40 amps?
8 AWG copper at 60 °C (NM-B cable), or 8 AWG THHN copper using the 75 °C column. Aluminum needs 6 AWG at 60 °C, or 8 AWG where the breaker and lugs are rated 75 °C.
What size wire for 50 amps?
6 AWG copper with NM-B cable, or 8 AWG copper THHN/THWN in conduit with 75 °C terminals. Aluminum: 4 AWG at 60 °C, or 6 AWG with 75 °C terminals.
How many amps can 12 gauge wire carry?
12 AWG copper is limited to a 20 A breaker by NEC 240.4(D), even though Table 310.16 lists 20 A (60 °C), 25 A (75 °C) and 30 A (90 °C).
What size wire for 100 amp service?
For a 120/240 V dwelling service, NEC 310.12 allows 4 AWG copper or 2 AWG aluminum (83 A at the 75 °C column). For a 100 A subpanel feeder, use Table 310.16: 3 AWG copper or 1 AWG aluminum at 75 °C.
What size wire for 125 amps?
For a 125 A feeder or subpanel at 75 °C: 1 AWG copper or 2/0 AWG aluminum. If it is the service for the whole dwelling, the 83% rule in 310.12 allows 2 AWG copper or 1/0 AWG aluminum.
What size wire for 150 amps?
For a 150 A feeder at 75 °C: 1/0 AWG copper or 3/0 AWG aluminum. For a 150 A dwelling service under 310.12: 1 AWG copper or 2/0 AWG aluminum.
How many amps can 8 gauge wire carry?
8 AWG copper is rated 40 A at 60 °C, 50 A at 75 °C and 55 A at 90 °C in NEC Table 310.16. Most 8 AWG circuits are protected at 40 A (NM-B) or 50 A (THHN/THWN with 75 °C terminals).
What is the ampacity of 6 AWG wire?
6 AWG copper: 55 A (60 °C), 65 A (75 °C), 75 A (90 °C). 6 AWG aluminum: 40 / 50 / 55 A at 60/75/90 °C.
How many amps can 14 gauge wire carry?
14 AWG copper is limited to a 15 A breaker by NEC 240.4(D), even though Table 310.16 lists 15, 20 and 25 A in the 60, 75 and 90 °C columns.
What gauge wire for 20 amps?
12 AWG copper. 14 AWG is capped at 15 A by 240.4(D), so it can never be used on a 20 A breaker. Aluminum needs 10 AWG for 20 A.
What is the ampacity of 18 AWG wire?
18 AWG is not in Table 310.16 and is not used for household branch circuits. Where the NEC allows it, 240.4(D) limits 18 AWG copper to 7 A (continuous load no more than 5.6 A) and 16 AWG copper to 10 A (continuous load no more than 8 A), and only with breakers or fuses listed and marked for that wire size, or Class CC, CF, J or T fuses.
Sources & method
- NFPA 70 (2023) — free read-only access
- NFPA — NEC 2023 cycle CMP-6 Second Draft Report (Table 310.16, SR-8432)
- NFPA — NEC 2023 cycle CMP-10 Second Draft Report (240.4(D), SR-8285)
- NFPA — NEC 2023 cycle CMP-2 Second Draft Report (210.19 voltage-drop note, SR-8042)
- NFPA — NEC 2023 cycle CMP-1 Second Draft Report (110.14(C), SR-7793)
- HELUKABEL — NEC 2023 Tables 310.16, 310.15(B)(1)(1), 310.15(C)(1) reprint
- NFPA 70 (2017) Chapter 9 Table 8 — conductor DC resistance
Results are estimates for general information. Found an error? It helps everyone — see our methodology.