Wire Gauge Calculator for Amps

To find wire gauge from amps, pick the smallest AWG whose ampacity at your insulation temperature meets the current. At the 75°C column, copper carries 20 A on 12 AWG, 35 A on 10 AWG, 50 A on 8 AWG, 65 A on 6 AWG, and 85 A on 4 AWG. So a 40-amp load lands on 8 AWG copper (50 A), leaving 10 A of headroom.

This wire ampacity calculator stores all three NEC temperature columns for copper and aluminum, so you can compare how much a hotter-rated insulation or a different metal changes the answer. Remember: ampacity is the raw limit — for continuous loads, apply the 125% rule (use the wire size calculator for that), and always size final terminations at 75°C.

Ampacity is the maximum current a conductor can carry continuously without its temperature exceeding the insulation's rating. NEC Table 310.16 lists three columns — 60°C, 75°C, and 90°C — because different insulations tolerate different heat.

• 60°C (TW, UF) — older or moisture-rated cable; the most conservative ampacity.
• 75°C (THW, THWN, XHHW, RHW) — the workhorse column and the rating of most modern breakers and lugs.
• 90°C (THHN, THWN-2, XHHW-2) — highest ampacity, but usable only for derating math unless every termination is rated 90°C (rare).

The higher the temperature rating, the more current the same copper can carry before its insulation degrades. But there is a catch: NEC 110.14(C) requires you to honor the lowest-rated component in the circuit. Since breakers and equipment terminals are almost always 75°C, the 90°C column is used to offset derating, not to load the wire harder at the terminals.

Copper outperforms aluminum at every gauge because it has lower resistivity. That is why a copper-to-aluminum swap always steps up one or two sizes for the same current — visible directly in this calculator when you toggle the material.

Aluminum conductors carry roughly 78% of the current of the same-gauge copper because of higher resistivity. The calculator's aluminum table reflects NEC 310.16, and the pattern is easy to remember: for a given ampacity, aluminum is usually two AWG sizes larger than copper.

• Copper 6 AWG (65 A) ↔ Aluminum 4 AWG (65 A)
• Copper 2 AWG (115 A) ↔ Aluminum 1/0 AWG (120 A)
• Copper 2/0 (175 A) ↔ Aluminum 4/0 (180 A)

Aluminum dominates on large feeders and service entrances (200 A residential services commonly use 4/0 aluminum or 250 kcmil) because it is far cheaper and lighter than copper. The trade-offs: aluminum expands and contracts more, so terminations must be torqued to spec and treated with antioxidant compound, and connectors must be rated AL-CU or CO/ALR. Modern AA-8000 series aluminum building wire has largely resolved the reliability problems of 1970s aluminum branch wiring.

Because aluminum has higher resistance, it also drops more voltage per foot — about 1.6× copper. On long aluminum runs, check the voltage drop calculator before committing to a gauge.

Example 1 — 40 A, copper, 75°C: smallest gauge with ≥ 40 A is 8 AWG (50 A). Headroom = 10 A. If this is a continuous 40 A load, apply the 125% rule (50 A required) — 8 AWG still works because its ampacity is exactly 50 A.

Example 2 — 90 A, copper, 75°C: 2 AWG carries 115 A and 3 AWG carries 100 A, so 3 AWG meets 90 A with 10 A headroom. Many installers jump to 2 AWG for margin and easier terminations.

Example 3 — 90 A, aluminum, 75°C: aluminum 2 AWG carries 90 A exactly (zero headroom), so 1 AWG (100 A) is the safer pick. This shows why aluminum steps up versus copper for the same current.

Example 4 — 55 A, copper, 90°C: at the 90°C column, 6 AWG carries 75 A, easily covering 55 A. But if terminations are 75°C, you must re-check the 75°C column (6 AWG = 65 A), which still passes. The 90°C rating only buys extra room for derating.

The table values assume 30°C ambient and no more than three current-carrying conductors bundled together. Exceed either and you must reduce ampacity:

• Ambient over 30°C — multiply by a correction factor (e.g., ~0.88 at 40°C, ~0.75 at 50°C for 75°C wire). Rooftops in sun can add 33°C to ambient (NEC 310.15(B)).
• 4–6 conductors in a raceway — multiply by 0.80.
• 7–9 conductors — multiply by 0.70; 10–20 conductors — 0.50.

This is exactly where the 90°C column earns its keep: you may compute derating from the higher 90°C ampacity and, as long as the final derated value does not exceed the 75°C termination ampacity, use it. For instance, 8 AWG THHN starts at 55 A (90°C); derated 0.80 for bundling gives 44 A, which still supports a 40 A circuit — whereas the 75°C 50 A value derated 0.80 gives only 40 A, right at the edge. The headroom this calculator reports is your first clue to whether a circuit survives derating.

Derating factors stack multiplicatively. A conduit on a sunny rooftop might combine a 0.75 ambient correction with a 0.70 bundling factor for eight conductors, leaving only 0.525 of the base ampacity. An 8 AWG THHN rated 55 A at 90°C would deliver just 29 A after both corrections — no longer adequate for even a 30 A circuit. This is why a wire that looks generous on the bare chart can fail in a hot, crowded raceway, and why rooftop solar conduit is a classic derating trap. Enter your load here to read the gauge and headroom, then apply your ambient and bundling factors before committing.

A worked case ties the temperature columns and material choice together. Imagine a 60-amp subpanel in a detached garage, fed from the house through 90 feet of buried conduit, and the panel expects 75°C terminations.

Start with ampacity. A 60-amp load reads directly off the 75°C column: copper 4 AWG carries 85 A and 6 AWG carries 65 A, so 6 AWG copper covers 60 A with headroom to spare. In aluminum, 4 AWG carries 65 A, so aluminum steps up to 4 AWG for the same duty. So far the bare chart says 6 AWG copper or 4 AWG aluminum.

Now the run length changes the story. At 90 feet one-way, voltage drop on 6 AWG copper at 60 A is meaningful, and buried conduit in warm soil invites a modest ambient consideration. Bumping to 4 AWG copper drops the loss well under the 3% target and gives the panel room to grow. The chart answers "will it overheat," this scenario shows why the final pick often lands one size larger once distance and future load enter. Confirm the drop with the voltage drop calculator and the breaker with the breaker size calculator.

1. Enter load current in amps. Use the device nameplate or breaker rating. Convert from watts if needed (watts ÷ volts = amps).
2. Choose insulation temperature. 75°C fits most modern THWN/XHHW installations; pick 90°C only if every termination is 90°C rated, or for derating headroom.
3. Select copper or aluminum. Watch the gauge jump when you switch to aluminum — it confirms the one-to-two-size step-up.

The result shows the AWG, that gauge's ampacity, and the headroom in amps. If headroom is small (under ~10%), consider the next larger size, especially where derating applies. For continuous loads, cross-check with the wire size calculator and confirm the breaker with the breaker size calculator.

• Using the 90°C column at 75°C terminals. The most common error; it can leave a wire under-protected relative to code.
• Ignoring the 125% continuous rule. Raw ampacity is not enough for loads that run 3+ hours.
• Reading the copper column for aluminum wire. Aluminum ampacities are 15–25% lower per gauge.
• Skipping derating. Heat and bundling routinely cut usable ampacity by 20–30%.
• Overlooking voltage drop. A gauge that passes ampacity may still drop too much voltage on a long run.

Ampacity answers "will it overheat?" — it does not answer "will voltage arrive intact?" Pair this tool with the voltage drop calculator for the full picture.

Panel and feeder work. Sizing 100 A and 200 A feeders in copper or aluminum is the classic use — a 200 A service is 4/0 aluminum or 2/0 copper at 75°C.

Appliance circuits. Ranges (40–50 A), EV chargers (40–60 A), water heaters (30 A), and HVAC (varies) each map to a specific gauge you can confirm here.

Solar and battery. DC ampacity uses the same table, but low system voltage makes voltage drop the usual constraint — use the 12V wire size calculator for DC runs.

Renovations with aluminum. When extending existing aluminum feeders, matching gauge and using proper AL-CU connectors is critical; this tool confirms the correct aluminum size for the load.

Individual THWN/XHHW conductors in conduit use the 75°C column, but the most common residential wiring method — NM-B cable (Romex) — is governed by a special rule. NEC 334.80 requires NM-B ampacity to be taken from the 60°C column of Table 310.16, even though the conductors themselves have 90°C insulation. This is because the cable jacket limits heat dissipation.

The practical result changes some familiar answers:

• 14 AWG NM-B — 15 A (60°C), the basis for 15 A lighting circuits.
• 12 AWG NM-B — 20 A (60°C), for 20 A receptacle circuits.
• 10 AWG NM-B — 30 A (60°C), for dryers and water heaters.
• 6 AWG NM-B — 55 A (60°C), still adequate for a 50 A range circuit.

Switch this calculator to the 60°C column to match NM-B installations, or leave it at 75°C for conduit-and-wire runs. The 90°C rating on the conductor insulation is not wasted — it still provides the derating headroom described above, so bundled or hot-location NM-B keeps more of its usable ampacity than 60°C-only insulation would. When in doubt about which cable method applies, size conservatively and confirm with your inspector; the difference between 60°C and 75°C sizing is one full gauge on many circuits.

Reading a spec sheet is straightforward once you know these columns: manufacturers print the insulation type (THHN, THWN-2, XHHW-2) and the corresponding ampacities, and you match the column to your termination rating and cable method. This is the same reference the wire size calculator and breaker size calculator draw on.