NEC Table 310.16 is the most-cited table in the electrical trade and the most frequently misused. The table number is only the starting point: continuous loads demand a 125% multiplier, hot attics and rooftops demand ambient-temperature correction, bundled conductors demand adjustment factors, and the termination temperature rating decides which column you're even allowed to read. This calculator applies all of it in the correct order and returns the smallest copper or aluminum conductor that survives your actual conditions — with the table built in so you can audit every answer.
For the printable chart version, see our NEC 310.16 Wire Ampacity Chart; for the full design workflow including voltage drop, use this tool together with the Voltage Drop Calculator.
Wire Ampacity Calculator (NEC 310.16)
125% continuous rule · ambient temperature correction · bundling adjustment · Cu & Al, 60/75/90 °C columns
Method: design current = load × 1.25 if continuous. Find the smallest conductor where table ampacity × ambient correction × bundling adjustment ≥ design current. Then verify the run passes voltage drop with the voltage drop calculator and pair with the right breaker.
Reading NEC 310.16 Correctly: The Four-Step Derate
The table assumes specific conditions: 30 °C ambient, no more than three current-carrying conductors in a raceway, and a stated insulation temperature rating. Real installations routinely violate all three assumptions, so the working method is:
Step 1 — design current. If the load runs three hours or more, NEC 210.19(A)(1) requires sizing at 125%: a 100 A continuous load is designed as 125 A. Step 2 — ambient correction. Table 310.15(B)(1) multipliers: a 40 °C attic cuts a 75 °C conductor to 0.88 of table value. Step 3 — bundling adjustment. Table 310.15(C)(1): seven to nine current-carrying conductors in one conduit drop ampacity to 0.70. Step 4 — termination rating. NEC 110.14(C) limits you to the column matching the weakest termination — almost always 75 °C for equipment under 100 A frames, even when the wire insulation is 90 °C THHN-2. The 90 °C column is legitimately used only as the starting point for derating, with the final result still capped by the 75 °C termination value.
Worked Example 1: 60 A Non-Continuous Feeder, Benign Conditions
A 60 A workshop subpanel feeder, non-continuous loads, copper, 75 °C terminations, ≤3 conductors, 30 °C ambient: no derating applies. Required table ampacity = 60 A. Reading the 75 °C copper column: 8 AWG (50 A) fails, 6 AWG (65 A) passes. Pair it with a 60 A breaker from our circuit breaker inventory and confirm the raceway fits with the conduit fill chart.
Worked Example 2: 100 A Continuous Load (The 125% Rule Bites)
A 100 A EV charger or commercial load running 3+ hours, copper, 75 °C, normal conditions: design current = 100 × 1.25 = 125 A. The 75 °C column gives 2 AWG (115 A) failing and 1 AWG (130 A) passing. Miss the 125% multiplier and you'd have "saved" money on 2 AWG that runs 15 A past its rating every day. The EV charger breaker chart walks this exact scenario for common EVSE sizes.
Worked Example 3: 200 A Continuous, Aluminum, 40 °C Attic
Design current = 250 A. Ambient correction at 40 °C for the 75 °C column = 0.88, so the required table ampacity = 250 ÷ 0.88 = 284.1 A. Reading the aluminum 75 °C column: 400 kcmil (270 A) fails, 500 kcmil (310 A) passes — corrected ampacity 310 × 0.88 = 272.8 A ≥ 250 A. At 30 °C the same load passes on 350 kcmil aluminum (250 A). One hot attic cost two conductor sizes; this is why routing matters as much as sizing.
NEC 310.16 Master Table (Built Into the Calculator)
Copper and aluminum ampacities in a raceway, cable, or earth, 30 °C ambient, ≤3 current-carrying conductors:
| 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 | 30 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 60 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 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 | 190 | 230 | 255 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
| 600 kcmil | 350 | 420 | 475 | 285 | 340 | 385 |
| 750 kcmil | 400 | 475 | 535 | 320 | 385 | 435 |
| 1000 kcmil | 445 | 545 | 615 | 375 | 445 | 500 |
Ambient Temperature Correction Factors (310.15(B)(1))
| Ambient | 60 °C column | 75 °C column | 90 °C column |
|---|---|---|---|
| ≤30 °C / 86 °F | 1.00 | 1.00 | 1.00 |
| 31–35 °C / 87–95 °F | 0.91 | 0.94 | 0.96 |
| 36–40 °C / 96–104 °F | 0.82 | 0.88 | 0.91 |
| 41–45 °C / 105–113 °F | 0.71 | 0.82 | 0.87 |
| 46–50 °C / 114–122 °F | 0.58 | 0.75 | 0.82 |
| 51–55 °C / 123–131 °F | 0.41 | 0.67 | 0.76 |
Common Circuit Quick Reference (Copper, 75 °C, Normal Conditions)
The everyday answers, before any corrections or adjustments:
| Breaker | Continuous load limit | Min. copper (75 °C) | Typical use |
|---|---|---|---|
| 15 A | 12 A | 14 AWG | Lighting, general receptacles |
| 20 A | 16 A | 12 AWG | Kitchen, bath, garage circuits |
| 30 A | 24 A | 10 AWG | Dryers, small EVSE, RV outlets |
| 40 A | 32 A | 8 AWG | Ranges, 32 A EV chargers |
| 50 A | 40 A | 8 AWG* | Ranges, 40 A EVSE, subpanels |
| 60 A | 48 A | 6 AWG | Workshop subpanels, 48 A EVSE |
| 100 A | 80 A | 3 AWG (2 AWG typical) | Subpanels, small services |
*50 A on 8 AWG copper assumes 75 °C-rated terminations; on 60 °C terminations (older equipment, some NM cable applications) 8 AWG is limited to 40 A and the circuit needs 6 AWG. This is the termination-rating rule in miniature — the column you may read is set by the equipment, not the wire jacket.
Bundling Adjustment: More Than 3 Conductors in One Raceway (310.15(C)(1))
| Current-carrying conductors | Adjustment factor |
|---|---|
| 1–3 | 1.00 |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
Neutrals carrying only unbalanced current don't count; neutrals in 3-phase wye systems feeding nonlinear loads (LED drivers, computers) do count — a trap that has cooked many office-building feeders.
Common Ampacity Mistakes
- Reading the 90 °C column because the wire says THHN. NEC 110.14(C) caps you at the termination rating — 75 °C for most lugs under 100 A frames. The 90 °C column is a derating starting point, not a final answer.
- Skipping the 125% continuous multiplier. EV chargers, commercial lighting, and any 3+ hour load require it. This alone is the most common residential undersizing error.
- Forgetting ambient correction in attics and on rooftops. A 40 °C attic strips 12% from a 75 °C conductor; a dark rooftop in direct sun can be worse. Route around heat or pay for it in conductor size.
- Ignoring bundling in big conduits. Ten circuits in one raceway halves every conductor's ampacity. Splitting into two raceways is often cheaper than doubling conductor size.
- Violating 240.4(D). Regardless of table ampacity, overcurrent protection is capped at 15 A / 20 A / 30 A for 14 / 12 / 10 AWG copper. Yes, even when 90 °C column math says 12 AWG "carries 30 A."
- Treating ampacity as the only check. A conductor that passes 310.16 can still fail voltage drop on long runs and still be wrong for the environment — see the PV wire vs. USE-2 vs. THHN guide for insulation selection, and our PV wire buyer's guide for solar-specific ratings.
Copper or Aluminum: The Practical Cutover Point
The table makes the trade concrete: a 100 A continuous load needs 1 AWG copper or 3/0 aluminum — the aluminum is two sizes larger, roughly twice the cross-section, and still typically 40% cheaper by the foot. Below about 60 A, copper's smaller diameter, easier terminations, and universal device compatibility outweigh the savings. At 100 A and above — service entrance, subpanel feeders, large PV interconnections — aluminum with AL-rated lugs and antioxidant compound is the industry default. Whatever the metal, the insulation system must match the environment: THHN/THWN-2 in dry conduit, XHHW-2 for wet and direct-burial raceways, and sunlight-resistant PV wire for exposed array wiring, as covered in the insulation selection guide.
When the Answer Is "Parallel Conductors"
Past roughly 400 A of design current, a single conductor becomes impractical — 600+ kcmil pulls are brutal, expensive, and sometimes unavailable. NEC 310.10(G) permits conductors in parallel at 1/0 AWG and larger, provided each parallel conductor is identical in length, material, cross-section, insulation, and termination. Two 4/0 copper conductors per phase share the load and are far easier to pull than one 1000 kcmil. The calculator's "Beyond 1000 kcmil" result is your cue to split the run in two and halve the per-conductor design current.
From Ampacity to a Complete Circuit
Ampacity is step one of four: correct conductor size, then voltage drop for run length, then the overcurrent device per the 125% rule and 240.6(A) standard sizes, then conduit fill for the raceway. Stock the conductors from our THHN building wire and Cerro Wire collections, or hit "Get this quoted" above and your calculated size, material, and conditions travel straight into the RFQ. For solar array conductors specifically, PV wire inventory carries the sunlight-resistant, 90 °C wet-rated stock.
Frequently Asked Questions
What size wire do I need for a 60-amp circuit?
For a non-continuous 60 A load with copper conductors and 75 °C terminations in normal conditions: 6 AWG copper (65 A). If the load is continuous, design current becomes 75 A and you need 4 AWG copper (85 A). Aluminum requires 4 AWG or 2 AWG respectively.
What size wire do I need for 100 amps?
Non-continuous, copper, 75 °C: 3 AWG (100 A) technically, but most installers step to 2 AWG. Continuous loads need 125%: 1 AWG copper or 3/0 aluminum at normal ambient. Hot attics and bundled conductors push it larger — run your exact conditions in the calculator.
Can I use the 90 °C column because my wire is THHN-2?
Not for the final answer. NEC 110.14(C) limits you to the termination temperature rating — typically 75 °C for equipment rated 100 A and below. You may start derating from the 90 °C value, but the corrected result may not exceed the 75 °C termination ampacity.
How much does a hot attic reduce wire ampacity?
At 40 °C (104 °F) ambient, a 75 °C-rated conductor keeps only 88% of its table ampacity; at 50 °C, 75%. A 6 AWG copper that carries 65 A in your basement carries 57 A in a 40 °C attic and 49 A in a 50 °C one.
Do I count the neutral when bundling conductors?
A neutral carrying only unbalanced current does not count as current-carrying. But a neutral in a 3-phase wye system with nonlinear loads (LED lighting, computers, VFDs) carries harmonic current and must be counted — a commonly missed adjustment.
When is aluminum acceptable instead of copper?
Aluminum is standard practice for feeders and service entrance at roughly 100 A and up, typically saving 30–50% on conductor cost at two sizes larger than copper. Use AL-rated lugs, antioxidant compound, and proper torque. Below 50 A branch circuits, copper's termination simplicity usually wins.
