NEC Wire Ampacity Tables: What Every Electrician Needs to Know

What Ampacity Actually Means

Ampacity is the maximum current a conductor can carry continuously under specific conditions without exceeding its temperature rating. It's not a suggestion — it's a hard limit based on physics. Push more current through a wire than its ampacity allows and the insulation heats up. Keep pushing and it degrades, melts, and eventually starts a fire. NEC ampacity tables exist to prevent exactly that.

The key phrase is “under specific conditions.” Ampacity isn't a fixed number for a given wire size. It changes based on the insulation temperature rating, the ambient temperature around the conductor, and how many conductors are bundled together. Understanding these variables is what separates a competent electrician from someone who just looks up a number in a table.

NEC Table 310.16: The Table You Use Every Day

Table 310.16 (previously 310.15(B)(16) in older code cycles) is the workhorse. It covers allowable ampacities for insulated conductors rated 0-2000V, in raceway, cable, or direct-buried, based on an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.

The table has three main columns based on insulation temperature rating:

• 60°C — TW, UF. These are the lowest-rated insulations. You rarely install 60°C wire anymore, but this column matters because many older devices and breakers are only rated for 60°C terminations. For circuits 100A and under, NEC 110.14(C)(1) generally requires you to use the 60°C column unless the equipment is listed and marked for higher temperatures.
• 75°C — THW, THWN, XHHW. This is the column most commercial and industrial work uses. Equipment rated for 75°C terminations (which is standard for anything above 100A, and increasingly common for smaller gear) lets you take advantage of this higher ampacity.
• 90°C — THWN-2, THHN, XHHW-2. The highest rated column. You can install 90°C wire (and you usually do — THWN-2 is the default wire type in most supply houses), but you can't use the 90°C ampacity for sizing unless your terminations are also rated for 90°C. The 90°C column is primarily used as a starting point for derating calculations.

The Numbers That Matter for Residential Work

For residential circuits using 75°C-rated copper conductors (the most common scenario), here are the ampacities you should have memorized:

• 14 AWG — 15A (15A breaker, lighting and general purpose)
• 12 AWG — 20A (20A breaker, kitchen, bath, garage, outdoor receptacles)
• 10 AWG — 30A (dryer circuits, water heaters, small AC units)
• 8 AWG — 50A (ranges, large AC units)
• 6 AWG — 65A (sub-panels, large equipment)
• 4 AWG — 85A (sub-panel feeds)
• 3 AWG — 100A (100A sub-panel or service)
• 2 AWG — 115A
• 1 AWG — 130A
• 1/0 AWG — 150A (150A service)
• 2/0 AWG — 175A
• 3/0 AWG — 200A (200A service)
• 4/0 AWG — 230A

For aluminum, the same 75°C column gives you roughly two sizes larger for the same ampacity. A 3/0 aluminum conductor carries 155A, compared to 200A for 3/0 copper. That's why a 200A aluminum service entrance uses 4/0 aluminum (205A), while a 200A copper service entrance uses 3/0 (200A).

THWN-2 vs. THHN: What's the Difference?

Walk into any electrical supply house and ask for building wire — you'll get THWN-2. In practice, most modern building wire is dual-rated THHN/THWN-2, meaning it carries both designations on the jacket. Here's what the letters mean:

• T — Thermoplastic insulation
• H — Heat resistant (75°C)
• HH — Higher heat resistant (90°C)
• W — Wet locations approved
• N — Nylon jacket (abrasion resistance)
• -2 — 90°C wet rating

THHN is rated 90°C in dry locations only. THWN-2 is rated 90°C in both dry and wet locations. Since the dual-rated wire handles both, it's the industry default. You get the 90°C rating for derating purposes and wet location approval in one wire.

Derating for Bundled Conductors

When you run more than three current-carrying conductors in a single raceway or cable, heat can't dissipate as effectively. NEC 310.15(C)(1) requires you to reduce the ampacity based on the number of conductors:

• 4-6 conductors — 80% of table value
• 7-9 conductors — 70% of table value
• 10-20 conductors — 50% of table value
• 21-30 conductors — 45% of table value
• 31-40 conductors — 40% of table value
• 41+ conductors — 35% of table value

This is where the 90°C column becomes critical. Start with the 90°C ampacity, apply the derating factor, and as long as the result doesn't exceed the 75°C (or 60°C) value for the termination, you're code-compliant. For example, 12 AWG THWN-2 has a 90°C ampacity of 30A. With six current-carrying conductors, derate to 80%: 30 × 0.80 = 24A. That still exceeds the 20A breaker on a typical 12 AWG circuit, so no upsizing is needed. Without the 90°C starting point, you'd be at 20A × 0.80 = 16A, and you'd have to upsize to 10 AWG.

Derating for High Ambient Temperature

NEC Table 310.16 assumes a 30°C (86°F) ambient temperature. If the conductors will be in a hotter environment — like an attic in summer, a boiler room, or a rooftop conduit run in Arizona — you need to apply a temperature correction factor from NEC Table 310.15(B)(1).

At 40°C ambient (104°F, common in attics), the correction factor for 75°C wire is 0.88. For 90°C wire, it's 0.91. Again, starting from the 90°C column gives you more headroom. If you have both high ambient temperature and bundled conductors, you apply both corrections — they multiply. A 12 AWG THWN-2 in 40°C ambient with 6 conductors: 30A × 0.91 × 0.80 = 21.8A. Still good for a 20A circuit.

Common Residential Wire Sizes: Quick Reference

For the DIY homeowner or apprentice wanting a practical summary:

• 14 AWG on 15A breaker — General lighting, bedroom and living room receptacles
• 12 AWG on 20A breaker — Kitchen countertop, bathroom, laundry, garage, outdoor receptacles (NEC requires 20A for these locations)
• 10 AWG on 30A breaker — Electric dryer (10/3 NM-B), water heater, small A/C unit
• 8 AWG on 40-50A breaker — Electric range/oven (8/3 or 6/3 NM-B), large A/C
• 6 AWG on 60A breaker — Sub-panel feed to garage or addition, EV charger
• 4/0 copper or 250 kcmil aluminum — 200A service entrance

When to Upsize Beyond the Table

Passing the ampacity table is the minimum. There are several situations where you should go larger:

• Long runs. Voltage drop on long runs may require upsizing even when ampacity is fine. NEC 210.19 recommends no more than 3% voltage drop on branch circuits. Our Wire Size Calculator checks both ampacity and voltage drop simultaneously.
• Motor loads. NEC 430 has separate rules for motor circuits. Conductors must be sized at 125% of the motor's full-load current, not the nameplate.
• Continuous loads. Per NEC 210.20(A), conductors supplying continuous loads (operating 3+ hours) must be sized at 125% of the continuous load, plus 100% of any non-continuous load.
• Future expansion. Running 12 AWG instead of 14 AWG costs a few dollars more per run and gives you 20A capacity instead of 15A. On new construction, the wire cost difference is negligible compared to the cost of re-pulling later.

The NEC ampacity tables are your starting point, not your ending point. Know the numbers, understand the derating factors, and always check voltage drop on runs over 50 feet. Use our Wire Size Calculator to verify your conductor sizing meets both ampacity and voltage drop requirements for any installation.