Wire Size & Ampacity Chart (NEC 310.16)

NEC 310.16 · Ampacity · Updated 2026-08-07

"What size wire do I need?" is really three questions stacked on top of each other: what ampacity does NEC Table 310.16 assign the conductor, what breaker does 240.4(D) actually allow on it, and do field conditions like bundling and distance knock the rating down. This guide answers all three for copper, with the full 60°C, 75°C, and 90°C columns, breaker-to-wire quick answers, and worked examples you can check by hand.

The chart assumes copper conductors, no more than three current-carrying conductors in the raceway, and ordinary ambient temperature. Stuff more conductors in the pipe and derating applies; run far enough and voltage drop becomes the sizing driver. And on every job, the adopted code text and your local inspector have the final say, not any chart.

The copper wire size chart (NEC 310.16)

NEC Table 310.16 lists allowable ampacities for insulated conductors in raceway or cable, based on 30°C ambient and not more than three current-carrying conductors. The three columns correspond to insulation temperature ratings: 60°C, 75°C, and 90°C. THHN, the wire on nearly every commercial job, is a 90°C conductor in dry locations, but you almost never get to protect at the 90°C number directly. The last column shows the small-conductor breaker caps from NEC 240.4(D), which override the table for 14, 12, and 10 AWG.

Read the chart in two moves. First, find the column your terminations allow: if the breaker and lugs are rated 75°C, use the 75°C column; if anything in the circuit is rated only 60°C, the 60°C column governs. Second, for 14, 12, and 10 AWG, apply the 240.4(D) cap - no matter what the columns say, ordinary branch circuits on those sizes are limited to 15 A, 20 A, and 30 A breakers. The 90°C column exists mostly as the starting point for derating math, not as a number you protect at.

Which temperature column do you actually use?

Three rules sort out the columns in practice, and they settle most arguments at the panel.

• Terminations set the ceiling. Breakers and lugs carry their own temperature ratings, and the circuit's usable ampacity cannot exceed the column matching the coolest-rated termination. On most modern equipment that means the 75°C column; some older or small equipment is limited to 60°C.
• The 90°C column is a derating launchpad. For THHN you take adjustment factors off the 90°C number - the conduit derating guide has the full factor table - then cap the result at the termination column.
• Small-conductor caps win last. After every other calculation, 14 AWG still cannot be protected above 15 A, 12 AWG above 20 A, and 10 AWG above 30 A in ordinary circuits, per 240.4(D).

Wire size by breaker size: quick answers

These answers assume non-continuous loads, three or fewer current-carrying conductors, normal ambient, and terminations rated as noted - the 50 A, 60 A, and 100 A rows in particular depend on 75°C terminations. Continuous loads, bundles, hot attics, and long runs each add a step that can push you up a size. Notice that the ground column follows the breaker, not the wire; the ground wire size chart walks through that logic and its edge cases.

Worked example: a 50 A circuit in a crowded conduit

1. Start at the 90°C column: The circuit is 50 A, wired in THHN, sharing an EMT run with other circuits for a total of 8 current-carrying conductors. The natural pick is 8 AWG, which shows 55 A in the 90°C column.
2. Apply the adjustment factor: Per 310.15(C)(1), 7 to 9 current-carrying conductors means 70 percent. 55 A x 0.70 = 38.5 A. That is below 50 A, so 8 AWG fails in this conduit even though the chart alone says it passes.
3. Step up a size and rerun the math: 6 AWG shows 75 A in the 90°C column. 75 A x 0.70 = 52.5 A, which covers the 50 A circuit.
4. Check the termination ceiling: 6 AWG at the 75°C column is 65 A, so the termination allows more than the derated 52.5 A. The lower number wins, 52.5 A stands, and 6 AWG is the answer. Bigger wire also eats raceway space - check the conduit fill chart before committing to the same run.

Worked example: when a 20 A circuit needs 10 AWG

Bundling bites hardest on small conductors. Say ten 2-wire, 20 A circuits share one raceway: that is 20 current-carrying conductors, which lands in the 10-to-20 band at 50 percent. Start 12 AWG THHN at the 90°C column: 30 A x 0.50 = 15 A, which cannot support a 20 A breaker. Move to 10 AWG: 40 A x 0.50 = 20 A, which exactly meets the requirement, and the 30 A cap from 240.4(D) is not the limiting factor here. Ten circuits of 12 AWG that looked fine on the chart become ten circuits of 10 AWG the moment they share a pipe - a real cost on a long run, and one of the strongest arguments for splitting raceways.

The 100 amp and 200 amp questions

For a 100 A breaker or feeder with 75°C terminations, the chart answers directly: 2 AWG copper at 115 A covers it, and 1/0 copper at 150 A buys headroom for derating or long runs. Watch the 60°C trap - 2 AWG drops to 95 A in the 60°C column, so equipment with 60°C-only terminations needs a larger conductor. The full Table 310.16 also lists odd-numbered sizes between the ones charted here, so a code-book check can sometimes save you a size.

A 200 A feeder is where this chart runs out: 2/0 copper shows 175 A at 75°C, which does not reach 200 A, so the general table pushes you to a conductor larger than 2/0. Residential services are a special case - the NEC contains a separate allowance that lets dwelling service conductors be sized smaller than the general table requires, which is why 200 A house services are commonly wired with conductors that look undersized against 310.16. That allowance has its own conditions and does not apply to ordinary feeders, so confirm the actual code text with your inspector before copying a neighbor's service.

Ampacity is not the only check: voltage drop

A conductor can pass every ampacity test and still starve the load at the end of a long run. The NEC's informational recommendation is to keep branch-circuit voltage drop to about 3 percent. For single-phase circuits, estimate VD = (2 x K x I x L) / CM, where K is roughly 12.9 for copper, I is the load in amps, L is the one-way length in feet, and CM is the conductor's circular mils. Here is a 20 A load at 120 V run 100 feet one way:

The ampacity chart happily approves 12 AWG for this 20 A circuit, yet the drop does not get under the 3 percent recommendation until 8 AWG - two sizes larger. Distance, not heat, sizes long runs. The voltage drop guide works the formula in both directions, including solving for the maximum run length a given size supports.

Common mistakes when reading a wire size chart

• Protecting at the 90°C number. THHN's 90°C rating is real, but terminations govern; the 90°C column is for derating math, not breaker selection.
• Forgetting the small-conductor caps. 12 AWG shows 25 A and 30 A in the higher columns, and the breaker is still limited to 20 A by 240.4(D).
• Ignoring the conductor count. Four or more current-carrying conductors in one raceway triggers derating, and the chart's numbers no longer apply as printed.
• Skipping the voltage drop check. Ampacity is a heat limit; a 100-foot run can demand two sizes more copper than the chart shows, as the example above proves.
• Sizing the ground by habit. The equipment grounding conductor follows the overcurrent device under 250.122, and it has its own rules when circuit conductors get upsized.
• Reading copper numbers for aluminum wire. Every number on this page is copper; aluminum conductors of the same size carry less and use a different K value (about 21.2) for voltage drop.
• Forgetting conduit fill. Upsizing for derating or voltage drop grows the conductor area, and a raceway that was fine at 12 AWG can bust the 40 percent fill limit at 8 AWG.

Run the whole calculation in the field. Conduit Fill & Bending Calc by Panda Taps puts this chart and the math around it on your iPhone and iPad: ampacity with derating from the correct column, voltage drop, conduit fill, box fill, and bend marking. It works offline in a basement or a trench and never asks for an account, so the answer is there while the inspector is standing next to you. Download Conduit Fill & Bending Calc on the App Store.

Frequently asked questions

What size copper wire do I need for a 100 amp breaker?

In the general 310.16 table, 2 AWG copper is rated 115 A in the 75°C column, which covers a 100 A breaker when the terminations are rated 75°C. Equipment with 60°C-only terminations lowers 2 AWG to 95 A, which does not qualify. Confirm termination ratings and any local amendments before pulling wire.

Can 12 AWG copper wire carry 25 or 30 amps?

Table 310.16 lists 12 AWG copper at 25 A in the 75°C column and 30 A in the 90°C column, but NEC 240.4(D) caps its overcurrent protection at 20 A in ordinary branch circuits. The higher numbers are still used as starting points for derating math. In practice, a 12 AWG branch circuit gets a 20 A breaker.

Why does the chart show three ampacities for each wire size?

The columns correspond to the 60°C, 75°C, and 90°C insulation temperature ratings, and the column you may use is limited by the coolest-rated termination in the circuit. THHN is a 90°C conductor in dry locations, so its 90°C number is the starting point for derating, while most breaker and lug terminations cap the final answer at the 75°C column.

Does the wire size chart account for voltage drop?

No. Table 310.16 only addresses heat, and a conductor can pass ampacity while dropping too much voltage on a long run. The NEC's informational recommendation is to keep branch-circuit voltage drop near 3 percent, which often forces a larger size than the ampacity chart alone requires.

Keep reading

• Conductor Derating in Conduit (NEC 310.15)
• Voltage Drop: the Formula, the 3% Rule, and Worked Examples
• Ground Wire Size Chart: EGC (250.122) & GEC (250.66)