The wire ampacity chart dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. For standard residential and light commercial copper wiring (14 AWG through 2 AWG), you will almost always use the 60°C column for circuits rated 100 amps or less, regardless of the fact that modern THHN wire boasts a 90°C insulation rating. Sizing wire based on the 90°C column for a standard 20A or 50A breaker is one of the most common—and dangerous—mistakes in DIY electrical work.
The Standard Wire Ampacity Chart (NEC Table 310.16)
The definitive source for conductor sizing in the United States is NFPA 70, the National Electrical Code (NEC), specifically Table 310.16 (formerly 310.15(B)(16)). The table below extracts the most frequently used copper conductor sizes for building wiring.
How to read this table: These values assume copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a single raceway or cable. The columns represent the temperature rating of the insulation and the termination points. TW and UF-B cables are rated for 60°C; THW and THWN are rated for 75°C; THHN and THWN-2 are rated for 90°C.
| AWG / kcmil | 60°C (140°F) TW, UF-B |
75°C (167°F) THW, THWN |
90°C (194°F) THHN, THWN-2 |
|---|---|---|---|
| 14 | 15 A | 20 A | 25 A |
| 12 | 20 A | 25 A | 30 A |
| 10 | 30 A | 35 A | 40 A |
| 8 | 40 A | 50 A | 55 A |
| 6 | 55 A | 65 A | 75 A |
| 4 | 70 A | 85 A | 95 A |
| 3 | 85 A | 100 A | 115 A |
| 2 | 95 A | 115 A | 130 A |
| 1 | 110 A | 130 A | 145 A |
| 1/0 | 125 A | 150 A | 170 A |
- 15A Circuit (Lighting/Receptacles): 14 AWG (60°C column). Note: Many pros standardize on 12 AWG to prevent voltage drop and allow future upgrades.
- 20A Circuit (Kitchen/Bath/Appliance): 12 AWG (60°C column).
- 30A Circuit (Dryer/RV Plug): 10 AWG (60°C column).
- 50A Circuit (Range/Hot Tub/Subpanel): 6 AWG (60°C column). See the 50A trap explained below.
Which Temperature Column Actually Applies to Your Breaker?
Looking at the chart above, you might assume that because you bought 12 AWG THHN wire (rated 90°C), you can run 30 amps through it on a 30A breaker. You cannot. The governing rule is NEC 110.14(C), which dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component—including the wire, the breaker lugs, and the device terminals.
Here is the hard rule for selecting your column:
- Circuits 100A or less: You must use the 60°C column. Standard residential breakers and receptacles are tested and listed at 60°C, even if the breaker casing is stamped with a 75°C rating.
- Circuits over 100A: You may use the 75°C column, provided the lugs and breakers are explicitly rated for 75°C (which most commercial and heavy residential panels are).
A very common DIY failure is wiring a 50A hot tub or range circuit with 8 AWG THHN. The 75°C column lists 8 AWG at 50A. However, because the circuit is under 100A, NEC 110.14(C) forces you to the 60°C column, where 8 AWG is only rated for 40A. To legally and safely wire a 50A breaker, you must use 6 AWG copper (rated 55A in the 60°C column). Always size the wire to the 60°C column, then verify the breaker size does not exceed that value.
How Derating Rows Modify the Base Value
The wire ampacity chart assumes you have a maximum of three current-carrying conductors (CCCs) in a conduit. If you pull multiple circuits through a single raceway, the wires heat each other up. Under NEC 310.15(C)(1), you must apply a derating factor.
Here is where the 90°C column finally becomes useful. You apply the derating percentage to the 90°C ampacity, not the 60°C ampacity.
Worked Example: You are pulling two 20A multi-wire branch circuits (4 hot wires, 1 shared neutral = 4 CCCs) through a single EMT conduit using 12 AWG THHN.
- Look up 12 AWG in the 90°C column: 30A.
- Find the derating factor for 4-6 conductors in NEC Table 310.15(C)(1): 80%.
- Multiply: 30A × 0.80 = 24A derated ampacity.
- Compare to the 60°C column (20A). Since 24A is greater than 20A, the 12 AWG wire is still legally permitted to be protected by a 20A breaker.
If you had pulled 7 to 9 conductors (derated to 70%), the math becomes 30A × 0.70 = 21A. Still safe for a 20A breaker, but you are running out of thermal headroom. For 10+ conductors (50% derating), 12 AWG drops to 15A, forcing you to upsize to 10 AWG wire or split the runs into separate conduits.
What the Ampacity Chart Cannot Tell You
The wire ampacity chart is strictly a thermal limit for the insulation. It does not account for the physics of electron flow over distance, nor does it guarantee the wire will physically fit in your termination lugs. Relying solely on this chart without considering the following factors will result in failing an inspection or experiencing equipment malfunction.
1. Voltage Drop (The Hidden Sizing Factor)
The NEC addresses voltage drop in Informational Note 310.15(B), recommending a maximum 3% drop on branch circuits and 5% total drop from the service entrance to the furthest outlet. The ampacity chart ignores distance entirely.
Worked Example: You are wiring a 120V, 20A branch circuit to a detached workshop 110 feet away. The ampacity chart says 12 AWG is perfectly fine for 20A. However, using the standard voltage drop formula VD = (2 × L × R × I) / 1000 (where L=110ft, R=1.98 ohms/kft for 12 AWG uncoated copper, I=16A continuous load):
- VD = (2 × 110 × 1.98 × 16) / 1000 = 6.96 Volts.
- Percentage Drop = (6.96 / 120) × 100 = 5.8%.
A 5.8% drop exceeds the 3% recommendation and will cause motors to run hot and lights to dim. To fix this, you must upsize to 10 AWG (or even 8 AWG), even though the ampacity chart says 12 AWG is thermally safe. For long runs, always calculate voltage drop before pulling wire. Consult resources like the OSHA electrical safety guidelines for proper grounding and installation practices when running feeders to outbuildings.
2. Physical Termination Limits
Ampacity charts deal in cross-sectional area, not physical dimensions. A 2 AWG copper wire might be thermally rated for 95A, but it may not physically bend into the tight radius of a standard 100A subpanel main breaker lug, or it might not fit under the screw terminal of a 60A GFCI breaker. Always check the manufacturer’s datasheet for the specific breaker or lug to verify the maximum AWG it can mechanically accept. If a wire is too large, you must use a listed mechanical lug reducer or pigtailing method, not simply strip back extra strands of copper to force a fit.
3. Local AHJ Overrides and Ambient Temperature
The baseline chart assumes an ambient temperature of 30°C (86°F). If you are routing NM-B cable through an attic in the Southwest US where ambient temperatures routinely hit 110°F (43°C), you must apply the ambient temperature correction factors found in the bottom of NEC Table 310.16. Furthermore, your local Authority Having Jurisdiction (AHJ) or city inspector always has the final say. Some municipalities mandate 12 AWG as the absolute minimum for all 15A and 20A receptacle circuits to mitigate voltage drop and reduce fire risk, entirely overriding the 14 AWG allowance in the national code. Always verify local amendments before buying wire in bulk.
