What Is NEC Table 310.16?
NEC Table 310.16 (titled “Ampacities of Insulated Conductors”in the 2023 edition, and formerly designated Table 310.15(B)(16) prior to the 2020 code cycle) lists the maximum current a conductor can carry continuously under standard conditions. It applies to conductors rated 0 through 2000 volts, installed in raceways, cables, or directly buried, with not more than three current-carrying conductors and an ambient temperature of 30 °C (86 °F).
You will find this table in Article 310 — Conductors for General Wiringof any NEC edition from 2020 onward. If you are working with an older code book, look for Table 310.15(B)(16)—the data is identical; only the table number changed.
This single table governs the vast majority of residential and commercial wire sizing decisions. Understanding it thoroughly will save you time on every job, reduce material costs, and keep your installations code-compliant. For a quick digital lookup, check out our NEC code reference tool.
Standard Conditions
Table 310.16 assumes an ambient temperature of 30 °C and no more than three current-carrying conductors in a raceway or cable. If your installation differs from either of these conditions, you must apply derating factors—covered later in this guide.
How to Read the Wire Size Chart
The table is organized into rows (wire sizes) and columns (temperature ratings). Here is how each part works:
Wire Gauge Column
The far-left column lists conductor sizes from 14 AWG (the smallest permitted for branch circuits) up through 2000 kcmil. Sizes follow the American Wire Gauge (AWG) system for smaller conductors, then switch to kcmil (thousands of circular mils) at sizes larger than 4/0 AWG.
Temperature Rating Columns
The table provides three ampacity columns based on insulation temperature rating:
- 60 °C column — Insulation types TW and UF. This is the most conservative rating and applies to many older installations and some equipment terminations.
- 75 °C column — Insulation types THW, THWN, XHHW, USE, and ZW. Most modern breakers and equipment have 75 °C-rated terminals, making this the column you will use most often.
- 90 °C column — Insulation types THHN, THWN-2, XHHW-2, and USE-2. While the wire itself can handle more current, your termination rating usually limits you to the 75 °C value. The 90 °C ampacity is primarily used as a starting point for derating calculations.
Choosing the Right Column
The column you use is determined by the lowest temperature rating in the circuit. For example, if you install THHN wire (rated 90 °C) but terminate it on a breaker rated 75 °C, you must use the 75 °C ampacity. Per NEC 110.14(C), conductor ampacity must be based on the temperature rating of the connected termination. For conductors 14 AWG through 1 AWG, most residential equipment terminals are rated 60 °C or 75 °C. Conductors 1/0 AWG and larger are generally permitted to use the 75 °C column. Check the NEC code reference for the full text of 110.14(C).
Copper Wire Ampacity Chart
The following table lists ampacity values for copper conductorsfrom NEC Table 310.16. A “Typical Breaker” column is included as a quick reference, though actual breaker sizing must follow NEC 240.4 and the specific overcurrent protection rules for your circuit.
| Wire Size | 60 °C (TW, UF) | 75 °C (THW, THWN) | 90 °C (THHN, XHHW-2) | Typical Breaker |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A |
| 12 AWG | 20A | 25A | 30A | 20A |
| 10 AWG | 30A | 35A | 40A | 30A |
| 8 AWG | 40A | 50A | 55A | 40A |
| 6 AWG | 55A | 65A | 75A | 55A |
| 4 AWG | 70A | 85A | 95A | 70A |
| 3 AWG | 85A | 100A | 115A | 85A |
| 2 AWG | 95A | 115A | 130A | 100A |
| 1 AWG | 110A | 130A | 145A | 110A |
| 1/0 AWG | 125A | 150A | 170A | 125A |
| 2/0 AWG | 145A | 175A | 195A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 175A |
| 4/0 AWG | 195A | 230A | 260A | 200A |
| 250 kcmil | 215A | 255A | 290A | 225A |
| 300 kcmil | 240A | 285A | 320A | 250A |
| 350 kcmil | 260A | 310A | 350A | 300A |
| 400 kcmil | 280A | 335A | 380A | 300A |
| 500 kcmil | 320A | 380A | 430A | 350A |
Important: NEC 240.4(D) limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A overcurrent protection regardless of the ampacity column used. Always verify breaker sizing rules before finalizing your design.
Aluminum Wire Ampacity Chart
Aluminum and copper-clad aluminum conductors have lower ampacity than copper at the same gauge. The following values also come from NEC Table 310.16. Note that the NEC does not list 14 AWG aluminum for general wiring; the smallest aluminum conductor in the table is 12 AWG.
| Wire Size | 60 °C (TW, UF) | 75 °C (THW, THWN) | 90 °C (THHN, XHHW-2) |
|---|---|---|---|
| 12 AWG | 15A | 20A | 25A |
| 10 AWG | 25A | 30A | 35A |
| 8 AWG | 35A | 40A | 45A |
| 6 AWG | 40A | 50A | 60A |
| 4 AWG | 55A | 65A | 75A |
| 3 AWG | 65A | 75A | 85A |
| 2 AWG | 75A | 90A | 100A |
| 1 AWG | 85A | 100A | 115A |
| 1/0 AWG | 100A | 120A | 135A |
| 2/0 AWG | 115A | 135A | 150A |
| 3/0 AWG | 130A | 155A | 175A |
| 4/0 AWG | 150A | 180A | 205A |
| 250 kcmil | 170A | 205A | 230A |
| 300 kcmil | 190A | 230A | 260A |
| 350 kcmil | 210A | 250A | 280A |
| 400 kcmil | 225A | 270A | 305A |
| 500 kcmil | 260A | 310A | 350A |
Aluminum Installation Notes
Aluminum conductors require anti-oxidant compound at terminations and connectors rated for AL/CU use. Aluminum expands and contracts more than copper under thermal cycling, so all connections must be torqued to manufacturer specifications. Many jurisdictions prohibit aluminum for 15A and 20A branch circuits due to historical connection failures.
Temperature Derating Factors
Table 310.16 is based on a 30 °C (86 °F) ambient temperature. When conductors operate in hotter environments—attics, rooftops, mechanical rooms, or solar conduit runs—you must multiply the base ampacity by a correction factor from NEC Table 310.15(B)(1). The higher the ambient temperature, the more you must reduce the allowable current.
| Ambient Temp | 60 °C Factor | 75 °C Factor | 90 °C Factor |
|---|---|---|---|
| 26-30 °C (79-86 °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 |
| 56-60 °C (132-140 °F) | --- | 0.58 | 0.71 |
Worked Example: Temperature Derating
Suppose you need to run 6 AWG THHN copper through a conduit on a rooftop where ambient temperature reaches 50 °C. Start with the 90 °C ampacity of 75A, then apply the correction factor:
Base ampacity (6 AWG, 90 °C): 75A
Correction factor at 50 °C for 90 °C column: 0.82
Derated ampacity: 75A × 0.82 = 61.5A
Since the terminals are 75 °C rated, verify 61.5A does not exceed the 75 °C value (65A). It does not, so 6 AWG is acceptable.
Use our wire ampacity chart calculator to apply temperature correction factors automatically.
Conduit Fill Derating
When more than three current-carrying conductors share a raceway or cable, heat dissipation is reduced. NEC Table 310.15(C)(1) requires you to reduce the ampacity by an adjustment factor based on the number of conductors. Equipment grounding conductors and neutrals that carry only unbalanced current are not counted.
| Number of Current-Carrying Conductors | Adjustment Factor |
|---|---|
| 1-3 | 100% |
| 4-6 | 80% |
| 7-9 | 70% |
| 10-20 | 50% |
| 21-30 | 45% |
| 31-40 | 40% |
| 41+ | 35% |
Worked Example: Conduit Fill Derating
You are running five current-carrying 10 AWG THHN copper conductors in a single conduit at normal ambient temperature (30 °C).
Base ampacity (10 AWG, 90 °C): 40A
Adjustment factor for 5 conductors: 0.80
Derated ampacity: 40A × 0.80 = 32A
Since the 75 °C column value for 10 AWG is 35A and 32A is below that, 10 AWG is acceptable.
When both temperature and conduit fill derating apply, multiply both factors together. Use our conduit fill calculator to verify your raceway sizing meets NEC Chapter 9 requirements as well.
Practical Wire Sizing Examples
Example 1: Residential 20A Kitchen Circuit
You are installing a 20-amp small-appliance branch circuit for kitchen countertop receptacles. The run is 45 feet from the panel, installed in NM-B cable through an interior wall at normal ambient temperature.
- Load: 20A breaker, potentially continuous (countertop appliances can run for hours).
- Wire selection: NM-B cable uses 60 °C column per NEC 334.80. At 60 °C, 12 AWG carries 20A—exactly matching the 20A breaker limit in NEC 240.4(D).
- Derating: Interior wall, normal temperature, only 2 current-carrying conductors in the cable. No derating required.
- Voltage drop check: At 45 ft on a 120V circuit, 12 AWG yields about 2.6% voltage drop at 20A—within the NEC-recommended 3%.
- Result: 12 AWG NM-B cable is the correct choice.
Verify with our wire sizing calculator.
Example 2: 30A Dryer Circuit at 240V with Long Run
A 30A, 240V clothes dryer is located 85 feet from the panel. The circuit runs through conduit in a conditioned basement.
- Load: 30A at 240V (the dryer nameplate confirms 5,400W / 240V = 22.5A, but the NEC standard circuit for dryers is 30A).
- Wire selection: At 75 °C, 10 AWG copper carries 35A, which exceeds the 30A breaker requirement.
- Derating: Basement is conditioned (under 30 °C), three current-carrying conductors (two hots and one neutral). No derating needed.
- Voltage drop check: At 85 ft on a 240V circuit, 10 AWG at 30A gives approximately 3.9% drop—exceeds the 3% recommendation. Upsize to 8 AWG, which brings voltage drop to about 2.4%.
- Result: Use 8 AWG copper (THHN or NM-B depending on wiring method) to keep voltage drop within limits.
Check the exact numbers with our voltage drop calculator.
Example 3: Commercial Panel Feeder with Conduit Fill Derating
A 200A feeder runs 120 feet from the main switchboard to a sub-panel in a commercial office building. The conduit also contains two additional 20A branch circuits (four extra current-carrying conductors) for a total of seven current-carrying conductors. Ambient temperature in the ceiling plenum is 40 °C.
- Starting point: 200A load, copper THHN conductors.
- Temperature derating (40 °C, 90 °C column): Factor = 0.91.
- Conduit fill derating (7 conductors): Factor = 0.70.
- Combined derating: 0.91 × 0.70 = 0.637.
- Required base ampacity: 200A ÷ 0.637 = 313.8A.
- Wire selection: From the 90 °C column, 350 kcmil provides 350A, which exceeds 313.8A. Also verify it does not exceed the 75 °C column value when checking terminal limits: 350 kcmil at 75 °C = 310A. Since 200A is well below 310A, the terminals are fine.
- Voltage drop: At 120 feet and 200A, check drop. 350 kcmil at 200A over 120 ft on a 480V 3-phase system yields roughly 1.1% drop—well within limits.
- Result: 350 kcmil THHN copper conductors.
The 125% Continuous Load Rule
NEC 210.20(A) and 215.3 establish one of the most critical—and most frequently misunderstood—requirements in conductor sizing: when a load operates for three hours or more continuously, the branch circuit and feeder must be sized at 125% of the continuous load. This means both the wire and the breaker must be rated to handle 125% of the continuous portion of the load plus 100% of any non-continuous portion. Continuous loads include commercial lighting, electric vehicle chargers, data center equipment, walk-in coolers, and any other load that runs for three or more hours without interruption.
Required Ampacity = (Continuous Load × 1.25) + Non-Continuous Load
Worked Example: Continuous Load Sizing
A commercial lighting circuit draws 16A continuously. The circuit also serves two receptacles that occasionally draw 4A total (non-continuous).
Required ampacity = (16A × 1.25) + 4A = 24A
At 75 °C, 12 AWG carries 25A — this passes, but barely.
A 10 AWG (35A at 75 °C) gives significantly more margin.
Breaker selection:The breaker must be rated for at least 24A. A 25A or 30A breaker would work mathematically. However, per NEC 240.4(D), 12 AWG wire is limited to 20A overcurrent protection. A 20A breaker only allows 20A ÷ 1.25 = 16A of continuous load, which leaves no room for the 4A non-continuous portion on top of 16A continuous.
Result:You must use 10 AWG wire with a 30A breaker to properly serve this circuit. The 10 AWG wire at 75 °C carries 35A (well above 24A), and the 30A breaker accommodates the full calculated load.
The 125% rule catches many electricians off guard because a circuit can pass ampacity calculations but still violate the continuous load requirement. Always ask: will this load run for 3+ hours? If yes, apply the 125% factor BEFORE selecting wire and breaker.
Use our breaker size calculator to automatically apply the 125% continuous load factor and find the correct wire and breaker pairing.
Common Wire Types Comparison
Choosing the right wire insulation type is just as important as choosing the right gauge. The insulation determines the temperature column you reference in Table 310.16 and dictates where the wire can be installed. The following table compares the most common wire insulation types you will encounter in residential and commercial work.
| Type | Temp Rating | Wet/Dry | Common Uses | Notes |
|---|---|---|---|---|
| TW | 60 °C | Dry/Wet | Older installations | Rarely specified today |
| THW | 75 °C | Dry/Wet | General wiring, feeders | Thicker insulation than THHN |
| THHN | 90 °C | Dry only | Most common building wire | Thin nylon jacket, used in conduit |
| THWN | 75 °C | Wet locations | Underground in conduit | Often dual-rated as THWN-2 |
| THWN-2 | 90 °C dry / 75 °C wet | Dry and wet | General purpose | Same wire as THHN/THWN dual-rated |
| XHHW | 90 °C dry / 75 °C wet | Dry and wet | Feeders, industrial | Cross-linked polyethylene |
| XHHW-2 | 90 °C | Dry and wet | Feeders, industrial | Wet rating maintained at 90 °C |
| USE-2 | 90 °C | Underground | Service entrance, direct burial | Not for interior use |
| NM-B | 90 °C wire / 60 °C system | Dry only | Residential (Romex) | Per NEC 334.80, use 60 °C column |
Most modern building wire is dual-rated THHN/THWN-2. This means a single wire roll handles both dry and wet locations at the appropriate temperature rating. When you buy “12 AWG THHN” at the supply house, check the jacket—it almost certainly says “THHN/THWN-2” on it.
Termination Rating Governs Ampacity
Remember NEC 110.14(C): The wire’s insulation rating doesn’t determine which ampacity column you use—the termination’s temperature rating does. Even 90 °C THHN wire connected to a 75 °C-rated breaker must use the 75 °C ampacity column. The 90 °C rating’s primary benefit is providing a higher starting ampacity for derating calculations.
For a complete interactive ampacity lookup by wire type and temperature rating, see our wire ampacity chart.
Common Wire Sizing Mistakes
Using the Wrong Temperature Column
This is the most common error. An electrician installs THHN wire (90 °C insulation) and reads the 90 °C ampacity column, but the breaker terminals are only rated 75 °C. Per NEC 110.14(C), you must use the lower temperature rating of the termination. Using the 90 °C value without derating as justification can result in an overloaded circuit and a failed inspection.
Ignoring Derating Factors
Skipping temperature or conduit fill derating is a code violation and a safety hazard. Conductors in a hot attic or a crowded conduit can overheat well before reaching their base-table ampacity. Always check ambient conditions and conductor count before finalizing your wire size.
Not Checking Voltage Drop
A conductor can be perfectly sized for ampacity yet cause problems if the run is long. NEC 210.19(A) Informational Note recommends no more than 3% voltage drop on branch circuits and 5% total (feeder + branch). Motors, sensitive electronics, and LED drivers are particularly affected by low voltage. Always run a voltage drop calculation for runs longer than 50 feet.
Oversizing Wire Unnecessarily
While undersizing is dangerous, excessive oversizing wastes money and makes installation harder. Larger wire costs more per foot, requires bigger conduit, and is harder to pull and terminate. On a large commercial project, one unnecessary wire size upgrade across dozens of circuits can add thousands of dollars. Size accurately using the tables and derating factors rather than guessing “one size up to be safe.”
Conclusion
NEC Table 310.16 is the foundation of conductor sizing for every residential and commercial electrical installation. By understanding how to read the three temperature columns, applying the correct derating factors for ambient temperature and conduit fill, and verifying voltage drop on longer runs, you can confidently select the right wire size for any circuit.
Remember the key principles: always use the temperature rating of the lowest-rated component in the circuit, apply both derating factors when they overlap, and check voltage drop on any run over 50 feet. These habits will keep your installations safe, code-compliant, and cost-effective.
Put this knowledge to work with our free calculators: wire sizing, wire ampacity chart, voltage drop, conduit fill, and breaker size calculator.