Conductor Sizing Made Simple: NEC Table 310.16 Explained

NEC Table 310.16 is the most referenced table on the Texas journeyman electrician exam. It is the go-to source for conductor ampacities, and you will use it for nearly every conductor sizing question on the Calculations portion. If you master this one table and the rules that go with it, you are covering roughly 30% of the Calculations section.

This guide breaks down how the table works, when to use each temperature column, and how to apply correction and adjustment factors correctly.

What Table 310.16 Tells You

Table 310.16 lists the allowable ampacities of insulated conductors rated up to and including 2000 volts. It covers conductors installed in raceways, cables, or directly buried in earth, based on an ambient temperature of 30C (86F). The table applies to installations with no more than three current-carrying conductors in a raceway or cable.

The table is organized into three main column groups based on conductor insulation temperature rating:

| Temperature Rating | Common Insulation Types | Column in Table | |---|---|---| | 60C (140F) | TW, UF | Left column | | 75C (167F) | THW, THWN, XHHW, USE | Middle column | | 90C (194F) | THWN-2, THHN, XHHW-2, USE-2 | Right column |

Each row lists a conductor size (from 14 AWG to 2000 kcmil) with the corresponding ampacity for copper and aluminum/copper-clad aluminum conductors.

The Three-Column Question: Which Column Do You Use?

This is where most exam mistakes happen. You might think you should always use the column that matches your conductor's insulation rating, but NEC 110.14(C) adds a critical rule about terminal temperature ratings.

The Terminal Temperature Rule (110.14(C))

The temperature rating of a conductor is only part of the equation. The terminals where the conductor connects (breakers, switches, equipment lugs) also have a temperature rating, and you must respect the lower of the two.

For circuits rated 100A or less, or for conductors #14 AWG through #1 AWG:

Use the 60C column for ampacity selection, unless the equipment is specifically marked for 75C terminations
Most residential panels, breakers, and devices are rated for 60C or 75C terminations
If the equipment is listed and marked for 75C, you can use the 75C column

For circuits rated over 100A, or for conductors larger than #1 AWG:

Equipment terminations are generally rated 75C
Use the 75C column for final conductor selection

When to Use the 90C Column

The 90C column is used only for applying temperature correction and adjustment factors. Here is the process:

Start with the 90C column ampacity (highest starting value)
Apply temperature correction factors
Apply adjustment factors for bundled conductors
Compare the result to the ampacity in the 60C or 75C column (based on terminal rating)
The conductor must satisfy both the derated 90C value AND the terminal temperature column value

This matters because using the 90C column as your starting point gives you more headroom when derating. The final selected conductor must still meet the requirements of the terminal temperature column.

Practical Example

You need to size a THHN copper conductor (rated 90C) for a 40A non-continuous load on a circuit with 75C-rated equipment terminations.

Using the 75C column: #8 AWG copper has an ampacity of 50A. This works.

Using the 90C column: #8 AWG copper has an ampacity of 55A. This also works, but you would use the 75C column value (50A) for comparison to the load because the terminals are rated 75C.

Both approaches select #8 AWG, but the distinction becomes critical when derating is involved.

Temperature Correction Factors

When the ambient temperature where the conductor is installed differs from the standard 30C (86F), you must apply temperature correction factors. These are found in Table 310.15(B)(1) (located directly below Table 310.16 in most NEC editions).

How Temperature Correction Works

If the ambient temperature is higher than 30C, the conductor can carry less current. If the ambient temperature is lower than 30C, it can carry more.

| Ambient Temp (C) | 60C Factor | 75C Factor | 90C Factor | |---|---|---|---| | 21-25 | 1.08 | 1.04 | 1.04 | | 26-30 | 1.00 | 1.00 | 1.00 | | 31-35 | 0.91 | 0.94 | 0.96 | | 36-40 | 0.82 | 0.88 | 0.91 | | 41-45 | 0.71 | 0.82 | 0.87 | | 46-50 | 0.58 | 0.75 | 0.82 | | 51-55 | 0.41 | 0.67 | 0.76 |

Example: A #10 AWG THWN-2 copper conductor (90C insulation) is installed where the ambient temperature is 45C. What is the corrected ampacity?

Base ampacity from 90C column: 40A Temperature correction factor for 90C at 45C: 0.87 Corrected ampacity: 40 x 0.87 = 34.8A

Adjustment Factors for Conduit Fill

When more than three current-carrying conductors are installed in a single raceway or cable, heat builds up and the ampacity must be reduced. These adjustment factors come from Table 310.15(C)(1).

| Number of Current-Carrying Conductors | Adjustment Factor | |---|---| | 4-6 | 0.80 | | 7-9 | 0.70 | | 10-20 | 0.50 | | 21-30 | 0.45 | | 31-40 | 0.40 | | 41+ | 0.35 |

What counts as a current-carrying conductor? Ungrounded (hot) conductors and grounded (neutral) conductors that carry current count. Equipment grounding conductors do not count. Neutral conductors that carry only unbalanced current on a single-phase circuit do not count as current-carrying in many cases, but neutrals on 4-wire three-phase wye systems that carry harmonic currents do count.

Combining Temperature Correction and Adjustment Factors

When both conditions apply (high ambient temperature AND more than three conductors in a raceway), you apply both factors to the base ampacity.

Example: Six #10 AWG THHN copper conductors in a conduit installed in a 40C ambient environment. What is the adjusted ampacity?

Base ampacity (90C column): 40A Temperature correction for 90C at 40C: 0.91 Adjustment for 6 current-carrying conductors: 0.80 Adjusted ampacity: 40 x 0.91 x 0.80 = 29.12A

Now check against the terminal temperature column. If the terminals are rated 75C, #10 AWG has an ampacity of 35A in the 75C column. Since 29.12A is less than 35A, the limiting factor is the derated ampacity of 29.12A. This conductor can safely handle loads up to about 29A.

Continuous Loads: The 125% Rule

For continuous loads (loads expected to operate for 3 hours or more), NEC 210.20(A) requires that the overcurrent device rating be not less than 125% of the continuous load. In practice, this means you need a conductor and overcurrent device rated for at least 125% of the continuous load current.

Example: A continuous load draws 32A. What is the minimum conductor size using THWN copper with 75C terminations?

Required ampacity: 32 x 1.25 = 40A Table 310.16, 75C column: #8 AWG copper = 50A (exceeds 40A) Minimum conductor size: #8 AWG

Putting It All Together: The Complete Conductor Sizing Process

Here is the step-by-step process for solving conductor sizing problems on the exam:

Step 1: Determine the load current. For continuous loads, multiply by 1.25.

Step 2: Identify the conductor insulation type and its temperature rating.

Step 3: Identify the terminal temperature rating of the equipment (60C or 75C).

Step 4: Check for derating conditions:

Is the ambient temperature above 30C? If yes, find the correction factor.
Are there more than 3 current-carrying conductors in the raceway? If yes, find the adjustment factor.

Step 5: If derating applies, divide the required ampacity by the combined correction and adjustment factors. This gives you the minimum base ampacity needed from Table 310.16.

Step 6: Select the conductor size from Table 310.16 using the appropriate column:

Use the 90C column to find a conductor whose derated ampacity meets the load
Verify the conductor also meets the terminal temperature column requirements

Step 7: Select the overcurrent device from the standard sizes in 240.6(A). The OCPD must not exceed the conductor ampacity (with the "next size up" exception from 240.4(B) for circuits 800A or less).

Common Exam Pitfalls

Using the wrong temperature column. Always check the terminal rating. Using the 90C column for final selection when the terminals are rated 60C or 75C will give you an undersized conductor.

Forgetting to derate for both conditions. If a problem mentions high ambient temperature and multiple conductors in a raceway, you need both factors.

Confusing current-carrying conductors with total conductors. Equipment grounding conductors and neutral conductors that carry only unbalanced current do not count for adjustment factor purposes.

Not rounding correctly. When dividing by correction and adjustment factors, the resulting ampacity must be met or exceeded by the table value. Do not round down.

Applying the continuous load factor at the wrong step. The 125% factor applies to the load current before you look up the conductor size. It is separate from temperature correction and adjustment factors.

Practice conductor sizing calculations with our free calculator

Sources:

National Fire Protection Association - NFPA 70: National Electrical Code, 2023 Edition
Texas Department of Licensing and Regulation (TDLR) - Electrician Exam Information
TDLR - Electrician Exam Statistics, Fiscal Year 2025
International Association of Electrical Inspectors (IAEI) - Key Updates in the 2026 NEC That Electrical Professionals Need to Know
Electrical Contractor Magazine - Building the Foundation: Key Changes to Articles 90-130 of the 2026 NEC