NEC Article 310 and Table 310.16 — Conductor Sizing for the Electrician Exam

NEC Article 310 governs conductors for general wiring — every wire you pull through conduit, run in cable, or bury in earth falls under its rules. The centerpiece of Article 310 is Table 310.16, the allowable ampacity table that electricians reference hundreds of times per week on the job and that exam writers test relentlessly.

If you can read Table 310.16, apply temperature correction factors, count current-carrying conductors, and work through a multi-factor derating problem, you will answer conductor sizing questions correctly on your licensing exam. This guide covers all of it.

What Article 310 Covers

Article 310 establishes the basic requirements for conductors used in general wiring: their allowable ampacities, insulation temperature ratings, permitted uses (dry, damp, or wet locations), and the adjustment and correction factors that reduce ampacity in challenging installation conditions.

Article 310 does not stand alone. It works in concert with Article 240 (overcurrent protection), Article 300 (wiring methods), and the equipment articles that specify minimum conductor sizes for specific loads. Conductor sizing for motors follows Article 430, which uses its own tables (430.248 and 430.250) instead of 310.16.

Understanding Table 310.16

Table 310.16 is officially titled "Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C, Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth."

That title tells you the two major conditions under which the table values are valid:

• The ambient temperature is 30°C (86°F)
• There are three or fewer current-carrying conductors in the raceway, cable, or direct-buried installation

When either of those conditions doesn't apply, you must apply correction or adjustment factors.

The Three Temperature Columns

Table 310.16 is organized into three conductor temperature rating columns:

The 90°C column shows the highest allowable ampacity but comes with an important limitation: you can only use 90°C ampacity where both the conductor and all connected terminals are rated for 90°C. Most circuit breakers, panels, and devices have terminals rated for 75°C, not 90°C.

NEC 110.14(C) governs terminal temperature ratings. For conductors rated 100A or less, terminals are typically rated for 60°C unless marked 75°C. For conductors rated over 100A, terminals are typically rated 75°C.

In practice, most residential and commercial circuit conductors are sized using the 75°C column because that's the terminal rating you'll encounter in standard equipment.

Key Ampacity Values to Memorize

These are the most exam-tested values — copper conductors, 75°C column:

Exam tip: Notice that 14 AWG at 75°C is listed as 20A, but NEC 210.19(A) and 240.4(D) restrict 14 AWG branch circuits to a maximum 15A overcurrent device. The table value is the conductor's ampacity — the code may limit you to a smaller overcurrent device.

Temperature Correction Factors

The footnotes below Table 310.16 provide correction factors to apply when the ambient temperature differs from the baseline 30°C (86°F).

Higher ambient temperatures reduce a conductor's ability to dissipate heat — so the allowable ampacity decreases. The correction factors are given for ambient temperatures from 10°C to 80°C in 5°C increments.

Selected correction factors (most exam-relevant):

How to apply: Multiply the table ampacity by the correction factor.

Example: You have a 6 AWG THHN conductor (90°C) installed in a boiler room where ambient temperature reaches 45°C. Table ampacity (90°C column) = 75A. Correction factor at 45°C, 90°C column = 0.87. Derated ampacity = 75 × 0.87 = 65.25A. Since terminals are likely 75°C, also check the 75°C column value (65A). The governing ampacity is the lower of 65.25A and 65A → 65A.

Adjustment Factors — More Than Three Conductors

When you bundle more than three current-carrying conductors together in a raceway or cable, heat cannot escape as efficiently. NEC 310.15(C)(1) requires applying an adjustment factor to reduce the allowable ampacity.

What Counts as a Current-Carrying Conductor?

This is one of the most-tested nuances on journeyman and master exams.

Count these:

• All ungrounded (hot) conductors
• Neutral conductors that carry current (see exceptions below)
• Grounded conductors on single-phase 2-wire circuits

Do NOT count these:

• Equipment grounding conductors (EGCs)
• A neutral conductor of a 3-phase, 4-wire, wye-connected system that carries only unbalanced current — but only when the load is balanced or the major portion of the load consists of linear loads [NEC 310.15(E)]
• A neutral that carries current from nonlinear loads (like electronic ballasts, VFDs, computers) does count because it can carry significant harmonic current

Exam tip: The EGC and the neutral question trips up many candidates. The bare or green equipment grounding conductor never counts. A white neutral wire may or may not count depending on the circuit type and load.

Applying the Adjustment Factor

Example: A 1¼" EMT contains 6 THHN conductors — three 10 AWG hots and one 10 AWG neutral (linear balanced load), plus two 8 AWG hots from a different circuit. How many conductors count for adjustment?

• Three 10 AWG hots: count (3)
• 10 AWG neutral (balanced linear load): does not count
• Two 8 AWG hots: count (2)
• Total current-carrying conductors: 5
• Adjustment factor for 4–6: 80% (0.80)

10 AWG THHN (90°C): 40A × 0.80 = 32A (limited to 35A terminal, so 32A governs) 8 AWG THHN (90°C): 55A × 0.80 = 44A (limited to 50A terminal, so 44A governs)

Combining Both Factors

When the ambient temperature differs from 30°C and there are more than three current-carrying conductors, apply both factors simultaneously. The NEC requires you to start with the 90°C column value when derating, because it gives you the most headroom before you hit terminal limits.

Formula: Derated ampacity = (Table 310.16 value, 90°C column) × (Correction factor) × (Adjustment factor)

Then compare to the 75°C column value (or terminal-rated column) — use whichever is lower as your maximum allowable current.

Worked exam example: Three 1/0 AWG THHN (90°C) conductors and three 4 AWG THHN (90°C) conductors are all in the same conduit. The installation is in a mechanical room where the ambient temperature is 40°C.

Current-carrying conductors: 6 total → adjustment factor = 0.80 Ambient 40°C → correction factor for 90°C column = 0.91

For 1/0 AWG (90°C table value = 170A): 170A × 0.91 × 0.80 = 123.8A 75°C column for 1/0 AWG = 150A Derated value (123.8A) < 150A → 123.8A governs

For 4 AWG (90°C table value = 95A): 95A × 0.91 × 0.80 = 69.2A 75°C column for 4 AWG = 85A Derated value (69.2A) < 85A → 69.2A governs

Common Insulation Types in Article 310

You do not need to memorize every insulation type in Table 310.16, but you should know the most common ones cold:

THHN — Thermoplastic, High Heat-resistant, Nylon-jacketed. Rated 90°C dry / 75°C wet. The standard choice for conductors in conduit on commercial and industrial jobs. When wet locations are a factor, check for THWN-2 rating.

THWN-2 — Same conductor as THHN in most cases; the "-2" designation means it's rated 90°C in both dry and wet locations.

XHHW-2 — Cross-linked polyethylene insulation, 90°C dry and wet. Often used in conduit for service conductors.

TW — Thermoplastic, rated 60°C wet and dry. Older type, rarely installed today but still appears on exams.

RHH / RHW-2 — Thermoset rubber insulation, 90°C. Common for service entrance conductors and wiring in high-heat environments.

USE-2 — Underground Service Entrance, rated 90°C. Used for underground service conductors from the meter to the panel.

Key wet location rule: XHHW (without the "-2") is rated 90°C dry but 75°C wet. Installers must check whether the installed location is classified as wet when specifying XHHW vs. XHHW-2.

Aluminum vs. Copper Conductors

Table 310.16 is split into two halves — copper (left) and aluminum or copper-clad aluminum (right).

Aluminum conductors of the same AWG have lower ampacity than copper. For example, at 75°C:

• 1/0 AWG copper: 150A
• 1/0 AWG aluminum: 120A

To carry the same current as copper, aluminum needs to be two AWG sizes larger as a general rule of thumb (though the actual relationship varies at larger sizes). Aluminum wiring requires AL-rated terminals and anti-oxidant compound at connections.

On the exam, pay attention to whether a question specifies copper or aluminum. If the question doesn't specify, assume copper.

Special Cases and Exam Traps

Rooftop conduit: NEC 310.15(B)(3)(c) adds an adder to the ambient temperature for raceways and cables installed on or in rooftops. If the raceway is within 7/8 inch of the roof surface, add 60°C (108°F) to the ambient temperature before looking up the correction factor. This dramatically reduces ampacity and is a common exam question.

Nipples: Per NEC 310.15(C)(1) Exception, conductors in a raceway nipple (24 inches or less) are not subject to the more-than-three-conductor adjustment factor. This exception appears on master exams.

Parallel conductors: NEC 310.10(H) permits conductors to be run in parallel (electrically joined at both ends) for 1/0 AWG and larger. Each parallel conductor must be the same material, AWG, insulation type, and length. The ampacity of parallel conductors is the ampacity of one conductor multiplied by the number of conductors per phase.

Conduit fill vs. ampacity: These are separate calculations. Conduit fill (NEC Chapter 9, Table 1) limits how many conductors physically fit in a raceway. Ampacity derating limits how much current those conductors can carry. Both apply — you need a conduit large enough to hold the conductors AND conductors large enough to carry the load after derating.

Conductor Sizing Procedure — Step by Step

1. Determine the load current. Calculate the continuous and non-continuous load. Size conductors for at least 125% of continuous loads [NEC 210.19(A)].

2. Find the minimum ampacity needed. The conductor must carry the required current after derating.

3. Identify installation conditions. What is the ambient temperature? How many current-carrying conductors are bundled? Is this a wet or dry location?

4. Start with the 90°C column if derating. Apply correction factors and adjustment factors to the 90°C value.

5. Check terminal limits. The final ampacity cannot exceed the 75°C (or 60°C) value for the conductor's AWG, based on terminal ratings [NEC 110.14(C)].

6. Select the minimum AWG where the derated ampacity meets or exceeds the required load current.

Practice This With Our Calculator

Derating calculations involve multiple multiplications and table lookups — the kind of calculation that eats time on a timed exam. Use our Wire Ampacity Calculator to practice applying correction and adjustment factors so the process becomes second nature before test day.

Related NEC Articles

Conductor sizing interacts with several other articles you'll be tested on:

• NEC 110.14(C) — Terminal temperature ratings that limit which column of Table 310.16 governs
• NEC 210.19(A) — Branch circuit conductor sizing (125% rule for continuous loads)
• NEC 240.4 — Protection of conductors by overcurrent devices
• NEC 310.15(B)(3) — Ampacity of conductors in raceways, cables, and earth — special conditions
• NEC 430.22 — Motor branch circuit conductors (Article 430 has its own sizing rules)

Key Takeaways for the Exam

Article 310 and Table 310.16 questions follow a predictable pattern. If you internalize these rules you can work through any conductor sizing question methodically:

• The 75°C column governs for most circuits because terminals are rated 75°C
• Use the 90°C column as the starting point when you must apply correction or adjustment factors
• Temperature correction factors apply when ambient temperature differs from 30°C (86°F)
• Adjustment factors apply when more than 3 current-carrying conductors share a raceway or cable
• Equipment grounding conductors never count toward the adjustment factor trigger
• The governing ampacity is always the lower of the derated value and the terminal-limited value
• Rooftop conduit adds up to 60°C to the ambient temperature — a major derating hit

Master these rules, memorize the key 75°C column values, and Table 310.16 problems will be some of the faster questions you answer on exam day.