NEC Table 310.16 — Wire Sizing and Ampacity: The Complete Exam Guide

No table in the National Electrical Code gets used more than Table 310.16. Whether you're sizing a branch circuit conductor, a feeder, or a service entrance cable, this table is your starting point. It appears on virtually every electrician exam — and the questions aren't just "what's the ampacity of 10 AWG wire." They test whether you know when to derate, which column to use, and how to apply correction and adjustment factors.

This guide walks through how to read Table 310.16 correctly and how to work through ampacity problems step by step.

---

What Table 310.16 Tells You

Table 310.16 lists the allowable ampacity of insulated conductors rated up to 2000V in the following conditions:

• Not more than three current-carrying conductors in a raceway, cable, or earth (or single conductor in free air per Table 310.17)
• Ambient temperature of 30°C (86°F)
• Based on conductor insulation temperature rating (60°C, 75°C, or 90°C)

The table lists ampacities for:

• Copper conductors (left side)
• Aluminum and copper-clad aluminum conductors (right side)

For each wire size (AWG or kcmil), there are three columns corresponding to conductor temperature ratings: 60°C, 75°C, and 90°C.

---

The Temperature Column Rule: NEC 110.14(C)

Here's the most important rule for using Table 310.16 correctly — and one of the most tested:

NEC 110.14(C) limits which column you can actually use based on the temperature rating of the equipment you're connecting to, not just the wire insulation.

The rule in plain terms:

• Equipment listed at 60°C → use the 60°C column
• Equipment listed at 75°C → use the 75°C column (maximum)
• Equipment listed at 90°C → use the 90°C column for derating calculations only, then the result must not exceed the 75°C ampacity

The practical result: Most commercial circuit breakers, lugs, and panels are rated 75°C. So in practice, most conductors are sized from the 75°C column, even if you're using 90°C-rated wire like THHN.

The 90°C column is primarily used as a starting point for applying correction factors — you can take the higher 90°C ampacity, apply derating factors, and as long as the final result doesn't exceed the 75°C ampacity of that conductor size, you're compliant.

---

Common Ampacity Values from Table 310.16 (Copper, 75°C)

Exam tip: Know the values for 14, 12, 10, 8, and 6 AWG copper at 75°C — these appear on exams most often. Also know that 14 AWG copper at 60°C is rated 15A even though the 75°C column shows 20A, because most residential wiring uses 60°C terminations.

---

Applying Correction Factors: High Ambient Temperatures

When the ambient temperature exceeds 30°C (86°F), the conductor must be derated using the correction factors in Table 310.15(B)(1).

Correction Factors for 75°C Conductors

How to Apply

Formula: Corrected Ampacity = Table Ampacity × Correction Factor

Example: 10 AWG THWN copper in a location with 45°C ambient:

• Table ampacity at 75°C: 35A
• Correction factor for 45°C, 75°C conductor: 0.82
• Corrected ampacity: 35 × 0.82 = 28.7A

---

Applying Adjustment Factors: More Than Three Conductors

When more than three current-carrying conductors are in the same raceway or cable, each conductor's ampacity must be reduced using the adjustment factors in Table 310.15(C)(1).

Adjustment Factors

What Counts as "Current-Carrying"?

Not all conductors in a raceway count toward the adjustment factor tally:

• Equipment grounding conductors do NOT count
• Neutral conductors carrying only unbalanced current in a 3-wire, single-phase system do NOT count
• Neutral conductors of a 4-wire, 3-phase wye system where the major loads are nonlinear (electronic ballasts, VFDs) DO count per the NEC's note

Exam tip: A common exam trap is including the grounding conductor in the count. EGCs don't count as current-carrying conductors for the derating calculation.

---

Applying Both Factors Simultaneously

When both a high ambient temperature AND more than three conductors are present, apply both factors:

Formula: Final Ampacity = Table Ampacity × Correction Factor × Adjustment Factor

Example: 8 AWG THWN copper, 45°C ambient, 6 current-carrying conductors in conduit:

• Table ampacity (75°C): 50A
• Correction factor (45°C, 75°C): 0.82
• Adjustment factor (6 conductors): 0.80
• Final ampacity: 50 × 0.82 × 0.80 = 50 × 0.656 = 32.8A

This is a key exam problem type — you must apply both factors and identify whether the resulting ampacity is sufficient for the intended load.

---

The "Reverse" Approach: Finding Minimum Conductor Size

Sometimes the exam gives you the load and asks for the minimum conductor size. In that case, work backwards:

Required table ampacity = Load Current ÷ (Correction Factor × Adjustment Factor)

Example: Load is 30A. Ambient is 40°C (0.88 for 75°C). 5 conductors in conduit (0.80).

• Required table ampacity = 30 ÷ (0.88 × 0.80) = 30 ÷ 0.704 = 42.6A
• From Table 310.16 (75°C copper): 8 AWG = 50A ✓ (10 AWG = 35A ✗)
• Minimum conductor: 8 AWG

---

Common Exam Mistakes on Table 310.16

1. Using the 90°C column for termination sizing — you can only use 90°C as a starting point for derating; the result must not exceed the 75°C value
2. Counting the EGC when determining adjustment factors — it doesn't count
3. Forgetting to apply correction factors when the problem mentions an elevated ambient temperature
4. Using copper values for an aluminum conductor question — always check which metal the problem specifies
5. Missing the minimum 14 AWG for 15A circuits (NEC 240.4(D)) — the overcurrent protection limit, not just Table 310.16, governs minimum conductor size

---

NEC 240.4(D) — Small Conductor Protection

Article 310 doesn't stand alone — NEC 240.4(D) sets maximum overcurrent protection for small conductors regardless of ampacity:

This means even though 14 AWG has an ampacity of 20A at 75°C, you cannot protect it with a 20A breaker. Maximum is 15A.

---

Ready to Practice Ampacity Problems?

GetLicenseReady's practice questions include dozens of Table 310.16 problems — conductor sizing, derating calculations, correction factors, and adjustment factors — to build the speed and accuracy you need on exam day.

Related reading: NEC Article 220 — Load Calculations | NEC Chapter 9 Conduit Fill Calculations | How to Pass the Electrician Exam on Your First Try