NEC 2023 Electrical Exam: Conductor Sizing & Ampacity (Article 310 / Table 310.16)

Article 310 governs conductor ampacity — the maximum current a conductor can carry continuously without exceeding its temperature rating — with Table 310.16 as the primary sizing reference for conductors in raceways or cables.

Key Points

• Ampacity = maximum continuous current a conductor can carry without exceeding its temperature rating.
• Table 310.16 applies to ≤3 current-carrying conductors in a raceway/cable at 30°C ambient, up to 2,000V.
• Table 310.17 applies to single conductors in free air — higher ampacity than Table 310.16.
• Ambient temperature above 30°C requires applying a correction factor from Table 310.15(B)(1)(1).
• More than 3 current-carrying conductors in a raceway requires an adjustment (bundling derating) factor.
• Conductor sizing must not exceed the temperature rating of the equipment terminals — usually the 75°C rule.
• Aluminum conductors require approximately 2 AWG sizes larger than copper for equivalent ampacity.
• 240.4(D) caps OCPD for small conductors: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A.
• Insulation type determines temperature column — TW = 60°C; THW/THWN = 75°C; THHN/THWN-2 = 90°C.
• AWG scale is inverse: larger number = smaller wire. 4/0 AWG is larger than 1 AWG.

Why It Matters: Undersized conductors overheat, degrade insulation, and cause fires. Oversized conductors waste material and may not fit terminations. Table 310.16 is one of the highest-frequency exam references — candidates who misread temperature columns, forget the 75°C terminal rule, or skip correction factors lose easy points. Field electricians use this table daily for every circuit they size.

Terms To Remember

Ampacity

Maximum continuous current a conductor can carry without exceeding its temperature rating.

Table 310.16

Ampacity table for ≤3 conductors in raceway/cable; 30°C ambient; up to 2,000V.

Table 310.17

Ampacity table for single conductors in free air; 30°C ambient; higher values than 310.16.

Temperature Correction Factor

Multiplier from Table 310.15(B)(1)(1) when ambient exceeds 30°C; always <1 above 30°C.

Adjustment Factor

Derating multiplier when >3 current-carrying conductors share a raceway (bundling).

75°C Terminal Rule

Size conductor using 75°C column when equipment terminals are rated 75°C, even if insulation is 90°C.

AWG

American Wire Gauge; inverse scale — higher number = smaller wire; 4/0 > 1 > 10 > 14.

kcmil

Thousands of circular mils; used for conductors larger than 4/0 AWG (e.g., 250 kcmil, 350 kcmil).

Circular Mil

Area of a circle with 1 mil (0.001 inch) diameter; unit for measuring conductor cross-section.

TW

60°C thermoplastic insulation; lowest ampacity column; dry/wet locations.

THW / THWN

75°C thermoplastic; dry/wet locations; most common conduit wiring.

THHN

90°C thermoplastic; dry only; highest ampacity column — but limited to 75°C at most terminals.

THWN-2

90°C thermoplastic; dry and wet; W = wet rated, 2 = 90°C in both conditions.

RHW

75°C thermoset rubber insulation; dry/wet; thermoset ≠ thermoplastic.

Compact Stranding

Used with aluminum conductors to reduce overall diameter; easier conduit fill.

NM Cable (Romex)

Nonmetallic sheathed cable for dry residential use; 90°C conductors derated to 60°C for ampacity.

Type UF Cable

Underground feeder; suitable for direct burial; NOT the same as NM (Romex).

Type USE Cable

Underground service entrance cable; direct burial permitted; 75°C rated.

Type AC Cable (BX)

Armored cable with fire-retardant paper wraps on individual conductors and a bonding strip.

Type MC Cable

Metal-clad cable; metal armor serves as EGC; no fire-retardant conductor wraps; 18 AWG to 2,000 kcmil.

Type NMC

Nonmetallic sheathed cable for dry/damp/corrosive locations; NOT suitable for direct burial or concrete.

Step Process Formula

Selecting the Correct Conductor Size — Step-by-Step

Basic Sizing Steps

• Step 1 — Determine calculated load current (amperes).
• Step 2 — Identify conductor material: copper or aluminum.
• Step 3 — Identify insulation type and its temperature rating (60°C, 75°C, or 90°C).
• Step 4 — Identify equipment terminal rating (usually 75°C for most equipment).
• Step 5 — Use the LOWER of the insulation column or terminal rating column in Table 310.16.
• Step 6 — Apply temperature correction factor if ambient temperature is above 30°C.
• Step 7 — Apply adjustment factor if >3 current-carrying conductors share the raceway.
• Step 8 — Select the conductor AWG whose derated ampacity meets or exceeds the load current.
• Step 9 — Verify overcurrent protection does not exceed conductor ampacity (240.4).
• Step 10 — For 14, 12, or 10 AWG copper, apply 240.4(D) hard OCPD limits.

Temperature Correction Formula

Formula: Corrected Ampacity = Table Ampacity × Correction Factor

Key Rule: As ambient temperature INCREASES above 30°C, ampacity DECREASES. Correction factors above 30°C are always less than 1.00. Correction factors below 30°C are greater than 1.00.

Example 1

Conductor: 1 AWG THW copper

Table Ampacity: 130A (75°C column)

Ambient: 55°C

Correction Factor: 0.75

Calculation: 130 × 0.75 = 97.5 → 97A

Example 2

Conductor: 1 AWG THW copper

Table Ampacity: 130A (75°C column)

Ambient: 110°F (≈43°C)

Correction Factor: 0.82

Calculation: 130 × 0.82 = 106.6 → 107A

Bundling Adjustment Factors

Note: Only current-carrying conductors count toward the total. Grounding conductors and neutrals carrying only unbalanced current do NOT count.

4 To 6 Conductors: 0.80 (80%)

7 To 9 Conductors: 0.70 (70%)

10 To 20 Conductors: 0.50 (50%)

Small Conductor Ocpd Limits 240 4D

Note: Hard limits regardless of Table 310.16 values. Example: 10 AWG THHN shows 40A in Table 310.16 (90°C column) — but 240.4(D) caps OCPD at 30A.

14 Awg Copper: Maximum 15A OCPD

12 Awg Copper: Maximum 20A OCPD

10 Awg Copper: Maximum 30A OCPD

Table 310 16 Key Values

Note: Selected ampacity values at 30°C ambient for common conductor sizes.

Copper 60C Tw

14 Awg: 15A

12 Awg: 20A

10 Awg: 30A

8 Awg: 40A

6 Awg: 55A

4 Awg: 70A

2 Awg: 95A

1 Awg: 110A

1/0 Awg: 125A

2/0 Awg: 145A

3/0 Awg: 165A

4/0 Awg: 195A

Copper 75C Thw Thwn Rhw

14 Awg: 20A

12 Awg: 25A

10 Awg: 35A

8 Awg: 50A

6 Awg: 65A

4 Awg: 85A

2 Awg: 115A

1 Awg: 130A

1/0 Awg: 150A

2/0 Awg: 175A

3/0 Awg: 200A

4/0 Awg: 230A

250 Kcmil: 255A

350 Kcmil: 310A

Copper 90C Thhn Thwn2

14 Awg: 25A

12 Awg: 30A

10 Awg: 40A

8 Awg: 55A

6 Awg: 75A

4 Awg: 95A

2 Awg: 130A

1 Awg: 150A

1/0 Awg: 170A

2/0 Awg: 195A

3/0 Awg: 225A

4/0 Awg: 260A

250 Kcmil: 290A

350 Kcmil: 350A

Aluminum 75C Thw Thwn Rhw

12 Awg: 20A

10 Awg: 30A

8 Awg: 40A

6 Awg: 50A

4 Awg: 65A

2 Awg: 90A

1 Awg: 100A

1/0 Awg: 120A

2/0 Awg: 135A

3/0 Awg: 155A

4/0 Awg: 180A

250 Kcmil: 205A

350 Kcmil: 250A

Insulation Type Quick Reference

Tw: 60°C; thermoplastic; dry/wet; lowest ampacity.

Thw: 75°C; thermoplastic; dry/wet; most common in conduit.

Thwn: 75°C; thermoplastic; dry/wet.

Thhn: 90°C; thermoplastic; dry only; size at 75°C column for most terminals.

Thwn 2: 90°C; thermoplastic; dry and wet; use 90°C column if terminals allow.

Rhw: 75°C; thermoset (rubber); dry/wet.

Rhh: 90°C; thermoset; dry only.

Xhhw: 90°C dry / 75°C wet; cross-linked polyethylene.

Xhhw 2: 90°C dry and wet; highest rating for wet locations.

Nm Romex: 90°C conductors — must use 60°C column for ampacity (NEC 334.80).

W Suffix: Conductor marked W is rated for wet/outdoor locations.

R Prefix: R = rubber (thermoset) insulation base material.

Cable Type Quick Reference

Nm Romex: Dry residential only; 90°C conductors derated to 60°C column.

Nmc: Dry, damp, corrosive — NOT direct burial, NOT embedding in concrete.

Ac Bx: Armored; fire-retardant paper wraps on conductors; bonding strip inside.

Mc: Metal-clad; armor = EGC; no fire-retardant wraps; 18 AWG to 2,000 kcmil.

Use: Underground service entrance; direct burial OK; 75°C.

Uf: Underground feeder; direct burial OK; 60°C; NOT called Romex.

Mv: Medium voltage (>2,000V); wet, dry, and direct burial all permitted.

Key Ac Vs Mc: AC cable has fire-retardant paper wraps on individual conductors; MC does not.

Worked Example 1

Scenario: 3-phase 480V circuit, 98A calculated load. Three copper conductors in RMC. Terminals rated 75°C.

Thhn: 90°C insulation but terminal = 75°C → use 75°C column. 3 AWG Cu @ 75°C = 100A ≥ 98A. Answer: 3 AWG.

Thw: 75°C column directly. 3 AWG = 100A ≥ 98A. Answer: 3 AWG.

Tw: 60°C column. 3 AWG = 85A (not enough). 2 AWG = 95A (not enough). 1 AWG = 110A ≥ 98A. Answer: 1 AWG.

Lesson: THHN and THW both land on 3 AWG because the 75°C terminal rule equalizes them. TW requires one size larger because its 60°C column values are lower.

Worked Example 2

Scenario: Replace 1 AWG TW aluminum with minimum THHN copper of equal ampacity.

Step 1: 1 AWG TW aluminum @ 60°C column = 75A.

Step 2: THHN copper but terminals are 75°C → use 75°C Cu column. Find ≥75A: 6 AWG = 65A (not enough); 4 AWG = 85A ≥ 75A.

Answer: 4 AWG THHN copper.

Lesson: Replacing aluminum with copper does not always mean going up in AWG. The 75°C terminal rule governs which column to use even for THHN.

Worked Example 3

Scenario: 4 AWG SE cable with 90°C aluminum conductors, terminated to 75°C equipment.

Step: 90°C insulation, but terminal is 75°C → use 75°C Al column. 4 AWG Al @ 75°C = 65A.

Answer: 65A per conductor.

Lesson: SE and USE cables often have 90°C insulation but are limited to 75°C ampacity at standard equipment terminals.

Common Confusions

• Students confuse Table 310.16 with Table 310.17 because both list ampacity values — 310.16 is for conductors in raceways or cables (≤3 conductors); 310.17 is for single conductors in free air, which has higher ampacity because heat dissipates more easily.
• Students confuse the insulation temperature rating with the terminal temperature rating because THHN is rated 90°C but is often sized using the 75°C column — NEC 110.14(C) requires using the lower of the insulation rating or the equipment terminal rating.
• Students confuse adjustment factors with correction factors because both reduce ampacity — correction factors apply when ambient temperature exceeds 30°C; adjustment factors apply when more than 3 current-carrying conductors share a raceway.
• Students confuse AWG direction because higher AWG numbers mean smaller wire — 14 AWG is smaller than 2 AWG; 4/0 AWG is larger than 1/0 AWG; conductors above 4/0 AWG are measured in kcmil.
• Students confuse NM cable ampacity with Table 310.16 values because NM conductors are 90°C rated — but NEC 334.80 requires NM cable ampacity to be based on the 60°C column, not the 90°C column.

Quick Questions

Question 1

Question: A 3-phase 240V circuit has a calculated load of 196A. Three RHW copper conductors in EMT. Terminals rated 75°C. What conductor size is required?

Correct Answer: 3/0 AWG copper. RHW is 75°C; terminals are 75°C → use 75°C Cu column. 3/0 AWG = 200A ≥ 196A. 2/0 AWG = 175A is insufficient.

What does Table 310.16 list for 10 AWG THHN copper, and what does 240.4(D) limit the OCPD to?

Question: What does Table 310.16 list for 10 AWG THHN copper, and what does 240.4(D) limit the OCPD to?

Correct Answer: Table 310.16 lists 10 AWG THHN at 40A (90°C column). NEC 240.4(D)(8) limits the OCPD to 30A maximum for 10 AWG copper regardless of insulation type.

A conductor's ampacity must be reduced when ambient temperature rises above what baseline?

Question: A conductor's ampacity must be reduced when ambient temperature rises above what baseline?

Correct Answer: 30°C (86°F). This is the baseline ambient temperature for Table 310.16 and 310.17. Any ambient above 30°C requires applying a correction factor less than 1.00 to reduce the table ampacity.

Exam Answer Frame

Style: 5-mark

Question: A calculated load is 173A. Identify the minimum size copper conductor permitted considering all insulation types. Explain the 75°C terminal rule and its effect on your answer.

Model Answer: Step 1 — Check Table 310.16 across all temperature columns for copper. At 60°C (TW): 3/0 AWG = 165A (insufficient); 4/0 AWG = 195A ✓. At 75°C (THW/THWN/RHW): 2/0 AWG = 175A ✓. At 90°C (THHN): 2/0 AWG = 195A ✓. Step 2 — The 75°C terminal rule (NEC 110.14(C)): most standard equipment terminals are rated 75°C. Even if the conductor insulation is 90°C (e.g., THHN), ampacity must be based on the 75°C column unless the equipment is specifically listed and marked for 90°C terminations. Step 3 — Result: The minimum conductor is 2/0 AWG copper (achievable with THW, THWN, RHW, THHN, or THWN-2 all sized using the 75°C column). Using TW (60°C only) requires 4/0 AWG — two sizes larger. The 75°C terminal rule is the reason THHN, despite being rated 90°C, provides the same minimum conductor size as THW in this scenario.