Conduit Fill and Wire Ampacity Reference
Why this matters
NEC Chapter 9 Table 1 governs conduit fill - how many conductors fit in a raceway. NEC Table 310.16 governs ampacity - how much current a given conductor can carry. Get these wrong and you've either crammed too many wires into too small a conduit (mechanical damage during pull) or undersized the conductor (overheating, fire risk). Both calculations come up on every electrical job above a single straight run, and both are heavily enforced at inspection.
Conductor ampacity - NEC Table 310.16
The base table for ampacity of insulated conductors in raceway, cable, or earth at 30 °C (86 °F) ambient.
Common copper THHN/THWN-2 (90 °C insulation) ampacities, NEC 310.16 75 °C column:
| AWG | Ampacity (75 °C) |
|---|---|
| 14 | 20 A |
| 12 | 25 A |
| 10 | 35 A |
| 8 | 50 A |
| 6 | 65 A |
| 4 | 85 A |
| 3 | 100 A |
| 2 | 115 A |
| 1 | 130 A |
| 1/0 | 150 A |
| 2/0 | 175 A |
| 3/0 | 200 A |
| 4/0 | 230 A |
Aluminum same insulation, NEC 310.16 75 °C column:
| AWG | Ampacity (75 °C) |
|---|---|
| 12 | 20 A |
| 10 | 30 A |
| 8 | 40 A |
| 6 | 50 A |
| 4 | 65 A |
| 3 | 75 A |
| 2 | 90 A |
| 1 | 100 A |
| 1/0 | 120 A |
| 2/0 | 135 A |
| 3/0 | 155 A |
| 4/0 | 180 A |
Aluminum is approximately 80% of copper ampacity for the same AWG.
The 60/75/90 column issue
NEC 310.16 has columns for three temperature ratings:
- 60 °C (older terminations, some residential)
- 75 °C (standard residential and commercial)
- 90 °C (modern THHN/THWN-2 insulation rating)
NEC 110.14(C) says you can only use the column matching the LOWEST-rated termination point in the circuit, even if the wire itself is rated higher. Most residential equipment terminations are 60 °C rated up to 100 A, 75 °C for 100+ A.
Practical impact:
- A #14 THHN wire (90 °C insulation) is rated 25 A at 90 °C
- But on a residential receptacle (60 °C termination), it's limited to the 60 °C column = 20 A
- A 20 A breaker is the maximum
This is why a "20 A" residential circuit uses #12 wire even though #14 is technically rated higher - terminations limit you.
Ampacity adjustment factors
The base ampacity assumes 30 °C (86 °F) ambient and 3 or fewer current-carrying conductors in a raceway. Real installs often violate these conditions; adjustments are required.
Temperature correction (NEC 310.15(B)): higher ambient temperature reduces ampacity. For typical 75 °C insulation:
- 86 °F (30 °C) = 1.00
- 95 °F (35 °C) = 0.96
- 104 °F (40 °C) = 0.91
- 113 °F (45 °C) = 0.87
- 122 °F (50 °C) = 0.82
- 131 °F (55 °C) = 0.76
- 140 °F (60 °C) = 0.71
A 50 A circuit in a 122 °F attic: 50 × 0.82 = 41 A effective. Conductors must be sized larger.
Multiple conductors in raceway (NEC 310.15(C)): 4+ current-carrying conductors require derating:
| # current-carrying conductors | Adjustment factor |
|---|---|
| 1-3 | 1.00 |
| 4-6 | 0.80 |
| 7-9 | 0.70 |
| 10-20 | 0.50 |
| 21-30 | 0.45 |
| 31-40 | 0.40 |
| 41+ | 0.35 |
A #12 conductor (25 A base at 75 °C, but limited to 20 A for typical residential receptacle): with 6 conductors in conduit, adjustment 0.80 × 25 = 20 A. Still 20 A maximum on the breaker.
Both adjustments multiply together if both apply.
Conduit fill - NEC Chapter 9 Table 1
The maximum percentage of conduit cross-sectional area that conductors can occupy:
- 1 conductor: 53%
- 2 conductors: 31%
- 3+ conductors: 40%
Conductor cross-section per AWG (NEC Chapter 9 Table 5 for THHN):
| AWG | THHN cross-section (in²) |
|---|---|
| 14 | 0.0097 |
| 12 | 0.0133 |
| 10 | 0.0211 |
| 8 | 0.0366 |
| 6 | 0.0507 |
| 4 | 0.0824 |
| 3 | 0.0973 |
| 2 | 0.1158 |
| 1 | 0.1562 |
| 1/0 | 0.1855 |
| 2/0 | 0.2223 |
| 3/0 | 0.2679 |
| 4/0 | 0.3237 |
Conduit internal cross-section (NEC Chapter 9 Table 4 for EMT):
| Trade size | 40% fill (in²) | 53% fill (in²) |
|---|---|---|
| 1/2" | 0.122 | 0.161 |
| 3/4" | 0.213 | 0.283 |
| 1" | 0.346 | 0.458 |
| 1-1/4" | 0.598 | 0.793 |
| 1-1/2" | 0.814 | 1.079 |
| 2" | 1.342 | 1.778 |
| 2-1/2" | 2.343 | 3.105 |
| 3" | 3.538 | 4.689 |
Worked example - sizing conduit
200 A service feeder using 3/0 AL conductors + #4 AL ground:
- 3× 3/0 conductors (L1, L2, N): 3 × 0.2679 = 0.804 in²
- 1× #4 ground: 0.0824 in²
- Total: 0.887 in²
At 40% fill (4+ conductors counted with ground):
- 1-1/2" EMT = 0.814 in² → INSUFFICIENT
- 2" EMT = 1.342 in² → FITS
Use 2" EMT for this feeder.
Common ampacity / fill mistakes
- Using wrong temperature column. Most residential terminations are 60 °C or 75 °C; ignore the 90 °C column for sizing breakers.
- Forgetting derating. 6 conductors in a conduit and using full base ampacity = overheating.
- Forgetting temperature correction. Conductors in an attic in summer aren't at 30 °C ambient.
- Counting wrong conductors as current-carrying. Equipment grounding conductors don't count; isolated grounded conductors and shared neutrals can.
- Undersized conduit. Pull damage during wire installation, especially on long runs with multiple bends.
- Mixing aluminum and copper terminations without proper antioxidant. Aluminum oxidizes; needs penetrating compound (NoAlOx, Ideal Noalox) at terminations.
Conductor types - what to use where
THHN/THWN-2: the workhorse insulation. Dry and wet locations, 90 °C rating. Pulls easily in conduit. Standard for branch and feeder conductors.
XHHW-2: cross-linked polyethylene insulation. Used in wet locations, conduit. Sometimes for service entrance.
RHW-2: rubber insulation, older code, harder to find now.
SE/USE cable: service entrance cable; integral concentric neutral. Above-grade or underground service feed.
UF cable: underground feeder; direct-bury rated. NM-B equivalent but with direct-burial insulation.
MC / AC armored cable: metal-clad / armor-clad. Used in commercial and where physical protection is needed; sometimes has integral ground via armor (AC) or separate (MC).
NM-B (Romex): for dry locations in residential construction. NOT for conduit installs (different rules).
NEC code references
- NEC 110.14(C) - termination temperature rules (the 60/75/90 column issue)
- NEC 310.15 - ampacity adjustment factors (ambient and bundling)
- NEC 310.16 - base ampacity tables
- NEC 310.12 - residential service conductor reduced ampacity (the "83% rule")
- NEC Chapter 9 Table 1 - conduit fill percentages
- NEC Chapter 9 Table 4 - conduit cross-section by trade size and material
- NEC Chapter 9 Table 5 - conductor cross-section by AWG and insulation type
- NEC 250.122 - equipment grounding conductor sizing
When wire seems oversized
References
- NEC Article 310 (conductor ampacity)
- NEC Article 110.14(C) (termination temperature)
- NEC Chapter 9 Tables 1, 4, 5 (conduit fill)
- NEC Article 358 (EMT)
- NEC Article 250.122 (equipment ground sizing)
- NEC Article 230 (services)
- Mike Holt Enterprises (best-in-class NEC training)
- Manufacturer data sheets (Southwire, Cerrowire, Encore Wire)