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
- On a 60 °C termination it drops to the 60 °C column = 15 A
- NEC 240.4(D) then caps the overcurrent device at 15 A for #14 copper regardless of which column you used
This is why a "20 A" residential circuit uses #12 wire: #14 copper is capped at 15 A by the small-conductor rule in 240.4(D), not by anything about the termination temperature.
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. From NEC Table 310.15(B)(1), 75 °C column:
- 86 °F (30 °C) = 1.00
- 95 °F (35 °C) = 0.94
- 104 °F (40 °C) = 0.88
- 113 °F (45 °C) = 0.82
- 122 °F (50 °C) = 0.75
- 131 °F (55 °C) = 0.67
- 140 °F (60 °C) = 0.58
A 50 A circuit in a 122 °F attic: 50 × 0.75 = 37.5 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 4/0 AL conductors + #4 AL ground:
- 3× 4/0 conductors (L1, L2, N): 3 × 0.3237 = 0.971 in²
- 1× #4 ground: 0.0824 in²
- Total: 1.054 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
You will open a panel and find conductors two sizes larger than the ampacity table calls for. Before you assume somebody overbuilt it, check whether one of these applies. Every one of them is a legitimate reason the wire is bigger than the breaker suggests.
Voltage drop on a long run. The NEC's ampacity tables are about heat, not about performance at the far end. Long feeders to a shop, a well, a barn, or a pole light get upsized so the load sees usable voltage. This is the most common reason.
Derating already applied. More than three current-carrying conductors in the raceway, or a hot ambient (attic, boiler room, rooftop in sun), forces the installer up a size or two to land back at the required ampacity after adjustment. The finished install looks oversized because the correction is invisible in the panel.
Terminal temperature limit. Equipment terminations are typically listed for the 60 or 75 degree Celsius column. A 90 degree conductor can be used for the derating math, but the final ampacity is capped by the lowest-rated termination. The wire gets sized to the terminal column, not the insulation column.
Continuous load. A load running three hours or more requires the conductor and the overcurrent device to be sized at 125% of the continuous load. Lighting runs, EV charging, and many commercial circuits land here.
Motor and equipment nameplate rules. Motor circuit conductors are sized from the table values in the motor articles, not the nameplate amps, and the overcurrent device is often larger than the conductor ampacity by design. A big wire on a small-looking breaker, or the reverse, is normal on motor circuits.
Aluminum. Aluminum carries less current per size than copper. A feeder that looks generous in aluminum is often exactly at minimum.
Planned future capacity. Somebody pulled for the panel upgrade that has not happened yet.
The one rule you must not break: an oversized conductor does not authorize an oversized breaker. The overcurrent device protects whatever is downstream and whatever equipment is terminated, and it stays at the size the load and the equipment call for. And when ungrounded conductors are increased in size (voltage drop being the usual reason), the equipment grounding conductor has to be increased proportionally as well. Upsizing the phase conductors and leaving the ground at minimum is one of the most commonly missed items at inspection.
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)