When sizing conductors for a new circuit, the American Wire Gauge (AWG) number only tells you the cross-sectional area of the bare copper. It does not tell you the physical footprint of the wire. For a 12 AWG wire, the bare copper diameter is 0.081 inches, but once you add THHN insulation, the overall diameter swells to 0.118 inches. That extra 0.037 inches per wire is the difference between a clean pull through a 1/2-inch EMT conduit and a jammed raceway that fails inspection.
This reference provides the exact awg wire diameter with insulation chart based on NEC Chapter 9, Table 5, allowing you to calculate conduit fill, bending radii, and physical clearances before you buy your materials.
The Master AWG Wire Diameter with Insulation Chart (NEC Chapter 9)
Before pulling wire, you need to know how to read this table. The Bare Copper column (sourced from ASTM B258) is used strictly for voltage drop and DC resistance calculations. The THHN/THWN-2 and XHHW-2 columns represent the maximum overall diameter of the insulated conductor, which you must use for conduit fill calculations per NFPA 70 (NEC) Chapter 9, Table 1. The Area column provides the cross-sectional square inches for the THHN profile, which is the exact number you divide into your conduit’s usable fill area.
| AWG Size | Bare Copper Dia. (in) | THHN/THWN-2 Overall Dia. (in) | XHHW-2 Overall Dia. (in) | THHN Cross-Sectional Area (sq in) |
|---|---|---|---|---|
| 14 AWG | 0.064 | 0.109 | 0.109 | 0.0093 |
| 12 AWG | 0.081 | 0.118 | 0.118 | 0.0109 |
| 10 AWG | 0.102 | 0.164 | 0.135 | 0.0211 |
| 8 AWG | 0.128 | 0.203 | 0.170 | 0.0324 |
| 6 AWG | 0.162 | 0.249 | 0.222 | 0.0487 |
| 4 AWG | 0.204 | 0.328 | 0.282 | 0.0845 |
| 2 AWG | 0.258 | 0.384 | 0.342 | 0.1158 |
| 1/0 AWG | 0.325 | 0.454 | 0.410 | 0.1618 |
Which Column Applies to Your Installation?
Choosing between the THHN and XHHW-2 columns depends entirely on the insulation jacket you are pulling through the raceway. Both are rated for 90°C in dry locations, but their physical profiles differ significantly due to the materials used.
THHN/THWN-2 (PVC with Nylon Jacket)
THHN uses a polyvinyl chloride (PVC) base insulation covered by a thin nylon outer jacket. The nylon makes the wire incredibly slick, which is great for pulling through long conduit runs. However, PVC requires a thicker application to achieve the same dielectric strength and heat resistance as modern alternatives. If you are pulling standard THHN from a big-box store, use the THHN/THWN-2 column.
XHHW-2 (Cross-Linked Polyethylene)
XHHW-2 uses cross-linked polyethylene (XLPE). Because XLPE is chemically more stable and highly resistant to heat and moisture, the insulation layer can be manufactured much thinner than PVC. Notice the chart above: at 10 AWG, THHN is 0.164 inches, but XHHW-2 is only 0.135 inches.
What About NM-B (Romex)?
You will not find NM-B cable in this specific diameter chart. NM-B is a multi-conductor cable assembly with an outer PVC jacket, not a single insulated conductor. When calculating conduit fill for short sleeves protecting NM-B (like a 10-foot drop down a block wall), NEC Chapter 9 requires you to treat the cable as a single conductor and use the major diameter of the oval cable to calculate the cross-sectional area, or simply refer to the manufacturer’s stated cable dimensions.
Derating, Conduit Fill, and What the Table Cannot Tell You
Knowing the physical diameter of your wire solves the mechanical problem of fitting it into the pipe. It does not solve the thermal problem of keeping it from melting. This is where ampacity derating and the limitations of the chart come into play.
How Derating Rows Modify the Base Value
When you bundle multiple current-carrying conductors in a single raceway, they trap heat. NEC 310.15(C)(1) mandates that you must reduce (derate) the allowable ampacity of the wire based on the number of conductors.
- 4 to 6 conductors: Multiply the base 90°C ampacity by 80%.
- 7 to 9 conductors: Multiply the base 90°C ampacity by 70%.
- 10 to 20 conductors: Multiply the base 90°C ampacity by 50%.
Example: You want to run four 10 AWG THHN circuits (8 current-carrying conductors total) in a single 1-inch EMT. The diameter chart confirms they will physically fit. However, the base 90°C ampacity of 10 AWG is 40A. Because you have 8 conductors, you must derate to 70% (40A x 0.70 = 28A). Since your breaker is likely 30A, this installation is a code violation. The diameter table got the wire into the pipe, but the derating table failed the circuit.
What the Table Cannot Tell You
While NEC Chapter 9, Table 5 is the legal standard for conduit fill calculations, it has blind spots that experienced electricians account for on the jobsite:
- Manufacturer Jacket Variance: The NEC lists the maximum allowable diameter for conduit fill calculations to ensure a worst-case safety margin. If you measure a spool of 12 AWG THHN from Southwire or Cerrowire with digital calipers, the actual outer diameter might measure 0.115 inches instead of the chart’s 0.118 inches. You must use the chart's larger number for official fill math, even if the physical wire is slightly smaller.
- Voltage Drop: This chart provides physical dimensions, not electrical resistance. If you are running a 6 AWG feeder 150 feet to a detached garage, the diameter chart won't warn you that you need to upsize to 4 AWG to maintain a 3% voltage drop limit. For resistance values, you must cross-reference NEC Chapter 9, Table 8.
- Bending Radius: The physical diameter dictates how tightly you can bend the wire without damaging the insulation jacket or snapping the copper strands. As a general rule, the minimum bending radius for single conductors in raceways is 4 to 8 times the overall insulated diameter, depending on the specific insulation type and whether it is shielded. A 1/0 AWG XHHW-2 wire (0.410" diameter) requires a minimum bending radius of roughly 3.3 inches, which is critical to know when pulling through tight junction boxes.
