Direct Code Answer
YES. Bare ground wires ALWAYS count toward conduit fill percentage.
One of the most widespread misconceptions among apprentice electricians and DIY installers is that bare copper grounding wires take up “zero space” or are exempt from National Electrical Code (NEC) raceway fill limits.
According to NEC Chapter 9, Note 2:
“Equipment grounding or bonding conductors, where installed, shall be included when calculating conduit fill. The actual dimensions of the equipment grounding conductor (insulated or bare) shall be used in the calculation.”
Bare Wire Area Dimensions (NEC Chapter 9, Table 8)
While insulated conductors (THHN, XHHW) use NEC Chapter 9, Table 5 for their dimensions, bare conductors use NEC Chapter 9, Table 8 (Conductor Properties).
Because bare wire has no outer plastic sheath or nylon jacket, its cross-sectional area is significantly smaller than insulated wire of the same gauge, but it is never zero.
Bare Copper Conductor Dimensions (NEC Table 8 Matrix)
| Wire Gauge (AWG / kcmil) | Bare Wire Stranding | Outer Diameter (in.) | Cross-Sectional Area (sq. in.) |
|---|---|---|---|
| #14 AWG | Solid / Stranded | 0.073 in. | 0.004 sq. in.0.004 sq. in. |
| #12 AWG | Solid / Stranded | 0.092 in. | 0.006 sq. in.0.006 sq. in. |
| #10 AWG | Solid / Stranded | 0.116 in. | 0.011 sq. in.0.011 sq. in. |
| #8 AWG | Stranded | 0.146 in. | 0.017 sq. in.0.017 sq. in. |
| #6 AWG | Stranded | 0.184 in. | 0.027 sq. in.0.027 sq. in. |
| #4 AWG | Stranded | 0.232 in. | 0.042 sq. in.0.042 sq. in. |
| #2 AWG | Stranded | 0.292 in. | 0.067 sq. in.0.067 sq. in. |
| 1/0 AWG | Stranded | 0.373 in. | 0.109 sq. in.0.109 sq. in. |
| 2/0 AWG | Stranded | 0.419 in. | 0.137 sq. in.0.137 sq. in. |
| 3/0 AWG | Stranded | 0.470 in. | 0.173 sq. in.0.173 sq. in. |
| 4/0 AWG | Stranded | 0.528 in. | 0.219 sq. in.0.219 sq. in. |
Practical Example: Why Bare Grounds Matter
Consider a tight conduit calculation: pulling 9 #12 THHN insulated circuit wires inside a 1/2″ EMT conduit.
- 1/2″ EMT 40% Max Allowed Fill Area: 0.122 sq. in.
- Area of 9 #12 THHN Wires: 9×0.0133=0.1197 sq. in. (Passes – under 0.122).
- Adding 1 Bare #12 Ground Wire:
- Total Area = 0.1197+0.006=0.1257 sq. in.
- Result: 0.1257>0.122→FAIL (Red tag inspection failure!).
Ignoring the single bare ground wire causes an illegal overfill condition that fails municipal inspection!
Physical Fill vs. Thermal Derating: Crucial Distinction
Electricians often confuse Physical Conduit Fill (NEC Chapter 9) with Thermal Ampacity Derating (NEC Article 310.15).
| Code Aspect | Are Bare Grounds Counted? | Reference Code Section |
|---|---|---|
| Physical Conduit Fill | YES | NEC Chapter 9, Note 2 & Table 8 |
| Thermal Ampacity Derating | NO | NEC Section 310.15(E) |
Why aren’t grounds counted for ampacity derating?
Under normal operating conditions, equipment grounding conductors carry zero current. Heat generated inside a raceway comes strictly from current-carrying phase (hot) and neutral conductors. Therefore, while ground wires occupy physical volume inside the pipe, they do not contribute to heat buildup.
Related Tools & Resources
- Calculate compliant wire loads: NEC Conduit Fill Calculator
- View complete trade size charts: Interactive Conduit Fill Table
- Learn more about derating: Conduit Fill Derating Rules (NEC 310.15 Guide)