Conduit Fill for Sub-Panel Feeders
Sub-panel feeder wire fill: 100A uses 3 AWG copper in 1-1/4 EMT, 125A uses 2 AWG in 1-1/4, 200A uses 2/0 in 2 inch. NEC 215 and 225 govern.
Wire fill for sub-panel feeders is governed primarily by NEC Article 215 (feeders) and the load calculation rules of NEC Article 220. Common residential sizes are 60A, 100A, 125A, and 200A. Typical 100A sub-panel feed uses 3 AWG copper THHN (4-wire including neutral and EGC) in 1-1/4 inch EMT or PVC. A 200A sub-panel feed uses 2/0 copper in 2-inch raceway. Detached structures must have a separate EGC and a new grounding electrode system per NEC 250.32.
Load characteristics
A sub-panel is sized to the calculated load it serves, not the main panel rating:
| Sub-panel rating | Typical use |
|---|---|
| 60A | Detached garage with light load |
| 100A | Workshop, ADU, large garage |
| 125A | Pool house, in-law unit |
| 200A | Full secondary dwelling, large addition |
Run NEC 220 load calculation on the loads served. Don't size purely to "future expansion" — oversized neutrals and conductors waste copper and cost.
NEC code references
- NEC 215 — feeder requirements (sizing, OCPD, neutral)
- NEC 220 — load calculation for feeders
- NEC 225 — outside branch circuits and feeders to other buildings
- NEC 250.32 — grounding at separate structures (4-wire feeder, separate GES)
- NEC 408.36 — overcurrent protection of panelboards
- NEC 310.16 — conductor ampacity tables
Conductor sizing
Use 75°C column of NEC Table 310.16:
| Sub-panel rating | Copper THHN | Aluminum XHHW-2 |
|---|---|---|
| 60A | 6 AWG | 4 AWG |
| 100A | 3 AWG | 1 AWG |
| 125A | 2 AWG | 1/0 |
| 150A | 1 AWG | 2/0 |
| 175A | 1/0 | 3/0 |
| 200A | 2/0 | 4/0 |
EGC size per NEC 250.122:
| Feeder OCPD | Copper EGC | Aluminum EGC |
|---|---|---|
| 60A | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
| 125A | 6 AWG | 4 AWG |
| 200A | 6 AWG | 4 AWG |
Neutral may be reduced per NEC 220.61 when the calculated neutral load is less than the ungrounded conductors — common in sub-panels with predominantly 240V loads.
Conduit sizing
Standard 4-wire feeder (2 hots + neutral + EGC):
| Feeder | Conductors | Total area | Min EMT | Min PVC Sch 40 |
|---|---|---|---|---|
| 60A | 3 × 6 + 1 × 10 EGC | 0.1733 in² | 1" (38%) | 1" (33%) |
| 100A | 3 × 3 + 1 × 8 EGC | 0.3009 in² | 1-1/4" (38.6%) | 1-1/4" (35.4%) |
| 125A | 3 × 2 + 1 × 6 EGC | 0.3719 in² | 1-1/2" (35.4%) | 1-1/2" (33.6%) |
| 150A | 3 × 1 + 1 × 6 EGC | 0.4424 in² | 1-1/2" (over 40%) | 1-1/2" (over 40%) |
| 200A | 3 × 2/0 + 1 × 6 EGC | 0.7038 in² | 2" (37.1%) | 2" (36.7%) |
Aluminum requires one trade size larger raceway in most cases.
Special requirements
- 4-wire feeder mandatory at sub-panels — neutral and EGC must remain separated downstream of the main bonding jumper.
- Detached structure: separate grounding electrode system (ground rod or Ufer) per NEC 250.32(A). Bond EGC to local grounding electrode at the sub-panel, but do NOT bond neutral to ground at the sub-panel.
- Outside feeder: NEC 225.30 limits the number of supplies to a structure. Single feeder per structure unless one of the exceptions applies.
- Disconnect at sub-panel: If the sub-panel has 6 or fewer circuit breakers it can serve as its own disconnect; otherwise a main breaker is required.
- OCPD location: Sub-panel feeder breaker lives at the upstream panel (main). The sub-panel itself can be a main-lug-only (MLO) panelboard or a main-breaker panel.
Worked example — 100A sub-panel in detached workshop, 80 ft from house
- Calculated load: 75A (per NEC 220 with workshop tools, lighting, receptacles)
- Feeder OCPD: 100A 2-pole breaker in main panel
- Conductors: 3 AWG copper THHN × 3 (L1, L2, N) + 8 AWG copper EGC
- Raceway: 1-1/4" PVC Schedule 40 underground (PVC is preferred direct-burial when sleeved at risers)
- Fill check: 3 × 0.0973 + 1 × 0.0366 = 0.3285 in² in 1-1/4" PVC Sch 40 (internal area 1.526, 40% = 0.610) = 21.5%, pass.
- Voltage drop: 80 ft × 75A on 3 AWG copper ≈ 2.5V (1.0%) — well under 3% limit.
- Grounding: Drive two ground rods at workshop, bond to sub-panel ground bar with 6 AWG copper (NEC 250.66). Neutral isolated from ground bar.
Use the Conduit Fill Calculator to verify any modification.
Quick reference
| Sub-panel | Copper feeder | EGC | Conduit (EMT) | Conduit (PVC) |
|---|---|---|---|---|
| 60A | 6 AWG × 3 | 10 AWG | 1" | 1" |
| 100A | 3 AWG × 3 | 8 AWG | 1-1/4" | 1-1/4" |
| 125A | 2 AWG × 3 | 6 AWG | 1-1/2" | 1-1/2" |
| 150A | 1 AWG × 3 | 6 AWG | 2" | 1-1/2" tight |
| 200A | 2/0 × 3 | 6 AWG | 2" | 2" |