Conduit Fill for Service Entrance
Service entrance wire fill: 200A dwelling uses 2/0 copper or 4/0 aluminum in 2-inch raceway per NEC 310.12 dwelling reduction. NEC Article 230 governs.
Wire fill for service entrance is governed by NEC Article 230 and the dwelling reduction in NEC 310.12. A standard 200A residential service uses 2/0 copper or 4/0 aluminum service-entrance conductors in 2-inch raceway (EMT, IMC, RMC, or PVC Sch 80 above grade). A 100A service uses 4 AWG copper or 2 AWG aluminum in 1-1/4 inch raceway. Underground laterals from utility transformer use USE-2/RHH/RHW-2 conductors in PVC.
Load characteristics
Service-entrance conductors carry the entire dwelling load. NEC 220.83 provides the dwelling calculation method:
| Service rating | Typical dwelling | Calculated minimum |
|---|---|---|
| 100A | Small home (<1500 sqft, gas heat) | <100A required |
| 150A | Older home modernized | 100-130A |
| 200A | Standard new construction | 130-180A |
| 320A (400A class) | Large home, all-electric | 200-300A |
Most new construction defaults to 200A service for headroom, even when calculated load is lower. The dwelling reduction (83%) accounts for diversity factors in residential loading.
NEC code references
- NEC 230 — services (overhead, underground, raceways, OCPD)
- NEC 230.42 — service conductor sizing
- NEC 230.79 — minimum service rating (100A minimum for one-family dwelling)
- NEC 250.24 — service grounding, main bonding jumper
- NEC 250.66 — grounding electrode conductor size
- NEC 250.102 — main bonding jumper
- NEC 310.12 — dwelling service and feeder reduction (83% rule)
- NEC 338 — SE and USE cable
Conductor sizing — dwelling reduction (NEC 310.12)
| Service rating | Copper SE | Aluminum SE (AA-8000) |
|---|---|---|
| 100A | 4 AWG | 2 AWG |
| 125A | 2 AWG | 1/0 |
| 150A | 1 AWG | 2/0 |
| 175A | 1/0 | 3/0 |
| 200A | 2/0 | 4/0 |
| 225A | 3/0 | 250 kcmil |
| 250A | 4/0 | 300 kcmil |
| 300A | 250 kcmil | 350 kcmil |
| 400A | 400 kcmil | 600 kcmil |
Neutral reduction per NEC 220.61: typically one size smaller for predominantly 240V dwellings.
Grounding electrode conductor per NEC 250.66:
| Largest service conductor | Copper GEC | Aluminum GEC |
|---|---|---|
| 2 AWG and smaller (Cu) | 8 AWG | 6 AWG |
| 1/0 or smaller (Cu) | 6 AWG | 4 AWG |
| 2/0 or 3/0 (Cu) | 4 AWG | 2 AWG |
| Over 3/0 to 350 kcmil (Cu) | 2 AWG | 1/0 |
| Over 350 to 600 kcmil (Cu) | 1/0 | 3/0 |
(GEC to ground rod is always sufficient at 6 AWG copper per NEC 250.66(A) regardless of service size.)
Conduit sizing
Three current-carrying conductors (2 hots + neutral) in service raceway. EGC is not run with service-entrance conductors — bonding is done at the main disconnect via the main bonding jumper.
| Service | Copper conductors | Total area | Min EMT | Min PVC Sch 80 |
|---|---|---|---|---|
| 100A | 3 × 4 AWG | 0.2472 in² | 1-1/4" (32%) | 1-1/4" tight |
| 125A | 3 × 2 AWG | 0.3474 in² | 1-1/4" (over 40%) | 1-1/2" (28%) |
| 150A | 3 × 1 AWG | 0.4119 in² | 1-1/2" (33%) | 1-1/2" (33%) |
| 175A | 3 × 1/0 | 0.4892 in² | 1-1/2" (over 40%) | 2" (24%) |
| 200A | 3 × 2/0 | 0.5811 in² | 2" (30.6%) | 2" (30%) |
| 200A aluminum | 3 × 4/0 | 0.7392 in² | 2" (over 40%) | 2-1/2" (28%) |
For 320A or 400A class service, expect 2-1/2 to 3-inch raceway.
Special requirements
- Raceway material: NEC 230.43 lists permitted methods. PVC Sch 80 is standard for above-grade exposed risers due to UV and impact resistance.
- Pipe sealing: Mast-head fitting on overhead service. Underground laterals use sealing bushings to prevent moisture migration into the panel.
- Service disconnect: Required in a readily accessible location, outside or just inside the building. NEC 230.70.
- Maximum 6 service disconnects: Per NEC 230.71 — though the 2020 NEC tightened this to a single service disconnect per service in many cases.
- Working space: NEC 110.26 — 36 inches in front, 30 inches wide, 6.5 ft headroom.
- Bonding: NEC 250.92 requires bonding around concentric/eccentric knockouts and use of bonding bushings.
Worked example — 200A overhead service, 25 ft service drop
- Service rating: 200A
- Conductors: 2/0 copper THHN/THWN-2 × 2 (hots) + 1/0 copper neutral
- Raceway: 2" RMC service mast (rigid metal, weatherhead, supported per NEC 230.27)
- Fill check: 2 × 0.2223 + 0.1855 = 0.6301 in² in 2" RMC (internal area 3.408, 40% = 1.363) = 18.5%, pass with huge margin (oversize for code minimum because 2" is the smallest standard mast and provides physical strength)
- GEC: 4 AWG copper to ground rod and water pipe per NEC 250.66
- Main bonding jumper: 2 AWG copper, factory-installed in 200A meter-main combo
Quick reference
| Service | Copper SE | Aluminum SE | EMT/RMC | PVC Sch 80 |
|---|---|---|---|---|
| 100A | 4 AWG | 2 AWG | 1-1/4" | 1-1/4" |
| 150A | 1 AWG | 2/0 | 1-1/2" | 1-1/2" |
| 200A | 2/0 | 4/0 | 2" | 2" Cu / 2-1/2" Al |
| 300A | 250 kcmil | 350 kcmil | 2-1/2" | 3" |
| 400A | 400 kcmil | 600 kcmil | 3" | 3-1/2" |