Commercial Three-Phase Service Entrance
Why this matters
A residential electrician moving into light commercial work loses jobs because the language of three-phase service ("277/480 wye", "240/120 high-leg delta", "CT cabinet", "free-air ampacity at the terminations") is not part of typical residential vocabulary. The transformer at the pole or pad, the meter base, the service disconnect, and the panelboard each follow rules that residential 120/240 single-phase work does not exercise. Quoting a 400A three-phase service entrance without understanding which configuration the utility offers, what the CT cabinet looks like, and how to size the entrance conductors and conduit produces install delays, failed inspections, and customer relationship damage. This article walks the actual configurations encountered in light commercial work in the US.
Common three-phase service configurations
Four configurations dominate US commercial:
| Configuration | Phase-to-phase voltage | Phase-to-ground (line-to-neutral) voltage | Common application |
|---|---|---|---|
| 208/120 wye | 208 V | 120 V | Small office, retail, residential apartment buildings |
| 480/277 wye | 480 V | 277 V | Mid-size commercial, lighting + HVAC, mid-rise |
| 240/120 delta with high leg | 240 V | 120 V (two phases), 208 V (high leg) | Small industrial, older commercial, mixed motor + lighting |
| 480 corner-grounded delta | 480 V | 480 V (two phases), 0 V (grounded leg) | Industrial; uncommon in light commercial |
The wye configurations are the most common modern installs. The delta-with-high-leg ("orange B phase") is a legacy configuration the utility may still serve for specific customers; it requires careful identification because connecting a 120V load to the high leg destroys the load.
What the utility actually delivers
The utility steps medium-voltage primary (typically 12.47 kV or 4.16 kV in distribution) down to the secondary the customer uses. For commercial:
- Pole-mount transformer bank (rural / older areas): three single-phase transformers in delta or wye configuration
- Pad-mount transformer (modern / commercial parks): single three-phase transformer on a concrete pad
The utility owns the transformer and the primary conductors. The customer owns from the secondary terminals downstream (service-entrance conductors, meter, disconnect, panelboard). The "demarcation point" varies by utility; sometimes the secondary side of the transformer is the customer's responsibility, sometimes the utility delivers to a CT cabinet on the customer's side.
Service-entrance components
| Component | Function | Typical sizing |
|---|---|---|
| Service-entrance conductors (SE) | Carry power from utility connection to first overcurrent device | Per NEC 230, 310 |
| CT cabinet (when metering at CT level) | Houses current transformers for utility metering | Utility-specified |
| Utility meter | Records consumption | Utility-supplied |
| Service disconnect | First load-side overcurrent device + means of disconnect | NEC 230.70-91 |
| Main breaker or fused switch | OCPD per NEC 230.90 | Sized for service rating |
| Distribution panelboard | Branches to individual loads | NEC 408 |
CT (Current Transformer) cabinet metering
For services over 200A (typical threshold; some utilities use 400A), the utility installs current transformers around the service conductors and reads current via a separate meter. The CT cabinet is a separately-mounted enclosure between the utility-side conductor entry and the service disconnect.
Key features:
- Utility-sealed (only utility can open without breaking the seal)
- CTs sized to the service rating (commonly 200:5, 400:5, 800:5, 1600:5 ratios)
- Test switch for meter verification (utility's tool)
- Conductor pull-through with no taps inside (CT cabinet is not for splices)
CT cabinets are utility-supplied or contractor-supplied per local utility's rule. Some utilities provide the cabinet and CTs; the contractor installs. Some require the contractor to purchase a listed cabinet from a specific list (Milbank, Eaton, Square D).
Sizing the service-entrance conductors
Per NEC 230 and 310:
| Service amps | Single set of copper SE | Single set of aluminum SE | Notes |
|---|---|---|---|
| 200 A | 2/0 AWG Cu, 4/0 AWG Al | 4/0 AWG XHHW or 250 kcmil | 75 C termination |
| 400 A | 500 kcmil Cu, 700 kcmil Al | n/a (parallel typical) | Or 2 parallel sets of 3/0 |
| 600 A | 2 sets of 350 kcmil Cu | 2 sets of 500 kcmil Al | Parallel typical |
| 800 A | 2 sets of 500 kcmil Cu | 2 sets of 700 kcmil Al | Parallel typical |
| 1200 A | 3 sets of 500 kcmil Cu | 3 sets of 700 kcmil Al | Parallel typical |
These are starting points; actual sizing depends on:
- 75 C versus 90 C termination ratings (most equipment is 75 C; conductors are typically rated 90 C but derated to terminal rating)
- Ambient temperature correction (NEC 310.15(B)(1))
- Adjustment factor for more than 3 current-carrying conductors in a raceway (NEC 310.15(C))
- Voltage drop (FPN: NEC recommends under 3 percent feeder, under 5 percent total)
The neutral can often be smaller than the phase conductors per NEC 220.61 if the neutral carries only the unbalanced portion of the load (which is typical for three-phase wye services). For 480/277 services with mostly balanced 480V three-phase loads (motors), the neutral can be much smaller.
Conduit sizing
The conduit must fit the conductors per NEC Chapter 9 fill rules:
- Single conductor: 53 percent fill
- Two conductors: 31 percent fill
- Three or more conductors: 40 percent fill
For SE applications, the typical configuration is three phase conductors plus one neutral plus a grounding electrode conductor (GEC), with the equipment grounding conductor (EGC) often a separate path. Three phase conductors plus neutral = 4 conductors = 40 percent fill rule.
| Conductor size | Conduit type | Typical conduit size for 3 + 1 |
|---|---|---|
| 2/0 AWG | RMC | 2 inch |
| 4/0 AWG | RMC | 2 1/2 inch |
| 500 kcmil | RMC | 4 inch |
| 700 kcmil | RMC | 4 inch |
For underground runs, use schedule 80 PVC at the riser or convert to PVC underground after the riser. Verify against the AHJ's preferred practice.
Service-entrance disconnect
NEC 230.70 requires the service disconnect to be located:
- Outside the building, OR
- Inside, nearest the point of entry of the conductors (typically within 5 feet of entry)
- Always readily accessible
References
- NFPA 70 (NEC) 2023, Article 230 (Services).
- NFPA 70 (NEC) 2023, Article 240 (Overcurrent Protection).
- NFPA 70 (NEC) 2023, Article 250 (Grounding and Bonding).
- NFPA 70 (NEC) 2023, Article 408 (Switchboards and Panelboards).
- NFPA 70 (NEC) 2023, Article 310 (Conductors for General Wiring).
- NFPA 70E (Electrical Safety in the Workplace).
- IEEE 142 (Grounding of Industrial and Commercial Power Systems).
- Utility service specification books (each utility publishes their own; e.g., PG&E EUSERC, Con Edison Bluebook).
- Manuall internal: Electrical Service Panel Upgrade, Electrical Three Phase Residential Light Commercial, Electrical Service Entrance Meter Base.