The service entrance is the most critical part of any electrical system. It determines the total capacity available for all loads in the building. Proper service entrance wire sizing ensures safe operation, code compliance, and adequate capacity for current and future electrical needs. Whether you are installing a new service or upgrading an existing one, understanding NEC requirements for service conductor sizing is essential.
Understanding Service Entrance Components
A complete service entrance consists of several components, each with specific sizing requirements. Understanding how these work together helps ensure proper system design.
Service Drop vs Service Lateral
Overhead service (service drop) brings power from the utility pole via aerial cables to a weatherhead on the building. Underground service (service lateral) brings power through buried conduit from a pad-mounted transformer or underground utility lines. The wire sizing requirements are similar, but installation methods and protection requirements differ significantly.
Service Entrance Conductors
Service entrance conductors run from the point of utility connection to the main service disconnecting means (main breaker or fused disconnect). These conductors are not protected by overcurrent devices until they reach the main breaker, making proper sizing critical for safety.
Grounding Electrode Conductor
The grounding electrode conductor connects the service to the grounding electrode system (ground rods, water pipe, concrete-encased electrode). Its size is based on the size of the service entrance conductors per NEC Table 250.66.
Service Conductor Sizing Per NEC
NEC Article 230 governs service entrance requirements. Service conductors must be sized for the calculated load but cannot be smaller than the minimum sizes shown in Table 310.12.
Standard Service Sizes
| Service Size | Copper Conductor | Aluminum Conductor | Typical Application |
|---|---|---|---|
| 100 Amp | 4 AWG | 2 AWG | Small homes, condos |
| 125 Amp | 2 AWG | 1/0 AWG | Moderate homes |
| 150 Amp | 1 AWG | 2/0 AWG | Medium homes |
| 200 Amp | 2/0 AWG | 4/0 AWG | Standard new construction |
| 320 Amp | 350 kcmil | 500 kcmil | Large homes |
| 400 Amp | 400 kcmil | 600 kcmil | Large homes, light commercial |
Temperature Rating Note
Residential Service Sizing
100 Amp Service
Once standard for residential construction, 100A service is now considered minimum and may not meet the demands of modern homes with central air conditioning, electric water heaters, and multiple high-power appliances. Many jurisdictions now require 200A minimum for new construction.
- Copper conductors: 4 AWG THWN or equivalent
- Aluminum conductors: 2 AWG THWN or equivalent
- Neutral: Same size as ungrounded conductors or per calculation
- Ground electrode conductor: 8 AWG copper minimum
- Suitable for: Small homes under 1,500 sq ft with gas appliances
200 Amp Service
The current standard for residential construction, 200A service provides adequate capacity for most single-family homes including central air conditioning, electric ranges, clothes dryers, and future additions like EV charging.
- Copper conductors: 2/0 AWG THWN or equivalent
- Aluminum conductors: 4/0 AWG THWN or equivalent
- Neutral: Can often be reduced to 1/0 copper or 3/0 aluminum
- Ground electrode conductor: 4 AWG copper minimum
- Suitable for: Most single-family homes up to 3,000+ sq ft
Cost Saving Tip
400 Amp Service
Large homes, homes with significant workshop or shop loads, or homes with multiple high-power systems may require 400A service. This is typically provided as two 200A services from a meter pack rather than a single 400A service.
- Usually configured as two 200A panels fed from a 400A meter main
- Each panel fed with 4/0 AWG copper or 250 kcmil aluminum
- Requires CT (current transformer) metering in many jurisdictions
- Suitable for: Large luxury homes, home workshops, multi-unit dwellings
Neutral Conductor Sizing
The neutral conductor in a service entrance carries the unbalanced load between phases plus the full load of any 120V circuits. NEC 220.61 allows the neutral to be sized at 70% of the ungrounded conductor capacity for loads over 200 amps, but this reduction requires careful load calculation.
| Service Size | Minimum Neutral (Copper) | Minimum Neutral (Aluminum) |
|---|---|---|
| 100 Amp | 4 AWG | 2 AWG |
| 200 Amp (if calculated) | 1/0 AWG | 3/0 AWG |
| 200 Amp (full) | 2/0 AWG | 4/0 AWG |
| 400 Amp (if calculated) | 250 kcmil | 350 kcmil |
Underground Service Installation
Underground services (service laterals) have additional requirements for conductor type and burial depth. USE-2 or XHHW-2 conductors are commonly used for underground installations.
Burial Depth Requirements
| Installation Method | Minimum Depth |
|---|---|
| Direct burial cable | 24 inches |
| Rigid metal conduit (RMC) | 6 inches |
| Intermediate metal conduit (IMC) | 6 inches |
| Schedule 80 PVC | 18 inches |
| Schedule 40 PVC with concrete | 18 inches |
| Under concrete slab 4 inch thick | 18 inches (from bottom of slab) |
Conduit Sizing
Service entrance conductors in conduit require adequate space for heat dissipation and pulling. Size conduit to allow no more than 40% fill with three or more conductors.
| Service Size | Minimum Conduit (PVC) | Minimum Conduit (RMC) |
|---|---|---|
| 100 Amp (aluminum) | 1-1/4 inch | 1-1/4 inch |
| 200 Amp (aluminum) | 2 inch | 2 inch |
| 200 Amp (copper) | 1-1/2 inch | 1-1/2 inch |
| 400 Amp (per run) | 2-1/2 inch | 2-1/2 inch |
Service Entrance Cable Types
SER (Service Entrance Round)
SER cable contains three insulated conductors plus a bare neutral wrapped with tape, all within a PVC outer jacket. It is commonly used for above-ground service entrance runs and as a feeder to subpanels.
SEU (Service Entrance Underground)
SEU cable is similar to SER but uses a flat configuration that is easier to route through walls. Despite the underground name, SEU is not suitable for direct burial and is used for above-ground service entrance installations.
USE (Underground Service Entrance)
USE cable is specifically designed for direct burial underground service laterals. It features moisture-resistant insulation suitable for wet locations and burial without conduit.
Common Confusion Warning
Service Upgrade Considerations
When upgrading from an older service (often 60A or 100A) to modern standards, several factors affect the project scope and cost.
Utility Requirements
Contact your utility company early in the planning process. They may need to upgrade the transformer serving your home, install a new meter base, or modify the service drop. Utility work can add significant time to the project and may involve fees.
Panel Location
Older homes often have panels in locations that do not meet current code requirements for working space or accessibility. A service upgrade may require relocating the panel, adding significant cost but improving safety and convenience.
Grounding System
Modern code requires more comprehensive grounding than older installations. A service upgrade typically requires installing new ground rods, bonding to water piping, and possibly installing a concrete-encased electrode if new concrete work is being done.
Commercial Service Sizing
Commercial services are typically three-phase and may be much larger than residential services. They require detailed load calculations per NEC Article 220 and often involve engineering calculations beyond basic wire sizing.
| Service Size (3-Phase) | Conductors per Phase | Typical Application |
|---|---|---|
| 200 Amp | 3/0 AWG per phase | Small retail, offices |
| 400 Amp | 500 kcmil per phase | Medium commercial |
| 600 Amp | 2 x 300 kcmil per phase | Large retail |
| 800 Amp | 2 x 400 kcmil per phase | Large commercial |
| 1200 Amp | 3 x 350 kcmil per phase | Large buildings |
Proper service entrance sizing ensures your electrical system can safely meet current and future needs. When planning a new service or upgrade, work with your utility company and local inspection department early in the process to understand all requirements. A properly sized service entrance is a long-term investment in your property safety and functionality.