Service Entrance Cable Sizing for 100A, 200A, and 400A

Service entrance conductors carry current from the utility transformer to your main breaker, and sizing them correctly is one of the most consequential decisions in any residential electrical installation. Get it wrong and you either waste money on oversized wire or risk overheated conductors that code inspectors will flag before the meter ever gets set. This guide walks through the applicable NEC rules, the typical conductor sizes for 100 A, 200 A, and 400 A services, and a worked example for the most common case: a 200 A residential service.

Always verify your installation against the current edition of the NEC adopted in your jurisdiction, your utility's service entrance requirements, and a licensed electrician before purchasing materials or pulling a permit.

How NEC 310.12 Changes the Math for Dwellings

The general ampacity tables in NEC Article 310 are based on continuous use at the conductor's rated temperature. For most circuits, that means you size the wire to handle 100% of the load continuously (or 125% for continuous loads on branch circuits and feeders).

Service entrance conductors for dwelling units get a different treatment under NEC 310.12. That section allows residential service conductors to be sized based on 83% of the standard ampacity table values, provided the service supplies a single-phase, 120/240 V, three-wire dwelling-unit service. The logic is that a home's total connected load almost never operates simultaneously at its theoretical maximum, so the code allows a calculated reduction.

In practice, the 83% rule means you divide the service amperage by 0.83 to find the minimum ampacity the conductors must have from the standard tables. For a 200 A service: 200 ÷ 0.83 = 241 A. You then look up a conductor whose Table 310.12 ampacity meets or exceeds 241 A.

This is why 4/0 AWG aluminum (rated 205 A at 75°C in Table 310.15(B)(16)) qualifies for a 200 A service under 310.12 even though 205 A is less than 200 A on its face. The code permits it because 200 × 0.83 = 166 A, which 4/0 Al exceeds. Utilities and local amendments vary, so confirm before ordering.

Typical Conductor Sizes by Service Rating

The table below reflects common practice using 75°C terminations and the NEC 310.12 dwelling allowance. Aluminum is the dominant material for service entrance cable because the cost difference over copper is significant at these large sizes.

These are starting points. Conductor length, burial depth, conduit fill, ambient temperature corrections, and utility requirements all affect the final selection. A long run from a pole-mount transformer to a detached workshop, for example, may require upsizing beyond what the service rating alone dictates.

For background on why aluminum conductors require upsizing compared to copper on equivalent circuits, and the differences in termination requirements, that article covers the full comparison.

Copper vs. Aluminum for Service Entrance

Copper remains the default choice for smaller conductors and most branch circuit wiring, but at service entrance sizes the economics shift sharply toward aluminum. A 4/0 aluminum SER cable runs roughly one-third the material cost of equivalent copper. Most meter bases and main breakers have aluminum-rated lugs, making the termination straightforward.

The tradeoffs are real. Aluminum expands and contracts more than copper with temperature cycling, which is why anti-oxidant compound and proper torque on terminals matter. Aluminum also requires larger conduit or larger cable because the wire itself is physically bigger at equivalent ampacity.

For a deeper look at the copper vs. aluminum decision, including termination best practices and where copper still wins, that comparison covers both materials across the full range of residential applications.

Worked Example: Sizing a 200 A Service Entrance

Here is the step-by-step process for a typical single-family home requesting a 200 A service.

Step 1 - Confirm the rule applies. Single-phase, 120/240 V, three-wire service to a single dwelling unit: NEC 310.12 applies.

Step 2 - Apply the 83% factor. 200 A ÷ 0.83 = 241 A. The conductor's standard table ampacity must be at least 241 A.

Step 3 - Select aluminum conductor. From NEC Table 310.12, 250 kcmil aluminum has a listed ampacity of 255 A at 75°C, which exceeds 241 A. However, 4/0 AWG aluminum is listed at 205 A in the standard table. Under the 310.12 allowance specifically, 4/0 Al is explicitly listed for a 200 A service. Most utilities and jurisdictions accept 4/0 Al SER or SER-type cable for a 200 A residential service because the NEC explicitly calls it out.

Step 4 - Check the neutral. For a standard 120/240 V single-phase service, the grounded neutral conductor can be sized per Table 310.12 as well, though the minimum is 4 AWG copper or 2 AWG aluminum per NEC 230.23 for most installations.

Step 5 - Verify with the utility. Your utility may require a specific cable type (SER vs. individual THWN conductors in conduit), a specific conductor size, or a particular meter base configuration. This step is not optional; utilities have disconnected services over non-compliant entrances.

400 A Services and Parallel Conductors

A 400 A service is common for larger homes, shops with heavy equipment, and EV charging combined with HVAC and electric cooking loads. Single-conductor solutions exist at 400 A, but they become unwieldy.

Single-conductor approach: 600 kcmil aluminum or 350 kcmil copper can reach 400 A under the dwelling service rule, but cable at these sizes is stiff, heavy, and difficult to pull. Conduit radius requirements become a real installation challenge.

Parallel conductors: NEC 310.10(H) permits conductors 1/0 AWG and larger to be run in parallel. Two sets of 3/0 AWG copper or two sets of 250 kcmil aluminum per phase is a common practical solution for 400 A services. Each set must be the same length, same conductor material, same AWG, same insulation type, and terminated in the same manner. For more on when and how to use parallel conductors, including the NEC requirements and common mistakes, that article covers the topic in detail.

Utilities often have strong preferences here, and some require a specific configuration. A 400 A installation warrants a pre-application conversation with your utility before buying materials.

Conduit and Physical Installation Considerations

Service entrance conductors typically run in one of two configurations: service entrance cable (SER or SEU) installed per NEC Article 230, or individual THWN-2 or XHHW-2 conductors pulled through rigid metal conduit (RMC) or intermediate metal conduit (IMC). Some utilities require rigid conduit from the weather head down to the meter base; others accept SER cable with the appropriate drip loop.

Conduit fill is calculated per NEC Chapter 9, Table 1, and the physical diameter of large service conductors makes conduit sizing a real constraint. Two sets of 250 kcmil aluminum for a 400 A service in a single conduit can require 2.5-inch or 3-inch rigid, depending on the number of conductors and insulation type.

Underground services use USE-2 or URD cable rated for direct burial, or THWN-2 conductors in schedule 80 PVC or rigid metal conduit. Burial depth requirements under NEC Table 300.5 vary by conduit type and location (under a driveway, under a building, in open ground).

Frequently Asked Questions

Can I use aluminum wire for a 200 A service entrance?

Yes. Aluminum is the standard material for residential service entrance conductors at 200 A. NEC 310.12 and most utility requirements explicitly accommodate 4/0 AWG aluminum SER cable for a 200 A single-phase residential service. Use anti-oxidant compound at terminations and ensure all lugs are aluminum-rated.

What size wire do I need for a 100 A service?

Under NEC 310.12, a 100 A single-phase residential service typically uses 2 AWG aluminum or 4 AWG copper. Apply the 83% factor: 100 ÷ 0.83 = 120.5 A required table ampacity. From the 310.12 table, 2 AWG aluminum is listed at 130 A, which satisfies the requirement. Confirm with your utility and local code amendment.

Does the 83% rule apply to subpanels and feeders?

No. NEC 310.12 applies specifically to service entrance conductors for single-family dwellings. Feeder conductors to a subpanel are sized per Article 215, which uses the full load calculation or the standard ampacity tables without the 83% shortcut. For sizing a subpanel feeder, the calculation follows a different path.

How is a 400 A service entrance typically wired?

Most 400 A residential installations use either a single large conductor (600 kcmil aluminum is common) or two parallel sets of smaller conductors (250 kcmil aluminum per phase is a frequent choice). The service may feed two 200 A main breakers in a split meter configuration, or a single 400 A main disconnect with a large distribution panel. Utility requirements vary significantly for 400 A, so the utility's engineering department should be contacted before design is finalized.