Sizing Wire for a 400 Amp Service: An NEC Calculation Guide

Answering First: What is the 400 Amp Service Wire Size?

For a 400-amp service, the most common 400 amp service wire size approach is to use parallel service entrance conductors. A typical configuration is two parallel sets of 250 kcmil aluminum conductors per phase (or comparable copper equivalents paralleled). If you choose a single conductor per phase rather than parallel runs, you must select a single conductor sized to meet the continuous ampacity required — that commonly means very large conductors (for example, in many practical installations a single-conductor solution requires conductors in the range of roughly 600 kcmil copper or on the order of 1000 kcmil aluminum, depending on the ampacity column and terminal ratings). The definitive answer to what size wire for a 400 amp service depends on several factors, including the calculated load per NEC Article 220, whether it’s a residential or commercial service, conductor material, and the terminal temperature ratings on the equipment. For dwelling units, the NEC permits a limited 83% sizing allowance for certain single‑phase dwelling services (see the dwelling services provisions), which significantly affects the final conductor calculation. Always consult the ampacity tables in NEC Table 310.16 for the edition adopted by your authority having jurisdiction and apply necessary adjustments before finalizing your wire size.

Understanding the Foundation: Service Load Calculation NEC

Before selecting any conductor, a precise service load calculation following NEC Article 220 is required. This calculation determines the actual demand of the installation, which is often less than the nameplate service rating. For a 400 amp residential service, the calculation includes general lighting, small-appliance circuits, laundry circuits, and specific loads such as HVAC systems, water heaters, and electric ranges. For a deeper dive, you can review how the 2023 NEC changes dwelling unit load calculations. The calculated load, not just the 400A rating, is the starting point for all subsequent steps.

Selecting Conductors: Copper vs. Aluminum Service Entrance Cable

The choice between copper and aluminum service entrance cable is a major decision point. Copper offers higher conductivity in a smaller size and simpler termination characteristics, while aluminum (or copper-clad aluminum) often provides a cost advantage for larger services like 400A. When selecting conductors, NEC Table 310.16 is the primary reference for ampacities. For service connections, electricians commonly use the 75°C (167°F) column because many lugs and terminals are listed at that temperature; however, you must confirm that the equipment terminals are listed for the temperature column you choose. Using a higher temperature column without appropriately listed terminals is a code violation.

The Core Calculation: Determining Your 400 Amp Service Wire Size

Once you have your calculated load, you can determine the conductor size. For a 400A service, you have two primary options: a single, very large conductor set or two (or more) smaller, paralleled sets.

Paralleling Conductors NEC: The Preferred Method

For services of 400A and larger, using parallel service entrance conductors is a common and practical method. It involves running two or more identical conductors per phase instead of one massive conductor. This method, governed by the paralleling conductors provisions in the NEC, makes the installation more manageable because smaller wires are more flexible and easier to pull and terminate. Per paralleling rules, all paralleled conductors in each phase must be the same material, length, insulation type, and circular-mil area. A key benefit for a 400 amp residential service is that, where the dwelling allowances apply, you can size conductors using the dwelling service provision (the commonly referenced 83% rule) and then paralleled conductors meet that per-run ampacity requirement.

Step-by-Step: Sizing Parallel Conductors for a 400A Residential Service

1. Apply the 83% Rule (where applicable): For a 400A residential service under the dwelling allowance example, calculate the minimum required ampacity: 400 A × 0.83 = 332 A. This is the total ampacity your conductors must be able to carry for that application.
2. Divide for Parallel Runs: If you plan two parallel sets per phase, divide the required ampacity by the number of parallel sets: 332 A / 2 = 166 A per set (before accounting for termination temperature ratings and any derating).
3. Consult NEC Table 310.16 and Termination Ratings: Look in the appropriate temperature column for a conductor with an ampacity at least equal to the per-set requirement and make sure the equipment terminals are listed for that temperature rating.
   • For Aluminum (e.g., XHHW-2 / USE‑2): 250 kcmil aluminum commonly shows ampacities that meet or exceed typical per-set requirements for many 400 A dwelling examples, but confirm the exact ampacity in the edition of Table 310.16 enforced by your AHJ and that the conductor type is listed for the application.
   • For Copper (e.g., THWN-2): 3/0 AWG copper has ampacities in the ~200 A range in many NEC editions, which is often adequate for a per-set target similar to the example above; again, verify the exact table values and terminal ratings.
4. Final Selection: Paralleling two sets of 250 kcmil aluminum conductors per phase is a common, cost‑effective choice for many 400 A residential services — but final selection must be verified against the ampacity table, applicable temperature column, terminal ratings, and any derating factors.

Correct application of the NEC rules and confirmation with the AHJ and the equipment manufacturer are critical to ensure the chosen conductors meet all requirements.

Critical Adjustments and Other Considerations

Sizing conductors isn’t just a table lookup. Professionals must account for conditions of use that can derate a conductor’s ampacity.

• Conductor Ampacity Adjustment: If you run conductors through locations with high ambient temperatures (above 30°C) or bundle more than three current‑carrying conductors in a raceway, you must apply correction and adjustment factors from the applicable NEC ampacity rules. Those derating factors can require a larger conductor size than the simple table lookup suggests.
• Voltage Drop Calculation for Services: For long service runs, a voltage drop calculation is important. The NEC recommends limiting voltage drop to 3% for feeders (and 5% total for feeder plus branch circuits combined) as a guideline for efficiency and equipment performance. Excessive voltage drop can cause equipment to operate poorly.
• Metering and Utility Requirements: Utilities specify meter base and metering equipment requirements for services. Utility practices vary by company and location: some utilities accept a Class 320 meter arrangement for certain 400 A services, others require a meter socket rated specifically for 400 A or CT metering. Do not assume a particular meter type — verify the utility’s metering and base specification before ordering equipment or installing the service.
• Service Disconnects: The main service disconnecting means and the number and location of service disconnecting means follow the service article requirements; some configurations allow multiple grouped disconnects (see the rules for two to six service disconnects in the NEC and consult your AHJ). Emergency disconnect and labeling rules also apply and are located in the service and emergency disconnect provisions — verify the exact provision enforced by your jurisdiction. For more on multiple disconnects, review how many service disconnecting means are allowed, and for labeling requirements see service equipment labeling guidance. All of these items fall under the scope of NEC Article 230 (Services) and related sections; confirm the exact code edition and local amendments with your AHJ.

Sizing Grounding and Bonding Conductors

Correctly sizing the grounding and bonding system is as important as sizing the service conductors. The sizes vary based on the service conductor size and the conductor material.

Grounding Electrode Conductor Sizing

The grounding electrode conductor (GEC) sizing is based on the largest ungrounded service conductor (or the equivalent area for paralleled conductors) and is taken from NEC Table 250.66 (use the edition adopted by your AHJ). When paralleled conductors create an equivalent largest ungrounded conductor area (for example two 250 kcmil aluminum runs give an equivalent area often treated in calculations as 500 kcmil), Table 250.66 is used to determine the GEC size. In many editions and common practice, the 500 kcmil equivalent example commonly results in a GEC such as #2 AWG copper or 1/0 AWG aluminum, but always verify the exact requirement in Table 250.66 for your adopted NEC edition before finalizing the GEC.

A Note on Sizing for Other Common Services

A related question is the ground wire size for a 200 amp service. Distinguish among the grounded conductor (neutral), the equipment grounding conductor (EGC) for fault‑current paths, and the grounding electrode conductor (GEC) that connects to ground rods or other electrodes. The GEC is sized from Table 250.66 based on the largest ungrounded service conductor. Consult the table for the adopted NEC edition to determine the exact GEC size for a 200 A service.

Sizing Wire for a 125 Amp Sub Panel

For a 125 A subpanel feeder you would normally consult the ampacity tables and the termination temperature rating. Typical conductor choices are 1 AWG copper or 2/0 aluminum in many NEC editions for a 125 A feeder, but the exact selection depends on conductor insulation (THHN/THWN‑2/XHHW‑2), terminal temperature ratings, ambient conditions, and any bundling or derating — so check the ampacity table and the equipment information before you finalize the conductor size.

Frequently Asked Questions (FAQs)

1. What size wire for a 400 amp service is most common for residential installations?

A commonly used configuration for a 400 amp residential service is parallel conductors — for example two sets of 250 kcmil aluminum conductors per phase is widely used. The final choice depends on your calculated load, the NEC ampacity tables, terminal ratings, ambient and bundling conditions, and the AHJ/utility requirements.

2. Why is using parallel service entrance conductors preferred over a single large wire?

Paralleling conductors (e.g., two 250 kcmil aluminum conductor runs) is often preferred because smaller conductors are more flexible, easier to pull, and easier to terminate. Paralleling is allowed by the NEC when the conductors in each parallel set are identical in material, length, insulation type, and circular‑mil area.

3. How does conductor ampacity adjustment affect my wire choice?

Conductor ampacity adjustment must be applied when ambient temperature correction factors or conductor bundling apply (NEC adjustment and correction factors). These adjustments reduce the ampacity you can use from the table and often require increasing conductor size. Always apply the NEC correction and adjustment rules to the ampacity from Table 310.16 (or the applicable table in the adopted code edition) before finalizing conductor selection.