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 involves using parallel service entrance conductors. Typically, this means two sets of 250 kcmil aluminum or 3/0 AWG copper conductors per phase. If using a single conductor per phase for a full 400A load (assuming a calculated load of 400A with no derating factors and without the 83% residential allowance), you would need at least 1000 kcmil aluminum or 600 kcmil copper. 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, and conductor material. For dwelling units, NEC 310.12 allows service conductors to be sized at 83% of the service rating, which significantly impacts the final calculation. Always consult NEC Table 310.16 for ampacity 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 NEC as detailed in Article 220 is non-negotiable. This calculation determines the actual demand of the installation, which is often less than the service rating itself. For a 400 amp residential service, the calculation includes general lighting, small appliances, laundry circuits, and specific loads like HVAC systems, water heaters, and electric ranges. Understanding how these calculations have evolved is critical, especially with recent code updates. 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. While copper offers superior conductivity in a smaller size, aluminum provides a significant cost advantage, making it the prevalent choice for larger services like 400A. When selecting conductors, NEC Table 310.16 is your primary reference for conductor ampacities. For service calculations, electricians almost always use the 75°C (167°F) column, as most lugs and terminals on service equipment are rated for 75°C. Using a higher temperature rating from the table without appropriately rated terminals is a common and dangerous 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, large conductor set or two smaller, paralleled sets.

Paralleling Conductors NEC: The Preferred Method

For services 400A and larger, using parallel service entrance conductors is standard practice. It involves running two or more smaller conductors per phase instead of one large one. This method, governed by NEC 310.10(G), makes the installation more manageable, as smaller wires are more flexible and easier to terminate. Per paralleling conductors NEC rules, all parallel conductors in each phase must be identical in material, length, insulation type, and cross-sectional area. A key benefit for a 400 amp residential service is found in NEC 310.12, which permits service and feeder conductors for dwelling units to have an ampacity of not less than 83% of the service rating.

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

1. Apply the 83% Rule: For a 400A residential service, calculate the minimum required ampacity: 400A × 0.83 = 332A. This is the total ampacity your conductors must be able to carry.
2. Divide for Parallel Runs: Since we are running two sets of conductors, divide the required ampacity by the number of parallel runs: 332A / 2 = 166A per conductor set.
3. Consult NEC Table 310.16: Look in the 75°C column for a conductor with an ampacity of at least 166A.
   • For Aluminum (XHHW-2): 250 kcmil aluminum is rated for 205A. This easily meets the 166A requirement. The 250 kcmil aluminum wire ampacity makes it a perfect fit.
   • For Copper (THWN-2): 3/0 AWG copper is rated for 200A, which also meets the 166A requirement.
4. Final Selection: The most common and cost-effective choice is paralleling two sets of 250 kcmil aluminum conductors for the ungrounded (hot) conductors and the neutral.

Correctly applying these NEC rules is critical. Master service calculations with our in-depth NEC code courses.

Critical Adjustments and Other Considerations

Sizing wire isn’t just about a table lookup. Professional electricians must account for conditions of use that can derate a conductor’s ampacity.

• Conductor Ampacity Adjustment: If you run conductors through areas with high ambient temperatures (above 86°F/30°C) or bundle more than three current-carrying conductors in a raceway, you must apply adjustment factors from NEC 310.15. This can force you to use a larger wire size.
• Voltage Drop Calculation for Services: For long service runs, a voltage drop calculation for services is crucial. The NEC recommends limiting voltage drop to 3% for feeders and 5% total for the branch circuit and feeder combined. Excessive voltage drop can cause equipment to malfunction and is inefficient.
• 400A Meter Base Requirements: The utility company dictates the specific 400A meter base requirements. Most require a Class 320 meter base (rated for 320A continuous, 400A maximum) for a 400A service. Always verify with the local utility provider before installation.
• Service Disconnects: A 400 amp disconnect is required. Per the 2023 NEC (section 230.85), this disconnect must be located outdoors for one- and two-family dwellings. It’s also important to understand how many service disconnecting means are allowed, as you may use up to six in some configurations. All equipment must be properly marked, so it’s vital to know how to comply with service equipment labeling requirements. All of these rules are governed by NEC Article 230 Services.

Sizing Grounding and Bonding Conductors

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

Grounding Electrode Conductor Sizing

The grounding electrode conductor sizing is based on the size of the service entrance conductors, as found in NEC Table 250.66. If you used parallel 250 kcmil aluminum conductors, the equivalent area per phase is the sum of their circular mil areas (2 x 250 kcmil = 500 kcmil). Based on this 500 kcmil equivalent area for the aluminum service conductors, NEC Table 250.66 requires a #2 AWG copper or #1/0 AWG aluminum GEC.

A Note on Sizing for Other Common Services

Many related questions arise for smaller services. When determining the ground wire size for 200 amp service, it’s crucial to distinguish between different types of “ground” wires. The term often refers to the Grounding Electrode Conductor (GEC), which connects the service equipment to the grounding electrode system (e.g., ground rods). The GEC size is determined by NEC Table 250.66. For a 200-amp service using 4/0 aluminum service conductors, the GEC must be at least #4 AWG copper or #2 AWG aluminum. The general term 200 amp ground wire size is ambiguous and should not be confused with the grounded (neutral) conductor or the equipment grounding conductors (EGCs) used for branch circuits. Similarly, for a 100-amp service, the typical 100 amp ground wire size for the GEC is #8 AWG copper, while the ungrounded (hot) and grounded (neutral) conductors would be much larger.

Sizing Wire for a 125 Amp Sub Panel

A related task is determining the wire size for 125 amp sub panel feeders. Using the 75°C column of Table 310.16, you would typically select 2/0 AWG aluminum (135A) or #1 AWG copper (130A), assuming no major derating is required. This demonstrates how the same principles apply to feeders and sub-panels throughout an electrical system.

Frequently Asked Questions (FAQs)

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

The most common configuration for a 400 amp residential service is using parallel conductors. This typically involves running two sets of 250 kcmil aluminum conductors for the ungrounded phases and the neutral, based on the 83% sizing allowance in NEC 310.12.

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

Using parallel service entrance conductors (e.g., two 250 kcmil aluminum wires instead of one 1000 kcmil aluminum wire) is preferred for several reasons. Smaller conductors are much more flexible, easier to pull through conduit, and simpler to terminate in lugs. This saves significant time and labor during installation. It also aligns with the common practice of using two 200A panels fed from a 400A meter-splitter combination.

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

Conductor ampacity adjustment, as required by NEC 310.15 for high ambient temperatures or for more than three current-carrying conductors in a raceway, reduces the safe current-carrying capacity of a wire. If your installation conditions require these adjustments, you will likely need to select a larger wire size than what is shown in NEC Table 310.16 to safely handle the load. For example, a conductor that is normally adequate might become undersized after applying a temperature correction factor.