Feeder Conductor Sizing: A Master Electrician’s Guide to the NEC Standard Method

Answer-First Summary: Feeder conductor sizing is a critical task for any journeyman or master electrician, governed by the National Electrical Code (NEC). The Standard Method, detailed in NEC Article 220, Part III, provides the rules for performing a detailed feeder load calculation. This process involves summing the loads from all connected branch circuits, applying demand factors for non-simultaneous use, and correctly accounting for continuous loads which must be calculated at 125% of their value. Proper wire size computation ensures the selected conductor has sufficient ampacity to handle the maximum expected load without overheating. This prevents insulation damage and fire hazards, making an accurate feeder conductor sizing calculation essential for safety and code compliance. Key considerations include the general lighting load, appliance loads, overcurrent protection, and potential voltage drop on long runs.

ALT Text for Featured Image: A licensed electrician pointing to a feeder cable inside an open electrical panel, with NEC code book and calculator nearby.

The Foundation: Branch Circuit vs. Feeder Conductor

In any electrical system, power flows from the utility through service entrance conductors to the main panel. From there, it’s distributed throughout a building. Understanding the distinction between a branch circuit and a feeder is fundamental for any licensed electrician.

• A Branch Circuit is the final leg of the journey, delivering power from the last overcurrent protection device (like a circuit breaker) directly to an outlet, light fixture, or a specific appliance. Think of it as a local road leading to a single house.
• A Feeder is a major artery. These conductors carry power from the service equipment to a subpanel or distribution panel that supplies multiple branch circuits. A common example is the wiring that runs from your main panel to a subpanel in a garage or workshop. Correctly performing the feeder conductor sizing for these arteries is crucial for the safety and reliability of the entire downstream system.

This article focuses on the “Standard Method” of feeder load calculation, as detailed in NEC Article 220, Part III. While the NEC provides optional methods for certain dwelling units, the standard calculation is the comprehensive approach required for most commercial and complex residential projects.

Understanding Feeder Load Calculation with the Standard Method (NEC Article 220, Part III)

The goal of a feeder load calculation is to determine the maximum load the feeder will likely experience. It’s not just a simple sum of all connected breakers. The NEC recognizes that not all loads will run simultaneously at full capacity and provides rules to create a realistic, safe, and efficient wire size computation.

Key Components of the Load Calculation

The first step is to tally all the loads the feeder will supply. These are broken down into categories within NEC Article 220:

• General Lighting and Receptacle Load: This is calculated based on the square footage of the area served, using the values in NEC Table 220.12. For non-dwelling units, specific receptacle loads are added per NEC 220.14(I).
• Appliance and Fixed Appliance Loads: This includes loads like disposals, water heaters, and other fixed equipment, typically taken at 100% of their nameplate rating.
• Dryer and Cooking Equipment Loads: NEC Table 220.54 and Table 220.55 allow for significant demand factors for these appliances, as it’s unlikely multiple units in a multifamily dwelling will all be operating at maximum heat at the same time.
• Motor Loads: These are calculated according to NEC Article 430; the feeder calculation includes the sum of full-load currents of all motors plus 25% of the highest rated motor.

Continuous vs. Noncontinuous Load

A critical distinction in any load calculation is whether a load is continuous or noncontinuous. A continuous load is defined by the NEC as a load where the maximum current is expected to continue for three hours or more. This includes things like commercial lighting, HVAC systems, or modern Electric Vehicle Supply Equipment (EVSE).

According to NEC 215.2(A)(1), feeder conductors must be sized to handle 100% of the noncontinuous load plus 125% of the continuous load. This 125% multiplier accounts for the additional heat generated over long periods and is a common area where calculation errors occur.

Applying Demand Factors

Demand factors are multipliers provided in the nec code book that allow you to reduce the total connected load. This is based on the principle of diversity—the probability that not all loads will be active at once. For example, NEC Table 220.42 allows you to apply a demand factor to the general lighting and receptacle load; the first portion is taken at 100%, but the remainder is taken at a lower percentage. Applying these factors correctly is key to rightsizing a system and avoiding the unnecessary cost of oversized conductors.

Mastering these complex rules is essential for any professional. For journeyman and master electricians looking to deepen their expertise, our advanced online electrical courses provide in-depth training on advanced load calculations and code interpretation.

Step-by-Step Guide to Feeder Conductor Sizing

Here is a simplified, step-by-step process for a standard method calculation:

1. Calculate and Categorize All Loads: Sum the volt-amperes (VA) for all loads supplied by the feeder, breaking them down into categories like general lighting, receptacles, appliances, and motors as defined in NEC Article 220. Identify which of these loads are continuous (expected to run for 3+ hours).
2. Apply Applicable Demand Factors: Before summing all loads, apply the demand factors permitted in NEC Article 220 to their respective load categories (e.g., general lighting demand factors from Table 220.42, range demand factors from Table 220.55). This provides the net computed load for each category.
3. Calculate Total Load with Continuous Load Multiplier: Sum the net computed loads from the previous step. The final feeder size is based on 100% of the noncontinuous portion of this load plus 125% of the continuous portion, as required by NEC 215.2(A)(1).
4. Determine the Required Ampacity: Divide the final calculated VA from the previous step by the system voltage (e.g., 240V for single-phase, or L-L voltage x √3 for three-phase) to find the required conductor ampacity in amperes.
5. Select the Conductor from NEC Table 310.16: With your required ampacity, consult the industry’s go-to wire ampacity chart, NEC Table 310.16. Choose a conductor from the correct temperature rating column (typically 75°C for most terminations) that has an ampacity equal to or greater than your calculated value.

Critical Considerations Beyond Basic Load Calculation

A complete feeder conductor sizing analysis doesn’t stop at the basic load calculation. Several other factors must be verified to ensure a safe and compliant installation.

Neutral Load Calculation (NEC 220.61)

The neutral conductor only carries the unbalanced portion of the load. Therefore, the NEC permits a separate neutral load calculation under section 220.61. For the portion of the neutral load exceeding 200 amps, a 70% demand factor can be applied. However, per NEC 220.61(B)(1), this reduction is prohibited for the portion of the load that is from electric discharge lighting or other non-linear loads (e.g., LED drivers, computer power supplies), which can produce harmonic currents that add up on the neutral.

Ampacity Adjustment and Conductor Correction Factors

The ampacity values in NEC Table 310.16 are based on specific conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. If your installation exceeds these, you must apply conductor correction factors for high ambient temperature or an ampacity adjustment for bundling more than three conductors. These factors reduce the conductor’s effective ampacity. For a detailed guide, see our article on how to calculate wire ampacity and derating.

Overcurrent Protection and Voltage Drop

The final selected conductor must be protected by an appropriately sized overcurrent protection device (OCPD), such as a circuit breaker or fuse. Additionally, for long feeder runs, voltage drop becomes a significant concern. While not a mandatory requirement, an informational note in the NEC (215.2(A)(1) Informational Note No. 2) recommends limiting voltage drop to 3% for feeders, with a total of 5% for the feeder and branch circuit combined, to ensure equipment operates efficiently. If your calculation shows excessive voltage drop, you must upsize the conductor, regardless of the load calculation result. Using a voltage drop calculator is highly recommended for any run over 100 feet. For example, upsizing the wire for a 100 amp wire size on a long run is a common requirement.

Key Takeaways for Feeder Sizing

• Always differentiate between a branch circuit and a feeder. Feeders supply subpanels and require calculations based on NEC Article 220.
• The Standard Method calculation is a detailed process of summing loads and applying specific demand factors.
• Continuous loads (operating 3+ hours) must be calculated at 125% of their rating for conductor sizing.
• Use NEC Table 310.16 to find conductor ampacity, but always check termination temperature ratings (usually 75°C).
• Do not forget to perform ampacity adjustments for ambient temperature and conductor bundling.
• The neutral load can often be reduced per NEC 220.61, but not for non-linear loads.
• Verify your final selection against voltage drop recommendations, especially for runs over 100 feet.

Related Resources

• Sizing Service Entrance Conductors per the NEC

Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition

Frequently Asked Questions (FAQ)

What is the standard method for feeder load calculation?

The standard method, outlined in NEC Article 220, Part III, is a detailed wire size computation that involves calculating the total load for all connected branch circuits, including general lighting, receptacles, appliances, and motors. It requires applying specific demand factors from NEC tables and calculating any continuous load at 125% of its value to determine the minimum required conductor ampacity.

How do you size a feeder for a 100 amp subpanel?

To size a feeder for a 100 amp subpanel, you must first perform a detailed feeder load calculation per NEC Article 220 to determine the actual expected load. For instance, even if the subpanel is rated for 100 amps, the calculated load might only be 75 amps. For a 75A continuous load, you would need a conductor with an ampacity of at least 93.75A (75A x 1.25). You would then select the appropriate wire size from the 75°C column of the wire ampacity chart in NEC Table 310.16 (e.g., #3 AWG copper or #1 AWG aluminum) and verify for voltage drop.

When do you apply the 125% rule for feeder conductor sizing?

The 125% rule is applied to any portion of the feeder load that is considered a “continuous load”—defined as a load expected to operate at maximum current for three hours or more. Per NEC 215.2, the feeder conductor ampacity must be at least 100% of the noncontinuous load plus 125% of the continuous load. This ensures the conductor can handle the sustained heat from long-running equipment.