Feeder Conductor Sizing using the Standard Method (NEC Art. 220)

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 — for non-dwelling occupancies use NEC Table 220.42(A); for dwelling units use NEC 220.41 (33 VA/m²) — and for specific receptacle counts see 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 when required by the NEC.
• Dryer and Cooking Equipment Loads: NEC Table 220.54 and Table 220.55 provide methods and demand factors for clothes dryers, ranges, and other cooking appliances, allowing realistic demand reductions in many installations.
• Motor Loads: Motor loads are calculated in accordance with NEC Article 430. For feeder sizing the NEC requires that the feeder include 125% of the largest motor’s full-load current plus 100% of each other motor’s full-load current (see Article 430 guidance). This is an important, code-required part of the standard feeder method.

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 equipment expected to run for prolonged periods.

Per the NEC the feeder conductor ampacity must be at least 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 NEC Article 220 that permit realistic reductions from the total connected load. The principle is diversity — not every load will be operating at once or at full value. For example, the general lighting and receptacle portion of a non-dwelling load can be calculated using unit values and demand factors in Table 220.42(A); cooking and laundry appliances have specialized tables of demand factors. Applying the correct demand factors is essential to rightsizing feeders and avoiding oversizing conductors unnecessarily.

Mastering these 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(A), range/dryer demand tables). 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 ampacity target is the noncontinuous portion plus 125% of the continuous portion, as required by NEC (see feeder sizing and 215.2 guidance).
4. Determine the Required Ampacity: Convert the final calculated VA to amperes (divide by system voltage — single-phase or three-phase) to find the required conductor ampacity.
5. Select the Conductor from NEC Table 310.16: With your required ampacity, consult NEC Table 310.16 (industry standard ampacity tables). Choose a conductor from the appropriate temperature-rating column (termination temperature limits govern the column selection — 75°C is commonly used for many terminations) with ampacity equal to or greater than your calculated value. For installations with elevated ambient or more than three current-carrying conductors, apply the correction and adjustment factors in 310.15 before finalizing conductor selection.

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 code-compliant installation.

Neutral Load Calculation (NEC 220.61)

The neutral conductor carries the unbalanced portion of the load. NEC 220.61 permits certain reductions in the neutral calculation for specific conditions (for example, 70% may be applied to parts of the calculated household range/dryer unbalanced load or to that portion of the unbalanced load in excess of 200 A under the permitted systems). The code also explicitly prohibits reductions for certain neutral portions, such as a 3-wire portion of a 4-wire wye system and segments consisting of nonlinear loads. Review 220.61 carefully before applying neutral reductions.

Ampacity Adjustment and Conductor Correction Factors

The ampacity values in NEC Table 310.16 assume a 30°C (86°F) ambient and not more than three current-carrying conductors in a raceway; if those conditions are not met you must apply the ambient temperature correction factors and conductor count adjustment factors in 310.15. These factors reduce the effective ampacity and are essential to apply whenever conditions differ from the table assumptions.

Overcurrent Protection and Voltage Drop

The final selected conductor must be protected by an appropriately sized overcurrent protective device (OCPD), such as a circuit breaker or fuse. Also verify the selected conductor is compatible with the OCPD ratings allowed by Article 240 and any motor or equipment requirements from other articles (e.g., Article 430 for motors).

Voltage drop is a practical performance issue for long feeder runs. The NEC provides informational guidance suggesting a maximum of 3% voltage drop for feeders (with a total of 5% recommended for feeder plus branch circuit) to preserve efficiency and equipment performance. If voltage drop is excessive on long runs, upsize the conductor to address it even when ampacity requirements alone would allow a smaller wire.

For long runs and critical loads, use a voltage-drop calculation and verify the chosen conductor meets both ampacity and voltage-drop performance.

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 from the NEC tables.
• Continuous loads (operating 3+ hours) must be calculated at 125% of their rating for conductor sizing (feeder ampacity must be at least noncontinuous load plus 125% of continuous load).
• Use NEC Table 310.16 to find conductor ampacity, but always check termination temperature ratings (often 75°C) and apply corrections for ambient temperature and more than three current-carrying conductors.
• Do not forget to perform ampacity adjustments for ambient temperature and conductor bundling when those conditions apply.
• The neutral load can often be reduced per NEC 220.61 in specific cases, but reductions are prohibited for certain circuits and for nonlinear load portions.
• Verify your final selection against voltage drop recommendations, especially for runs over 100 feet; upsize conductors where necessary to meet voltage-drop and performance goals.

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 conductor sizing process that involves calculating the total load for all connected branch circuits, applying the appropriate demand factors from the NEC tables, and sizing any continuous load at 125% of its value to determine the minimum required conductor ampacity for the feeder.

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

To size a feeder for a 100 amp subpanel, perform the standard feeder load calculation per NEC Article 220 to determine the actual expected load. If the continuous portion of the calculated load is 75 A, apply the 125% multiplier to that continuous portion (75 A × 1.25 = 93.75 A). Then choose a conductor whose ampacity (from the appropriate temperature column in NEC Table 310.16 after any required derating) is equal to or greater than 93.75 A and verify voltage drop for the full run. Conductor selection needs to consider termination temperature limits, bundling or ambient adjustments, and required overcurrent protection, not a single default AWG size for every situation.

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 in the NEC as a load expected to operate at maximum current for three hours or more. Per NEC 215.2 and the feeder sizing requirements, the feeder conductor ampacity must be at least the noncontinuous load plus 125% of the continuous load to account for sustained heating.