Feeder Sizing for Multiple Motors: An Electrician’s Guide to NEC 430.24

Correctly performing feeder sizing for multiple motors is a critical skill for any journeyman or master electrician. Governed by the National Electrical Code (NEC), these motor load calculations ensure that conductors can safely handle the combined load, preventing overheating and potential hazards. The fundamental rule, found in NEC 430.24, requires sizing the feeder conductor so that its ampacity is not less than the sum that includes: 125% of the Full Load Current (FLC) of the highest rated motor (per 430.6), plus 100% of the FLCs of all other motors on the feeder. In addition, NEC 430.24 requires adding 100% of any non‑motor noncontinuous loads and 125% of any continuous non‑motor loads on the feeder when present. These additions account for both starting conditions and steady‑state continuous loads, and they keep the feeder within safe thermal limits.

Understanding the Core Principles of Motor Feeder Sizing

In any industrial or commercial setting, it’s rare to find a single motor operating in isolation. More often, multiple motors run from a single feeder circuit. Correctly sizing this feeder is not just about compliance; it’s about safety and reliability. Article 430 of the NEC, often called the “motor article,” provides the essential rules for these installations.

The primary principle behind NEC 430.24 is to accommodate the worst‑case scenario for conductor heating and voltage stress: the moment the highest rated motor starts while all other motors on the same feeder are already running, plus any other non‑motor loads that are supplied by the same feeder. Because motors draw significantly higher inrush currents on startup than during normal operation, the 125% multiplier on the largest motor’s FLC (together with 100% contribution of the other motors and the applicable treatment of non‑motor loads) ensures the feeder has enough capacity to handle both transient and continuous stresses. When determining the FLC values to use, consult NEC 430.6 to know when to use the NEC tables and when nameplate values apply.

The Step‑by‑Step Wire Size Computation per NEC 430.24

For a journeyman electrician or master electrician, executing a precise wire size computation is a mark of professionalism. Follow these steps to ensure your feeder is sized correctly according to the NEC.

1. Determine Motor Full Load Current (FLC)
   The first and most critical step is to find the FLC for each motor connected to the feeder. NEC 430.6(A)(1) directs that, for many common motors, you use the FLC values listed in the tables in Article 430 (such as Table 430.248 for single‑phase or Table 430.250 for three‑phase motors) for conductor and certain device sizing. However, there are exceptions where the motor nameplate FLA is used (for example, for motors built for low speed (<1200 rpm), for certain high‑torque or specialty motors, and for particular multispeed motors). Also note that nameplate FLA is required for selecting separate motor overload protection in many cases. Check 430.6 and the specific motor application to determine which source applies.
2. Identify the Highest Rated Motor
   Review the FLC values you gathered for all motors. The “highest rated motor” is the one with the highest FLC (not necessarily the highest horsepower). This motor’s FLC will be the one multiplied by 125% in the feeder ampacity formula.
3. Calculate the Total Minimum Conductor Ampacity
   Apply the conductor formula from NEC 430.24 (and include non‑motor loads if present):

   Minimum ampacity = 125% of the highest motor FLC + 100% of each other motor FLC + 100% of non‑motor noncontinuous loads + 125% of non‑motor continuous loads

   For example, if a feeder supplies three motors with FLCs of 50 A, 40 A, and 32 A and there are no additional non‑motor loads, compute: (50 × 1.25) + 40 + 32 = 134.5 A. If non‑motor loads exist on the feeder, include them per the above rule. This result is the minimum conductor ampacity required before considering temperature corrections, conduit fill, or adjustment factors for multiple current‑carrying conductors.

4. Select the Conductor Size Using Article 310 Ampacity Tables
   With the minimum required conductor ampacity calculated, consult the ampacity tables in Article 310 (for example, Table 310.16 for conductors in raceways when the conditions for that table apply) and use the column corresponding to the termination temperature rating at the equipment per NEC 110.14. Do not assume a temperature column — the allowable ampacity must be selected based on the terminal rating for the connected equipment. Using the earlier example of 134.5 A and a 75°C termination rating, a 1/0 AWG copper conductor (150 A at 75°C in the applicable table) would meet the ampacity requirement. Always verify ambient temperature correction and adjustment factors in 310.15 if your installation has more than three current‑carrying conductors or higher ambient temperatures.

Sizing the Feeder Overcurrent Protection Device (OCPD)

Sizing the feeder OCPD follows the provisions of NEC 430.62. The feeder protector rating is tied to the maximum permitted rating (or setting) of the largest motor’s branch‑circuit short‑circuit and ground‑fault protective device as allowed by NEC 430.52, plus 100% of the FLCs of the other motors on the feeder. Because NEC 430.52 gives different permitted maximum ratings depending on the type of device used (fuse, inverse‑time breaker, instantaneous‑trip device, etc.) and on motor characteristics, you must check 430.52 and the manufacturer’s overload relay data to determine the appropriate permitted branch‑circuit device rating for the largest motor. After determining that permitted maximum, add the FLCs of the other motors; the resulting total is the maximum rating allowed for the feeder OCPD under 430.62.

Note: you cannot assume a single percentage (for example, 250%) for every situation — 430.52 identifies different permitted values for different devices and conditions. Once the allowable maximum rating is calculated, the selected OCPD must also coordinate with conductor ampacity, upstream and downstream protection, and equipment ratings. If the calculated allowable maximum does not exactly match a standard OCPD size, choose a device consistent with the NEC limits, coordination needs, and conductor protection requirements (Article 240). Consult the applicable NEC tables and equipment manufacturer’s information when performing the device selection.

Critical Considerations and Modern Influences

While the NEC provides the foundational rules, a master electrician must also consider other factors that influence a motor feeder’s performance and safety.

• Voltage Drop: Long conductor runs can lead to voltage drop, causing motors to run inefficiently and overheat. After sizing for ampacity, use a voltage‑drop calculation to ensure the motor sees acceptable voltage at full load (commonly recommended to keep drop near 3% for motors, though project requirements vary).
• Variable Frequency Drives (VFDs) / Power Conversion Equipment: For power conversion equipment the feeder and branch conductors are sized based on the power conversion equipment’s input current. NEC 430.122 requires conductor ampacity of not less than 125% of the rated input current to the power conversion equipment. Where multiple converters are on one feeder, treat the converters as the loads — use the converters’ rated input currents (or nameplate/control‑plate input currents) and apply the appropriate NEC provisions and listing instructions (see 430.122 and 430.130). In short, use the drive/control rated input current rather than the motor FLC when the converter is the connected load on the feeder.
• Load Types: These calculations are aimed at continuous motor duty as covered in the NEC. If noncontinuous or intermittent motor loads are present, NEC has provisions (for example, specific multipliers and exception conditions) that may affect the conductor sizing and feeder demand; consult 430.22(E), 430.24, and 430.26 for permitted adjustments and demand factor options under engineering review or authority having jurisdiction approval.
• Short‑Circuit Current Rating (SCCR): All equipment on the circuit (circuit breakers, panels, motor controllers, etc.) must have an SCCR sufficient to withstand available fault current at their terminals. Ensure all equipment and conductors are coordinated and that available fault current has been documented at the equipment line terminals.

Staying current with these rules is essential for professional growth and safety. For those looking to master industrial wiring or motor controls, consult NEC Article 430, Article 310 for conductor ampacities, and the equipment manufacturer’s instructions. Additional training and manufacturer coordination are often required for complex industrial systems.

Related Resources

• How the 2023 NEC Simplifies Motor Conductor Sizing

Frequently Asked Questions (FAQ)

What is the basic rule for feeder sizing for multiple motors?

The basic rule in NEC 430.24 is to size the feeder to the sum that includes 125% of the FLC of the highest rated motor plus 100% of the FLCs of all other motors, and to add any non‑motor loads per the section (100% for noncontinuous nonmotor loads; 125% for continuous nonmotor loads).

Do I use the motor nameplate FLA or the NEC code book for motor load calculations?

For many conductor and device selections NEC directs the use of the FLC values in Article 430 tables. However, nameplate FLA is used in certain situations (e.g., separate motor overload protection, and for specific motor types such as low‑speed or high‑torque motors where the tables do not apply). Check NEC 430.6 to determine which value to use for your specific motor and application.

How does a VFD affect the wire size computation for a motor feeder?

When a feeder supplies power conversion equipment, use the converter’s rated input current to size conductors. NEC 430.122 requires sizing conductors for at least 125% of the converter input current for a single converter load. For multiple converters, apply the appropriate provisions using the rated input currents of the converters, and follow listing or manufacturer instructions where provided.

What’s the difference between sizing the feeder conductor and the overcurrent protection device (OCPD)?

Feeder conductor sizing (NEC 430.24) is a thermal ampacity calculation that ensures the conductor will safely carry the combined motor and permitted non‑motor loads. Feeder OCPD sizing (NEC 430.62) is limited by the maximum permitted rating (or setting) of the largest motor’s branch‑circuit short‑circuit and ground‑fault protective device (determined in 430.52) plus 100% of the other motors’ FLCs. Because 430.52 gives different permitted values by device and motor type, determining the feeder OCPD rating requires consulting those provisions and the manufacturer data rather than using a single blanket percentage.