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 to have an ampacity of at least 125% of the Full Load Current (FLC) of the highest rated motor, plus the sum of the FLCs for all other motors on the circuit. This wire size computation accounts for the high inrush current when the largest motor starts while others are running. Properly applying this rule by using the FLC values from the NEC code book tables—not the motor’s nameplate FLA—is essential for determining the final conductor ampacity and ensuring a safe, compliant installation.

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: the moment the highest rated motor starts while all other motors on the same feeder are already running. A motor draws a significantly higher current (inrush current) upon startup than during normal operation. The 125% multiplier applied to the largest motor’s Full Load Current (FLC) ensures the feeder has enough capacity to handle this surge without a significant voltage drop or tripping the overcurrent protection device (OCPD). This calculation is foundational for any professional performing motor load calculations.

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 code book.

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. Crucially, NEC 430.6(A)(1) mandates that you use the FLC values from the tables in Article 430 (such as Table 430.248 for single-phase or Table 430.250 for three-phase motors), not the Full Load Amperage (FLA) listed on the motor’s nameplate. The NEC tables provide standardized, conservative values to ensure safety even if a motor is replaced with a different, less efficient model of the same horsepower. The nameplate fla is reserved for sizing the motor’s individual overload protection, not the conductors. For a deeper dive into these calculations, our three-phase electrical calculations guide is an excellent resource.
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 value, which may not always be the motor with the highest horsepower rating. This motor’s FLC will be the one multiplied by 125%.
3. Calculate the Total Minimum Conductor Ampacity
   Apply the formula from NEC 430.24:

   Minimum Ampacity = (FLC of highest rated motor × 1.25) + (Sum of FLC of all other motors)

   For example, consider a feeder supplying three motors with FLCs of 50A, 40A, and 32A.

   • Highest FLC: 50A
   • Calculation: (50A × 1.25) + 40A + 32A = 62.5A + 40A + 32A = 134.5A

   The feeder conductor must have a minimum ampacity of 134.5A.

4. Select the Conductor Size Using NEC Table 310.16
   With the minimum required conductor ampacity calculated, turn to NEC Table 310.16 to select the appropriate conductor size in American Wire Gauge (AWG). You must use the column that corresponds to the temperature rating of the terminals. For most motor applications, this will be the 75°C terminal rating column. Using our example of 134.5A, you would look in the 75°C column of a wire ampacity chart and select the conductor size that meets or exceeds this value. In this case, a 1/0 AWG copper conductor (rated for 150A) would be required.

Sizing the Feeder Overcurrent Protection Device (OCPD)

Sizing the conductor is only half the battle; you must also size the feeder’s branch circuit protection. This calculation is covered in NEC 430.62 and follows a different rule than conductor sizing. The rating of the feeder OCPD is determined by taking the maximum permitted rating of the largest motor’s branch-circuit short-circuit and ground-fault protection device (as determined from NEC 430.52) and adding the FLC (not FLA) of all the other motors on the feeder.

Let’s use the same motors from our wire-sizing example (FLCs of 50A, 40A, and 32A) to illustrate. First, you must determine the a maximum permitted rating for the branch-circuit protection for the largest motor (50A FLC). Assuming an inverse-time breaker is used, NEC 430.52 allows a maximum rating of 250% of the FLC (50A x 2.5 = 125A). The feeder OCPD calculation is: 125A (max permitted branch OCPD) + 40A + 32A = 197A. Per NEC 430.62(A), the rating of the feeder OCPD shall not be greater than this calculated value. Since 197A is not a standard OCPD size according to NEC 240.6, you must select the next standard size down. Therefore, a 175A breaker or fuse would be selected. Understanding the different types of protective devices, like a motor-rated switch, is also vital for a complete system design.

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, it’s wise to use a voltage drop calculator to ensure the voltage at the motor stays within an acceptable range (typically 3-5%).
• Variable Frequency Drives (VFDs): The increasing use of VFDs changes the calculation. For a feeder supplying a single VFD, conductors are sized for at least 125% of the VFD’s rated input current (per NEC 430.122). When a feeder supplies multiple VFDs, the principle of NEC 430.24 is applied: size the conductors for 125% of the largest VFD’s rated input current plus the sum of the rated input currents of all other VFDs. In all cases, the VFD’s input current is used instead of the motor’s FLC because the VFD is the load on the feeder. To learn more, read our popular article, What is a VFD (Variable Frequency Drive)?
• Load Types: These calculations primarily apply to a continuous duty motor. If a feeder supplies a mix of continuous and non-continuous load motors, the standard calculation is typically sufficient and conservative. However, interlocking or demand factors may be permitted under specific conditions per NEC 430.26.
• Short-Circuit Current Rating (SCCR): All equipment on the circuit, including the panel and protective devices, must have an SCCR sufficient to withstand the maximum available fault current. This is a critical safety consideration to prevent catastrophic equipment failure.

Staying current with these complex rules is essential for professional growth and safety. For those looking to master every aspect of industrial wiring, you can enhance your industrial electrical skills with our motor controls courses. These comprehensive online electrical courses are designed to elevate your expertise.

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, detailed in NEC 430.24, is to calculate the sum of the Full Load Currents (FLCs) of all motors, but with one modification: the FLC of the motor with the highest rating is first multiplied by 125%. The resulting total amperage is the minimum ampacity required for the feeder conductor.

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

For sizing conductors and overcurrent devices, you must use the Full Load Current (FLC) values found in the tables of the NEC code book (e.g., Table 430.250). The nameplate FLA (Full Load Amps) should only be used for selecting the motor’s specific thermal overload protection.

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

When a feeder supplies a Variable Frequency Drive (VFD), the wire size computation uses the VFD’s rated input current instead of the motor’s FLC. For a single VFD, conductors must be sized for at least 125% of its rated input current (NEC 430.122). If the feeder supplies multiple VFDs, you calculate 125% of the largest VFD’s input current plus 100% of the sum of the other VFDs’ input currents, similar to the standard motor calculation.

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

Feeder conductor sizing (NEC 430.24) is based on 125% of the largest motor’s FLC plus 100% of the others. In contrast, the feeder OCPD sizing (NEC 430.62) is based on the maximum permitted rating of the largest motor’s branch-circuit protective device (per NEC 430.52) plus 100% of the other motors’ FLCs. This often results in a lower-rated OCPD than the conductor’s total ampacity might suggest.