Motor Feeder Calculator: Sum of Loads Method

Accurately determine the minimum ampacity for conductors supplying multiple motors based on National Electrical Code (NEC) guidelines.

Enter Motor Loads

What is a “Feeders for Several Motors are Calculated Using the Sum Of” Calculation?

When an electrical circuit supplies power to more than one motor, the conductor (or feeder) providing that power must be sized correctly to handle the combined load safely. The phrase “feeders for several motors are calculated using the sum of” refers to the standard methodology prescribed by the National Electrical Code (NEC), specifically in section 430.24. This calculation ensures the feeder can withstand the continuous operational current of all motors running simultaneously, plus the additional strain of the largest motor starting up, which draws a high inrush current.

This method is not a simple sum. It involves adding 125% of the Full-Load Current (FLC) of the largest motor to the sum of the FLCs of all the other motors in the group. This ensures a safety margin, preventing overheating and potential fire hazards. This calculation is a fundamental skill for electricians and engineers designing industrial and commercial electrical systems.

The Formula for Calculating Feeders for Several Motors

The core principle for sizing feeders for several motors is to account for both the continuous load and the peak demand during motor startup. The formula is as follows:

Total Feeder Ampacity = (FLC of Largest Motor × 1.25) + (Sum of FLC of all other motors)

Alternatively, and more simply for calculation, this can be expressed as: (Sum of FLC of ALL motors) + (FLC of Largest Motor × 0.25). Our calculator uses this second, equivalent formula.

Formula Variables

Variable	Meaning	Unit	Typical Range
Total Feeder Ampacity	The minimum required current rating for the feeder conductor.	Amperes (A)	10A – 2000A+
FLC	Full-Load Current: The current a motor draws while operating at its rated horsepower.	Amperes (A)	1A – 1000A+
FLC of Largest Motor	The highest FLC value among all motors in the group.	Amperes (A)	Typically 5A or higher in multi-motor setups.

Practical Examples

Example 1: Small Workshop

A workshop has three motors with the following Full-Load Currents from NEC tables: 50A, 40A, and 32A.

• Inputs: 50A, 40A, 32A
• Largest Motor FLC: 50A
• Calculation: (50A × 1.25) + 40A + 32A = 62.5A + 40A + 32A = 134.5A
• Result: The feeder conductor must have an ampacity of at least 134.5A.

Example 2: Industrial Process Line

An industrial line has four motors: one large 100A motor, two medium 25A motors, and one small 10A motor.

• Inputs: 100A, 25A, 25A, 10A
• Largest Motor FLC: 100A
• Calculation: (100A × 1.25) + 25A + 25A + 10A = 125A + 60A = 185A
• Result: The feeder must be sized for a minimum of 185A. For a more detailed guide on this, you can check our Motor Load Calculation Guide.

How to Use This Motor Feeder Calculator

1. Add Motors: Click the “Add Motor” button for each motor in your group.
2. Enter FLC Values: In each input field that appears, enter the Full-Load Current (FLC) in Amperes for one motor. It is critical to use the FLC values from NEC tables (e.g., Table 430.250 for three-phase motors), not the Full Load Amperes (FLA) from the motor’s nameplate.
3. Calculate: Press the “Calculate Ampacity” button.
4. Interpret Results: The calculator will display the total required feeder ampacity, which is the minimum size for your conductor. It also breaks down the sum of all currents, the largest motor’s current, and the 25% bonus amount for clarity. The chart provides a visual representation of these components.

Key Factors That Affect Feeders for Several Motors Calculation

• Motor FLC Values: The accuracy of the calculation depends entirely on using the correct FLC values from the NEC tables for the given motor horsepower and voltage.
• Continuous vs. Non-continuous Loads: The standard calculation assumes continuous duty. If motors are for intermittent duty, demand factors might apply, but for general safety, the continuous duty calculation is standard.
• Other Loads: If the feeder also supplies non-motor loads (like lighting or heaters), those loads must be added to the calculation, typically at 100% for non-continuous loads and 125% for continuous loads. Our calculator focuses purely on motor loads.
• Voltage Drop: For long feeder runs, you must also perform a voltage drop calculation to ensure the voltage at the motor terminals is sufficient. This may require a larger conductor size than the ampacity calculation alone suggests.
• Ambient Temperature: If the conductors are in a high-temperature environment, their ampacity must be derated, potentially requiring a larger wire size.
• Number of Conductors: If more than three current-carrying conductors are in a single raceway or cable, their ampacity must also be adjusted, as detailed in our Conductor Ampacity Guide.

Frequently Asked Questions (FAQ)

Why is it 125% of the largest motor and not just the sum of all motors?

The extra 25% accounts for the high inrush current the largest motor draws when it starts. This prevents the feeder’s overcurrent protection (breaker or fuse) from tripping unnecessarily when the biggest motor kicks on while others are already running.

Should I use the motor nameplate FLA or NEC table FLC?

You must use the Full-Load Current (FLC) values from the tables in NEC Article 430 (e.g., 430.248, 430.250). These are standardized values for conductor sizing, whereas the nameplate FLA is used for sizing the motor’s specific overload protection.

What if I have two motors of the same “largest” size?

You only apply the 125% multiplier to one of them. The calculation becomes: (Largest Motor FLC × 1.25) + (FLC of the other “largest” motor) + (Sum of all smaller motors).

Does this calculator work for both single-phase and three-phase motors?

Yes, the calculation principle is the same. The key is to use the correct FLC values from the appropriate NEC table for your motor type (single-phase or three-phase) and voltage.

What about overcurrent protection for the feeder?

Sizing the feeder’s overcurrent protective device (OCPD) is a separate calculation (NEC 430.62). It is generally sized based on the rating of the largest motor’s branch-circuit OCPD plus the sum of the FLCs of the other motors. Explore this in our Overcurrent Protection Explained article.

What if other loads like lighting are on the same feeder?

You must add the amperage for those loads to the motor calculation. Continuous non-motor loads are added at 125% of their value, and non-continuous loads are added at 100%.

How does a Variable Frequency Drive (VFD) change this calculation?

If a motor is supplied by a VFD, you typically use the VFD’s rated input current instead of the motor’s FLC in the calculation. See our Guide to VFD Sizing for more details.

Is this calculated ampacity the final wire size I should use?

Not necessarily. This ampacity is the *minimum* required. You must still account for voltage drop, ambient temperature correction, and conductor bundling adjustments, all of which might require a larger wire size.