15.3 Motor Overload Protection & Feeder Calculations for Multiple Motors (NEC 430.32 & 430.24)

15.3 Motor Overload Protection & Feeder Calculations for Multiple Motors (NEC 430.32 & 430.24)

Quick Answer: Under NEC 430.32, motor overload protection is sized strictly from the motor nameplate current rating, not table values: 125% of nameplate FLC for motors with a marked Service Factor of 1.15 or greater or a marked temperature rise of $40^\circ\text{C}$ or less, and 115% of nameplate FLC for all other motors. For multi-motor feeders under NEC 430.24, feeder conductors must have an ampacity of not less than 125% of the highest-rated motor FLC plus the sum of the FLCs of all other motors ($1.25 \times I_{\text{largest}} + \sum I_{\text{others}}$). Under NEC 430.62(A), feeder short-circuit protection is sized from the largest branch SCGF device plus the sum of other motor FLCs, rounding DOWN to the next lower standard rating if non-standard.

Motor circuits present unique challenges both at the individual motor level (thermal overload protection) and at the distribution level (sizing feeders serving multiple motors). Overheating is the primary cause of motor insulation breakdown and winding fires, resulting from mechanical binding, extended acceleration cycles, low line voltage, or the loss of one phase on a three-phase system (single-phasing). At the feeder level, calculating conductor ampacity and selecting overcurrent protective devices requires a distinct mathematical procedure that accounts for diversity and starting transients.

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Motor Overload Protection Fundamentals (NEC 430 Part III)

An overload is an operating condition in which current exceeds normal full-load rating due to mechanical overloading, locked rotor, or abnormal voltage, but does not involve a short circuit or ground fault. Overload currents typically range from $110\%$ to $600\%$ of rated current.

Overload Sizing: The Nameplate Exception (NEC 430.6(A)(1))

While conductors and fuses/breakers are sized from NEC Tables 430.247–430.250, NEC 430.6(A)(1) mandates that motor overload protection must be based on the actual motor nameplate full-load current:

The Two Sizing Tiers of NEC 430.32(A)(1)

For continuous-duty motors rated more than 1 horsepower, separate overload devices (such as thermal overload relay heaters or electronic overload relays) must be sized according to the two specific criteria of NEC 430.32(A)(1):

1. The 125% Tier (High Thermal Resilience):
   • Motors marked with a Service Factor (SF) of 1.15 or greater; OR
   • Motors marked with a temperature rise of $40^\circ\text{C}$ or less. $\text{Maximum Overload Trip Setting} = \text{Nameplate FLC} \times 1.25$
2. The 115% Tier (Standard Thermal Resilience):
   • All other motors (e.g., motors with a Service Factor of 1.0, or marked with a temperature rise greater than $40^\circ\text{C}$). $\text{Maximum Overload Trip Setting} = \text{Nameplate FLC} \times 1.15$

What Is Motor Service Factor?

The Service Factor (SF) represents a multiplier indicating the percentage of continuous mechanical overload a motor can safely deliver without sustaining thermal damage, assuming rated voltage and frequency. A motor with an SF of $1.15$ can deliver $115\%$ of its rated horsepower continuously. Because an SF $1.15$ motor has greater copper mass and higher-grade insulation, the Code permits overload devices to be set higher ($125\%$ vs. $115\%$).

Modifying Overload Protection (NEC 430.32(C))

If the overload relay selected according to the standard $125\%$ or $115\%$ limits is insufficient to start the motor or carry the load, NEC Section 430.32(C) permits installing the next higher size overload relay or setting the trip point up to the following absolute maximum modification caps:

• Motors with SF $\ge 1.15$ or temp rise $\le 40^\circ\text{C}$: maximum $140\%$ of nameplate FLC.
• All other motors: maximum $130\%$ of nameplate FLC.

Number of Overload Units Required (NEC Table 430.37)

Under NEC Table 430.37, for any three-phase AC motor, three overload units (one in each ungrounded phase conductor) are required. In the past, older code editions permitted two overload units; however, if one phase opens on a wye-delta transformer supplying a three-phase motor, current in one winding can increase to $230\%$ of normal. Three overload units guarantee detection regardless of system transformer connections.

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Multi-Motor Feeder Conductor Sizing (NEC 430.24)

In industrial plants and commercial buildings, a single feeder raceway frequently supplies a motor control center (MCC) or distribution panel feeding multiple motors. How are the feeder conductors sized?

The 125% Largest Motor Rule (NEC 430.24)

Under NEC Section 430.24, conductors supplying two or more motors shall have an ampacity not less than:

1. 125 percent of the full-load current rating of the highest-rated motor in the group; plus
2. The sum of the full-load current ratings of all other motors in the group.

$\text{Feeder Minimum Ampacity} = (1.25 \times I_{\text{largest, table}}) + \sum I_{\text{other motors, table}}$

Critical Feeder Conductor Sizing Rules:

1. Highest-Rated Motor Is Based on Amperes, Not Horsepower: The "largest motor" is defined under NEC 430.17 as the motor with the highest Table FLC. If a feeder powers a 10 HP, 208V single-phase motor ($55\text{ A}$ from Table 430.248) and a 15 HP, 460V three-phase motor ($21\text{ A}$), the 10 HP motor is the "largest motor" ($55\text{ A} > 21\text{ A}$).
2. Tied Highest Ratings: If two or more motors in the group have identical largest currents, only one motor is multiplied by $1.25$. All remaining motors are added at $100\%$.
3. Feeders Supplying Motors Plus Other Loads (NEC 430.24(1)–(3)): If the feeder also supplies non-motor loads (lighting, heating, receptacles), the ampacity must satisfy: $\text{Feeder Ampacity} \ge (1.25 \times I_{\text{largest motor}}) + \sum I_{\text{other motors}} + (1.25 \times I_{\text{continuous loads}}) + I_{\text{noncontinuous loads}}$

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Multi-Motor Feeder Short-Circuit Protection (NEC 430.62)

Selecting the overcurrent protective device for a motor feeder is one of the most critical calculation skills tested on the Minnesota licensing exam. It differs sharply from branch circuit calculation.

The Sizing Formula (NEC 430.62(A))

Under NEC 430.62(A), a feeder supplying a specific fixed motor load shall have a protective device with a rating or setting not greater than:

1. The largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor of the group (calculated under Table 430.52 and Section 430.52(C)(1) Exception No. 1); plus
2. The sum of the full-load currents of all other motors of the group.

$\text{Feeder OCPD Max} = \text{Largest Permitted Branch SCGF Device} + \sum I_{\text{other motors, table}}$

THE CRITICAL DIFFERENCE: Rounding DOWN on Feeders

Pay close attention to the language of NEC 430.62(A): "shall not have a rating or setting greater than..."

• On Branch Circuits (NEC 430.52(C)(1) Ex. 1), if a calculation yields a non-standard rating, you are permitted to ROUND UP to the next higher standard rating.
• On Feeders (NEC 430.62(A)), you MUST ROUND DOWN to the next lower standard rating listed in NEC 240.6(A) if the calculation does not match a standard size! The Code does not provide a general "next size up" exception for motor feeders.

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Step-by-Step Worked Feeder & Overload Calculations

Example 1: Motor Overload Relay Heater Sizing

A continuous-duty, 3-phase, 460-volt, 20 HP motor has a marked nameplate current of $25.0\text{ A}$, a service factor of $1.15$, and a temperature rise of $40^\circ\text{C}$. The Table 430.250 full-load current is $27.0\text{ A}$. What is the maximum initial trip current setting for the separate overload protection device?

Step 1: Identify Governing Current

• Under NEC 430.6(A)(1), overload protection is sized from nameplate current ($25.0\text{ A}$), NOT Table FLC ($27.0\text{ A}$).

Step 2: Determine Overload Percentage (NEC 430.32(A)(1))

• Marked Service Factor is $1.15$ (meets the $\ge 1.15$ threshold).
• Multiplier = $125\%$ ($1.25$).

Step 3: Calculate Overload Setting

$\text{Max Overload Setting} = 25.0\text{ A} \times 1.25 = \mathbf{31.25\text{ A}}$

(Note: If the service factor had been $1.0$, the multiplier would be $115\%$, yielding $25.0\text{ A} \times 1.15 = 28.75\text{ A}$.)

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Example 2: Feeder Conductor Sizing for Multiple 3-Phase Motors

An industrial feeder supplies three 460-volt, 3-phase squirrel-cage induction motors:

• Motor 1: 30 HP ($I_{\text{FLC}} = 40\text{ A}$ from Table 430.250)
• Motor 2: 15 HP ($I_{\text{FLC}} = 21\text{ A}$ from Table 430.250)
• Motor 3: 10 HP ($I_{\text{FLC}} = 14\text{ A}$ from Table 430.250) All terminals are rated for $75^\circ\text{C}$. What is the minimum required feeder conductor ampacity and the smallest copper THHN conductor permitted?

Step 1: Identify the Highest-Rated Motor

• Motor 1 has the largest full-load current: $40\text{ A}$.

Step 2: Apply the NEC 430.24 Feeder Formula

$$\begin{aligned} \text{Feeder Ampacity} &\ge (1.25 \times I_{\text{largest}}) + \sum I_{\text{others}} \\ &= (1.25 \times 40\text{ A}) + 21\text{ A} + 14\text{ A} \\ &= 50\text{ A} + 35\text{ A} = \mathbf{85.0\text{ A}} \end{aligned}$$

Step 3: Select Conductor Size from NEC Table 310.16

• Review $75^\circ\text{C}$ copper column:
  • 4 AWG copper has an allowable ampacity of $85\text{ A}$ ($85\text{ A} \ge 85.0\text{ A}$ — acceptable).
• Conclusion: Sizing requires 4 AWG THHN copper conductors.

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Example 3: Feeder Overcurrent Protective Device Sizing

For the three motors in Example 2, an inverse-time circuit breaker will be installed as the feeder overcurrent protective device. Each branch circuit is protected by an inverse-time circuit breaker sized at the maximum standard rating permitted by NEC 430.52(C)(1) Exception No. 1. What is the maximum standard rating permitted for the feeder circuit breaker?

Step 1: Calculate Branch SCGF Device for Each Motor (Table 430.52 @ 250%)

• Motor 1 (40A): $40\text{ A} \times 2.50 = 100\text{ A}$. (100A is a standard size under NEC 240.6(A)). Branch breaker = $100\text{ A}$.
• Motor 2 (21A): $21\text{ A} \times 2.50 = 52.5\text{ A}$. Next standard size up under Exception 1 = $60\text{ A}$.
• Motor 3 (14A): $14\text{ A} \times 2.50 = 35.0\text{ A}$. (35A is a standard size). Branch breaker = $35\text{ A}$.

Step 2: Identify the Largest Permitted Branch Device

• The largest branch protective device in the group is Motor 1's breaker: $100\text{ A}$.

Step 3: Apply the NEC 430.62(A) Feeder Formula

$$\begin{aligned} \text{Feeder OCPD Max} &= \text{Largest Branch Device} + \sum I_{\text{other motors}} \\ &= 100\text{ A} + 21\text{ A} + 14\text{ A} \\ &= 100\text{ A} + 35\text{ A} = \mathbf{135.0\text{ A}} \end{aligned}$$

Step 4: Apply the Rounding DOWN Rule (NEC 430.62(A))

• Review standard ratings in NEC 240.6(A): $100, 110, 125, 150, 175\text{ A}$.
• Sizing cannot exceed $135\text{ A}$. Rounding up to 150A is strictly prohibited by Section 430.62(A).
• The next lower standard rating is $125\text{ A}$: $\text{Maximum Permitted Feeder Breaker} = \mathbf{125\text{ A}}$

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Practical Exam Scenarios & Trap Avoidance

Trap 1: Multiplying All Motors by 125% on a Feeder

• Exam Trap: Sizing a multi-motor feeder by multiplying the sum of all motor currents by 1.25: $(40 + 21 + 14) \times 1.25 = 75 \times 1.25 = 93.75\text{ A}$.
• Correction: Under NEC 430.24, only the single highest-rated motor is multiplied by $1.25$. All other motors are added at $100\%$.

Trap 2: Rounding UP the Feeder OCPD

• Exam Trap: Calculating $135\text{ A}$ for a feeder breaker and selecting a 150A breaker.
• Correction: Under NEC 430.62(A), feeder protective devices must not exceed the calculated value. You must ROUND DOWN to 125A.

Trap 3: Using Table FLC for Overload Sizing

• Exam Trap: A question gives both nameplate current and Table FLC and asks for the overload heater size.
• Correction: Overload sizing is the ONLY calculation in Article 430 that uses nameplate current. Conductors and breakers use Table FLC.