How to Size a Motor Circuit Breaker with NEC Article 430

Understanding the Core Principles of Motor Circuit Protection

Sizing a circuit breaker for a motor is a unique challenge compared to other types of loads. The fundamental issue is a motor’s need for a large amount of current to start, known as motor inrush current, which can be six to eight times its normal operating current. A standard circuit breaker sized only for the running load would trip every time the motor starts. However, the circuit still requires robust protection against dangerous short circuits and ground faults. NEC Article 430 provides a detailed roadmap for navigating this dual requirement.

A crucial distinction within NEC 430 is the separation of two types of protection:

• Short-Circuit and Ground-Fault Protection (SCGFP): Covered in Part IV of Article 430, this is the role of the circuit breaker or fuse at the start of the branch circuit. Its job is to protect conductors and components from high-level, dangerous fault currents.
• Motor Overload Protection: Covered in Part III of Article 430, this is typically handled by a separate overload relay or heater element, often integrated into the motor starter. Its purpose is to protect the motor itself from overheating due to excessive loads during operation.

The motor circuit breaker sizing we are discussing here focuses exclusively on short-circuit and ground-fault protection.

A Step-by-Step Guide for Motor Circuit Breaker Sizing

Following the rules in the NEC Handbook ensures every installation is safe and reliable. This step-by-step process breaks down how to perform the necessary load calculations for proper motor breaker size.

1. Step 1: Find the Motor Full-Load Current (FLC)

   This is one of the most critical and often misunderstood steps. For sizing branch-circuit protection, you must not use the Full-Load Amperes (FLA) listed on the motor’s nameplate. Instead, NEC 430.6(A)(1) directs you to use the FLC values found in Tables 430.247 through 430.250. These tables provide standardized current values based on motor horsepower, voltage, and phase. Specifically:

   • Use Table 430.248 for a single-phase motor.
   • Use Table 430.250 for a polyphase motor (three-phase).

   The nameplate FLA is used later for sizing the separate motor overload protection devices.

2. Step 2: Choose Your Protection Device

   NEC Table 430.52 lists several types of overcurrent protective devices. The most common for circuit breaker applications are the inverse time circuit breaker and the instantaneous trip circuit breaker. An inverse time breaker reacts slowly to small overcurrents (like motor startup) but quickly to large short circuits, making it a versatile choice for motor branch circuit protection. An instantaneous trip breaker, or motor circuit protector (MCP), is designed only for short-circuit protection and must be used as part of a listed combination starter that includes separate overload protection.

3. Step 3: Apply Percentages from NEC Table 430.52

   Once you have the FLC from the NEC tables and have selected your breaker type, you’ll use Table 430.52 to find the maximum sizing percentage. For the most common types of motors (AC polyphase squirrel-cage, single-phase), the percentages are:

   • Inverse Time Circuit Breaker: 250% of the FLC from the NEC Table.
   • Instantaneous Trip Circuit Breaker: 800% of the FLC from the NEC Table (for most motors).

   It is important to note that while the motor service factor is a critical piece of information on the nameplate, it primarily influences the sizing of the overload protection device, not the short-circuit protection device. According to NEC 430.32, a motor with a service factor of 1.15 or greater can have its overload device sized up to 125% of the nameplate FLA.

4. Step 4: Select the Breaker Rating (The “Next Size Up” Rule)

   After multiplying the FLC by the percentage from Table 430.52, the result may not correspond to a standard breaker size listed in NEC Table 240.6(A). In this specific case for motor circuits, NEC 430.52(C)(1) Exception No. 1 permits you to use the next higher standard size overcurrent device. For instance, if your calculation results in 85A, and 85A is not a standard size, you are permitted to select a 90A breaker. This is a crucial exception that applies specifically to motor short-circuit protection and helps ensure the motor can start without tripping the device. However, there are upper limits. For example, if a motor will not start with a breaker sized at 250%, the NEC allows increasing the size, but not to exceed 400% for an inverse time breaker on a motor with an FLC of 100A or less.

Practical Example: Sizing a Breaker for a Polyphase Motor

Let’s apply these steps to a common scenario: sizing an inverse time breaker for a 25 horsepower, 460-volt, three-phase, squirrel-cage, continuous duty motor.

1. Find FLC: According to NEC Table 430.250, the FLC for a 25 hp, 460V, three-phase motor is 34A.
2. Choose Device: We will use an inverse time circuit breaker.
3. Apply Percentage: From NEC Table 430.52, the multiplier for an inverse time breaker is 250%.Calculation: 34A (FLC) x 250% = 85A
4. Select Breaker Rating: 85A is not a standard size listed in Table 240.6(A). Applying the “next size up” rule from 430.52(C)(1) Exception No. 1, we select the next standard size, which is a 90A circuit breaker.

Key Considerations for Accurate Motor Breaker Sizing

Beyond the basic calculation, several other factors are essential for a complete and compliant motor circuit design. Staying informed on recent code changes, such as those that simplify motor conductor sizing and protection, is crucial for professionals.

• Conductor Sizing is Separate: The branch-circuit breaker sized using Table 430.52 is for short-circuit protection only and does not protect the conductor against overloads. Conductor sizing is a separate calculation, typically requiring an ampacity of at least 125% of the motor FLC per NEC 430.22. In our 25 hp example, the conductors would need an ampacity of at least 42.5A (34A x 125%).
• Feeder Protection: If you are supplying multiple motors from a single feeder, the rules for feeder protection are different. NEC 430.62 specifies sizing the feeder breaker by taking the rating of the branch-circuit device for the highest rated motor in the group and adding the FLC of all other motors.
• Motor Control Circuits: The protection of motor control circuits is another distinct area covered in Part VI of Article 430, which has its own set of rules for overcurrent protection.
• Specialized Applications: Always be aware of specific motor applications that may have unique requirements. For instance, the use of reconditioned motors or specialized equipment like pool pumps, which have seen recent updates to GFCI protection requirements, demands careful review of all applicable NEC sections.

Advancing Your Expertise in Motor Systems

Correctly applying NEC Article 430 is a hallmark of a knowledgeable electrician. As motor technology advances with intelligent protection systems and increased automation, a deep understanding of these foundational principles is more important than ever. To further your skills and master the complexities of motor controls and protection schemes, consider enhancing your knowledge. Specialize in motor controls with our advanced online electrical training courses.

Primary Sources

• NFPA 70, National Electrical Code (NEC), Article 430: Motors, Motor Circuits, and Controllers
• NEC Table 430.52: Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices
• NEC Tables 430.248 & 430.250: Full-Load Current for Single-Phase & Polyphase AC Motors
• NEC Table 240.6(A): Standard Ampere Ratings for Fuses and Inverse-Time Circuit Breakers

Frequently Asked Questions (FAQ)

Why is motor breaker sizing different from other circuit breaker sizing?

Motor breaker sizing is different to accommodate the high motor inrush current that occurs during startup, which can be 6-8 times the normal running current. NEC Table 430.52 allows for a much higher trip setting for short-circuit protection to prevent nuisance tripping while still protecting against dangerous faults.

Can I use the motor’s nameplate FLC for motor circuit breaker sizing?

No. For sizing the branch-circuit short-circuit and ground-fault protection device, NEC 430.6(A)(1) requires using the motor full-load current (FLC) values from Tables 430.247-430.250. The nameplate current rating is used to size the separate motor overload protection devices.

What is the difference between an inverse time circuit breaker and an instantaneous trip circuit breaker in motor applications?

An inverse time circuit breaker provides both overload and short-circuit protection, with a delayed trip on minor overcurrents and a fast trip on major faults. An instantaneous trip circuit breaker provides only short-circuit and ground-fault protection and must be paired with a separate overload relay as part of a listed combination motor controller. The sizing percentages in NEC Table 430.52 are also different: typically 250% for an inverse time breaker and 800% for an instantaneous trip breaker.

Does the breaker sized with Table 430.52 also provide motor overload protection?

No, it does not. The device sized according to NEC Table 430.52 is for branch-circuit short-circuit and ground-fault protection only. Motor overload protection, which protects the motor from overheating, must be provided separately as specified in Part III of NEC Article 430.