Motor overcurrent protection is a critical aspect of electrical safety and functionality. It safeguards motors and their circuits from damage due to excessive current flow, which can result from overloads, short circuits, or ground faults. Understanding how to properly size and apply overcurrent protection devices is essential for electrical professionals and a frequent topic on NEC electrical exams.
Example Motor Overcurrent Protection Questions on NEC Electrical Exams
A 50 hp, 200-volt, three-phase, Design B motor in a water treatment plant needs a time-delay Class CC fuse. If the initial rating is inadequate for starting current, the maximum allowable fuse rating is _____ amperes.
A robotics engineer is installing a 15 hp, 575-volt, three-phase, squirrel cage, Design B motor in an automated assembly line. The maximum permitted rating for a _____ ampere dual element (time-delay) fuse would provide adequate branch-circuit protection.
A paper mill uses three 60 hp, 575-volt, three-phase, synchronous motors on a circuit. The maximum rating of a _____ ampere inverse time circuit breaker would provide adequate short-circuit and ground-fault protection.
How to Identify a Motor Overcurrent Protection Question on NEC Electrical Exams
Key phrases to look out for in the motor overcurrent protection question:
- Motor horsepower (hp) and voltage
- Motor type (e.g., Design B, squirrel cage, synchronous)
- Overcurrent protection device type (e.g., fuse, circuit breaker)
- Terms like "short-circuit protection," "ground-fault protection," or "branch-circuit protection"
When you spot these elements:
- Confirm it's a motor overcurrent protection question, not a refrigerant compressor or motor overload question.
- Refer to NEC Article 430.52 and 430.250
Motor Overcurrent Protection Articles: NEC 430.52 and 430.250
To correctly apply NEC Article 430 for motor overcurrent protection, follow these steps:
Identify Key Information in the Question
- Motor specifications (horsepower, voltage for each motor)
- Motor type (DC, single-phase, two-phase, or 3-phase)
- Motor design (squirrel cage, synchronous, Design B, etc.)
- Number of motors on the circuit
- Type of overcurrent protection device
- Any indication of motor starting difficulties
Reference Relevant NEC Articles and Tables
- Table 430.250: Find full-load current (FLC) for 3-phase motors
- For DC motors: Table 430.247
- For single-phase motors: Table 430.248
- For two-phase motors: Table 430.249
- Table 430.52(C)(1): Maximum ratings for motor branch-circuit short-circuit and ground-fault protective devices
- NEC 430.53(C)(4): Rules for group motor installations
- Article 430.52(C)(1)(a): Sizing up to standard ampere ratings for overcurrent device
- Article 430.52(C)(1)(b): Rules for sizing when motor can't start
- Article 240.6: Standard ampere ratings for fuses and fixed-trip circuit breakers
Calculation Process
- Determine the full-load current (FLC) from the appropriate table
- Find the multiplier from Table 430.52(C)(1). If motor can't start, use Article 430.52(C)(1)(b) for adjusted multiplier
- For single motor: Multiply FLC by the appropriate multiplier. If there are multiple motors: Apply multiplier to largest motor's FLC, then add FLC of remaining motors based on NEC 430.53(C)(4)
- Adjust to next standard size using Article 240.6, if necessary
Walkthrough for a NEC Electrical Exam Motor Overcurrent Protection Question
Question:
A paper mill uses three 60 hp, 575-volt, three-phase, synchronous motors on a circuit. The maximum rating of a _____ ampere inverse time circuit breaker would provide adequate short-circuit and ground-fault protection.
Identify Key Information:
- Three motors, each: 60 hp, 575 V, 3-phase, synchronous
- Device is an inverse time circuit breaker
- Has no trouble starting the motor with overcurrent device
Reference Relevant NEC Articles and Tables:
- Table 430.250 for full-load current (FLC) of 3-phase motors
- Table 430.52(C)(1) for maximum ratings of protective devices
- Article 430.53(C)(4) for group motor installations
- Article 240.6 for standard ampere ratings
Calculation Process:
a. Determine FLC from Table 430.250:
- For a 60 hp, 575 V, 3-phase synchronous* motor, FLC = 49 A
b. Find the multiplier from Table 430.52(C)(1):
- For a synchronous motor with an inverse time circuit breaker, multiplier = 250%
c. Apply the rule for multiple motors [NEC 430.53(C)(4)]:
- Largest motor: 49 A × 250% = 122.5 A
- Add FLC of remaining motors: 49 A + 49 A = 98 A
- Total: 122.5 A + 98 A = 220.5 A
d. Adjust to next standard size based on 430.52(C)(1) and using Article 240.6:
- The next standard size above 220.5 A is 225 A
*A very common mistake is that people miss the fact that it is a synchronous motor and instead use the values for squirrel cage or wound rotor motor. Please read through the question thoroughly.
Walkthrough for a NEC Electrical Exam Motor Overcurrent Protection Question That Can’t Start
Question:
A 40 hp, 200-volt, three-phase, Design B motor in a water treatment plant needs a time-delay Class CC fuse. If the initial rating is inadequate for starting current, the maximum allowable fuse rating is _____ amperes.
Identify Key Information:
- One motor: 40 hp, 200 V, 3-phase, Design B (squirrel cage)
- Time-delay Class CC fuse required
- Maximum allowable fuse rating if the initial rating is inadequate for starting current
Reference Relevant NEC Articles and Tables:
- Table 430.250 for full-load current (FLC) of 3-phase motors
- Article 430.52(C)(1)(b) for sizing when motor can't start
- Article 240.6 for standard ampere ratings
Calculation Process:
a. Determine FLC from Table 430.250:
- For a 40 hp, 200 V, 3-phase motor, squirrel cage FLC = 120 A
b. Since the question states this initial rating is inadequate for starting current, we apply Article 430.52(C)(1)(b):
- This allows us to increase the rating to the next higher standard rating, not to exceed 400%
e. Calculate the maximum allowable fuse rating: 120 A × 400% = 480 A. Adjust to next standard size below* using Article 240.6:
- The next standard size below 480 A is 450 A
*Please note that we are rounding down for this standard size, instead of rounding up in the first question because the code states that in no case should the overcurrent device exceed the value calculated. This is common for questions where the motor is unable to start due to the overcurrent device.