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. Overcurrent protection for motors is unique due to inrush current. Most loads are protected from overload and short-circuit/ground-fault with a single OCP device. Often, this is not the case for motors as they are protected from overload and short-circuit overcurrents with separate devices. Understanding how to properly size and apply overcurrent protection devices is essential for electrical professionals and a frequent topic 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.
Key phrases to look out for in the motor overcurrent protection question:
When you spot these elements:
To correctly apply NEC Article 430 for motor overcurrent protection, follow these steps:
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.
a. Determine FLC from Table 430.250:
b. Find the multiplier from Table 430.52(C)(1):
c. Apply the rule for multiple motors [NEC 430.53(C)(4)]:
d. Adjust to next standard size based on 430.52(C)(1) and using Article 240.6:
*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.
A 40 hp, 200-volt, three-phase, Design B motor in a water treatment plant is being protected from short-circuit by a time-delay fuse. If the initial rating is inadequate for starting current, what is the maximum allowable fuse rating?
1. Determine FLC from Table 430.250: For a 40 hp, 200 V, 3-phase motor, squirrel cage FLC = 120 A
2. 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 225%
3. Calculate the maximum allowable fuse rating: 120 A × 225 % = 270 A. Adjust to next standard size below* using Article 240.6: The next standard size below 270 A is 250 A
*Please note that we are rounding down for this standard size, instead of rounding up because the code states that in no case should the overcurrent device exceed the value calculated when using the higher percentages permitted to account for inrush current.
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