By Mike Holt
The most fundamental requirement in any electrical system is proper overcurrent protection of conductors and equipment. According to the NEC Article 100, overcurrent is the condition where the current in amperes is greater than the rated current of the equipment or conductors, resulting from an overload, short circuit, or ground fault.
An overcurrent protection device protects the circuit by opening the device when the current reaches a value that will cause an excessive or dangerous temperature rise in conductors.
Most overcurrent protection devices respond to both, short-circuit or ground-fault current values as well as overload conditions. Typically a 30 ampere circuit breaker will open in xx seconds when the fault value is xx, xx seconds at xx, and xx seconds at xx. For fuses, they are listed to open Xxxxxx.
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In addition, overcurrent protection devices must have an interrupting rating sufficient for the maximum possible fault-current available on the line side terminals of the equipment [110.9].
Typically conductors must be protected against overcurrent in accordance with their ampacity after ampacity adjustment [240.4]. For example, a 10 AWG conductor is protected by a 30A breaker or fuse. But in some applications, overcurrent protection is not accomplished by a single protection device; therefore this rule does not apply [240.4(D)].
For example, 430.31 specify that overload devices must be provided to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. 430.32 specify that overload protection device be sized between 115% to 140% of the motor nameplate current rating.
Where as 430.51 require the branch-circuit short-circuit and ground-fault protection device to be sized sufficiently larger enough so that the motor branch-circuit short-circuit and ground-fault protective device is capable of carrying the motors starting current. This is accomplished by sizing a circuit breaker at 250% of motor full load current rating in accordance with 430.52.
Example: 5-hp, 230V single-phase motor with a service factor of 1.16 and a nameplate current rating of 28A.
The overload protection device must be sized no more than 125 percent of motor nameplate current in this example [430.32(A)(1)].
Overload Setting: 28A x 1.25 = 35A
The short-circuit and ground-fault protection device is to be sized no greater than 250 percent of motor full load current rating [430.52(C)(1)].
Branch Circuit Short-Circuit Ground-Fault
Device: 28A x 2.50 = 70A
In addition, a motor is permitted to be protected against overload, short circuit and ground fault by a single protection device, but they device must be sized in accordance with the overload rules in 430.32 [430.55]. For this example, a 35A fuse can be used.
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