Overcurrent Protection & Available Fault Current
What are the three important ratings that must be properly applied when selecting an overcurrent protective device?
Ampere rating, voltage rating, and interrupting rating.
Reference: NEC 110.9, 240.6, 240.83, 240.85
True or False: The voltage rating of an OCPD must be equal to or greater than the system voltage.
True.
Reference: NEC 110.9, 240.85
Why can a motor branch circuit require separate short-circuit/ground-fault protection and overload protection?
Motor starting current can be much higher than normal running current, so the short-circuit/ground-fault OCPD must allow starting while separate overload protection protects the motor from sustained overloads.
What table is used to determine the full-load current of a 230-volt, single-phase motor?
Table 430.248.
What law is the foundation for the various methods used to calculate available fault current?
Ohm's Law
True or False: Short-circuit calculations are performed as though overcurrent protective devices are replaced by copper bars.
True.
If a conductor's ampacity does not correspond to a standard circuit breaker or fuse rating, what NEC section permits the next higher standard rating under specific conditions?
NEC 240.4(B).
What type of voltage rating does a fuse have?
A straight/full voltage rating.
What NEC section provides the overload protection requirements for continuous-duty motors greater than 1 horsepower?
NEC 430.32(A)(1)
A 10-hp, 230-volt, single-phase motor has a Table 430.248 current of 50 amps. What minimum conductor ampacity is required at 125%?
62.5 amps.
Calculation: 50 × 1.25 = 62.5 A
What is the first step in determining available fault current at points throughout an electrical system?
Draw a one-line diagram showing all sources of available fault current.
What NEC section requires overcurrent devices to have an interrupting rating sufficient for the available fault current?
NEC 110.9.
What is the definition of interrupting rating?
The highest current at rated voltage that a device is identified to interrupt under standard test conditions.
Reference: NEC Article 100
A properly rated straight-voltage circuit breaker can be used on which systems: 120 V, 120/240 V, 208Y/120 V, 240 V delta, 480Y/277 V, 480 V, 600Y/347 V, and 600 V?
All of the listed systems, when the breaker is properly rated and applied.
Reference: NEC 240.85
A motor has a 1.15 service factor. What percentage of its rated horsepower does the service factor represent?
115%, or 1.15 times its rated horsepower.
A 10-hp, 230-volt, single-phase motor has a 50-ampere full-load current. What maximum dual-element time-delay fuse is permitted at 175%?
90 amps.
Calculation: 50 × 1.75 = 87.5 A → next standard size = 90 A.
What are two major sources of fault-current contribution?
The utility source and on-site generators.
Also: Running motors can contribute significant fault current.
What NEC section requires the overcurrent devices, impedance, component SCCRs, and other circuit characteristics to be selected and coordinated so protective devices can clear faults without extensive damage?
NEC 110.10.
A downstream panel has 45,000 amperes of available fault current. What minimum interrupting rating is required for a fully rated circuit breaker?
45,000 amperes.
Reference: NEC 110.9
Slash-rated circuit breakers are permitted on what type of systems?
Solidly grounded systems where the voltage to ground does not exceed the lower voltage rating of the breaker.
Reference: NEC 240.85
A motor has a nameplate current of 32 amps and a 1.15 service factor. What is the maximum overload protection setting under the basic 125% rule?
40 amps.
Calculation: 32 × 1.25 = 40 A
Reference: NEC 430.32(A)(1)
A 100-kVA, three-phase transformer has a 480-volt primary and a 208-volt secondary. What are the maximum primary and secondary OCPD sizes using the calculations from the lesson?
300-amp primary and 350-amp secondary.
Reference: NEC 450.3(B)
A transformer has a marked impedance of 3%. What multiplier is used for the transformer impedance to establish the worst-case available fault current?
0.9.
Calculation: 3% × 0.9 = 2.7%
A circuit has 60,000 amps of available fault current at the beginning of a 50-foot 480-volt, three-phase feeder bus. Approximately how much available fault current is calculated at the end of the run using the lesson's calculation?
Approximately 49,200 amps.
A circuit breaker is marked 600 volts AC. Can it automatically be used on a 600-volt DC system?
No. The device must have a DC voltage rating equal to or greater than the system voltage.
A circuit breaker marked 480 volts is being considered for a 480/277-volt solidly grounded wye system, a 480-volt earth-grounded delta, and a 480-volt high-resistance-grounded system. Can the properly rated breaker be used on all three?
Yes.
A 25-hp, 460-volt, three-phase motor has a Table 430.250 full-load current of 34 amps. What is the maximum instantaneous-trip circuit breaker setting at 800%?
272 amps.
Calculation: 34 × 8 = 272 A
Reference: NEC 430.52(C)(3)
Why must special consideration be given to transformer overcurrent protection when the transformer is energized?
Because magnetizing inrush current occurs on the primary and can be significantly higher than normal operating current.
A 1,000-kVA, 480/277-volt, three-phase transformer has a 3% impedance. Using the 0.9 impedance multiplier and ignoring motor contribution, approximately what is the available fault current at the secondary terminals?
Approximately 44,556 amps.
At one point in a system, the available fault current is 25,385 amps. A downstream motor controller has a calculated multiplier of 0.648. What is the available fault current at the motor controller?
16,449 amps.
Calculation: 25,385 × 0.648 = 16,449 A