National Electrical Code Basics: Overcurrent Protection Part 1
Overcurrent can destroy electrical circuits and equipment. Learn about overcurrent and overcurrent protective devices in Part 1 of this National Electrical Code series.
National Electrical Code (NEC) Article 240 generalizes overcurrent protection and protective devices to avoid damage to the electrical systems. Factors contributing to overcurrent are a demanding environment, overload, general deterioration, and accidental damage to the electrical components.
Image used courtesy of Eaton
An Overview of Overcurrent
Overcurrent may be overload or short-circuit.
•Overload is excessive current constrained to the regular circuit path without insulation breakdown.
Overload may be temporary or continuous.
○Examples of temporary overload are motor starting currents and transformer inrush currents. These currents are harmless everyday occurrences – protective devices do not act in response to them.
○Continuous overload includes many loads in a circuit, overloaded equipment, and worn motor bearings. These currents may fluctuate between two and six times the operating magnitude, and the protective devices open the circuits to prevent damage and fires.
•Short-circuit comprises a current flowing through abnormal paths. Short-circuit – or fault – currents are much larger than operating currents – may be above 50 kA. Short-circuit currents may severely impair conductors and equipment, producing insulation damage, conductor melting, gas ionization, arcing, fires, and enormous magnetic field stresses capable of distorting the busbars.
Removing an overload within a few seconds is generally enough to ward off damages. In contrast, not drawing a short circuit within a few thousand of a second – not many cycles – may damage the electrical components.
National Electrical Code Article 240
Article 240 specifies general requirements for overcurrent protection and overcurrent protective devices. Table 240.3 addresses article numbers covering overcurrent protection for specific circuits and equipment. For example, use Article 210 to protect branch circuits, Article 430 for motors, motor circuits, and controllers, Article 440 for air-conditioning and refrigerating equipment, and Article 450 for transformers and transformer vaults.
Overcurrent Protective Devices
The indispensable overcurrent devices are fuses and circuit breakers. They limit the current in any wire to the maximum permitted by the National Electrical Code. The NEC employs the term “Ampacity” to express the maximum safe current for any particular type and size of wire.
•Fuses. Fundamentally, a fuse is a short piece of metal of a kind and size that melts when the current exceeds a preset magnitude – the ampere rating – with potentially destructive power.
Fuses are reliable, stable, safe, and do not require regular maintenance or testing. They have high interrupting ratings – can withstand high short-circuit currents without ripping apart.
UL categorizes the fuses in classes. Some classes are L, RK1, RK5, T, J, CC, CD, G, K1, K5, and H.
Current-limiting fuses clear a short-circuit current in less than one-half cycle – about 0.00833 seconds in a frequency of 60 Hz – averting short-circuit currents from building up to their potential values. The potential short-circuit current will emerge by replacing the fuse with a conductor of the same impedance. Figure 1 shows the concept of current limitation.
Figure 1. The current-limiting capability of a fuse. Image used courtesy of Lorenzo Mari
The area under the curve characterizes the amount of energy delivered to the circuit under fault conditions. Thermal energy and magnetic forces are proportional to the current magnitude squared. Reducing the current ten times will reduce the energy to 1/100th of the available level.
•Circuit breakers. The four significant types of circuit breakers – depending on the medium to extinguish the arc – are air, oil, SF6, and vacuum. We’ll focus on low-voltage, molded-case circuit breakers.
A low-voltage molded-case circuit breaker is an uncomplicated mechanism that responds to overloads and short circuits. The arc extinguishing medium is air.
Circuit breakers sense circuit current in two ways: by producing heat – thermal circuit breakers – or by creating a magnetic field – magnetic circuit breaker.
Thermal circuit breakers use a heating element in series with the load, located close to a bimetallic strip. The bimetallic strip, mechanically connected to the circuit breaker contacts, will bend or warp to open the contacts if the current is higher than the rated value. There is a time delay before the circuit opens, depending on the overcurrent magnitude. This type of circuit breaker protects against overloads.
Magnetic circuit breakers connect a coil in series with the load. The magnetic field established by the load current around the coil attracts the metal arm of a solenoid that opens the contacts.
These circuit breakers have a short delay in opening the contacts when an overcurrent occurs – they are instantaneous circuit breakers employing the forces coming from the short-circuit high current for the switching function.
The thermomagnetic circuit breakers employ the thermal and magnetic methods simultaneously, combining the short-circuit protection, overload protection, operational switch, and disconnecting means into one piece of equipment.
Current limiting circuit breakers do not employ fuses. They operate at ultra-high speed to limit the short-circuit current before reaching its maximum potential peak value in the first half cycle – they interrupt the arc by opening the contacts at an ultra-high rate.
Keep the short-circuit current to a minimum without impairing the ability of the overcurrent protective device to detect and clear the fault correctly.
Most overcurrent protective devices are labeled with two current ratings
- Standard ampere rating
- Interrupting rating
Protective Device Standard Ampere Rating
Overcurrent protective devices are rated in amperes. Each unit has a particular ampere rating. Table 240.6(A) exhibits the standard ampere ratings for fuses and fixed-trip inverse time circuit breakers – fixed-trip means that you cannot change the current settings, and inverse time means the more the short-circuit current magnitude, the less the time to clear the fault.
The standard figures vary between 15 A and 6 000 A. Fuses have five additional standard ampere ratings: 1, 3, 6, 10, and 601 A – itemized in Section 240.6(A).
Section 240.6(A) permits using fuses and inverse time circuit breakers with nonstandard ampere ratings.
When the long-time pickup setting is adjustable – you can change it – there are two options
- With accessible external adjusting means (Section 240.6(B)). The ampere rating is the maximum setting possible. The rationale is that anybody may change the setting
- With restricted access adjusting means (Section 240.6(C)). The ampere rating is permitted to be equal to the adjusted current setting. The justification is that only qualified personnel may change the setting
Protective Device Interrupting Rating
The interrupting rating expresses the capacity of a protective device to bear a short-circuit current.
Section 110.9 states that the interrupting rating of the overcurrent device, at nominal voltage, must be equal to or higher than the short-circuit current available at the line terminals of the device.
A short-circuit current surpassing the protective device capability may slightly break the piece or produce an explosion, depending on the current magnitude – the latter may pose a severe safety hazard.
Section 110.10 states that the overcurrent protective device, the circuit impedance, the short-circuit current available, and other circuit characteristics must be selected and coordinated to minimize damage to the circuit components.
The requirement in Section 110.9 refers only to the overcurrent protective device and not to its capability to protect the circuit components. Therefore, selecting elements with sufficient short-circuit withstand ratings is essential and coordinating them with the protective device to avoid exceeding those ratings.
The short-circuit withstand rating of a component is the maximum short-circuit current it can resist. Exceeding this rating may damage the piece.
Some circuit characteristics to ponder are
- Wire length. The longer the wire, the lower the available fault current (AFC)
- Wire size. The larger the wire size, the higher the AFC
- Wire type. Copper wire causes higher AFC than aluminum
- Cable or raceway. AFC is different when conductors are in a cable compared to the same conductors in a raceway.
- Raceway type. Conductors in a ferrous metal raceway cause less AFC than the same conductors in a plastic raceway
Fuses must interrupt a minimum of 10 kA at 125V, with some exceptions. Most current-limiting fuses have an interrupting capacity of 200 kA or 300 kA.
Current-limiting fuses allow specifying components with low short-circuit withstand ratings for systems with high short-circuit levels.
Section 240.60(C) requires marking the fuse interrupting rating on the fuse barrel where other than 10 kA. This marking is not mandatory for fuses used for supplementary protection – such as when applied to protect lower-rated circuit breakers.
Most branch-circuit, molded-case circuit breakers have an interrupting rating of 10 kA. Larger circuit-breakers are 14 kA or higher. Current limiting circuit breakers may handle 200 kA.
These ratings, for a particular design, vary with the rated voltage, decreasing as the applied voltage increases – a current limiting circuit breaker may have an interrupting rating of 200 kA at 240 V, 150 kA at 480 V, and 100 kA at 600 V.
Section 240.83(C) requires marking the circuit breaker interrupting rating on the circuit breaker where other than 5 kA. This marking is not mandatory for circuit breakers employed in supplementary protection. A circuit breaker without a marked interrupting rating has a default value of 5 kA.
Protective Device Voltage Rating
In addition to the current ratings, overcurrent protective devices have a voltage rating. This rating must be equal to or higher than the nominal system voltage. A misapplied overcurrent device will not be capable of interrupting the rated short-circuit current.
Section 240.83(E) requires marking the circuit breakers with the voltage rating. Section 240.85 covers straight and slash voltage ratings in circuit breakers.
•Straight voltage ratings. Circuits may use these devices when the nominal voltage between any two conductors does not exceed the circuit breaker´s straight voltage rating. For example, a circuit where neither the phase voltage nor the line voltage exceeds 240 V may use a straight 240V circuit breaker. See figure 2.
Figure 2. 208Y/120V system. Image used courtesy of Lorenzo Mari
Use a straight-rated circuit breaker every time there is a chance of line voltage appearing across only one pole.
•Slash voltage ratings. These circuit breakers display two voltage markings separated by a slash – some typical markings are 120/240 V, 208Y/120 V, 480Y/277 V, and 480/277 V.
The lower figure is the nominal voltage to the ground, and the higher number is the nominal voltage between two ungrounded conductors. One pole will clear the line-to-ground overcurrents, and two or three poles will clear the line-to-line overcurrents.
Slash voltage circuit breakers are only suitable for solidly grounded wye systems, like the one shown in figure 2 – they are not appropriate for floating, resistance-grounded, or corner-grounded, delta-connected systems.
For example, a 480 V, 3-phase, corner-grounded, delta-connected system cannot use a 480Y/277 V circuit breaker because the voltage of two phases to the ground exceeds 277 V.
The correct application is 480 V or 600 V straight voltage ratings. Furthermore, 3-phase, corner-grounded, delta circuits can only use two-pole circuit breakers marked 1ɸ-3ɸ. See figure 3.
Figure 3. 480 V, 3-phase, corner-grounded delta. Image used courtesy of Lorenzo Mari
Likewise, a 240 V, delta-connected system – floating – cannot use a 120/240 V circuit breaker because there is no connection to the ground. This situation requires 240 V, 480 V, or 600 V straight voltage ratings. See figure 4.
Figure 4. 240 V delta system. Image used courtesy of Lorenzo Mari
Noteworthy is the 3-phase, 4-wire delta system. The middle of one phase is tapped and grounded, creating a neutral and causing the high leg voltage to the ground to be √3 times the phase-to-neutral voltage.
For example, a 120/240 V, 3-phase, 4-wire delta system has a voltage of √3 x 120 V = 208 V in the high leg. A 120/240 V slash-rated breaker is inadequate for the high leg because the voltage to the ground exceeds 120 V. See figure 5.
Figure 5. 120/240V, 3-phase, 4-wire delta system. Image used courtesy of Lorenzo Mari
This system may use a circuit breaker with a 240 V straight voltage rating because the line and phase voltages don’t exceed 240V.
Fuses don’t have slash voltage ratings. They are total voltage-rated devices suitable for any electrical system arrangement.
Summary of Key Takeaways
- Overcurrents may be overloads or short-circuits. Overloads are currents constrained to the regular circuit paths and may be temporary or continuous.
- Short-circuit currents flow through abnormal paths and have a sizeable destructive power.
- Fuses and circuit breakers protect the electrical systems by clearing overloads and short-circuits.
- Overcurrent protective devices have current and voltage ratings.
- Current-limiting devices clear the fault in less than ½ cycle, limiting the fault’s energy supplied to the circuit.
- Use the straight voltage rated circuit breakers where the slash voltage rated devices do not qualify.