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Short-Circuit Current

Jan 1, 2001 12:00 PM, By Mike Holt

Understanding the role of short-circuit current is critical to the reliability of your electrical system and to the safety of the people who maintain it.

When designing or modifying a distribution system, you need to ensure proper breaker sizing and coordination. To do so, you'll need to understand the role of available short-circuit current (SCA).

Available SCA is the current (measured in amperes) available at a given point in the electrical system. When determining the available SCA, first calculate SCA at the secondary terminals of the utility transformer. Then, calculate it at the terminals of the service equipment, branch-circuit panel, and branch-circuit load.

Available SCA differs at each point of the system. SCA is highest at the utility transformer and lowest at the branch-circuit load. The greater the circuit impedance (utility transformer and the additive impedances of the circuit conductors), the lower the available SCA.

Factors that determine the available SCA at the utility transformer include the system voltage, transformer kVA rating, and transformer impedance. Properties that determine the circuit impedance include the conductor material (copper versus aluminum), size, and length.

The impedance of the circuit increases as you move farther from the utility transformer. Therefore, the available SCA is lower downstream from the utility transformer.

Interrupting rating. Overcurrent protection devices, such as circuit breakers and fuses, must have an ampere interrupting rating (AIR) sufficient for available SCA current in accordance with the NEC Secs. 110-9 and 240-1. Unless marked otherwise, the ampere interrupting rating for branch-circuit circuit breakers is 5000A [Sec. 240-83(c)] and 10,000A for branch-circuit fuses [Sec. 240-60(c)].

Protection of electrical components. Electrical equipment, components, and conductors must have an SCA (withstand) rating that will permit the circuit overcurrent protective device to clear a fault without extensive damage to any of the components of the system [Secs. 110-9, 110-10, 250-2(d), 250-90, 250-96(a), and Table 250-122 Note].

If the available SCA exceeds the equipment/conductor SCA rating, the thermal and magnetic forces can cause the equipment to explode, and cause the circuit and grounding conductors to vaporize. Worse, it can easily result in serious injury or even death. The only solution to the problem of excessive available fault current is to:

Install equipment that has a higher short-circuit rating, or Protect the components of the circuit by a current-limiting protection device such as a fast-clearing fuse, which can reduce the let-thru energy.

Now that you have a basic understanding of the role SCA plays in electrical system design, you can have more confidence that your designs will result in long service life and safe operation.

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