Additional Current Limiting Devices; Defining Branch Circuits - ABB MS116 Manual

MA N UA L MOTOR START ER G UI D E

7.2.10 Additional current limiting devices

In addition to transformers, other devices, such as some types of fuses and circuit-breakers, also serve to limit the
available fault current within a facility. The published let-through energy I 2 t and peak let-through I can be used to
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determine the load-side available fault current. The higher of these two ratings is the available fault current beyond
the device.
Using our example above, we know that our facility has a maximum available fault current of 63 kA. Let's assume that
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for a 63 kA fault at 480 V AC, a current-limiting circuit-breaker as has an I t of 30 kA²s, and an I of 27 kA. The available
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fault current downstream from this circuit-breaker is 30 kA.
In this example, the required SCCR for all components installed beyond this circuit-breaker must be equal to, or
greater than, 30 kA at 480 V AC.

7.3 Defining branch circuits

Electrical distribution within a facility requires the coordination of many circuits to loads. Beyond the point of the
service entrance, all circuits leading away are considered feeders or feeder taps, until just ahead of a load. The circuit
between the load-side terminals of the final overcurrent protective device and the load itself is called the branch cir-
cuit. This also means that the branch circuit protective device is actually part of the feeder circuit, not the branch.
The figure below shows an example of this.
Feeder vs. branch circuits
Figure 36: Defining branch circuits.
Manual motor starters are suitable for branch circuit protection when tested as Type E or F combination motor con-
trollers. However, these devices cannot be used for providing protection of feeder circuits. Feeder circuit protection
is typically provided using either fuses, UL 489 / CSA C22.2 No.5 molded case or UL 1066 / CSA C22.2 No.31 power
circuit-breakers.
The term "branch circuit' applies regardless of the type of load. Common load types for industrial and commercial
applications include motors, heaters, and lamps. The requirements for motor branch circuits are more intense than
other load types, so the following section reviews these requirements in detail.
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