Selective Coordination
Eaton has helped developed a white paper on Selective Coordination which is available on NEMA's web site as well as on this page. Selective Coordination, as described in the 2005 National Electric Code (NEC) Article 100, states "Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings." Essentially in a selectively coordinated system only the breaker directly supplying the overloaded/faulted part of the system will open, thus allowing the rest of the system to operate. Since the choice of the overcurrent protective device, their ratings or settings is a task of the design professional, a better understanding of the options and choices is required. Watch this video to learn more about selective coordination.
Achieving selective coordination is straightforward when using the following selection calculator. Follow these steps to ensure that selective coordination is in place:
| Step 1a: | When the Utility Fault Current is provided by the local utility enter the value here. Transformer KVA and impedance data is not required to continue. |
| Step 1b: | If the Utility Fault Current is not provided, enter the transformer kVA and % impedance data. |
| Step 2: | Select Primary, Secondary Voltage & Phase data from the dropdown boxes. |
| Transformer calculations will automatically populate selected fields based upon embedded formulas in the calculator. | |
| Step 3: | Select Conductor Type and Raceway Characteristics from the drop down box. |
| Step 4: | Select conductor size between secondary main Molded Case Circuit Breaker (MCCB) and branch MCCB from AWG drop down box. |
| Step 5: | Enter the length (feet) of one phase conductor between secondary main MCCB and branch MCCB or the length of conductor between the step-down transformer and panelboard with built-in secondary main MCCB and branch breakers. [Note: See NEC 240.21 regarding tap rules for transformer secondary conductors.] |
| Step 6: | Enter the number of conductors per phase between secondary MCCB & branch MCCB. |
| Step 7: | Select secondary transformer MCCB type from drop down box. |
| Step 8: | Select the desired branch MCCB type from downstream branch breaker drop down box. |
| Step 9: | Calculator will display coordination level and indicate if there is selective coordination between selected breakers. Note: Selected coordination is based on utilizing appropriate adjustable trip settings for all selected breakers. If the calculator indicates no selective coordination, then vary parameters such as conductor length, secondary, branch breakers to create a coordinated system. |
Notes: Selective coordination will be achieved with the indicated frames WHEN utilizing appropriate adjustable trip settings. In some cases the calculator shows large frame branch breakers almost as large as or the same as the main breaker frame size. The long delay and the short delay adjustments must be properly set to coordinate for low-level overloads/faults.
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