Mike Dolt Enterprises
Mike Dolt Enterprises
User: VxA, Location: Atlantic Provinces (Canada)
Asked on: Friday, April 17, 2009 at 10:22 am EDT
Answered on: Sunday, May 12, 2024 at 4:58 pm EDT
Question: Hi,This application is a 13.2/4.16kV 3-Phase Delta-Wye transformer feeding a Medium Voltage MCC line-up in an industrial (supervised) facility. The transformer is oil filled (outdoor).The location of the transformer is such that I would need to run the secondary conductors outside for 50' or so (they will be physically protected as per NEC). Then I would like to enter the electrical room and run for 25ft or less to the proposed location of the Medium Voltage MCC.The primary protection (which is tied into secondary side ground fault detection to allow for quicker reaction in event of secondary ground fault) keep the Arc Flash levels quite low (Cat 1). As a result, I would like to not have incoming protection on the MV MCC and simply let the individual cubicle fused disconnects do the job. There are 5 such fused disconnect switches.Questions;1) because the overall secondary run is > 25ft, am I able to avoid having secondary protection installed outdoors (or at nearest indoor location to the conductor entrance)? Sections 240.21(C) (3) & (4) talk about this. It is an industrial application.2) if questions (1)'s answer is yes, can I avoid having incomng protection on the MV MCC so long as I use 6 or less individual OCPDs in the MCC, and I size the secondary conductors such that they are protected by the OCPDs? Section 450.3(A) discusses this.Your help would be appreciated.Thanks,
Answer 1
Yes, you can avoid having secondary protection installed outdoors or the nearest indoor location to the conductor entrance.According to NEC Section 240.21(C)(3) and (4), for industrial occupancies, secondary conductors over 25 feet long and outdoors are allowed without secondary protection if the following conditions are met: The conditions of (C)(3) are not present (e.g., no apparent electrical hazard). The secondary conductors are mechanically protected from physical damage. The secondary conductors are sized to be protected by the primary protection device.In your case, the transformer is outdoor and the secondary conductors will be physically protected. The primary protection (which is tied into secondary ground fault detection) provides adequate protection. Therefore, you meet the conditions to avoid secondary protection outdoors.
Answer 2
Yes, you can avoid having incoming protection on the MV MCC if the following conditions are met: There are six or fewer individual OCPDs in the MCC. The secondary conductors are sized to be protected by the OCPDs.NEC Section 450.3(A) states that individual OCPDs for motor circuits can be omitted if the secondary conductors are protected by the primary overcurrent device within an enclosure accessible only to qualified persons. In your case, you have six individual fused disconnect switches in the MCC, and the secondary conductors are sized to be protected by them. Therefore, you meet the conditions to omit incoming protection on the MV MCC.
Conclusion:
Based on the information provided, you can avoid having secondary protection installed outdoors and incoming protection on the MV MCC under the following conditions: The secondary conductors are physically protected. The secondary conductors are sized to be protected by the primary protection device. There are six or fewer individual OCPDs in the MV MCC.* The secondary conductors are sized to be protected by the OCPDs. Flag for review