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   some distribution transformers and secondary service lines, particularly in neighborhoods with older distribution systems and underground systems.

•   PEV charge rate, or level, is the PEV characteristic that most influences the overload risks posed to service transformers from PEV adoption. Increased charge durations, due to larger battery sizes, can also impact thermal aging.

•   Each transformer’s remaining capacity per customer is one of the strongest indicators of the potential risk that a transformer may exceed its thermal ratings. This metric incorporates a number of key factors including the transformer’s existing demand, thermal ratings, and the number of customers served.

•   Assets near the load are most susceptible to system overloads from PEV clusters as the potential benefit of spatial diversity decreases.

•   PEV clustering will occur randomly throughout a system. While it may indicate an increased risk of higher than average loading levels, PEV clustering alone does not signify the likelihood of negative impact as other PEV load characteristics must also be taken into account.

•   Transformers characterized by low capacity per customer are the most likely to be affected by PEV adoption. In particular, transformers lower than 25 kVA are expected to be the most susceptible to overloading as they typically have lower amounts of capacity, which can be quickly consumed by one or more PEVs.


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NEC [National Electrical Code]. 1996. NEC Handbook, NFPA 70. Article 625, Electrical Vehicle Charging System Equipment, Section 625-2. Quincy MA: National Fire Protection Association.

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