A 480VAC, 3-phase, 4-wire wye-connected electrical distribution in an existing facility is protected against short circuits by "fully-rated" overcurrent protective devices. The circuit breakers at the service equipment have a 42kA interrupter rating. The calculated fault current for the existing electrical system at the service equipment -- fed from a 1,500kVA transformer (480VAC secondary, 5.75% Z) was calculated to be about 31,378A. The utility is going to change the transformer that feeds the building from 1,500kVA to 2,500kVA. The calculated fault current for the system fed from a new 2,500kVA transformer (480VAC, 5.75% Z) is approximately 52,298A. Purchasing new distribution equipment that is "fully rated" is out of the question; the existing electrical equipment must remain. Motor contribution during short-circuit conditions is negligible, and is connected on branch circuits fed from an MCC downstream of the service equipment. If "series rated" protection is provided, which of the following minimum requirements must it meet?

A) A fusible switch that uses current-limiting fuses with 65kA interrupter ratings shall be installed ahead of the existing circuit breakers in the service equipment. This can be accomplished by a field design and installation.
B) A current limiting circuit breaker with a 65kA interrupter rating shall be installed ahead of the existing circuit breakers in the service equipment. This can be accomplished by a field design and installation.
C) A fusible switch that uses current-limiting fuses with 65kA interrupter ratings is permitted to be installed ahead of the circuit breakers in the service equipment. This installation must be designed by an electrical engineer engaged primarily in electrical design and maintenance, and field installed per the engineer's stamped and documented information.
D) The system may remain as is.

Answer: C)

Explanation: Per a 2005 NEC change, series rated overcurrent protection is permitted in existing installations under engineering supervision of an engineer engaged primarily in the design and maintenance of electrical installations. This would be appropriate for an existing installation where the owner can’t afford to replace existing electrical equipment with new fully rated electrical equipment [240.86(A)]. Another option would be to purchase and install factory-designed (engineered) tested combinations [240.86(B)]. If possible, the best option may be to install fully rated electrical equipment.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.