The existing NEC requirements for these feeder taps dictate that the ampacity of the secondary tap conductors be at least 1/3 of the overcurrent device protecting the feeder conductor multiplied by the primary-to-secondary voltage ratio. At first, this sizing seems reasonable when considering that the feeder circuit device is being asked to provide short-circuit protection for the smaller tap conductors. But, this is often extremely conservative and frequently results in a conductor sized much larger than is actually required according to the laws of physics. By using formulas that have been widely utilized by IEEE, the Canadian Electrical Code, and the IEC, much smaller conductors can be installed. This will provide significant cost savings for electrical distribution systems, allowing North American manufacturers to be more competitive in the global marketplace.
An example would be helpful. Assume a feeder conductor is a 3/0 with an ampacity of 200 amperes, and protected with a 200 ampere overcurrent protective device. Also assume a one-to-one voltage ratio for simplicity. According to the 2005 NEC, the smallest 25 foot secondary tap conductor would be a 4 AWG with an ampacity of 85 amperes, even if it were only supplying a 10 ampere load. (Three times the ampacity of a 6 AWG, with an ampacity of 65 only gives 195 amperes, which doesn?t meet the 200 ampere requirement.) According to the physics formula, and UL standards, a 200 ampere Class J fuse will protect a 10 AWG conductor for faults up to 200,000 amperes. (Maximum I 2 t let-through of a 200 ampere Class J fuse at 600 volts with 200,000 amperes available is 300 x 10 3 ampere squared seconds, while the short-circuit withstand of a 10AWG copper conductor is 303 x 10 3 ampere squared seconds.) As we can imagine the cost savings here will be substantial, and within the safety umbrella of internationally accepted standard physics formulas.
