Q Our firm has furnished and installed a complete electrical system to a sound stage facility. The building is about 140 feet by 200 feet and stands about 50 feet at the peak. The construction of the building is a complete structural steel framing bolted together and is finished with metal exterior walls and roof system. A four-wire, 120/208V, 3,000A electrical service has been installed for this building. We have installed in parallel four sets of 750 kcmil copper conductors, six conductors per phase. The structural steel has been bonded as per Article 250-104 (d) of the National Electrical Code by use of a 3/O AWG copper wire from the building framing to the grounded conductor of the main. The electrical installation of the building has been inspected by the New York Board of Fire Underwriters and has met all the requirements of the National Electrical Code. I have been informed by the local fire marshal that the building must also comply with the 1030.1 (g) of the New York State Uniform Fire Prevention and Building Code, which states: "Metal roofs, veneers and sidings on buildings shall be made electrically continuous and shall be grounded in at least two locations on the opposite sides of the building in conformity with Article 250 of reference standard RS-51-2 (National Electrical Code, NFPA 70)." According to the fire marshal the building must also comply with Article 250-96 (a) of the National Electrical Code, which states: "Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings and other metal noncurrent-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel or similar coating shall be removed at threads, contact points and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary." The size of the bonding for the roof and sidings that the fire marshal has requested is 3/O AWG copper or equivalent. His request is based on Table 250-66 and that the "fault current likely to be imposed" on the roof and sidings is the actual service main and therefore the 3/O AWG copper sizing. The electrical equipment is floor mounted on a slab and no supports were necessary to the building structure and/or sidings. The service-entrance feeders are underground and come into the bottom of the equipment through the slab. The service is physically attached to the structure only by way of the structural steel bond as mentioned above. My understanding of "fault current likely to be imposed" on the roof and sidings is the overcurrent devices supplying electrical equipment that is mounted on the metal structure and the metal roof and sidings. This would be lighting fixtures, air conditioning, safety disconnects, etc., and using Table 250-66 the largest size bonding required for the installations of this project would therefore be #8 AWG copper or equivalent. My questions are as follows: What size (AWG or equivalent) bonding jumper is required to ensure bonding of the metal roof and sidings that will comply with 1030.1 (g) of the New York State Uniform Fire Prevention and Building Code stated above? How do I determine this bonding jumper size?
A First, the quoted requirement is to ground the metal roofs and so forth in at least two locations in conformity with the Article 250. This cannot be accomplished because such a requirement does not exist in Article 250. Similar rules are provided for mobile homes, recreational vehicles and park trailers, but such requirements are found only in Articles 550, 551 and 552 respectively, not in Article 250. Of these references, only 550-11(c)(4) refers to opposite ends of the building. This section and Sections 551-56 and 552-57 consider the exterior metal panels to be bonded by attachment to each other and to the chassis. The chassis is required to be bonded with a minimum #8 AWG copper conductor. The reference to 250-96(a) is not applicable because of the phrase "that are to serve as grounding conductors." The metal frame of a building is prohibited by Section 250-136(a) from serving as a required grounding conductor. Therefore, no sizing rules for a conductor that is not required should be expected to be provided in the Code.
Since the "requirement" that is being imposed is not supported by the NEC, the only possible sizing rule that can be applied is the one coming from the authority who imposed the requirement. The actual fault current likely to be imposed on the metal building is minimal because such a path presents a relatively high impedance. Even a fault in some equipment attached to the metal roof or siding is not likely to impose a high fault current on this metal skin because the grounding conductors that are required will have a much lower impedance. Maintaining a low impedance ground return path is a requirement of 250-2(d) and in order to have such a low impedance, the grounding conductors must be run with or enclose the circuit conductors as required by Sections 250-134, 250-118 and 300-3(b). Building steel, metal studs, metal roofing and metal siding cannot provide the low impedance path because they do not follow the circuit conductors. Certainly high currents could be imposed on the metal skin by lightning or perhaps by some utility accident, but the NEC does not specifically cover these situations.