Q A local utility recently insisted that I provide a larger service entrance conductor than the Code seems to require.
The application is as follows: A new 10,000-sq.-ft. metal building was constructed for use only as a warehouse for storing pallets of equipment and parts. Sixteen 250W high-bay lights were installed on four 120V AC breakers. About 20 120-V AC receptacles were installed on about eight 20A breakers. Some bathroom lights, emergency and exit lights, and other small miscellaneous loads were installed. There are no significant motor loads, air conditioning or HVAC equipment. The total connected load on the main panelboard is estimated at about 12.0 KVA. Based on this, the connected load would be less than 15A per phase. A pad-mounted, utility transformer serves the new warehouse. This transformer is rated 75KVA, three-phase. The secondary is 480/277V AC, wye-connected. This transformer feeds a 400A, three-phase, four-wire panelboard with main lugs only through an underground conduit. A 100A breaker in the main panelboard feeds a 480/277V to 208/120V transformer and this transformer supplies a 208/120V panelboard for lighting and receptacles. A 200A spare breaker was installed in the main panelboard, but no load connected.
Paragraph 230-42 of the 1999 NEC deals with the required size and ratings of service entrance conductors. Subparagraph (a) states that the ampacity of the service entrance conductors before the application of any adjustment factors shall not be less than the sum of the non-continuous loads plus 125% of continuous loads. Assuming the loads to be 100% continuous, we say the NEC dictates service conductors have ampacity not less than 18A.
Article 240 deals with overcurrent protection. Paragraph 240-21(c) (4) deals specifically with protection for transformer secondary conductors. Paragraph 240-21(d) deals specifically with protection for service entrance conductors. We were unsure of whether to use (c) or (d), since the service entrance conductors are technically transformer secondary conductors. However, close reading seems to indicate these two sections have the same requirements. It appears the service entrance conductors may be considered protected by a grouping of no more than six breakers in the main panel, so long as the ampacity rating of the service entrance conductors exceeds the sum of the trip settings of all installed breakers. We believe this section requires that the service entrance conductors to have a minimum rating of not less than 300A and must therefore be a minimum size 350kcmil. We also believe that, based on this, the customer could have removed the 200A spare breaker from the main panel and size the service entrance conductors with a minimum rating of 100A, or a No. 2 AWG. The utility disagreed and insisted that the service entrance conductors be sized to carry the full load of the 400A panel. They required the customer to install 500kcmil service entrance conductors.
A The transformer secondary conductors are most likely service lateral conductors, so the overcurrent protection requirements are given in Sections 240-21(d) and 230-91. One to six devices may be used. The rating or combined ratings must not be less than the load calculated according to Article 220. In addition, according to Section 230-42, the ungrounded conductors must be at least as big as the required main disconnect. Based on your load, this minimum size would be 60A according to Section 230-79(d). Although you would be permitted to have conductors, a main disconnect, and an overcurrent device as small as 60A, 230-91 requires the overcurrent device to provide overload protection for the service conductors. According to 230-91, Exception No. 3, the two main devices you mention are permitted to exceed the rating of the service entrance conductors. Even though the disconnects are larger than the 60A size required, the conductors are still required by Section 230-42 to be only as big as the required size. The provision for future expansion without capacity for future expansion is not necessarily a Code violation, so the 60-A conductors could be terminated on the 400A main lugs. However, if there were only one main, perhaps a 400A molded-case circuit breaker in the panel, the conductors would have to be sized so they could be protected by the 400A device. The utility is not covered or necessarily bound by the Code requirements. They may have other rules. Many utilities require a minimum size conduit or a minimum size service. In your case, it appears that the utility does not want to serve a 400A panel unless the implied 400-A capacity is provided. This is well within their authority in most jurisdictions.