In the November 2011 issue, I wrote about the rewrite of Section 250.30(A)(1) through (A)(8) in the 2011 National Electrical Code (NEC). Last month, I discussed the grounding and bonding procedures using the grounded conductor at the service equipment and outside utility transformer. In this issue, I discuss the grounded conductor when it is routed from the secondary side of a separately derived system (SDS) to equipment-supplying loads in a facility.
As required by Section 250.30(A), the grounded conductor must not be used to ground the noncurrent-carrying parts of load-side equipment of the SDS. Section 250.142(B) clarifies this general rule.
Grounding and bonding an SDS is similar to grounding and bonding the service equipment, except it can take place at the SDS, which, in this case, is the source or the first disconnecting means as permitted per 250.30(A)(1)(a) or (b). For example, consider a transformer having 4/0 AWG THWN copper secondary conductors supplying a panelboard with a 225-ampere main. If grounding and bonding was performed at the SDS or at the panelboard (first disconnecting means), a 2 AWG copper conductor is selected as outlined by the provisions of Section 250.30. The 2 AWG copper conductor is selected from Table 310.15(B)(16) that is sized from Table 250.66 and based on 4/0 copper transformer secondary conductors. Note that this is a legal installation. Section 408.36(B) permits a panelboard to be supplied through an SDS.
If the ungrounded phase conductors do not exceed 1,100 kcmil copper or 1,750 kcmil aluminum, the grounding and bonding conductors are selected from the size required by Table 250.66. Otherwise, apply the 12-percent rule. For example, if the ungrounded phase conductors are rated at 2,000 kcmil, then you multiply 2,000 by 12 percent, which equals 250 kcmil (2,000 12 = 250). A 250 kcmil can be used to ground and bond the items as outlined in Section 250.30. Be aware that this article does not address the paralleling of the ungrounded phase conductors. This general rule is so important to designers, installers and inspectors that it is repeated in Sections 250.24(C)(1), 250.24(D)(1), 250.30(A)(3)(a), and 250.102(C)(1) in the 2011 NEC.
In this article, the system-bonding jumper (SBJ) is used to ground and bond the X0 in the transformer to the transformer enclosure and the structural steel of the facility. Assuming that the transformer secondary conductors are 4/0 copper, the SBJ would be 2 AWG copper-—sized from the 4/0 conductors in accordance with Table 250.66. The grounding and bonding can be done at the first disconnecting means (panelboard) instead of at the transformer.
The requirements for installing and using the supply-side system-bonding jumper (S-SBJ) are located in Section 250.30(A)(2) and 250.30(A)(8) as sized in accordance with Section 250.102(C)(2). The S-SBJ can be used to ground and bond the transformer’s enclosure to the panel-board’s enclosure. The S-SBJ also can be used to ground and bond the raceway from the SDS to the panelboard. As previously noted, this bonding jumper is sized based on the largest of the ungrounded phase conductors and selected from Table 250.66.
The grounding electrode is selected from (1) metal water pipe, (2) structural metal (3) or other electrodes per Sections 250.50 and 250.52. The chosen electrode grounds and bonds the SDS to earth ground as Section 250.30 requires. As previously stated, this earth-ground connection can be made either at the transformer (SDS) or at the first disconnecting means.
The grounding-electrode conductor is installed either at the transformer or at the first disconnecting means (panelboard) and sized per Table 250.66, just as the other grounding and bonding items of the SDS. As sized and selected above, the grounding electrode must be a 2 AWG copper conductor.
What is required?
Based on the ungrounded phase conductors being 4/0 copper, then the SBJ, GEC and S-SBJ must at least be a 2 AWG copper conductor. The grounding electrode is selected from the selection requirements outlined in Section 250.30(A)(4). Note that the grounding and bonding method must not create parallel paths per 250.30(A)(1), Ex. 2.
The listed sections document the grounding and bonding methods that are necessary to select the appropriate size conductors to ground and bond an SDS. If these conductors are sized as outlined in this article and as required by the NEC, then, under normal operating conditions, the grounding and bonding conductors will safely clear a ground-fault, should one occur. The grounding and bonding procedures outlined for your review present a documented guideline for grounding and bonding SDSs.
I’ll discuss grounding and bonding from one structure to another in the next issue.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.