JaeI receive many questions on how to design and install the electrical system for a separately derived system, including the use of the grounded neutral conductor. Naturally, grounding and bonding the secondary side always is a major issue as well as how the neutral conductor is used. For example, how is it installed when used as both a neutral and an equipment-grounding conductor on the transformer’s secondary side? Another popular question is, “How does an installer locate and install the transformer?”
Article 250 of the National Electrical Code (NEC) covers grounding and bonding an individual transformer, and in my opinion, the best procedure depends greatly on how the secondary conductors of the transformer are enclosed in a raceway. In other words, determine if they are enclosed in a nonmetallic or metallic raceway.
Consider a 10-foot run (in metal raceway) of three ungrounded 4/0 AWG THWN copper conductors per phase. The run extends from the secondary side of a transformer and terminates to a 225-amp circuit breaker located in a panelboard supplying various types of loads. The 4/0 copper conductors require a specific size copper grounding-electrode conductor to be sized and installed to properly ground the transformer to earth ground.
For safety and clearing of ground faults, the system-bonding jumper, -equipment-bonding conductor (if needed) and grounded conductor (neutral) must be at least the same size as required by specific sections of the Code.
The grounded conductor to serve as a neutral must be sized in accordance with 220.61, 310.15(B)(4)(C), 215.2(A)(1), Ex. 2, and Example D3(a) of Annex D. For the sake of determining the actual neutral load, let’s say the calculation produced a 4-AWG copper conductor. But, when used as both a neutral and equipment-grounding conductor, the provisions in Table 250.66 must always take precedence. Application of this table clearly demands that a minimum size 2-AWG copper EGC be selected and used, not the calculated number four neutral that is too small.
A 4-AWG copper conductor is not considered adequate to return the amount of the fault-current that a 4/0 AWG conductor can deliver to the lugs of the circuit breaker in the panelboard. For an installer to better understand these Code requirements pertaining to the sizing of this conductor, as well as other grounding conductors, the installer must review 250.142(A)(3), 250.30(A)(1), 250.30(A)(3), 250.30(A)(7), 250.30(A)(8)(a) and Table 250.66 of the 2008 NEC.
After reviewing 250.142(A)(3), it is clear to the user of the Code that the grounded conductor can be used as a current-carrying conductor (neutral) as well as an equipment-grounding conductor on the supply side of the electrical system. Section 250.30(A)(1) contains the requirements for sizing and selecting the system-bonding jumper, while 250.30(A)(3) deals with sizing and selecting the grounding-electrode conductor.
Section 250.30(A)(7) covers the type(s) of grounding electrode(s) permitted to be used to earth ground the secondary side of the transformer.
Applying the Code
All the elements needed to appropriately bond and ground the secondary side of the separately derived system are sized and selected per Table 250.66. For example, running 4/0 copper conductors from the panelboard to the secondary side of the transformer requires the bonding and grounding elements to be sized as follows:
1. System bonding jumper per 250.30(A)(1); No. 4/0 copper requires 2 AWG copper.
2. Grounding-electrode conductor per 250.30(A)(3); No. 4/0 copper requires 2 AWG copper.
3. Grounding electrode per 250.30(A)(7); metal structural is used.
4. Grounded-neutral conductor per 250.30(A)(8)(a); No. 4/0 copper requires 2 AWG copper.
5. Equipment-bonding jumper (if needed) per 250.102(C); No. 4/0 copper requires 2 AWG copper.
A 600-volt-or-less, dry-type transformer per 450.13(A) can be located on an open wall, column or structure and is considered accessible but not necessarily readily accessible. On the other hand, a 50-kVA, 600-volt-or-less, rated transformer can be installed in a hollow space of a building that is not permanently closed in by the structure. Wherever installed, the transformer must meet the ventilation requirements of 450.9 and a clearance of 450.21, which requires that the transformer be installed, so temperature surrounding the transformer is not excessive. Heat buildup in the windings can cause a fire hazard and damage elements of transformer and building materials. Blocking ventilating openings of the transformer by walls or other obstructions that can cause heating problems is prohibited.
Finally, select the appropriate sections from Articles 250 and 450 for bonding and grounding, and install a separately derived system. Then size grounding elements, and install accordingly.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.