Grounding a 75 kVA Separately Derived System
A 75 kVA dry-type transformer is installed in a commercial building. The primary is 480 V, three-phase, three-wire delta; the secondary is 208Y/120 V, three-phase, four-wire. The secondary conductors are 250 kcmil copper, 75°C, run 15 ft in EMT to a 250 A main-breaker panel. The building has a metal water service pipe entering the building and structural steel columns. The electrician plans to bond the secondary neutral to the panel enclosure and to the water pipe at the panel, and to run a 6 AWG copper grounding electrode conductor to the steel. The transformer has a 125 A primary breaker. The inspector is concerned about the system bonding jumper location and the size of the grounding electrode conductor.
1. Size the grounding electrode conductor and the system bonding jumper for this transformer, and identify the code basis.
Read the worked answer
The secondary is a separately derived system, so the grounding and bonding rules of NEC 250.30 apply.
Grounding electrode conductor
NEC 250.30(A)(5) requires a grounding electrode conductor from the grounded conductor of the separately derived system to the grounding electrode system. The size is based on the largest ungrounded secondary conductor using Table 250.66.
- Largest ungrounded secondary conductor: 250 kcmil copper
- Table 250.66, copper 250–500 kcmil: 2 AWG copper
The proposed 6 AWG grounding electrode conductor is undersized. It must be at least 2 AWG copper.
System bonding jumper
NEC 250.28(D)(1) requires the system bonding jumper to be sized using Table 250.66, based on the largest ungrounded conductor of the separately derived system. For 250 kcmil copper, the minimum is 2 AWG copper.
The 6 AWG conductor does not satisfy either the grounding electrode conductor or the system bonding jumper requirement.
2. Where must the system bonding jumper be installed, and may the neutral be bonded again at the 250 A panel? Explain.
Read the worked answer
The system bonding jumper must be installed at one point only: at the transformer, or at the first disconnecting means on the secondary, but not at both. NEC 250.30(A)(1) permits either location, but the bonding jumper and the grounding electrode conductor connection must be arranged so that the neutral is grounded at only one point.
If the system bonding jumper is installed at the transformer, a supply-side bonding jumper must run from the transformer to the 250 A panel, and the neutral must be isolated from the panel enclosure at the panel. If the bonding jumper is installed at the panel, the neutral must be isolated at the transformer.
Bonding the neutral again at the 250 A panel would create a parallel path for neutral current through the equipment grounding conductor and the building metal. That is objectionable current under NEC 250.6 and can energize metal parts, interfere with ground-fault sensing, and overheat equipment grounding conductors. The electrician's plan to bond at both the panel and the water pipe is not acceptable as described.
3. What grounding electrodes must be used, and how is the water pipe connection made?
Read the worked answer
NEC 250.50 requires all available grounding electrodes to be bonded together into a single grounding electrode system. At this building, the available electrodes include the metal underground water service pipe and the structural steel columns. If a concrete-encased electrode or ground ring is present, it must be included as well.
Water pipe
The connection to the metal water service pipe must be made within 5 ft of the point where the pipe enters the building, before any meter or insulated joint, under NEC 250.52(A)(1). Because a water pipe is not permitted to serve as the only grounding electrode, a supplemental electrode is required under NEC 250.53(D)(2).
Structural steel
The structural steel must be bonded to the grounding electrode system. The connection must be accessible and made with a listed clamp or fitting suitable for the steel.
The grounding electrode conductor must be sized under Table 250.66 based on the largest ungrounded secondary conductor, which is 2 AWG copper here. Each electrode connection must be made with a listed fitting, and the system must be bonded together.