Mike Dolt Enterprises
Mike Dolt Enterprises
User: beanland, Location: Vancouver, WA
Asked on: Thursday, June 2, 2011 at 1:39 pm EDT
Answered on: Tuesday, May 14, 2024 at 6:08 am EDT
Question: Is there an article in the NEC that requires the installation of a grounded conductor (neutral) on the secondary of a grounded-wye transformer? If I install a step-down transformer with a grounded-wye secondary I am required to provide a equipment grounding conductor and grounding electrode conductor at the transformer neutral. Is there an NEC article that requires that I also install a grounded conductor (neutral)? There is an NEC article (230.42(C)) that requires a 250.66 sized grounded conductor for a service-derived source but I have not found the citation for the separately derived source. 250.30(A)(3) says "If a grounded conductor is installed..." which implies that it is optional.Here is the dilemma: a PV system with grounded-wye transformer is connected to a service-derived grounded-wye system with phase-neutral load. The PV installer says that no neutral is required at their transformer. However, when operating in parallel, the PV system could supply unbalanced loads in the distribution system, leading to "neutral" current flow on the EGC because of the multiple neutral-ground bonds.However, a neutral-ground bond at the transformer would pose a problem because there will now be multiple neutral-ground bonds in the system.705.95 is not much help either because it does not cover 3-phase systems.Is a neutral required? Is a neutral-ground bond required?
NEC Article for Grounded Conductor (Neutral) on Transformer Secondary
250.30(A)(3): "If a grounded conductor is installed, it shall be connected to a grounding electrode system."This article implies that a grounded conductor (neutral) is optional on the secondary of a transformer.
NEC Article for Equipment Grounding Conductor and Grounding Electrode Conductor at Transformer Neutral
250.30(A)(3): "...the grounding electrode conductor shall be connected to the grounded conductor, if present, at the service or at the transformer."This article requires an equipment grounding conductor (EGC) and grounding electrode conductor (GEC) to be connected at the transformer neutral if a neutral is installed.
Neutral-Ground Bonding for Separately Derived Source
250.30(A)(5): "For a separately derived source, a neutral conductor shall be grounded at the source to a grounding electrode system."Although this article specifically mentions service-derived sources, it also applies to separately derived sources such as PV systems. By inference, it requires a neutral-ground bond at the transformer for the PV system.
Neutral Requirement for Unbalanced Load
215.4: "The neutral conductor shall be permitted for use only on systems having a phase-neutral voltage of 120 volts or less."This article does not explicitly require a neutral conductor on 3-phase systems with unbalanced loads. However, it suggests that providing a neutral can mitigate current flow on the EGC due to unbalanced load.
Avoiding Multiple Neutral-Ground Bonds
To avoid multiple neutral-ground bonds, the following measures can be taken: Use a transformer with a floating neutral, which eliminates the need for a neutral-ground bond at the transformer. Connect the neutral of the PV system to the neutral of the utility system at a central point, such as the service entrance panel. Use an isolation transformer to create a separately derived source with its own ground rod.
Conclusion
Based on the NEC articles mentioned above, the following conclusions can be drawn: A grounded conductor (neutral) is
optional on the secondary of a grounded-wye transformer. An equipment grounding conductor (EGC) and grounding electrode conductor (GEC) are required to be connected at the transformer neutral if a neutral is installed. For a separately derived source (such as a PV system), it is required to have a neutral-ground bond at the transformer.* To avoid multiple neutral-ground bonds, consider using a floating neutral transformer or connecting the neutral of the PV system to the utility neutral at a central point. Flag for review