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Neutral Conductor Value Engineering in 3-Phase String-Inverter Systems

Value engineering is the process of maximizing the ratio between function and cost in commercial and industrial (C&I) or utility solar applications. By identifying low- or no-cost ways to improve energy harvest, for example, Pure Power Engineering can value-engineer a project by increasing its revenue-generating capabilities. Provided we maintain safety and reliability, our engineering team can also value-engineer a project by reducing its costs.

Neutral conductor sizing and specification is a good example of the latter. Here I provide some pro tips for identifying applications where there is no need for—or benefit from—the installation of a neutral conductor. Most importantly, the guidance provided here will ensure that your systems are futureproof, safe and reliable.

Original Equipment Manufacturer (OEM) Considerations

A neutral is a current-carrying conductor that is intentionally connected to ground. In North America, this grounded current-carrying conductor is color-coded white or gray for easy identification. While electricians and inspectors are accustomed to seeing a neutral conductor in three-phase electrical installations, specific string inverter manufacturers may or may not require a neutral. 

Because neutral requirements vary by manufacturer, it is important to review the OEM’s listed installation manual to determine whether a value engineering opportunity exists. Some three-phase string inverters do not require a neutral conductor to operate or require a neutral for voltage sensing only. This is due to the fact that PV inverters typically output balanced three-phase power.

As an example, the installation manual for Chint Power Systems’ CPS SCA-series grid-tied PV inverter states: “The neutral conductor is optional.” Yaskawa Solectria Solar’s installation manual contains similar language: “The neutral conductor from the inverter to point of interconnection (POI) is optional.” 

Note that some OEMs specifically allow for the installation of a bonding jumper in place of a neutral conductor. Per Yaskawa Solectria Solar, its inverter uses the neutral for voltage sensing only, meaning it does not carry current. The manual notes that if the authority having jurisdiction (AHJ) approves, “you can install a jumper across the EGC and the neutral terminal” to “satisfy the inverter [voltage] sensing purposes.”

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National Electrical Code (NEC) Considerations

While the AHJ has the final say regarding code interpretation, multiple sections in the NEC support the strategic elimination or downsizing of the neutral or “grounded” conductor. First and foremost, Section 110 specifically states that, “Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.” 

Since the inverter installation manual is part of the product listing, installing the equipment in accordance with the manufacturer’s guidelines will ensure that the installation is safe and free from hazards. If the OEM’s manual states that a neutral is optional, the product listing ensures that the neutral is not required for the practical safeguarding of persons or property. In the event that an AHJ requires a neutral in an application where it will not carry current, provisions in Article 705 limit the size of the neutral conductor. 

In NEC 2017, this allowance is found in Section 705.95(B): “A conductor used solely for instrumentation, voltage detection, or phase detection and connected to a single-phase or 3-phase interactive inverter, shall be permitted to be sized at less than the ampacity of the other current-carrying conductors and shall be sized equal to or larger than the equipment grounding conductor.” While relocated and slightly revised, this provision also appears in NEC 2020. Per Section 705.28(C), “a neutral used solely for instrumentation, voltage detection, or phase detection shall be permitted to be sized in accordance with 250.102.”

Electrical Design Considerations

Specific details about the electrical design are also relevant when specifying and sizing neutral conductors. These relate primarily to inverter architecture, system interconnection and grid service configuration. Central inverter applications, for example, likely provide little opportunity or incentive for neutral conductor value engineering, as these designs typically use prefabricated skid-based power stations. 

Atypical grid voltages may also complicate neutral sizing and specification. In our experience, the vast majority of C&I services are wye-grounded, four-wire configurations. While grounded Delta configurations are rare, this atypical service type may require special attention.

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Considerations for Adequacy and Future-Proofing

In order to maintain functionality while driving down costs, it is important to consider long-term system adequacy and future operations and maintenance activities. Where allowed by the manufacturer, Pure Power Engineering supports the strategic elimination or size reduction of neutral conductors to individual inverters. Where allowed by Code, we support the strategic reduction of the neutral conductor size. For a multitude of reasons, however, we do not recommend eliminating neutral conductors to inverter aggregation panelboards. 

First, monitoring equipment and auxiliary power supplies are often powered from inverter panelboards and require a neutral reference. Second, when it comes time to swap out the inverters 10 years down the road, you may need a neutral reference for the replacement equipment. Lastly, if an AHJ insists at the final inspection that you install a neutral—even though the inverter does not need a neutral—the scope of the rework will not extend throughout the facility but rather be limited to the inverter output circuits.

In supply-side interconnections, we also recommend maintaining a full-sized neutral at the point of interconnection (POI) based on a holistic cost-benefit analysis. Technically, these conductors are on the utility side of the service. As an AHJ, utilities often have requirements that do not necessarily adhere to the NEC. The last thing you want to find out at the final inspection is that the utility requires a full-sized neutral, as this re-work will necessitate another shutdown. Given that this is often a relatively short conductor run, the benefit of downsizing the neutral to the POI is very small compared to the potential costs associated with a last-minute shutdown.

Looking for more pro tips for your C&I solar projects? Contact Pure Power Engineering to learn more about our value-engineered design and construction drawing services.