Is a Solar PV System Separately Derived?

Short answer: It depends on whether or not a transformer or auto-transformer is used. Most grid interactive PV systems will not be considered separately derived.

Confusion about a separately derived system can lead to improper grounding and equipment selection among other things. In this article I will explain what a separately derived system is and some examples.

Many PV installers, plan reviewers and inspectors believe that a PV system is separately derived. It’s making it’s own energy so it must be right? While it can be, in most cases it’s not.

The NEC defines separately derived as:

An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.

NEC 2014

Determining if a system is separately derived requires a look at the neutral or grounded conductor. A simple way to tell if the system is separately derived is to check whether the neutral is tied together with the neutral of the existing system.

For example, a wye-delta transformer would be considered separately derived because the neutral from the wye side is not tied together with the neutral of the system. A wye-wye transformer, however, would not be considered separately derived because the neutrals are usually tied together.

A transformerless inverter (all modern inverters) in a grid tie configuration will not be separately derived unless a transformer is used to step up/step down the voltage and that transformer isolates the neutral on one side of the transformer by using a delta-wye or delta delta transformer. The transformer itself would isolate the neutral on the inverter side (secondary side) from the existing system conductors which will result in a separately derived system as defined by the NEC.

This concept goes for generators and off-grid PV systems as well. A bonded neutral generator will be considered separately derived while a floating neutral will not (see this article from electricgeneratorsdirect.com for more). Off-grid PV systems use an auto-transformer to derive a neutral so they will be considered separately derived.

To summarize, your residential PV system is most likely not considered separately derived while commercial systems utilizing a transformer may be. Off-grid systems will more than likely be considered separately derived.