Retrofitting Non-Isolated Inverters in Legacy Arrays: Page 3 of 3
Inside this Article
Based on this understanding of existing Code requirements and factoring in the relevant changes introduced in NEC 2017, I recommend the following practices when retrofitting non-isolated inverters in place of legacy transformer-isolated inverters.
1. There is no need to replace existing USE-2/RWH-2 cables with PV Wire. PV systems installed more than 3 years ago are unlikely to have PV Wire cable whips or source-circuit conductors. It is not possible to retrofit these modules with PV Wire, nor is it necessary. The USE-2/RHW-2 cable installed within these arrays is perfectly good and is safe for the operating life of the PV system. NEC 2017, the most recently adopted Code edition, supports this practice.
2. If the existing array has white wires for the previously grounded conductors, simply re-identify these as ungrounded conductors. Whereas one pole of legacy transformer-isolated PV arrays is connected to ground via a fuse, both poles of non-isolated PV arrays are balanced on either side of the ac ground reference. This means that a PV array operating at 300 Vdc has a voltage to ground of 150 Vdc for both the positive and the negative poles. In other words, neither conductor is at ground potential even though the circuit is referenced to ground through the grounded ac service transformer. Since neither pole of the array is intentionally grounded, service personnel should re-identify any dc conductors with a white marking or insulation, since these wires will no longer be at ground potential.
Prior to the introduction of non-isolated inverters, installers commonly used white markings to identify intentionally grounded conductors in a PV array. This practice was intended to meet NEC Section 200.6, which includes a special allowance for re-identifying grounded single conductors in PV systems with a white marking [200.6(A)(6)]. Where existing USE-2/RHW-2 conductors are identified in this manner, service personnel can simply remove or cover the white marking. In the event that existing conductors have white insulation, I recommend re-identifying these white conductors by some suitable means rather than removing and reinstalling new conductors, based on the precedence that 200.6(A)(6) sets for the re-identification of small PV system conductors.
3. Installing a dc disconnect that opens both positive and negative poles of the PV array will bring the existing system into full compliance with NEC 2017. While not required in existing installations, the safest and best approach is to replace the dc disconnect on each inverter with one that opens both the positive and negative poles of the PV array. You can easily rewire even the standard Square D HU361 disconnect used on many thousands of systems to open both positive and negative conductors. This practice makes the inverter much safer to service in the event of a ground fault. Fortunately, most replacement inverters on the market today have an integral dc disconnect that opens both poles. It is very easy and straightforward, therefore, to upgrade an existing PV array for full compliance with the newest edition of the NEC.
—Bill Brooks / Brooks Engineering / Vacaville, CA / brooksolar.com