PV EGC sizing

[rickb101260]

Tried searching for a thread but no joy. I'm trying to better understand sizing the PV EGC for an ungrounded system using a Solaredge Inverter. 2014 NEC 690.47(C)(3) states I can size and combine my EGC on the AC side to the largest ungrounded conductor which in this system is 10awg. Yet as far as I can make out, the DC side is sized at 8awg copper minimum 2014 NEC 250.166(C). This makes no sense to me at all. Thoughts?

 

[jaggedben]

You poor soul that's still on the 2014 NEC.

Ok, is your AHJ enforcing 690.47(D)? If yes, and they/you want that GEC run separately from the circuit conductors, then that's where your 8awg per 250.166 applies. The EGC between array and service can be 10awg if it's separate or if they are not enforcing the array electrode requirement. Arguably it can be 14awg between array and inverter.

If the AHJ wants/allows you to combine the array GEC and EGC, then your 8awg applies to the combined conductor.

FWIW, all the GEC requirements are gone in the 2017 NEC.

 

[SolarAlternativesDesign]

On a similar topic, for a battery backup system, would the circuit from the battery inverter to backed up loads panel require a GEC per 250.66? I suppose the neutral would somehow be bonded to the GE even in backup mode.
Also, I suppose the ground wire from service panel to the battery inverter switch would be the same rating.

 

[jaggedben]

On a similar topic, for a battery backup system, would the circuit from the battery inverter to backed up loads panel require a GEC per 250.66? I suppose the neutral would somehow be bonded to the GE even in backup mode.
Also, I suppose the ground wire from service panel to the battery inverter switch would be the same rating.

The feeder to the backup loads panel needs an EGC sized to 250.122 or maybe 250.102, but not 250.66.

The system (inverter or battery lead) may or may not need a GEC (that is, additional to the service GEC). It depends how its configured. You are correct that in most cases the backup neutral is not switched and therefore remains grounded at the service. A truly off-grid system would need the neutral grounded and bonded somewhere other than a service panel, probably at the inverter or the first disconnect. And then there may be DC grounding, although that's increasingly uncommon.

 

[SolarAlternativesDesign]

The system (inverter or battery lead) may or may not need a GEC (that is, additional to the service GEC). It depends how its configured.

Thank you. Could you also explain the type of configuration in which a GEC would not be needed?

 

[Carultch]

Thank you. Could you also explain the type of configuration in which a GEC would not be needed?

GEC's are required for "separately derived systems". Anytime you go through an isolation transformer, or a transfer switch that switches the neutral, such that you need to re-establish the reference to ground, that is what becomes a separately derived system.

The kind of inverter that doesn't require a GEC, is a transformerless inverter or non-isolated inverter. It has both polarities ungrounded, and at equal and opposite voltages-to-ground. Most string inverters built today, are this kind. This kind of inverter doesn't isolate the DC side from the AC side, with a transformer, like the alternative kind of inverters do. The absolute voltage of the DC polarities is governed by the absolute voltage of the AC waveform's midline. Of course, that is zero, due to the AC grid being grounded. We used to call these ungrounded systems. We now call them functionally grounded, because they are grounded through the functionality of the power electronics as a result of ultimately being connected to a grounded grid. A truly ungrounded system would have transformerless inverters connected to an ungrounded AC grid.

When inverters are built with an isolation transformer, either internal or external, they required a GEC, in order to satisfy separately-derived-systems grounding requirements where it established the bond between ground and the grounded polarity. This used to be the way nearly all inverters were built, until transformerless inverters and "ungrounded" systems became more common. Of inverters built today, you usually see this on central inverters, and rarely see it on string inverters. One polarity would be grounded through a GFCI fuse/breaker, and the other would be ungrounded. The transformer inside the inverter would allow the power electronics to produce a waveform, that had its troughs at ground and its crests at the full voltage it could produce. It would use the transformer, to remove the DC offset of this waveform, in order to feed it to a grounded AC grid.

 

[jaggedben]

Thank you. Could you also explain the type of configuration in which a GEC would not be needed?

I think you were asking about GECs and battery systems...

Most battery configurations these days are not going to require a GEC. That most likely includes any ESS (e.g. Powerwall or Enphase Encharge) where the battery is fully contained in a listed assembly (so there is no field installed DC wiring) and you are connecting to an already grounded AC system. Even something like LG Chem with Solaredge doesn't require new grounding because the battery is functionally grounded through connection through the inverter as Carultch described above. And note that in general the NEC doesn't really require battery circuits to be grounded.

I think it is easier to answer the opposite of your question, explaining the type of configuration in which a GEC would be needed. That would most likely be a simple (one might say 'old school') system with, say, a field assembled AGM battery bank. There are a couple reasons you might want to ground the negative on such a system. One is if there is any DC utilization, to provide a ground fault path. The other is if parallelled with a PV array that doesn't have sophisticated ground fault detection, then grounding one conductor is required. (see 690.41) Once you are grounding a field installed conductor you will need a GEC, pretty much by definition. Most likely we are talking off-grid systems here. Note that such a system would have to either be off-grid DC or use an isolated inverter.

 

[SolarAlternativesDesign]

Thank you both for the excellent explanation. I'm glad to have confidence in my grounding design on these battery backup systems.