OCPDs in battery circuits of EG4 18Kpv.

[PVGeezer]

First posting to this forum. While I have 35+ years of professional PV and battery system design experience, I'm retired now so a bit rusty on my NEC, especially the recent Code cycles.

I'm designing the battery side of my 2x EG4 18kpv system, which is currently operating well in grid tied mode fed by 9s x 8p Znshine 450's. My county is still on the 2014 Code cycle, so it looks like I have some options for my battery system design that might not be available to folks on more recent Code revisions. I'd like to build up my own LiFePo4 battery system using 304ah prismatic cells and JK BMS's.

Here's my question. Neither polarity of the battery lugs on the EG4 inverters are grounded. This is just fine per NEC 250 VIII because it operates under 60Vdc. So far so good.

However, article 240 requires OCPDs for all ungrounded current carrying conductors. So my conclusion is that to be Code compliant I either need to use a listed two pole breaker, two single pole breakers, or two battery fuses for each battery. Not one only for the positive conductor, as is shown in the EG4 18Kpv manual and is commonly being done by members of this forum.

Am I missing something here? If so, can someone point to the Code section allowing only a single OCPD in the positive leg when both positive and negative conductors are ungrounded?

 

[wpns]

I'd compare the pricing to the EG4 PowerPro WallMount batteries, which have builtin breakers and tie into the RSD system on the 18Kpv to shut down the panels, inverters, and batteries simultaneously.

That'll make things a lot easier in the future when your AHJ has upgraded their requirements and you want to change something...

 

[PVGeezer]

That is a valid observation. I was actually looking at the EG4 wall mounts today. Not sure but they also appear to only have breakers on the positive conductors. Can anyone confirm that for me?

In my case the batteries will be located indoors about 25-30 feet from the (outdoor mounted) inverters. So my battery conductors will have a much higher exposure to possible ground faults than the recommended EG4 installation where the batteries are adjacent to the inverters. I (not to mention my AHJ) would be uncomfortable with those long ungrounded negative conductors without over current protection. And I don't want to end up adding additional OCPDs to EG4s battery systems, might as well DIY at that point.

So, back to my original question. Does anyone know of a Code justification allowing the (ungrounded) negative battery conductors to safely forgo over current protection?

 

[EnergizerNate]

That is a valid observation. I was actually looking at the EG4 wall mounts today. Not sure but they also appear to only have breakers on the positive conductors. Can anyone confirm that for me?

In my case the batteries will be located indoors about 25-30 feet from the (outdoor mounted) inverters. So my battery conductors will have a much higher exposure to possible ground faults than the recommended EG4 installation where the batteries are adjacent to the inverters. I (not to mention my AHJ) would be uncomfortable with those long ungrounded negative conductors without over current protection. And I don't want to end up adding additional OCPDs to EG4s battery systems, might as well DIY at that point.

So, back to my original question. Does anyone know of a Code justification allowing the (ungrounded) negative battery conductors to safely forgo over current protection?

I do not, and I am fusing the negative (and positive) lines from my bus bars back to my inverter. I am only fusing the positive lines of each battery attaching to the busbars, mostly because I don't trust the Chinese breaker in each battery and I think fusing both lines would be overkill at a per battery level. Not the best parallel, but my PV panels are fused on the pos and neg of each string, but not at each panel.

 

[wpns]

I was actually looking at the EG4 wall mounts today. Not sure but they also appear to only have breakers on the positive conductors.

Hmm, pretty sure it's a 2-pole breaker, but I can't say for sure if it's 2 poles in parallel on the positive lead only. OTOH, I'm not sure as a sealed package that you could tell the difference. Flip the breaker (or have the breaker tripped by the RSD) and there's no potential between the positive and negative leads.

 

[PVGeezer]

Hmm, pretty sure it's a 2-pole breaker, but I can't say for sure if it's 2 poles in parallel on the positive lead only. OTOH, I'm not sure as a sealed package that you could tell the difference. Flip the breaker (or have the breaker tripped by the RSD) and there's no potential between the positive and negative leads.

I looked at one of the video teardowns and its pretty clear that the breaker protects the positive buss only. So if I went with the EG4 wall mounts I'd probably add a set of OCPDs for both polarity conductors externally. The Midnite MNBC product line would I think probably be the easiest Code compliant way to do that but it's certainly not a cheap solution, on top of the already expensive wall mount product itself (relative to DIY anyhow).

You do raise a valid point regarding RSD integration though. I hadnt considered the potential advantages of that for the battery side of things.

Im not a fan of RSD for the PV side. I hate the idea of needing to have a bunch of electronics in the array field. So my array is out in my cow pasture with an electric fence around it. PV has no stored energy and is current limited by its nature, so if a module or conductor ever did experience a ground fault its not going to cause any harm except to itself. First responders can just leave it be. At worst, Id just let it arc till it got dark and then disconnect the mc connectors in that string.

Batteries capable of many kiloamps of fault current located in a building are a completely different thing in my mind. Lots of stored energy there to be respectful of. The stored energy in a 100kwh battery system is about the same as in 3 gallons of gasoline.

 

[PVGeezer]

I do not, and I am fusing the negative (and positive) lines from my bus bars back to my inverter. I am only fusing the positive lines of each battery attaching to the busbars, mostly because I don't trust the Chinese breaker in each battery and I think fusing both lines would be overkill at a per battery level. Not the best parallel, but my PV panels are fused on the pos and neg of each string, but not at each panel.

I think you're right to add OCPDs to both battery to inverter conductors.

I understand your overkill concern on the individual battery circuits but I have yet to find a Code justification for not having fuses or breakers in those circuits too. Consider that if you have multiple batteries in parallel and there is a fault in one of those conductors it could be backfed by the combined fault current of the rest of the batteries in the system. Even if there are breakers on the positive legs of each battery if the fault is resistive those other breakers might not clear and the faulted conductor could see up to the combined current rating of all the other battery breakers in the system. Not good.

Re fusing the PV side, unless you have a string combiner with 3 or more strings it's not necessary. The fuses won't clear the fault anyway because string short circuit current is well below the fuse rating. Thats why most inverters are designed with no more than two parallel strings per MPPT.

 

[wpns]

I'm still trying to figure out how the power leaks out of the battery if you have the positive lead disconnected inside the battery. Something inside the battery shorts to ground, I guess?
If you're going to add all kinds of external stuff, I'm not sure the EG4 WallMount batteries are for you, you'll be losing the plug-and-play advantages of the batteries, and trying to add safety stuff that's more reliable than the EG4 parts is a lot of work. Not that code might not require you to do stuff like that.

 

[EnergizerNate]

I think you're right to add OCPDs to both battery to inverter conductors.

I understand your overkill concern on the individual battery circuits but I have yet to find a Code justification for not having fuses or breakers in those circuits too. Consider that if you have multiple batteries in parallel and there is a fault in one of those conductors it could be backfed by the combined fault current of the rest of the batteries in the system. Even if there are breakers on the positive legs of each battery if the fault is resistive those other breakers might not clear and the faulted conductor could see up to the combined current rating of all the other battery breakers in the system. Not good.

Re fusing the PV side, unless you have a string combiner with 3 or more strings it's not necessary. The fuses won't clear the fault anyway because string short circuit current is well below the fuse rating. Thats why most inverters are designed with no more than two parallel strings per MPPT.

I see your point on fusing both lines of each battery. I don't see people doing it and they seem to pass inspection all the time, so I guess I didn't even consider it enough to talk myself into it (except for the whole bank to inverter).

Good point on the PV Isc and the fuses. I don't have parallel strings technically until my 6 strings reach my inverter and connect to 3 MPPTs directly on the inverter. I think (it's been a year since I did it) that NEC 2017 was a little more explicit about fusing the ungrounded negative on the PV lines, so that's why I did it. I also tend to over engineer things...

 

[PVGeezer]

You have to wind up with two ground faults, one on each polarity. The first one just creates a grounded system in what was an ungrounded one. The second one completes the circuit. Best way to understand it is to sketch the circuit and start throwing ground paths at it in various locations and trace the current flows from all sources. Also consider that those ground paths might be resistive so they don't produce high enough fault currents to trip the OCPDs.

And yep you can certainly argue that this kind of dual fault scenario might be a low probability event. But it does happen, hence the Code requirements for all ungrounded conductors to be protected.

Ive not personally seen it in battery systems but I have seen it in PV arrays where multiple backsheet isolation failures have occurred. A couple of houses in Germany (European electrical practice is for circuits to be ungrounded) had bad roof fires back in the oughts from this. Having been involved in some of that at the time I'm very cautious about it now. Call me paranoid I guess.

I think you might already need external OCPDs if you want to use more than three wall mounts. If you look at the EG4 battery to 18kpv wiring diagrams (not the ones in the wall mount quick start guide, the ones on the EG4 website) there is a note requiring external fusing when using more than 3 wall mounts in parallel (positive leg only though).

That makes perfect sense as the busbars are apparently rated for 600A and the breakers are 200A). For some reason that requirement is omitted in the wall mount quick start guide. I've asked EG4 for an explanation.

 

[PVGeezer]

I see your point on fusing both lines of each battery. I don't see people doing it and they seem to pass inspection all the time, so I guess I didn't even consider it enough to talk myself into it (except for the whole bank to inverter).

Good point on the PV Isc and the fuses. I don't have parallel strings technically until my 6 strings reach my inverter and connect to 3 MPPTs directly on the inverter. I think (it's been a year since I did it) that NEC 2017 was a little more explicit about fusing the ungrounded negative on the PV lines, so that's why I did it. I also tend to over engineer things...

Your comment prompted me to take a look at the 2017 Code. I retired about the time NEC 2017 was being revised so I'm not very familiar with it. I didn't see anything new in article 690.9 that would require OCPDs in PV source circuits (unless of couse you are paralleling 3 or more) but I did see some interesting language in 690.9 which allows OCPDs in only one ungrounded leg if the system is equipped with ground fault protection in accordance with 690.41. The justification stated is that the GFP in combination with one set of OCPDs is sufficient protection, which makes sense as the GFP should shut down the circuit after the first fault in an ungrounded system is detected and before a second fault can result in excessive current flow.

So that's interesting although not directly pertinent to my question which was regarding battery systems not PV. I took a quick look at the new (to me) section 706 covering ESSs and didn't immediately see anything similar there that would justify not having OCPDs for both circuit polarities, but I'll keep digging, maybe it shows up in 2020 or 2023.

 

[PVGeezer]

There is a change in the 2023 NEC article 706.31 covering over current protection for ESS's. In the 2017 and 2020 Code revisions adherence to article 240 (which requires OCPDs in all ungrounded conductors) is a requirement. That sentence has been removed in the 2023 cycle, don't know why.

So maybe that's the justification for EG4 not having OCPDs in the negative leg? Not many jurisdictions are on 2023 yet if any and anyhow I'm not so sure omitting the negative leg OCPDs is the right thing to do in my case.

BTW, there is a requirement in 706.31 for OCPDs at the ESS end of the inverter circuits when the circuits pass through a wall, which mine will, so I'm going to need additional fuses or breakers there anyway, regardless of whether I go DIY using 2014 code or with listed ESS's using a later Code cycle.