Feedback on my DIY bifacial panel unistrut ground mount design?

[Hedges]

"Listed for use in PV systems"
That seems to say DC listed breakers, and diodes, are not allowed as backfeed or overcurrent protection unless explicitly listed for PV.

You quote "assumed overcurrent device rated in accordance with 690.9(B) shall be used when applying Table 250.122"

Does that mean for a 60A circuit, assume 60A breaker (even though there isn't one) and use 10 awg ground wire?!
That can't be correct - do you know what it really means?

 

[Think About It]

"Listed for use in PV systems"
That seems to say DC listed breakers, and diodes, are not allowed as backfeed or overcurrent protection unless explicitly listed for PV.

You quote "assumed overcurrent device rated in accordance with 690.9(B) shall be used when applying Table 250.122"

Does that mean for a 60A circuit, assume 60A breaker (even though there isn't one) and use 10 awg ground wire?!
That can't be correct - do you know what it really means?

My post was only meant to address that the EGC does not have to be the same size as the other conductors when those conductors are larger due to voltage drop. This is an exception for PV as the EGC commonly has to be upsized.

I take the first part to mean that if no OCPD in circuit then the 125% rule from 690.9(B) must be applied for the purposes of table 250.122. So if you had a 60A circuit and no OHCP, you would need a 75A OCHP, therefore 75A would be the number you would use in table 250.122, which would be #8 CU. If you had a 48A circuit, no OCPD, #10 CU would work. If a 60A OCPD is used in the circuit then #10 CU would work.

Edit: I just read this over and my head is spinning. LOL

 

[Hedges]

If you had a 48A circuit, no OCPD, #10 CU would work. If a 60A OCPD is used in the circuit then #10 CU would work.

Edit: I just read this over and my head is spinning. LOL

If I had 48A circuit and no OCPD, I wouldn't like the idea of #10 ground wire carrying that much current continuously. Which was the point I was getting at.

I thought I read somewhere that both PV wires and ground wire had to be sized for Isc x 1.56, which would be conservative.

Oh, and since shorted wires in a PV system won't blow a fuse, you have to size ground that big all the time. After all, OPCD is required to be at least Isc x 1.56

 

[Think About It]

If I had 48A circuit and no OCPD, I wouldn't like the idea of #10 ground wire carrying that much current continuously. Which was the point I was getting at.

I'm not disagreeing with you. I'm just stating what the code is allowing. Personally I like the idea of putting fuses at the source. I alsot think that if a string is fused at the source, is 300 VDC or less, and 30A or less, that we should throw out all the extraneous wiring requirement (like metal conduit) and use "normal" AC wiring methods.

I thought I read somewhere that both PV wires and ground wire had to be sized for Isc x 1.56, which would be conservative.

I've seen that number (1.56) somewhere before. Please let me know of you can find the reference.

 

[Hedges]

The math was 1.25x (normal margin for thermal OCP) and 1.25x for cloud edge effects on PV panels.

Here's a circular reference for you:

Code - 1.56 rule or conduit/temp rating or both?

I am trying to understand the electrical code and see that wire ampacity can be derated by several factors, including number of wires in conduit, ambient temp, the 80% conductor rule and solar 125% Isc rule (1.56 rule) When I add all these up the ampacity requirements become a bit ridiculous...

diysolarforum.com

Somewhere, it came from NEC.

Circuit Sizing and Current | UpCodes

Explore a searchable database of US construction and building code. Code regulations are consolidated by state and city for easier navigation.

up.codes

 

[Hedges]

I alsot think that if a string is fused at the source, is 300 VDC or less, and 30A or less, that we should throw out all the extraneous wiring requirement (like metal conduit) and use "normal" AC wiring methods.

Just 9kW of arc available. And DC, so it can draw a long flame without extinguishing. What could go wrong?

Fusing at the source would be good if the fuse can blow. It can't.
The only way to blow a fuse in a PV system is to have two, maybe three or more strings backfeed into one that is individually fused, and shorted.

Ground fault can find parallel shorts to ground and turn off SCC. Arc fault can do the same for series arcs. But an arc might continue anyway.
If you have RSD and if arc-fault triggers it, have a better chance. Not clear to me that happens.
My TriPower has a "fault" relay which I'm considering wiring to the external RSD keep-alive transmitter, so that might do what I want.

 

[AxelHeyst]

I ran a model to estimate 'meaningful' power loss due to wire gauge. If production is greater than loads, and battery SoC is full, then the losses aren't meaningful.

Worst case is winter:




Total loss at 8awg is 2,500Wh, but meaningful loss for 8awg is 1,125Wh.
Total loss at 6awg is 1,600Wh, but meaningful loss for 6awg is 710Wh.
Total loss at 4awg is 1,000Wh, but meaningful loss for 4awg is 450Wh

So the delta is 260Wh power loss for going up to 6awg. (Summer delta is 130Wh).
The delta is 675Wh for going up to 8awg.

One panel produces 2,300Wh in a winter day. 8awg meaningfully loses half of one panel, 6awg loses a third of a panel, and 4awg loses a fifth of a panel.

Of course, this is just a very rough model of demand load. Some use-days could incur greater meaningful power loss for various reasons.

Okay, I'm off to grok the last several comments and get up to speed on the nec language. What a treasure trove for learning the ins and outs of this stuff.

 

[AxelHeyst]

Pictures of the completed rack. Thank you everyone for the comments with this!


By the way: Yes, the lower edge is close to the ground and snow will accumulate there in winter. In a normal winter, this will occur four times and the accumulated snow will melt by noon. In the worst winter we've had in recent memory (aka this one), it will occur six times and the snow will be melted by the next day, assuming no one is there to push it off. Crushing loads of snowmelt>ice doesn't happen here. I could have designed it higher off the ground, but I judged it not worth the extra effort and expense.

By the way, you might notice a scratch in one of the middle panels. Bobcat prints! The neighborhood bobcat came over and climbed up the thing to check it out shortly after I got all the panels up. Apparently glass is slippery so it left dusty claw marks all up the center. They brushed off easily enough.

 

[AxelHeyst]

I have what I think is a dumb question regarding grounding and lightning protection, the SCC GFP system, and my combiner boxes. This is a case of thinking I knew what I was doing, getting the stuff, and going 'wait, how does this fit...'

I have reviewed filterguy's document on grounding and his document on solar panel ground fault protection systems. I believe page 4 of his GFP document most closely matches my system.

This is my system diagram:


This is my combiner box:


My SCCs are Midnight Solar Classic 150s, which I'll run in Follow-Me mode. I intend to use the internal GFP setup. Per the SCC manual, p18:

"If the internal grounding jumper is installed in a Classic [what you do when you are using the unit's GFP system], the battery negative and DC source negative must not be connected to the system grounding conductor anywhere in the system. Grounding of these circuits will defeat the GFP function."

My read of that statement is:

• At no point outside of my master SCC do I connect PV negative or battery negative to ground.
• At my master SCC, the PV and battery negative will connect to the internal grounding jumper. I will run ground from that jumper to my main ground at the AC panel (which is a ground wire bound to metal water pipe, I believe).
• The bare copper ground wire I'm bonding to my PV panel frames and rack will run all the way to that single ground point (or, because of the layout of my equipment room, I'll connect it proximately to the master SCCs internal grounding jumper).

Okay, that seems simple enough. My diagram reflects this. (Although, is it correct that I'm taking the DC ground to the inverter DC ground lug, or should I be wiring the DC ground directly to the earth ground near the AC panel where it's grounded to a pipe? I'm seeing conflicting things on different diagrams, and I don't understand the relationship between the inverter DC ground lug and the AC ground. Are they connected electrically? I think so, but I'd rather not assume and the manual doesn't say.)

Okay we're getting to my question. When I look in that combiner box, there's a ground wire connection coming out of the bottom of the arrester, the blue device on the right. You can see some examples of this in this thread on these style combiner boxes, and here is a screenshot of a pic wattmatter posted there:


What do I do with that? Isn't that ground wire connected to the PV negative, and thus a no-no per the SCC's GFP system?

 

[AxelHeyst]

I think my DC side ground system should actually look something more like this...

Still not sure about the ground connection coming out of those combiner arrestors.

 

[Hedges]

What do I do with that? Isn't that ground wire connected to the PV negative, and thus a no-no per the SCC's GFP system?

The surge arrestor should be DC open between all wires, except when a surge voltage appears. It would clamp PV+, PV-, Ground all within some voltage limits. That should be fine. Midnight sells their own version like that, with MOV between the nodes.

 

[AxelHeyst]

I see. So then electrically things should be wired like this.

 

[Hedges]

Does Midnight allow battery negative to be grounded? I thought grounding was required by code for higher battery voltages (e.g. 48V charged above 50V), and should be able to clear battery fuse. But I think (not certain) Midnight PV(-) goes through to battery(-).

What Midnight Classic apparently does have is magnetic coupling to PV(+), which avoids the issue of shorted FET causing battery overcharge. But if PV(-) does go through to battery, I'd think that would be incompatible with a second ground bond.

I've bought a Midnight Classic but not put it to use yet.

 

[AxelHeyst]

No - per the Midnight classic manual, "the battery negative and DC source negative must not be connected to the system grounding conductor anywhere in the system. Grounding of these circuits will defeat the GFP function."

 

[Shimmy]

I see. So then electrically things should be wired like this.
View attachment 144310

I hate to say this... but I think you need a separate wire for the array ground. The combiner SPD and SCC grounds can be tapped if you really want to, but preferrably the tap is actually the chassis ground connection point.

All of your enclosures (shunt, inverter, DC disconnect, AC disconnect will need ground connections if they are metal. (This is one of the reasons wireway can be really nice.)

 

[Marcaroni]

Hello, this is my first post, but have gained from this forum for some time now.

I'm helping a neighbor upgrade his system. He's getting (12) new bifacial panels. I'm designing a ground rack for him. If you'd care to, I'd love any feedback. See any problems with my design? Think it'll blow over? Gut check? Any feedback is much appreciated.

• Snow loads are minimal. It'll snow a foot at most here, and then melt.
• Wind can be stiff, but we have no hurricanes or tornadoes here.
• All of the fittings are standard unistrut fittings.
• Fitting attachment to the piers is deep jbolts embedded, not drilled expansion bolts.
• The only non-unistrut stuff besides the concrete is the diagonal cross braces, which I'm thinking I'll use something like ~1-2"dia galvanized metal fence pipe.

Okay lets see if I can insert images correctly...
View attachment 132260View attachment 132261View attachment 132262View attachment 132263View attachment 132264View attachment 132265View attachment 132266View attachment 132267View attachment 132268View attachment 132269

 

[Marcaroni]

Can you please tell me the vendor where you bought the unistrut solar panel clamps?" No one seems to know anything and I can't find them (except in a Unistrut pdf.) .thanks

 

[Wv lew]

Impressive. Thanks for sharing.