Converting 50kW system from micro inverters to 8x SMA inverters

[pclausen]

My current SMA 7.7s do clip quite a bit. Here's a screenshot from Feb 27, 2020:



The top one is a 3x12 array with 36 Talesun 270 Watt panels (9,720 watts).

The bottom one is a 2x12 + 1x8 array with 32 Talesun 275 Watt panels (8,800 watts).

I suppose the area being clipped is pretty small compared to the area under the curve, but I would think it adds up to several percent a year. Also keep in mind this was in February when the sun is pretty low in the sky.

I'll definitely check out the SMA Sunny Designer to see what it comes up with. I got my base info in there already, but want to double check my array tilt angles to get them just right and also enter my cable details.

It is currently showing my annual yield as 87,956 kWh where my actual with the micro inverters (and the 2 SMA 7.7s) was 67,200 kWh in 2020 and 67,535 kWh in 2021. So I definitely left a lot on the table from the failed inverters, associated cable loss @ 240V, and the micro inverters being rated at 250 Watts where a lot of my panels are 280 watts.

[bcroe]

I would worry more about failed equipment and wiring loses, than
that level of clipping. Bruce Roe

[nerdralph]

I'll definitely check out the SMA Sunny Designer to see what it comes up with. I got my base info in there already, but want to double check my array tilt angles to get them just right and also enter my cable details.

It is currently showing my annual yield as 87,956 kWh where my actual with the micro inverters (and the 2 SMA 7.7s) was 67,200 kWh in 2020 and 67,535 kWh in 2021.

Keep in mind Sunny Designer doesn't factor winter snow cover. Here in Nova Scotia I find subtracting 30% from the winter production numbers roughly compensates for the snow.

[pclausen]

You guys must get a ton of snow up there. Here in VA we typically get 2 or 3 events each winter, and its typically only a few inches, although over the years there have been cases where we got a foot or more. I try to keep up with it and broom it off the ground mount arrays, but sometimes it gets away from me, or it falls during the night.

I got the superstrut "wall" put together and the one inverter already inside, mounted to it. Temporarily wiring of course until I get the wireway and what not installed:



So it will look something like this once the other inverters are mounted:



The gap in the top row is where the 225A sub-panel will go. They got the 6.0s and 7.0s on a pallet for me, just waiting on some more 7.7s to show up to ship the order. Hopefully this coming week.

I'm looking at this 4x4 10' long wireway:

https://www.gordonelectricsupply.com/p/Unity-44120Gflnk-4X4X120-Flt-Lid-Wwy/6175789

Would there be an issue with laying both AC and DC wires in the same wireway? That way I could get away with just having a single wireway.

[peakbagger]

I think you now need a internal divider to run AC and DC in the same wireway. Its standard wireway with an add on internal separator to keep the DC wires separate from the AC. I am unsure if when you run the wires in through the side of the wireway if you have extend the conduit inside to the right compartment (which would be a PITA).

I did find this reference from 2017

Microsoft Word - 2017 NEC Code - Ch 3 Article 300 (1).docx (allstarce.com)

(1) 1000 Volts, Nominal, or Less. Conductors of ac and dc circuits, rated 1000 volts, nominal, or less, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway. Secondary wiring to electric-discharge lamps of 1000 volts or less, if insulated for the secondary voltage involved, shall be permitted to occupy the same luminaire, sign, or outline lighting enclosure as the branch-circuit conductors

[pclausen]

Thanks for that reference. So based on that, I should be able to have my AC and DC wires share the same wireway. I plan to bundle the AC and DC wires separately, with the AC wires running along the upper portion of the wireway and the DC wires along the lower portion. I'll look at adding a separator, although I can see that being a bit of a pain, depending on how deep/tall it is, which could interfere when wires need to cross from above or below.

My next challenge will be how to transition the existing conduits that terminate outside the building, to connect them to the wireway mounted to the superstrut rack.

Here's a visual:



The conduit marked AC will go away since the 7.7 mounted outside will move to the inside. I then plan to dig up the 2 DC conduits on the far right and extend them underground over to where all the other conduits come up into the sub-panel, that will also be relocated to the inside.

The ETH (Ethernet) conduits will go away as well.

So that leaves me with 4 3/4" conduits and a 1 1/2" conduit, that all need to got to the wireway on the inside.

I'm thinking I would terminate all 5 conduits into a junction box and then run a single 2", from it, to the wireway on the inside. I would mount the junction box level with the wireway on the inside, so that I would just need a short piece of conduit to shoot out the back of the junction box into the wireway.

The tricky part will be cutting the existing conduit without damaging the wires that are inside them. But I'm thinking this tool would do it: (It handles up to 2.5" pipe)



Does that all sound like a reasonable approach?

Thanks

[nerdralph]

You guys must get a ton of snow up there. Here in VA we typically get 2 or 3 events each winter, and its typically only a few inches, although over the years there have been cases where we got a foot or more. I try to keep up with it and broom it off the ground mount arrays, but sometimes it gets away from me, or it falls during the night.

It's not so much the total volume, but where we don't get really cold winters lots of the snow is wet and sticky so it doesn't slide off.

[bcroe]

Sounds like a plan. I would keep the AC and DC wiring far apart. Here
I run direct buried cables under the slab, then turn straight up through
the slab, no outside appearance. Cut a conduit with wires in it, been
done with care. Bruce Roe

[pclausen]

I picked up some wireway runs from a local supplier. I decided to make 2 horizontal runs so that I just need 2 short 3/4" EMT drops from each inverter down into the wireway. So this is how it will be configured:



Also got 30' of 300 MCM copper and a 225A breaker since I'll be over 200A during peak production:



Since its raining today in Virginia, I figured it would be the perfect day to shut down the entire solar plant and migrate everything inside. After isolating the solar plant from the mains and disconnecting DC at the arrays, I pulled the SMA 7.7 from the outside wall and disconnected all the AC wires going to the 225A sub-panel:



I then removed the sub-panel and SMA bracket along with the 3/0 Copper cables that will be replaced with the 300 MCM:



I then installed the 7.7 below the other 7.7. Given the different mounting system used on the 40 vs. 41 SMAs, I had to add another strut in order to mount it properly. I also installed the subpanel in the center as planned:



Next I added 2.5" conduit between the meter base and sub-panel and pulled the 300 MCM through it, along with the existing 3/0 Neutral and 1/0 Ground:



Pulling that 300 MCM was no fun at all and I still have to connect it on both ends, which will be even less fun. All in all, not bad for day 1 with a trip to Lowe's to pick up parts.

Btw, cutting the conduit with that pipe cutter was a breeze:



I got a bigger one for cutting the 1.5" conduit going up the wall, so that should be pretty easy too. Hopefully I'll get all the remaining outside work done tomorrow, which will mainly be installing the junction box in approximately the same position the sub-panel used to be.

Pricing on electrical supplies are pretty insane these days. The 300 MCM was over $10/ft and the 4x4 wireway was even worse. Those little Tee's alone were over $70 a pop. Completely nuts.

[pclausen]

Sounds like a plan. I would keep the AC and DC wiring far apart. Here
I run direct buried cables under the slab, then turn straight up through
the slab, no outside appearance. Cut a conduit with wires in it, been
done with care. Bruce Roe

I plan to bundle the DC along the roof inside each wireway and the AC along the floor. I'll use these stick on base pads that you can run zip ties through, to keep everything in place, once all the wires are laid in the wireways. In addition to using 300 MCM from the subpanel to the meter base, I'll use #6 for all the AC runs between the inverters and 40A breakers. The array wiring is all #10. Hopefully that will minimize wire loss as much as possible.

[pclausen]

Got the 300 MCM connected to the 225A breaker, but my God that was a royal pain:



If I had been able to bring it up from below, it would have been no big deal, but bending 300 MCM coming from the side like this, not so much.

[pclausen]

Made decent progress over the last few evenings after work. Got the 300 MCM terminated at the shop meter base: (in case you're wondering, the shop meter base is mine and does not belong to the POCO. Their meter base is at the main residence that in turn feeds both the house and shop, through a 400A fused disconnect)



Got a junction box installed in place of the sub-panel and redid the conduit up into it. Not the prettiest since I had to work with what was already there:



I ran a 2" conduit through the wall behind the junction box into the wireway on the inside and then extended all the 10 AWG cables through it:



I used thick walled butt splices and double walled heat shrink on all the cable extensions. I picked up a crimp tool especially calibrated for use on non-insulated butt splices.



Inside view with the 2 SMA 7.7's wired up, semi final and all the micro inverter strings very temporarily wired up just so that I could get all the panels back online again until the other SMAs arrive:



I was hoping that by switching to 3/4" EMC, I would be able to fit the 10 AWG PV wire through, but no luck. So I had to convert to 10 AWG THHN just before entering the EMC conduit.



Why SMC did not have at least one 1" knock-out in the base is beyond me. Yes there are 2 3/4" knock-outs below the MPPT inputs and one on the side, but it would be double the hassle to cut twice the amount of EMT conduit and drilling the extra holes in the wireway for each inverter.

Anyway, the inverters are scheduled to arrive tomorrow, so I'll finally be able to start converting everything over permanently. Picked up some 6 AWG for all the AC wiring between the inverters and sub-panel today. So I should be all set to hopefully get most of the arrays converted over this weekend.

[bcroe]

Those knockouts are usually not where you need them. I solved that with
metal punches the sizes I use.

The splices look OK to me. Do check them for warmth with max power
running a few times. Bruce Roe

[peakbagger]

Just in case, the big wire looks like it may be aluminum, make sure you use aluminox or similar product on all connections and definitely retorque the connections after the system has run for awhile.

[pclausen]

I thought about making the holes bigger, but the aluminum casing is pretty thick. I don't think my cheap Harbor Freight punch set would have been up to the task. Good tip on checking the splices for heat under load.

Yeah, the MCM 500 that runs up to the house is aluminum. I was pretty liberal with aluminox (or whatever the anti-oxidation stuff I picked up a Lowe's was called) back when I initially ran that wire back in 2017. I also used the same product on the copper MCM 300 this weekend, just to be on the safe side. I'll retorque it all for sure.

One of the reasons for upgrading the wiring between the solar sub-panel and the meter was, was that the 3/0 copper I used before would get quite hot with 200A running through it during peak production. The 300 MCM should run nice and cool by comparison I would think. The MCM 500 aluminum runs underground the 200' distance up to the house where the POCO meter is.

MCM 500 over 200 ft with 250V and 200A is going to drop about 5 volts, or 2%. Nothing I can really do about that, but at least I can minimize any losses before hitting the shop meter base.