Is My New System Undersized?

[PV4Me]

Quick update -- floated the idea of moving the inverters closer to the MSP with the installer and his feedback was generally positive but he did mention the following caveats:

1. It will not be so easy to upgrade and add more capacity in the future in terms of inverters and panels as that will require more wire runs.
2. The DC circuits of the 10 panel strings have good voltage on them but the circuits with 5 panels may be a little low.
3. I will no longer have AC power out by the arrays. (Currently AC power at the arrays is used to power the wi-fi antenna that allows me to connect to my inverters, but if I am moving the inverters closer, that should be a moot point). Any other reason I would want to have AC power at the arrays that I'm missing?
4. It will be more expensive than running the CU cable for the AC leg the long distance as I will need DC combiner boxes, a lot more wiring (he suggested #6 AWG), and also according to him I will need DC disconnect switches at the arrays to be in compliance with NEC. He said currently having the inverters at the arrays is compliant already since the inverters have DC and AC disconnecting means to them already but moving the inverters away from the arrays will require DC disconnect switches.

He suggested we could do the following:

- DC combiner & disconnect switches at the array
- Inverters mounted in basement, relatively close to MSP
- New 200a subpanel at inverters location
- AC run back to meter main 200 amp line side connection fused AC disconnect


Also in the process of reaching out to PG&E to ask about the line voltage conditions and make them aware of my new setup. Thank you everyone.

[bcroe]

My preference would have been to run 3 strings of 8 panels on each inverter, assuming the inverter allows them
to be combined to a single feed. When my trench was dug, I threw in direct burial 12-3 to bring limited AC out to
the array. Conduit around it brings it out of the ground to boxes, with GFI.

STILL WAITING to hear AC voltage readings at the meter, and at the inverters when active. A meter reading at
night might give a hint of the PoCos impedance contribution to the problem. Bruce Roe

[tyab]

If you move the inverters you have to have a DC disconnect within 5' (? - need to reread that code section to be sure) of where it enters your building. Also you have to make sure your DC setup is compliant with Rapid Shutdown. Also I think that the parts of the conduit in your basement would have to be metallic - not PVC (I would have to review the code sections to be sure). The total costs for this may exceed the upgrades sticking with your existing setup. Just wanted to toss this out so you could discuss it with your installer. And yes, there are advantages to have AC at your ground mount - maybe a PoE power weather station or lights easy to plug in, misc electrical tools, etc. You have to decide this. Note that for me personally having AC at the site was worth it to deal with the AC losses but that is a choice you have to decide.

One of the the things I'm also concerned about is how your system is currently tied into the the service. You just mentioned this above with the installers idea of a local 200amp sub panel with a line side connection.. How is your system currently connected to the service? Given it was an upgraded setup, I'm guessing that previously you had a load side connect. This means your service panel (most likely 200 amps) had a circuit breaker (OCPD) in it - usually in the lower right corner. And the 200 amp circuit breaker for that service panel would have been downgraded from 200amps to whatever was needed. In any case a load side connect is the most common way to connect systems. However your system now is 96 amps, which needs a 125 amp OCPD. And for the vast majority of residential customers it is not possible to have code compliant 125 amp back fed OCPD unless you have a somewhat uncommon 400 amp (not a 200/200 split) panel. For systems over 80 amps of output generally you are looking at a line side tap. So do you have a line side connection currently?

Since your in PG&E land - PG&E has very specific requirements for a line side tap. This document helps:

https://www.pge.com/includes/docs/pdfs/shared/customerservice/nonpgeutility/electrictransmission/handbook/TD-6999B-048.pdf

I fear that your current interconnect may not be compliant and could be a serious fire risk.

This is not the first thing that has raised questions. Previously a poster suggested that you pay an independent licensed electrician to come inspect everything and given all of the information you have provided so far I would agree with that prior poster. Maybe $200 bucks to get a solid independent review of what you have and what is and is not code compliant. The main thing I want to put out to you is - this system is connected to your home which protects your family. Might be time to make sure your home is correctly protected from non-compliant items that could lead to fire risk on your home.

[PV4Me]

Thank you everyone. I was out of town for a few days for work so I apologize for the delays.

Definitely plan on getting an independent review of the work performed just to see if there are any issues I need to raise with the installer.

For those wondering about the voltage at night, I measured it to be 247v at the main service panel, this morning at 5:45 while it was still dark out. Will report back with what it is during the day.

[PV4Me]

Today is a very cloudy and overcast day here and I checked the voltage again at 11:30 am today and it was still at 247v at the main service panel.

[bcroe]

Today is a very cloudy and overcast day here and I checked the voltage again at 11:30 am
today and it was still at 247v at the main service panel.

I made a few measurements which varied, the 240 meter voltage here would rise 6V to 10V when the inverters pushed
back 15KW. This rise is of course in addition to that in my own wiring. Its more that I expected; the pole transformer is at the
end of the drive with about a 200' PoCo pair underground, feeding a 200A box. Bruce Roe

[PV4Me]

Thank you for the input Bruce.

I am leaning towards going with the longer DC run implementation vs the longer AC run implementation but my reservation is regarding the possibility of upgrading in the future.

I am thinking that using a larger cable in the buried conduit and keeping the AC run longer may make it easier to upgrade in the future versus trying to run a lot of smaller DC cables in the conduit. I know the combiner boxes will reduce that to some degree, but will that make expanding in the future harder?

My installer says upgrading will still be possible even if we run DC the longer length because there will still be the existing 100A sub-panel at the arrays that I can use in the future if needed.

Question for the pros on here: if I use a large enough cable and keep the AC run longer, is the only difference the higher cost of using a thicker conductor for that long run? As in, can I minimize losses to the same degree as if I were to use DC for the long run if my conductor size is large enough?

Thanks again everyone!

[tyab]

Here are some more thoughts. Regardless of long run AC or DC, your system currently has to use 125 amp AC OCPD and most likely this will be a line side tap. This allows up to 100 amps of inverter output and currently you are at 96 amps - unless you toss in a 4 panels with microinverters are you already maxed out. To go higher inverter output you would need to design the grid interconnect to use a 150 amp OCPD which would allow up to 120 amps of inverter output. At this point you are breaking out of the PG&E residential size and into commercial size - their crossover is 30KW of PV output (need to read the PGE docs again- it may be 30KW of inverter output - not sure but in any case you are on the edge). Which means you have to pay PG&E all engineering fees and all interconnect fees - these can be sizable. Also you will be no longer be under a residential billing plan.

Do you REALLY need that much output? With your existing system you are going to be under NEM 2.0 and PG&E only pays you $0.04 per KWh for overgeneration at your NEM true up.

Your interconnect to the grid must conform to 705.12 (lots of stuff in there). One thing to be aware of is 705.12(D)(2)(3). Thus if your guy is thinking about a 200 amp sub panel in the basement - that is too small. (have him read all subsections very carefully). For the fused disconnect for the line side tap - my reading of 705.12(D)(2)(2) says that you need a fused disconnect rated for 325 amps with 125 amps fuses in it (this is assuming you have a 200 amp service) - thus you are looking at a 400 amp disconnect - make sure to use one on the PG&E approved list. Again your guy needs to really understand 705.12 to make sure your existing system is code compliant and more importantly safe for your home and be aware of all PG&E requirements.

Given your using string inverters, you can oversize your system as long as you stay within the string input requirements of the inverters. This means you are clipping some of the max input but are increasing the overall daily output. See bcroe's system for an example. If you do the long run with DC you would have to plan for that now and not later.

Again - do you really need more output?

If you run the long run DC, your entire system is now subject to Rapid Shutdown of those DC circuits. This is new to CA as of Jan 1 2014 and is covered under 690.12. You are subject to this since you previously said that you have conduit running through the basement - DC conductors that are more than 5 ft on/in a building must be protected by 690.12. This may be somewhat expensive and I hope your installer fully understands the ramifications of Rapid Shutdown. In addition you have to comply with 690.13 - 690.17 all related to DC conductors. Lots of new stuff in there for the 2014 code you are now under - mostly related to DC disconnects. This is in addition to the rapid shutdown. Be aware that even for the AC long run your installer needs to review 690.16 carefully for your DC strings.

Thus I see these options:

1. Plan for a inverter peak output increase - adding more inverters. Most likely 120 amp output. Make sure PG&E does not consider you a commercial setup. Make sure all AC output can handle that. Make sure if your doing a long run of DC that you have the necessary conductors sized correctly and have rapid shutdown covered. Long run AC or DC - this will be expensive either way.

2. Plan for daily output increase by oversizing but stay with existing inverters. Long run DC means you need to have rapid shutdown taken care of and you will have to plan for the string upgrades. Long run AC makes this easier - no change from existing fixed interconnect.

3. No more upgrades (you have a large residential system now). Long run DC means you need rapid shutdown covered plus everything to move inverters. Long run AC means larger wires.

No matter the direction you want to go - make sure your grid interconnect is 705.12 compliant and for your ground mount relevant 690 sections.

Oh and I hate to bring this up but your AHJ may require a 690.47(D) GEC on your ground mount since you are now having to file a permit for the changes. Have your installer talk to the AHJ and see if they are enforcing 690.47(D). - some are not. Compared to the other stuff you are looking at - its not much expense if you have to put one in but there are a number of code requirements from 250.52 that have to be followed.

[PV4Me]

Wow tyab, thank you for taking the time to write all that out, that is some great information! I need to read it over a couple of times to fully understand it, but in the meantime here is a picture that the installer sent me of what he is proposing.

Thank you again!!


IMG_1733 copy.png

[tyab]

Great photo - helps.

So your running a separate 100 amp AC branch circuit to a subpanel on the ground mount. I understand why you may want have AC out there - its useful. Those AC wires are running in the same conduit as the DC wires. I believe that is not allowed via NEC 690.31(B) - better have your guy check on that - PC source/output circuits must be separated from separate systems.

I don't know if the SMA DC disconnects on the ground mount disconnects are part of the rapid shutdown system - there are a lot of SMA experts that hang out here - hopefully they will pipe in. You may find that completely complying with rapid shutdown may be the single reason to have AC be the long run.

200 amp sub panel in basement is too small. Let me walk you through this. Each inverter must have have a 40 amp OCPD (circuit breaker). So you have this sub panel by the inverters that has 3 x 40 amp OCPD protecting the inverter output. It need 120 amps of OCPD protecting the wires feeding it. We have to round up to next available OCPD trade size which is 125 amps, so now all of the wire feeding it have to be rated for at least 125 amps. This sub panel must comply with 705.12(D)(2)(3)(a) which requires that the sub panel itself be rated for 245 amps (125 + 40 + 40 +40). Problem is - that is not trade size. Panels over 225 amps are getting expensive. Talk to your installer and have him review that code section very carefully.. Now there is another section 705.12(D)(2)(3)(b) that would allow you to get away with a 225 amp sub panel but it requires a single backfeed - but you have three. There is a common misconception that a panel can be marked "NO LOADS ALLOWED" to allow the (b) clause to be used but it is technically not code and most AHJ's do not allow it. Note that you have the same issue if you have the inverters at the ground mount - so this issue does not matter if you run the long run DC or AC.

The service fused disconnect may also have to be 400 amps for similar reasons - even though you only have 125 amps fuses in it. Note that if you have a single 400 amp service (not a split 200/200 which is common for large homes like you have) then you may need to have a 600 amp rated disconnect. Again make sure your installer clearly understands what PG&E and the AHJ will require here.

Keep us in informed about how this all works out. If you are finding all of this NEC stuff interesting, might be worthwhile to pick up an NEC book - nfpa.org - about $100 to buy one or if you make a free account you can read the NFPA 70 2014 online for free but its kind of a pain to use. Remember, CA is on the 2014 code - not 2017.

Good luck.