Noob needs more help (pics inside)

[diegodog]

Wire will be in conduit the entire run.

If I hit the line with the tiller or plough I would think there will be much bigger issues. I just assume I get crispy if I do it. It is not in a farming area so hopefully I don't do anything that outrageous but .....

[max2k]

I'm thinking burrowing/chewing animals.....or how about freeze/thaw action....or how about digging or plowing.... a manufacturing defect......you get the picture.

OP would need to put wires in SCH80 PVC conduit and dig the trench deep enough so that would help against most of that. Manufacturing defects (wire broken under insulation) can start arch and burn out entire run at the defect spot possibly damaging PVC conduit. Not sure how fuses would help in that case though- until it shorts out completely of course. While ominous nobody reported this kind of problem here yet so these must be extremely rare.

[DanS26]

OP would have unsplit wires coming from terminal box(es) at array all the way to the inverters There's just no chance for any arching along the way.

I'm thinking burrowing/chewing animals.....or how about freeze/thaw action....or how about digging or plowing.... a manufacturing defect......you get the picture.

[max2k]

Sorry to hear that.

IMO a fused "pass through" box at the array for each string would be a wise investment. A fault anywhere along your 6 strings from array to the building as you have it designed now will cause an arcing/blazing mess and shut down your entire production. Not worth the risk IMO.

OP would have unsplit wires coming from terminal box(es) at array all the way to the inverters There's just no chance for any arching along the way.

[max2k]

10-4 - My lingo is not helping. By unplugging the panels I meant going to each individual MC 4 connector just to be sure. I will try to not speak so generically next time. Either way this is a great reminder.

no worries, you're coming up to speed very quickly. You just need to break the string in 1 spot, no need to disconnect all 10. It doesn't matter where you'd break it too- any convenient location will do.

Of course, if you manage to get under remaining voltage somehow it will still be dangerous but it would be hard to do- you'd need to get inside another MC4 connector and then reach out to the other end of the unbroken part of the string and get yourself connected there as well. IMO at that point after so many efforts you'd deserve what is coming . What makes this way safe is that panels have 2 ends and all connected in series with closed connectors- to get inside MC4 you need to unplug it first breaking string yet again. There's just no exposed contacts there except in the terminal box.

You'll even be fine if someone would turn AC/DC disconnects back at the house as the string will still be broken. I always prefer to work if I can ensure no idiot can turn things back on ignoring warning signs. I saw it happened once- someone just came, took the sign off and turn the switch ON. Luckily person who was working saw it and was not touching anything at the moment. He expressed his opinion about that in a very colorful way .

[diegodog]

No, unplugging at that voltage / current will create DC arch which will melt whatever you unplugged. The correct way to go about this:
- disconnect your AC first. This will stop current consumption from your array by inverters dropping current on DC side close to 0 as well.
- disconnect DC using inverter's DC disconnects, just in case
- go to the array and break the string you're going to work on somewhere in the middle by disconnecting one of the MC 4 connectors. This will ensure no matter what you touch in that string won't be able to shock you as you have 2 breaks now- one at the inverter and one somewhere in the string so your body can't close both breaks at the same time.

If you don't do the last step the array will remain energized as long as there's any kind of sun light and will produce 450V DC with enough current to kill you so be careful.

10-4 - My lingo is not helping. By unplugging the panels I meant going to each individual MC 4 connector just to be sure. I will try to not speak so generically next time. Either way this is a great reminder.

[DanS26]

Unfortunately is too late.

I do like the idea of the fused box at the array so I can shut down the panels if necessary, however can't you just unplug the panels if something occurs between the panels and inverter to take all current off the home run?

Thanks for the post and information !

Sorry to hear that.

IMO a fused "pass through" box at the array for each string would be a wise investment. A fault anywhere along your 6 strings from array to the building as you have it designed now will cause an arcing/blazing mess and shut down your entire production. Not worth the risk IMO.

[max2k]

Unfortunately is too late.

I do like the idea of the fused box at the array so I can shut down the panels if necessary, however can't you just unplug the panels if something occurs between the panels and inverter to take all current off the home run?

Thanks for the post and information !

No, unplugging at that voltage / current will create DC arch which will melt whatever you unplugged. The correct way to go about this:
- disconnect your AC first. This will stop current consumption from your array by inverters dropping current on DC side close to 0 as well.
- disconnect DC using inverter's DC disconnects, just in case
- go to the array and break the string you're going to work on somewhere in the middle by disconnecting one of the MC 4 connectors. This will ensure no matter what you touch in that string won't be able to shock you as you have 2 breaks now- one at the inverter and one somewhere in the string so your body can't close both breaks at the same time.

If you don't do the last step the array will remain energized as long as there's any kind of sun light and will produce 450V DC with enough current to kill you so be careful.

[max2k]

... Each inverter has 25 amps output. ....

Instead of that- how about using Midnight MNPV6 Disco AC Micro Combiner? It can handle 3 inverters with up to 60A breakers each (OP needs only 30 really) with output bus rated to 168A combined. It has handle which simply switches all those internal breakers on or off mechanically at the same time. No subpanel required, I believe it can be wired directly to MSP 80A breaker for PV system in MSP. IIRC 80A breaker is listed for continuous 80A current, no need to apply 125% rule. Besides, in OP case that breaker will never see more than 60A in real life. OP just need to replace main breaker in MSP with 160A one to satisfy MSP bus bar requirements.

[diegodog]

If you have already purchased your inverters and wiring.......then ignore the comments below.......

A design using just two inverters is possible using just 5 wires carrying DC from combiner boxes at the array to the inverters at the building. Combine 3 strings each into two home runs. Instead of 13 wires of 107 ft each (6 pos 6 neg 1GEC) you then only need 5 wires of 107 ft (2 pos 2 neg 1GEC). Wire savings alone will almost pay for an inverter plus you can use a fused combiner box at the array making for a much safer installation.

The Fronius inverters will accommodate this design...not sure if the SMA inverters will handle combined strings.

You seem to be cost conscious so you may want to redesign to save lots if not too late.

Unfortunately is too late.

I do like the idea of the fused box at the array so I can shut down the panels if necessary, however can't you just unplug the panels if something occurs between the panels and inverter to take all current off the home run?

Thanks for the post and information !

[diegodog]

Tyab + Butch I'm going to need a second on that. LOL! I need a little time to digest before I have a reply.

My absolute immediate concern is getting the wiring to the inverter (including ground) from the panels before the winter comes. Which is soon northern ME. Once the wiring is all complete up to the inverter everything else can be done in -30 and 10 feet of snow. The main service line runs in the attic (from the back corner (meter) to the front corner (MSP)) so I wont have to bury the lines going from the inverter to the MSP I will run it along with he main line that supplies the barn now if it comes down to that). I think I'm very close with the help from the above postings to being able to get the necessary wire from the panels to the inverter and have it grounded properly.

You can wire each inverter individually to its own 30A 'double' MSP breaker using 10 AWG essentially creating 3 completely independent solar systems.

It appears this is a no go. If I only had 20 panels and 1 inverter could I do it with a 30A double breaker? I start to get confused from that point as I would think if I had the space I could do it. This probably means I'm missing a key detail somewhere along the way.

Thanks again guys for all the help.

[DanS26]

If you have already purchased your inverters and wiring.......then ignore the comments below.......

A design using just two inverters is possible using just 5 wires carrying DC from combiner boxes at the array to the inverters at the building. Combine 3 strings each into two home runs. Instead of 13 wires of 107 ft each (6 pos 6 neg 1GEC) you then only need 5 wires of 107 ft (2 pos 2 neg 1GEC). Wire savings alone will almost pay for an inverter plus you can use a fused combiner box at the array making for a much safer installation.

The Fronius inverters will accommodate this design...not sure if the SMA inverters will handle combined strings.

You seem to be cost conscious so you may want to redesign to save lots if not too late.

[diegodog]

Thanks Max2K!

It's clear to me now!

I had the 10 awg PV wire on the brain because the panels have the 10 awg pv wire on them, so I boxed my thinking off and just ran (wildly) with that idea. I figured if its whats on the panels and is plenty heavy enough to make that distance with little no no loss why not play it safe. However, with the amount of line I will be running I'm going to run the THWN-2 wire as you suggested. The way I was going to do it is foolish and wasteful. Thanks a million for the great advice.

You will also notice those concrete blocks and 6x6's I used for my stand and mount. It's built like that out of pure ignorance. When you don't know what your doing you go BIG to over compensate the lack of knowledge. If mother nature wants to take my panels for a ride she is gonna have to work!

I will run the the EGC green wire and let the electrician decide. No way I want to fight with a wire pull.

I'd connect this last 6AWG piece not to the inverter but to the building grounding rod directly 'bonding' building rod with array grounding rods. I'd connect inverter ground to building MSP grounding bar with normal green EGC wire. Here again local AHJ might just require the green EGC to extend all the way from inverter to your panels even if it doesn't make much sense. Since you want to pass the inspection you will have to have it there.

10- 4. I understand the above. Luckily I have no real inspection. The electric company said they are only checking their equipment, they don't care what I do with mine. There are 0 inspectors here. We have built the barn (pictured here) and a house on the other side of the field and the only inspection was an old timer, who never even got out of his car, to make sure we have vent pipes for the plumbing! So your really at the mercy of the electrician which is another reason I want to understand what I'm putting on my property and why I like to go as far as I can before I call anyone in.

Thanks again Max2K. I grasp what needs done now to get to the inverter and it makes logical sense to me.

I will be back later with a comedy called from the inverter to the MSP (took me a minute to figure out MSP) and see what you guys think.

Once again I am very thankful to you guys(gals) and this resource. Its quite and amazing amount of knowledge and help received here.

[tyab]

This should be doable with a load side connection on his 200 amp MSP and yes we have to use a sub-panel. The sub-panel has to have a 225 amp busbar to make it work - here is the layout.

- Downrate the MSP to a 125 amp OCPD (circuit breaker).
- Install a 100 amp OCPD in the lower right corner.
- Run a properly sized feeder (105 amps + adjustments - thank you 2014 code! - 2011 code would have required 2/0 copper) from the 100 amp OCPD to MLO sub panel that is 225 amps.
- Have a 225 amp MLO (main lugs only) subpanel. If you want it can be a OCPD subpanel but then you are paying the high cost to installed a 100 amp main breaker in this - less expensive to use a MLO panel.
- Put in three 35 amp OCPD one per inverter in the subpanel. No location requirements for these in the subpanel but convention would be lower.

Details:

First the SMA 6000 has a rated AC output of 25 amps (from the datasheet). Thus are total output of 75 amps has to be dealt with but its different between the MSP and the sub panel.

Starting from the main service panel.

We are going to backfeed a total of 75 amps from the three inverters. 75 * 1.25 = 93.75, round up and we use a 100 amp OCPD in the lower right corner. This falls under 705.12(D)(2)(3)(b) and thus we have to down rate the MSP breaker from 200 amps to 125 amps. These breakers are not cheap, but that is the way it goes - either that or go to a line side tap.

You run a properly sized feeder (105 amps plus adjustments) to the MLO subpanel. 105 amps since we will have backfeed OCPD of 105 amps total between the thee 35 amps OCPD's

The main lugs only subpanel has to be rated for 225 amps. Why? You have a 100 amp OCPD feeding the subpanel and in the subpanel you have 3 x 35 amps OCPD backfeeding. This requires us to follow 705.12(D)(2)(3)(c) and thus we are forced to use a 225 amp subpanel.

Each inverter has 25 amps output. 25 * 1.25 = 31.25, round up to next trade size and use a 35 amp OCPD for each inverter.

Have three 35 amps OCPD's in the subpanel - no location requirement since we are under 705.12(D)(2)(3)(c).

Now what happens if you need a safety switch between the MSP and the subpanel? This is messy since a 200 amp safety switch does not cover - we are 5 amps over according to the NEC. That means a 400 amp switch. Ouch. It may be less expensive to put a 100 amp main breaker in the MLO subpanel and then you can use a 200 amp safety switch. Hopefully you don't need this and can save that cost.

There are a bunch of sticker requirements for the downrated MSP breaker, the 100 amp breaker you put in the MSP in the lower right corner, outside AC disconnect. etc.

[max2k]

...
With my limited understanding of things I need to get DC power to the one side of the inverter.

yes, to one of its 3 sides at the bottom- SMA has plugs on all 3 sides. Internally DC wires are on the left side and AC output wires are on the right side, both under black plastic cover at the bottom. I wired DC to mine through bottom left holes, this way water won't be able to get in there. You'll also need to put wires in the conduits so plan accordingly.

So a straight shot to the inverter from right off the panels seems like the safest most straight forward option for a rookie. 12 wires (6 strings x 2 wires). I like the idea of stepping the wire down a size (after checking the charts for distance) to save a few $$. Since the DC disconnect is on the inverter I'm assuming If I kept it 10awg I would go from the panel to the DC disconnect/inverter with no equipment in between. If I step down a size on the wire I would need a junction box after the panels then a straight shot to the DC disconnect/inverter. Is this a correct line of thinking? If so it makes sense to me and I can get this line buried before the snow hits.

I'm not sure where are you getting this 10awg from- panel wires are very different and much more expensive type called PV wire. Your longest run sounds like 160' + 107' + 60' = 327'. This run can be done with cheaper THWN-2 wire. To switch from one type wire to another you need to install some terminal box at the array where PV wires would go in but conduit with THWN-2 would go out into the trench. I'm not recommending particular brand/model because I have no experience with those. In order to have less than 3% of losses on that distance you need your wire to have resistance less than 451V / 8.8A * 0.03 = 1.5 Ohm for both wires: 327' x 2 = 654' total distance. 14 AWG has 1.6 Ohm over that distance:https://www.cirris.com/learning-cent...lculator-table which is acceptable IMO as this is the distance from the far most corner of the array. Inverter connector can take 14 gauge wire. If you use 12 gauge for DC run it would have about 1 Ohm resistance over the same distance which will introduce 2% loss. 3/4' PVC SCH80 conduit would allow you to have 12 x 12 gauge wires or 17 x 14 gauge wires inside. Here comes $1 million question- do you need EGC green wire in that conduit or not. I'd try to ask your electrician as it is up to your local rules really. From pure academic point of view it's not needed but your local AHJ can sometimes bend the space and laws of physics a little. I think pulling additional wire after you already put 12 other in the conduit would be much more difficult.

Panel's wires are PV type wire while the 'home run' wires are THWN-2 so somewhere at the array you need a box with terminals switching from one to another..
I don't understand this one. Does this mean I cannot connect the dc disconnect/inverter up to the PV wires coming off the panels directly? I have to switch them to THWN-2? Was this statement made assuming the inverter was at the panels and not the building? I may also be completely lost and not getting what your putting down.

with inverter at the building your DC run in the trench/conduit is done using THWN-2 wire while panels use Photo voltaic (PV) wire- much thicker sun / abrasion resistant / expensive wire. You don't want to run PV wire in the conduit.

For grounding I'm using 6 awg all around. I will hook all the panel frames together with 6 awg. Once the panels are hooked together I will send (1x) 6 awg to a grounding rod in the field by the panels. Then I will send (1x) 6 awg to a 2nd grounding rod in the field by the panels but making sure they are the proper distance apart. And finally I will send (1x) 6 awg along the run to the barn to go to the inverter.

I'd connect this last 6AWG piece not to the inverter but to the building grounding rod directly 'bonding' building rod with array grounding rods. I'd connect inverter ground to building MSP grounding bar with normal green EGC wire. Here again local AHJ might just require the green EGC to extend all the way from inverter to your panels even if it doesn't make much sense. Since you want to pass the inspection you will have to have it there.