Noob needs more help (pics inside)

[ButchDeal]

Here is my "learned enough to be dangerous post of the day" Thank to Max and Tyab for getting me thinking on this track and learning a ton!

6 strings of panels. Each string is 451 volts 8.8 amps
SMA inverter (x3) 25A max out put on the AC side


My panels will put off a max AMP of 6 strings x 8.8A. That is 52.8 AMPS (at absolute full throttle).

Why would I double the amount needed up to 100 AMP for something that will max out and most likely never hit at 52.8 AMP. I can see leaving some safety up to 60 AMP but going to 100 AMP and crippling my MSP from 200A to 125A seems like a bad idea.

What am I missing?

If someone is on the side of follow the book go 100AMP can you please explain exactly the risk of how the 52.8 AMPS on the 60 AMP breaker causes disaster and how the 100 AMP breaker would perform better? Is there a scenario which you could throw out to help me see why one would go 100AMP

Because you are changing volts in the inverter.

here is the equations Amps X Volts = Watts

on your strings you have 451V and 8.8A = 3,968.8 watts
You are converting that to AC 240V so it is 16.54 amps on the AC side.
16.54a X 8 strings = 132 amps (there is loses and clipping in the inverters)

Also keep in mind the volts will not always be 451.

[Mike90250]

Ah, you meant loops in the wire, not the PVC conduit. That's better. Always use big service loops, just in case...

[max2k]

Here is my "learned enough to be dangerous post of the day" Thank to Max and Tyab for getting me thinking on this track and learning a ton!

6 strings of panels. Each string is 451 volts 8.8 amps
SMA inverter (x3) 25A max out put on the AC side

We have the 25A x 3(inverters) = 75A * 1.25 for safety = 93.75A - 100 AMP breaker will be needed in the MSP

My MSP is 200A I multiply 1.2 x 200 and get 40A as the Max I can connect to the MSP which does not work (I need to do 100 AMP in the MSP) according to the above calculation.

Here is my question and the part I'm not getting.

My panels will put off a max AMP of 6 strings x 8.8A. That is 52.8 AMPS (at absolute full throttle).

Why would I double the amount needed up to 100 AMP for something that will max out and most likely never hit at 52.8 AMP. I can see leaving some safety up to 60 AMP but going to 100 AMP and crippling my MSP from 200A to 125A seems like a bad idea.

What am I missing?

If someone is on the side of follow the book go 100AMP can you please explain exactly the risk of how the 52.8 AMPS on the 60 AMP breaker causes disaster and how the 100 AMP breaker would perform better? Is there a scenario which you could throw out to help me see why one would go 100AMP

Thanks again!!!

since you're converting one DC voltage to another AC voltage the only thing which remains constant is the power (minus losses). Let's assume your 6 strings can produce 451 V x 8.8 A x 6 = 23,812 Watt. When converted to 240 V AC (L1/L2) it would produce current: 23,812 W / 240 V ~ 99 A. In reality inverters will be the limiting factor as each is rated to 25 A max so they can produce 75 A max together, this is the max current you can possibly feed back to MSP. Now the standard breaker for that is probably 80A and I believe you don't need 125% upgrade as 80A is its continuous rating. So, here you have MSP with supposedly 200 bus bar and backfeed 80A solar breaker. You can 'overload' your bus bar by 40A so you'd need to change your main breaker to 160A so it could accommodate your 80A backfeed: 160A + 80A = 240A.

BTW, I looked at the pics of your MSP and either I'm reading it wrong or it has 125A bus bar inside. Hope more experienced members would correct me there.

What is the model # of your panels? I'm trying to get their exact specs.

[diegodog]

Yikes!

I totally get the math on this one and get it now. Thanks Max and Butch.

I also prob wrongly assumed the bus was 200 due to the breaker size. If the bus bar is 125 then it appears I have a hot mess on my hands.

The panels are Hyundai Heavy Industry - HIS-M305TI

Thanks again guys.

[max2k]

Yikes!

I totally get the math on this one and get it now. Thanks Max and Butch.

I also prob wrongly assumed the bus was 200 due to the breaker size. If the bus bar is 125 then it appears I have a hot mess on my hands.

hope this won't be that bad. I'd make sure I get at least 200 A MSP given it's a new construction.

The panels are Hyundai Heavy Industry - HIS-M305TI

now we're talking- these are 305W rated panels so the STC power 60 of them can possibly produce: 305 x 60 = 18, 300 Watt which on AC side will produce: 18,300 / 240 = 76.25A. Your inverters would still be 'clipping' it to 75A but I doubt you'll ever see it- that 18,300W is the 'rating' in reality you won't see them above 90% (16.5kW) very often.

[diegodog]

Is there any issue to installing the panels 2-3 weeks before any electrical hook up is done (besides using up life prior to being able to capture the energy)? I would like to get the panels placed and mounted before the wind comes.

[max2k]

Is there any issue to installing the panels 2-3 weeks before any electrical hook up is done (besides using up life prior to being able to capture the energy)? I would like to get the panels placed and mounted before the wind comes.

none that I'm aware of, especially if you know how are you going to do grounding and how you going to connect them in strings afterwards: if you place them close to each other it could make it difficult to access middle ones later. OTOH you have ground mounted array so you should be able to access any of them at any time from the back. The good recommendation here was also to connect '+' and '-' MC4 connectors of each panel to each other to prevent their oxidizing. While it will effectively 'short' the panel nothing bad will happen to it as panels won't ever exceed its Isc current and those connectors designed to sustain that easily. After all panels don't 'produce' energy, it was there to begin with so they can't possibly 'overheat'.

On my roof mounted array I used rails and according to the mfg specs had to put special 'lugs' to the rails to wire grounding wire through. The lugs and grounded wires ended up under the panels and I had to complete the grounding wiring along with panel installations as it would be impossible to do later without removing panels first.

[diegodog]

Sweet I'm waiting on the proper galvanized hardware to get here and then I'm going to throw those panels up and get that job out of the way. There are certain things that are much better to do when its not -20. I came up with that question from My brother-in-law telling me I can't do it "because the electricity has no where to go". I have verified it a few places and the only fool that has ever heard of that is my brother-in-law. Although it did scare me enough to not do it until I confirmed it a bunch of times (LOL).

In the process of diagramming the wire plan up to the inverter. Also looking into the MSP bus bar and what exactly I'm dealing with there. Also shopping for a nice "box" to make the switch from PV to THWN-2 wire in.

I'll continue to post here and throw up pictures as I go along.

Thanks again for everything

[max2k]

... My brother-in-law telling me I can't do it "because the electricity has no where to go". ...

Naysayer, eh? That's entire different level of confusion and while he is technically correct- it doesn't have anywhere to go it also doesn't matter .

Just keep in mind panels are always 'ON' when the Sun is shining on them and be safe. 45V voltage from a single panel is not dangerous but when you connect more in series it quickly gets there.

[tyab]

"Now the standard breaker for that is probably 80A and I believe you don't need 125% upgrade as 80A is its continuous rating"

This is incorrect and does not follow 690.9(B). 690.9(B) is very clear. 690.8(A)(3) tells use to use the inverter continuous output current which is 25 amps per the datasheet. You have three of them so the max output is 75 amps. "Overcurrent device rating shall be not less than 125 percent of the maximum currents calculated in 690.8(A)." There is an exception for continuous rated OCPD's but you will not find them in residential switchboard gear. Thus we end up with a 100 amp OCPD.for the MSP interconnection.

[max2k]

"they can produce 75 A max together, this is the max current you can possibly feed back to MSP. Now the standard breaker for that is probably 80A and I believe you don't need 125% upgrade as 80A is its continuous rating"

This is incorrect and does not follow 690.9(B). 690.9(B) is very clear. 690.8(A)(3) tells use to use the inverter continuous output current which is 25 amps per the datasheet. You have three of them so the max output is 75 amps. "Overcurrent device rating shall be not less than 125 percent of the maximum currents calculated in 690.8(A)." There is an exception for continuous rated OCPD's but you will not find them in residential switchboard gear. Thus we end up with a 100 amp OCPD.for the MSP interconnection.

Can we say he doesn't have continuous load then? His system won't be producing at 75A more than couple hrs a day (if that) so it clearly falls under non continuous category and 125% requirement should not apply.

[foo1bar]

Can we say he doesn't have continuous load then?

No.

His system won't be producing at 75A more than couple hrs a day (if that) so it clearly falls under non continuous category .

No. 3 hours a day would be continuous.


I believe that standard practice (and what would be expected by an inspector) is that it'd be 125% of the inverter outputs. So 75A * 125% = 93.75A, which means 100A breaker.

[max2k]

No.


No. 3 hours a day would be continuous.


I believe that standard practice (and what would be expected by an inspector) is that it'd be 125% of the inverter outputs. So 75A * 125% = 93.75A, which means 100A breaker.

Well, that sucks- OP would need 100A backfeed breaker and downgrade main one to 140A to get under 240A limit on the bus bar.
Are there 225A bus bars available for 200A MSP? That would allow him to have (225 * 1.2) - 100 = 170A main breaker. I have a feeling neither 140A nor 170A are standard size breakers.

[ButchDeal]

OP really needs a line side tap.

[foo1bar]

I also prob wrongly assumed the bus was 200 due to the breaker size.

I would assume it is at least 200.
It may even be 225A bus

If the bus bar is 125 then it appears I have a hot mess on my hands.

Very doubtful.
I wouldn't say it's impossible, but it's really unlikely. I'd be pissed at the electrician if they did a 200A main breaker in a 125A bus-bar panel.
(and wonder how the heck it happened)

IF you can figure out what the panel model number/name is, you can find out what the bus capacity is.

IF you have a 225A bus bar with a 200A main breaker you could have a larger main breaker.
225A * 1.2 = 270A
270A - 100A = 170A.
There isn't 170A main breaker, so you'd go with a 150A (*IF* you have a 225A bus bar)


If you don't have 225A bus bar (or can't determine if you do)
then you can either upgrade the panel to one with a 225A bus bar.
OR you live with a 125A main breaker.
(I'm a cheapskate - I'd live with 125A main. I grew up with a 60A service. )