DC trunkage

[electrofelon]

Because that's too big or too small?
The house has four 200A SQ D Panelboards, three in the main EC and one in the Mechanical Room.

Ok I guess I misunderstood what was going on. I thought it was a 200 amp service and it was claimed that the 600 A disco was needed for a series rating. Seems like the 600 amp size is needed anyway.

 

[electrofelon]

So you could put in a DC combiner and squeeze 60-65KW out of a 1/0 CU, but as JB said I dont think you will find an inverter to handle that. Plus you would not have multiple MPPT's which is going to cost you some production. It seems like a lot of trouble and compromise to go through to avoid redoing a measly 250 foot run.

Is the run pipe the whole way? If so what about pulling out the existing conductors and pulling in new strings? Fronius' largest is a 15 KW and that will do 1kv strings, two mppts per inverter (you dont have to utilize both). Do the math, see how many you can fit.

 

[Open Neutral]

It's direct burial under the length of the driveway. Replacing it is Not On The Table.

So the real question is: What can we do with what's there? How does that compare to the potential capacity of the panels?

The whole reason for feeding the house at all, vs. just 100% grid-tie back through the barn meter, is to supply loads & recharge Powerwalls {or equivalent} in the house during long outages, to save on generator runtime. I don't yet know enough about the Powerwalls esp. their maximum charge current.

 

[wwhitney]

I don't yet know enough about the Powerwalls esp. their maximum charge current.

The data sheet is here:

https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall%202_AC_Datasheet_en_northamerica.pdf

It lists maximum continuous real power 5 kW, apparent power 5.8 kVA. I assume charging should have a power factor close to 1. So the continuous charging current would be a maximum of 20.8A per Powerwall.

Cheers, Wayne

 

[Open Neutral]

Ok I guess I misunderstood what was going on. I thought it was a 200 amp service and it was claimed that the 600 A disco was needed for a series rating. Seems like the 600 amp size is needed anyway.

I thought the dedicated 100KVA transformer was a clue that this is not a Levittown 2BR.

 

[jaggedben]

It's direct burial under the length of the driveway. Replacing it is Not On The Table.

So the real question is: What can we do with what's there? How does that compare to the potential capacity of the panels?

The whole reason for feeding the house at all, vs. just 100% grid-tie back through the barn meter, is to supply loads & recharge Powerwalls {or equivalent} in the house during long outages, to save on generator runtime. I don't yet know enough about the Powerwalls esp. their maximum charge current.

The energy storage doesn't place an additional requirement on what you need out of the unused feeder. Basically, what you can do with the feeder is output 80% of the ampacity of the feeder, for solar inverter nameplate output.

Tesla puts a limit on how much solar inverter output you can couple per Powerwall in an offgrid situation. (I believe its 7.6kW per Powerwall.) Other ES systems usally do too. So the money available for energy storage may limit the PV system size, independently of the feeder. It is also otherwise optional to size the PV with respect to tue energy storage, you could size it to annual usage or other considerations.

 

[electrofelon]

It's direct burial under the length of the driveway. Replacing it is Not On The Table.

So the real question is: What can we do with what's there? How does that compare to the potential capacity of the panels?

The whole reason for feeding the house at all, vs. just 100% grid-tie back through the barn meter, is to supply loads & recharge Powerwalls {or equivalent} in the house during long outages, to save on generator runtime. I don't yet know enough about the Powerwalls esp. their maximum charge current.

This not going to be a cheap system, I'm sure we'll over 100k, so I don't see why there can't be money in there to get the proper conduit (s) out there. Maybe talk to a directional borer? What happens when one of those direct buried conductors fail as they often do? I would megger them for sure.

The way to get the absolute most power out of that feeder is to use a transformer to step up to 600 volts for the wire run, then use a 100% breaker and enclosure to use it 100% of it's rating. Or you could use the conductors at their 90 degree ampacity (if so rated) and Transition to 75 degree sized conductors for your terminations. Then of course you would have match the sum of your inverter output current to get as close as possible to that value. This beats DC by a long shot because your mppt voltage is going to be a lot lower than your cold temp open circuit voltage.

I think you could get there with 600vac, even without any of those "tricks" since you would have 120A x 600v = 72kw which would likely be around 80-85 kw of panels.

 

[Open Neutral]

This not going to be a cheap system, I'm sure we'll over 100k, so I don't see why there can't be money in there to get the proper conduit (s) out there. Maybe talk to a directional borer? What happens when one of those direct buried conductors fail as they often do? I would megger them for sure.

It's the disruption, more than the money. (It's on Bainbridge Island; if/when I get there again, I'll invite you over....)
Yes, we will megger them first.

The way to get the absolute most power out of that feeder is to use a transformer to step up to 600 volts for the
wire run, then use a 100% breaker and enclosure to use it 100% of it's rating.

What I've failed to convey is we only want a fraction of the output going to the house. The rest will be ordinary grid tie through the barn meter.
One question is: can we run 2 inverters off a common solar array? Or must we split the panels into multiple arrays?
\

 

[Open Neutral]

The energy storage doesn't place an additional requirement on what you need out of the unused feeder. Basically, what you can do with the feeder is output 80% of the ampacity of the feeder, for solar inverter nameplate output.

If we have smaller inverters at the house, that would limit the feeder current.
There's no reason to push more just to be sent out to PSE; we can do that in the barn.

Tesla puts a limit on how much solar inverter output you can couple per Powerwall in an offgrid situation. (I believe its 7.6kW per Powerwall.)

I'm reading the Tesla manuals I can find, but did not yet see that number. Thanks.

 

[jaggedben]

One question is: can we run 2 inverters off a common solar array? Or must we split the panels into multiple arrays?
\

Sure you can. Practically speaking, you probably would anyway. There is no requirement for physical separation of panels based on electrical design.

 

[jaggedben]

Now that I think about it, you'll probably need a reasonable argument to invoke one of the exceptions for having two services feed one building. Probably possible.

And you shouldn't expect to be able to change on the fly which PV feeds which meter. It will be essentially two projects that happen to share a building.

 

[Open Neutral]

Now that I think about it, you'll probably need a reasonable argument to invoke one of the exceptions for having two services feed one building. Probably possible.

Well, the meter on the barn currently having solar is enthusiastically labeled about grid tie. The old house meter was as well. I assume if/when we run DC to the house to feed inverters there, the same will be true again.

And you shouldn't expect to be able to change on the fly which PV feeds which meter. It will be essentially two projects that happen to share a building.

My question is:
Can I have one solar array, i.e one source of DC, and feed that to 2 inverters: one big one in the barn, a smaller one in the house?

 

[electrofelon]

Well, the meter on the barn currently having solar is enthusiastically labeled about grid tie. The old house meter was as well. I assume if/when we run DC to the house to feed inverters there, the same will be true again.



My question is:
Can I have one solar array, i.e one source of DC, and feed that to 2 inverters: one big one in the barn, a smaller one in the house?

Arays don't usually have "one source of DC" usually they consist of many strings. You could theoretically combine them all into one circuit but that would have many disadvantages and you probably couldn't find an inverter to do it

 

[Open Neutral]

Arays don't usually have "one source of DC" usually they consist of many strings. You could theoretically combine them all into one circuit but that would have many disadvantages and you probably couldn't find an inverter to do it

The ones I have seen were all in series. Granted, they were not acre-size but rather rooftop. Since I looked seriously, I know there's been a move towards "microverters" on ?each panel? but....

 

[wwhitney]

In general it's series/parallel.

Each panel produces some DC voltage (say 30V). A string inverter wants its input voltage in some range (say 390V - 510V for a 240V AC inverter). So in this example each series string in the array has to be 13 to 17 panels long. If you have 30 panels, you use two series strings of 15. If the inverter has only one DC input, you parallel the two series strings.

That's a bit of a simplification, and my numbers are off a bit, but that's the idea.

Cheers, Wayne

 

[mikeames]

I was wondering about bringing DC back to the house vs locating the inverters by the arrays.
If the wire is rated at [say] 300V AC, what's the Code say about DC, given the AC is in RMS?
………..
Equivalent.
…………


Most comments are not accurate.
There is a definite difference between current capacity of a given size of conductor when passing energy on DC versus AC.

Agreed but to be fair the OP was vague. The question was NOT specifically about power, It was about VOLTAGE since that's what was specified with 300v AC not the size of the conductor. There is no difference. Voltage is voltage. The heating effect (RMS) and the amount of power for a given voltage waveform IS different but 300V or actually 600v (assuming 0v sine) Peak-2-Peak is the same voltage as 300v flatline. Obviously the amount of power from the two will be different.

 

[jaggedben]

...
My question is:
Can I have one solar array, i.e one source of DC, and feed that to 2 inverters: one big one in the barn, a smaller one in the house?

In a word, no. You'll have to allocate the solar panels to belong to separate systems. Multiple inverters on some kind of common bus would interfere with each other's operation, violate their listings and warranties, etc...

 

[Open Neutral]

In a word, no. You'll have to allocate the solar panels to belong to separate systems. Multiple inverters on some kind of common bus would interfere with each other's operation, violate their listings and warranties, etc...

Thanks, that was exactly what I was curious about.

 

[Open Neutral]

In general it's series/parallel.
...
If the inverter has only one DC input, you parallel the two series strings.

But there's no guarantee that 2 sets of panels will have the same output voltage. I suspect that's part of the move towards one inverter/string.

 

[wwhitney]

But there's no guarantee that 2 sets of panels will have the same output voltage. I suspect that's part of the move towards one inverter/string.

My understanding is that in practice this is a non-issue, but perhaps those with some actual experience could confirm. : - )

Cheers, Wayne