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Planning a 24.3 kW Grid Tied System, some help on some questions.

thescoutranch

New Member
Joined
Jul 13, 2022
Messages
25
Hello all,

Being a novice at Solar, I have been reading and searching for info and answers on this site for the last month, thank you all who have asked and answered questions before me. I have learned so much but still have a way to go. I think I have a better idea of some questions to ask now. I have revised my solar plan so many times from where I started.

Background:
Since we have access to 1:1 net metering here, and some plans let you roll you credits forward monthly (but never get cash), my goal is to replace all my electric consumption with this system. We do not have any tier rates. If we have any extended blackouts, we have a 7000W generator in the RV for basic needs.

I will follow up with the data, that I have put together for myself, later, but I have been thru the last 3 years of electric bills, been to PVwatts (fantastic tool) to see the expected power generated for my proposed system. Yes, I know we use a lot of electric, but we have a pool pump, hvac (5 months old), kitchen fridge, beer fridge, chest freezer, garage has a minisplit, Air compressor, tools, etc, our RV stays plugged in for battery maintenance, all lights are led, ceiling fans, wife cannot tolerate heat. House is 2700 sqft. I don't expect electric rates to go down, only up.
I have also been thru the conductor size/voltage loss calculator and the conduit % full calculator - wonderful tools.
Created a 6 page excel spreadsheet to estimate costs for myself.

Currently, I think I like this iteration of my plan, but I have a couple of questions I am not sure about. First, a couple of reasons why I came up with some of the features in this plan. Any comments are welcome.
  • I am in Central TX (hot and sunny), and the main panel and meter are on the west side of the house and get some horrible heat in the afternoon; mounting the inverters out at the array would allow them to be mounted underneath on the north side of the array providing them a cooler operating environment due to shade from the panels. Also, we get a constant breeze out there, so some additional cooling.
  • Using a 5kW inverter for each series of (9)450 watt panels (~4k best case scenario) doesn’t work the inverter at it’s max rating.
  • If an inverter dies, I only lose 1/6 of my power while I wait on parts.
Questions:
If I use a circuit breaker panel out at the array location to feed the 350 ft run to the house, would I calculate the wire size using the 3 circuit breaker ratings (ie 3x20, 60 amps), or do I still need to use the Max Output Current values from the inverters (24 x 3, 72 amps)?

Is there anything fundamentally wrong with the layout of my system? I would be using UL listed parts for all components.

Does a “Grid Tied Only” inverter have the auto shutdown feature (when you lose grid power) required for Solar Systems tied to the grid, or is a different piece of hardware required? Am I looking for a specific UL listing for this feature?

So far, I have seen listings and people talk about:
“Off grid” inverters – Cannot connect to grid without some safety measures/hardware, does not need grid power to work, solar and batteries only?
Inverters with “anti-islanding” – not sure what this is
“Hybrid” Inverters – I assume these are to be used with a solar/grid/battery backup?
“Grid Tied Only” inverters – solar and grid only, no batteries?
Am I correct with these descriptions, if not, help me out, please.

Thanks,
Epp

Edited to remove pdf and upload jpg
Solar Information - TX Plat - Visio.jpg
 
Last edited:
Using a 5kW inverter for each series of (9)450 watt panels (~4k best case scenario) doesn’t work the inverter at it’s max rating.
Your DC to AC ratio is less than 1 to 1. Most installs use a ratio of 1.25 to 1 for the most cost effective mix of components. There is also an NEC backfeed formula that may limit the relationship of backfeed breakers to the bus bar capacity of your main service panel. If you are using a sub panel at the array the calculation may be different. Also inverters are considered a continuous load and the breakers should be rated at 125 per cent of their expected output. Finally, consult a voltage drop table for a 700 ft 240 volt circuit and size the wire accordingly. Most likely the wire size needed to get voltage drop to acceptable level may be greater than the circuit breaker needed for the expected current.
 
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Hello all,

Being a novice at Solar, I have been reading and searching for info and answers on this site for the last month, thank you all who have asked and answered questions before me. I have learned so much but still have a way to go. I think I have a better idea of some questions to ask now. I have revised my solar plan so many times from where I started.

Background:
Since we have access to 1:1 net metering here, and some plans let you roll you credits forward monthly (but never get cash), my goal is to replace all my electric consumption with this system. We do not have any tier rates. If we have any extended blackouts, we have a 7000W generator in the RV for basic needs.

I will follow up with the data, that I have put together for myself, later, but I have been thru the last 3 years of electric bills, been to PVwatts (fantastic tool) to see the expected power generated for my proposed system. Yes, I know we use a lot of electric, but we have a pool pump, hvac (5 months old), kitchen fridge, beer fridge, chest freezer, garage has a minisplit, Air compressor, tools, etc, our RV stays plugged in for battery maintenance, all lights are led, ceiling fans, wife cannot tolerate heat. House is 2700 sqft. I don't expect electric rates to go down, only up.
I have also been thru the conductor size/voltage loss calculator and the conduit % full calculator - wonderful tools.
Created a 6 page excel spreadsheet to estimate costs for myself.

Currently, I think I like this iteration of my plan, but I have a couple of questions I am not sure about. First, a couple of reasons why I came up with some of the features in this plan. Any comments are welcome.
  • I am in Central TX (hot and sunny), and the main panel and meter are on the west side of the house and get some horrible heat in the afternoon; mounting the inverters out at the array would allow them to be mounted underneath on the north side of the array providing them a cooler operating environment due to shade from the panels. Also, we get a constant breeze out there, so some additional cooling.
  • Using a 5kW inverter for each series of (9)450 watt panels (~4k best case scenario) doesn’t work the inverter at it’s max rating.
  • If an inverter dies, I only lose 1/6 of my power while I wait on parts.
Questions:
If I use a circuit breaker panel out at the array location to feed the 350 ft run to the house, would I calculate the wire size using the 3 circuit breaker ratings (ie 3x20, 60 amps), or do I still need to use the Max Output Current values from the inverters (24 x 3, 72 amps)?

Is there anything fundamentally wrong with the layout of my system? I would be using UL listed parts for all components.

Does a “Grid Tied Only” inverter have the auto shutdown feature (when you lose grid power) required for Solar Systems tied to the grid, or is a different piece of hardware required? Am I looking for a specific UL listing for this feature?

So far, I have seen listings and people talk about:
“Off grid” inverters – Cannot connect to grid without some safety measures/hardware, does not need grid power to work, solar and batteries only?
Inverters with “anti-islanding” – not sure what this is
“Hybrid” Inverters – I assume these are to be used with a solar/grid/battery backup?
“Grid Tied Only” inverters – solar and grid only, no batteries?
Am I correct with these descriptions, if not, help me out, please.

Thanks,
Epp

Edited to remove pdf and upload jpg
I thought mine was a long way from the main panels at 150 feet. Yikes!
 
Finally, consult a voltage drop table for a 700 ft 240 volt circuit and size the wire accordingly. Most likely the wire size needed to get voltage drop to acceptable level may be greater than the circuit breaker needed for the expected current.
I thought on VAC lines you are supposed to use the run for the voltage drop calculations (350ft), and for VDC lines you used the round trip distance (700ft) for the voltage drop. Is this incorrect?

In my planning, I used 60 amps @ 240vac (each leg, coming out of circuit breaker panel 1) over the 350ft run to the house to size my wire at <3% voltage drop.
Was this incorrect?

“Also inverters are considered a continuous load and the breakers should be rated at 125 per cent of their expected output.”

I assume my selection of the 20 amp breakers would fulfill this since the panels would only have the Wattage potential (442 vdc into inverter, converted to 240 vac) of 16.85 amps at 240 VAC. Once you account for inverter losses, panels are not at optimum angle ( it’s a fixed array, have not decided on 20 or 25° angle yet), panels not at STC, line losses in connection points and pv cables to inverters, I am expecting at least a 7% loss of power potential to the inverters which would drop the 16.85 amps to below 16 amp.
The only wild card is the temp coefficient, but that would occur in the winter when the array angle would be the least efficient. Georgetown TX has a 0 degF record low in ~1899, but 2021 saw 10 degF, so I definitely don’t want to ignore temp coefficient.

Thanks
Epp
 
In my planning, I used 60 amps @ 240vac (each leg, coming out of circuit breaker panel 1) over the 350ft run to the house to size my wire at <3% voltage drop.
Was this incorrect?
I think you are correct that AC voltage drop can use the actual tables. One table I checked let you input AC or DC
....,I assume my selection of the 20 amp breakers would fulfill this since the panels would only have the Wattage potential (442 vdc into inverter, converted to 240 vac) of 16.85 amps at 240 VAC. Once you account for inverter losses, panels are not at optimum angle ( it’s a fixed array, have not decided on 20 or 25° angle yet), panels not at STC, line losses in connection points and pv cables to inverters, I am expecting at least a 7% loss of power potential to the inverters which would drop the 16.85 amps to below 16 amp.
As long as your local inspector buys it you should be okay. You may also be able to get by with a 60 Amp breaker feeding each of the sub panels.
You mentioned 5kW inverters and three give you some redundancy. Is there a more cost effective solution using on or two larger inverters. I know that was not one of you questions but the answer might be useful for other readers.
The othervthing not mentioned is the size of your main panel but you have the option to do a line side tap (meter base).
 
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You mentioned 5kW inverters and three give you some redundancy. Is there a more cost effective solution using on or two larger inverters. I know that was not one of you questions but the answer might be useful for other readers.
I also looked at two inverters per array and one big inverter per array.
With six inverters that are slightly oversized, if I lose one inverter I only lose 1/6 of my generation. With the other two inverters being slightly oversized I can reroute if there is a long lead time on a replacement inverter.

With two inverters per array I would lose 1/4 of my power generation and even with the remaking inverter slightly oversized I might be able to adjust the power output until the replacement came in.

With one inverter per array if the inverter goes out then I am belly up until replacement comes in.

I come from a jet engine testing background so I always love redundancy to keep things going.

I think two inverters per array might be the sweet spot for my set up, but still checking prices and specs on inverters.

Epp
 
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