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My first post .... this is a great forum to solidify my research. I am installing a 6kW array of 20 panels in a field at 250 feet from the house..... Planning 2 strings to a SunnyBoy 6.0-US (with secure power feature). Running 396VDC at 8 amps (10AWG - 125% is 10amps and 5VDC drop) seems like a better solution than 240VAC at 25amps (6AWG - 125% pushes you to 31 amps and 7VAC drop). Running DC allows the SunnyBoy secure power option closer to the house. Running a 14AWG extension cord 250 feet into the field would be rather silly. However, I have a bit of concern of running 400VDC (18" below grade in conduit) across the field. Located in Oregon with minimal lightning danger. Am I on the right track? Appreciate the help...
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Run the 400VDC the longer distance. That allows you to use smaller wire and have the secure power outlet close to your house. -
That's what I have, a DC rated throw switch at the pole, DC wires and nice dry cool inverter in my basement out of harms way.Comment
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THANKS gang... appreciate knowing I'm on the right track. I will plan the additional DC disconnect at the PV array. I like safety first!!!!
While I have your expertise... Can you comment on the following plan. Would really appreciate some expertise on wire sizing and run lengths.
20 SolarWorld SW290 panels. (Standard Conditions 1000W/M2, 25C, AM 1.5 : Voc= 39.6VDC , Vmpp = 31.9VDC, Isc = 9.75A, Impp= 9.2A)
DC disconnect for 2 strings of 10 panels at array. 8' 5/8" grounding rod at PV array.
250 feet of 10AWG THWN copper to Inverter (burried 18" deep in PVC conduit)
396Voc at Isc= 9.75 amps, upscaling 125% is 12.2amps and 10AWG would have 6VDC drop over 250 feet. (Do I need to calculate 500 feet because wire resistance is sum of running to inverter and back to PV array... ie both directions?)
SunnyBoy 6.0-US mounted on exterior basement wall. Built in DC disconnect inside SunnyBoy. (AHJ does not require AC disconnect at SunnyBoy.... should I add????)
Should I add grounding rod at SunnyBoy (8' 5/8" rod)?
75 feet 8AWG (2 conductor stranded with ground) to main disconnect panel. Routed in PVC conduit above ground but secured under porch (crawl space).
40amp OCPD inside disconnect panel. (Disconnect Panel has my meter base and 200 amp breaker to "sub-panel" that feed my house.
If above looks good, the last hurdle is ground racking. Looking for a simple pile driven system for low profile single panel portrait mount.Comment
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Code requires that you up the Isc by 1.56, so 9.75 A requires conductors with an ampacity of 15.21 A. With two strings, you'll have four current carrying conductors, so there is an 80% adjustment factor to the base rating. For 10 awg, that works out to 35 * 0.8 = 28 A, so you still have plenty of ampacity.
For voltage drop, use the Imp, and figure the string voltage will drop about 10% from the STC Vmp as the array heats up. With 10 awg, that would put loss over 250 ft at 1.6% or so.
For the run to the main disconnect, you need three conductors plus ground... two hots, a neutral, and ground. The neutral doesn't have to be full sized since it is not truly a current carrying conductor; it must be at least as big as the EGC required, 10 AWG. It might just be easier to make it 8 AWG.
You should plan to run copper that bonds the electrode at the ground mount array with the house grounding system. Some ground mount systems use steel in the concrete piling as the electrode, instead of a copper rod.
If you drop another electrode at the inverter, it absolutely must be bonded with the ground electrode that is used for the existing AC service. If you have a good AC ground already, this serves no purpose. If your service panel is grounded through just a water pipe or something like that, then yeah, you'll probably need to add a rod.
At peak output, the 6 kW inverter will produce 25 A @ 240 V. That is about 1% drop with 8 AWG over 75 ft. Here is a decent calculator to use, which specifies that you enter one-way distance.
With your inverter on the exterior wall, you might be able to avoid rapid shutdown requirement. If you have more than 5 ft of DC run inside the building, you'll need a shutdown system.Last edited by sensij; 06-08-2017, 01:12 PM.CS6P-260P/SE3000 - http://tiny.cc/ed5ozxComment
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Here are two points you need to know.
1. Higher voltage is always less expensive to implement, and is the most efficient.
2. Volt for volt, DC is always more efficient. A 240 VDC circuit is more efficient than a 240 VAC circuit.
This is why utilities use such high voltage. On point to point transmission they use High Voltage DC (HVDC).
Makes your decision a No Brainer.Last edited by Sunking; 06-08-2017, 04:27 PM.MSEE, PEComment
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I don't know what that particular inverter is equipped with. My installer originally only broke the hot side (of
the 2 systems) with a 600VDC switch. I had it off later while correcting some of his wiring issues, but the
circuit through the return blew the inverter GFI fuse. I got another (cheap E+ay) switch so that either system
could have BOTH conductors disconnected by their own switch. Bruce Roe PVdiscon.jpg
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SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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Just a note here. Standard design practice is 3% voltage loss. Gets really expensive and impracticable below 3% on longer distances. Example by code may only require 12 AWG, but at 3% require 1/0, and at 2% 250 MCM. 12 AWG cost 12-cents/ft, 1/0 @ $1.60/ft, and 250 MCM @ $6.55/ft. Copper is expensive and semiprecious metal.
MSEE, PEComment
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Everything installed, AHJ signed off and generation approved by utility. It was a fun project. I'll start a new post and add some pictures but in a nutshell, I increased to 7kw with 24 panels.
24 SolarWorld SW290 panels. (Voc = 475VDC and Vmpp = 383VDC Impp = 9.2amps)
Two DC disconnect for 2 strings of 12 panels at array with 8 foot 5/8" grounding rod at PV array.
300 feet of 10AWG THWN copper to Inverter (4 wires buried 24" deep in 3/4" PVC conduit)
SunnyBoy 7.0-US mounted on exterior basement wall. Built in DC disconnect inside SunnyBoy.
75 feet 8AWG (3 conductor stranded with ground) to main disconnect panel. Routed in 1" PVC conduit above ground but secured under porch (crawl space).
40amp OCPD inside disconnect panel. (Disconnect Panel has my meter base and 200 amp breaker to "sub-panel" that feeds my house.
Ground mounted system on Patriot Solar racking.
Total system cost $13K. With Oregon & Federal incentives of $9.9K the break even should be 3 years.
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Congrats. Lotta work. Lotta decisions.Comment
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Damn, I wish I could get 75% of my solar array paid for by tax payers. Minnesota has some good incentives, but if only if you get your power from an investor owned power company. I get my power from a CO-OP so no state incentives. I'll have about $9,500 or $1.50 per watt into my system before federal tax credit. It will be an 8 year payback for me so not a great investment.Comment
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