Help me make sure I understand running DC high voltage wires buried in conduit back to inverter from remote (125ft) array
- Thread starter cdherman
- Start date
MajicDiver
Solar Enthusiast
Exactly! Working around live grounded 120/240 VAC always bothers me, I’m far more comfortable with floating 400-500 VDC. In most situations all that would happen if you got a tool across both DC legs is an arc and burned tool tip, you would have to be doing some stupid stuff and not using your head at all to get shocked
Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
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The 400-500V to 600VDC PV array is not floating in operation. Non-operating, for some systems, it is.
It is typically either a negative-ground array, with maybe 1A fuse or breaker to interrupt the grounding. That will prevent a fire but not prevent electrocution.
Or, transformerless inverter (either off-grid or on-grid) is is probably referenced to AC voltage. In some cases it also has AC voltage riding on it.
AC tends to be more lethal because 60 Hz is near optimum to disrupt heart rhythm. Ask me how I know.
DC of course can also cause burns. The hazard voltage and current is a bit higher than for AC.
I used rigid conduit because I only wanted to do 12" coverage not 18" required for PVC. But steel was cheaper 20 years ago.
Quattrohead
Solar Wizard
I'm sorry but this is just complete and absolute rubbish.
Quattrohead
Solar Wizard
Are you doing a permitted install, a half decent install or DIY lash up?
For a DIY lash up you could use direct burial 10 AWG and call it a day
For a DIY lash up you could use direct burial 10 AWG and call it a day
MajicDiver
Solar Enthusiast
I have checked for AC on mine repeatedly over the past 7/8 months after reading the threads on the subject and have never seen any sign of any.
Would you be willing to explain or provide a reference explaining “probably referenced to AC”?
Quattrohead
Solar Wizard
Stick your digging device through any pair of cables carrying a decent amount of power and let me know how it goes for you.
Quattrohead
Solar Wizard
And why is using cable that is specifically designed for burial and will carry the current that the OP wants rubbish?
I'm sorry I was a bit harsh in my reply to you but touching anything over 24 volts is starting to get dangerous. Maybe sometime I will tell you the story about my worst electric shock and it was only from 24 volts ac.
hwy17
Anti-Solar Enthusiast
Voltage drop doesn't matter for arrays.
10 awg will lose less of your power on the wires, but 12 or 14 awg is also acceptable.
10 awg will lose less of your power on the wires, but 12 or 14 awg is also acceptable.
Because I understand how electric circuits work, I know that the amount of power being carried by a cable has no effect on what happens if you cut into it. You have to get in the circuit to be shocked by it. A single point of contact with a shovel on an ungrounded system won't hurt you. Even if you hit both conductors, the current will flow through the shovel head between the wires. Even if the system negative is ground referenced, it is likely to be through a charge controller with a GFP system that will open that ground connection and leave the PV side ungrounded.
Also, PV is a current source, which means as soon as fault current starts flowing, the voltage starts dropping. But a grounded 120 volt circuit is a voltage source and will try to deliver all it can into a fault.
I stand by what I said.
Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
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- 20,695
What model?
I don't think I see AC either for my Sunny Boys. Mostly I'm using transformer type, but have used TL, also TriPower which is transformerless.
It is my understanding that if I used SB across two hot legs of 3-phase, then it would carry AC common mode. Across 208V of 120/208Y I expect 104Vrms.
Maybe referenced to DC level Vpeak not directly to the AC signal, maybe referenced to midpoint of AC signal.
The SMA transformerless inverters one power-up do a test where the ground one end of PV array, probably then other end. That is to check for current indicating a fault to ground somewhere.
After that they do not continuously test, but PV array gets galvanic connection through transistors and inductors to grid L1 & L2. If you checked voltage from any point of array (except midpoint), I think you would see DC voltage. If you connected a load, I think you would see current. And DC current would flow into utility transformer which doesn't like that.
Page 2:
"No galvanic isolation The product is not equipped with a transformer and therefore has no galvanic isolation. •Do not operate grounded PV modules together with the product. If grounded PV modules are connected to the product, an event will occur which will appear on the product display. The event will also be displayed, along with the associated message, in the event list on the user interface of the product."
"PV modules with a high capacity to ground must only be used if the coupling capacity of all PV modules does not exceed 2.5 μF."
The latter is due to AC currents flowing through the capacitance.
Current is directly flowing from PV panels through buck converter, or in the case of these models apparently buck/boost, into AC lines. If part of PV string was grounded, current could flow through that connections because there is a path to the AC transformer which is bonded to ground.
Only if current exceeds 1A (GFCI fuse in my old Sunny Boys) or 0.5A (breaker used for GFCI of Midnight Classic), or whatever detection threshold yours has.
Depending on the impedance your body presents, you could get a bad shock and hundreds of watts delivered without tripping GFCI.
MajicDiver
Solar Enthusiast
Growatt MIN-11400-TL-XH-US (Version 2)
cdherman
New Member
Lets see if I can redirect the conversation back to my needs. (all you worried about how I burry this -- I'll do it right when the times comes!)
I need to understand getting HV DC from a solar array back to a different location for the inverter. Trying to understand what is involved, what ratings and rules must be followed.
hwy17 -- your comment stimulates thought. Of course some drop in voltage, so long as it doesn't overheat things, is really moot. The MPPT inverter is adjusting to various voltage and amperage of the circuit all the time. Really a sweet point to make. Thanks! Too much of a drop means heat, though as long as that heat is spread over 125' (and not due to a poor connection etc) then heat is manageable.
Someone else asked if the online calculator was accommodating for one or both ways. The calculator specifically asked for "one way distance" so I think it was doing the calculation correctly.
Someone else suggested a single strand of 14 THHN as the pull wire for future pulls. Makes sense. Relatively small and slick. Someone also suggests steel elbows. I was leaning to either the real "wide circumference" bends, or perhaps even custom bending a very large bend. But have not worried about the burial aspects just yet. Like I said about. I want to understand the electrical first -- how to bury right comes after the layout is clearer.
hwy17
Anti-Solar Enthusiast
You can technically use UF-B too.
I bought 14awg UF-B for my 200ft distance. Then when I got the trench dug, I realized I couldn't go this far and not put some conduit in for the future. So now I have one run of 14awg UF-B and one empty 3/4" PVC conduit buried together.
I bought 14awg UF-B for my 200ft distance. Then when I got the trench dug, I realized I couldn't go this far and not put some conduit in for the future. So now I have one run of 14awg UF-B and one empty 3/4" PVC conduit buried together.
MarkSolar
Solar Enthusiast
You need to calculate the Vmax and Vmin that your array will output, you can do that either using online tables or calculations. Vmax and Vmin are dependent on the max and min temperatures you expect to see in your area, you can use your own experience, or if you need to submit documentation for a permit you can get those numbers from the ASHRAE handbook. Once you know the Vmin you can select the wire size based on %voltage drop you want. That's typically 2-3% but you can get it as low as you want by spending more money on bigger wire. The Vmax will determine the insulation rating on the wire. There is no requirement to derate the wire since you are burying it so there are no high temperatures involved, but you need to use the NEC table to choose an adequate wire size based on current. Since this is treated as a feeder you will use NEC table 310.12(A) to find the required wire size. You can't use anything smaller that called out in the table, but if it's not adequate for voltage drop you can of course go larger. You need to use the wire required at 75C in the table because the equipment on the end (breakers, enclosures, etc) is rated for 75C, and code requires you use the lowest rated device to size the wire.
You asked earlier about what kind of wire. I prefer XHHW aluminum for this application. It will be about half the cost and weight of copper, and even though the XHHW insulation is tougher than THWN, it is more flexible so it's easier to work with at the ends. It's also resistant to chemicals if that matters in your application. It is slightly more expensive than THWN, maybe 15% or so.
Mike 134
Solar Enthusiast
Just an FYI Yes you might have to derate because it is a continuous load therefore can only be loaded to 80%
MarkSolar
Solar Enthusiast
It's a bit confusing because there is an 83% rule that allows a feeder to be 83% of the required ampacity. So if you need 100A you can select a feeder size that carries 83A. You're correct that for continuous service (>3 hours) you need to upsize the wire by 120%. But those two things cancel out, so if you just use the table you get the right wire choice for continous service.
He's talking about the 125% rating for continuous loads. The breaker and wire has to be rated at 125% of the load current. Consequently, a breaker can only be loaded to 80% of the rating.
The 83% rule for feeders only applies to feeders carrying the entire load of a residence. If, for example, you separate the air conditioner load onto a breaker located outside at the meter, then the feeder going inside to the main panel has to be rated at 100%.
cdherman
New Member
You last folks talk about breakers and 75c ratings and such. But for a DC HV feed to an array of panels, its not clear to me we are talking the same applications.
It would indeed seem relevant to have a DC fuse in a HV DC circuit. No one that I know of has proper DC "breakers" -- lots on eBay and amazon that get miserable reviews. I would rather have a small supply of fuses for the DC circuits. A DC circuit of known amps etc will seldom overload. You don't need to have breaker you can reset again and again. The fuse will melt with a reliable degree of accuracy to protect stuff. Inform me why the talk about a "breaker".
As for Vmax and Vmin etc. I will try and find the itty bitty details pertaining to temperature. Do I really need to question the rated volts and watts that much of a PV panel? Perhaps so?? I don't really give a hoot about Vmin. I assume panel ratings are based on some arbitrary ambient temp. Say 20c/72f -- so they would actually produce LESS when hotter (this is what I think I know).... And since bright sun at the proper angle is practically never going to occur, especially in cooler months, just why do I need to fuss over this detail?????
It would indeed seem relevant to have a DC fuse in a HV DC circuit. No one that I know of has proper DC "breakers" -- lots on eBay and amazon that get miserable reviews. I would rather have a small supply of fuses for the DC circuits. A DC circuit of known amps etc will seldom overload. You don't need to have breaker you can reset again and again. The fuse will melt with a reliable degree of accuracy to protect stuff. Inform me why the talk about a "breaker".
As for Vmax and Vmin etc. I will try and find the itty bitty details pertaining to temperature. Do I really need to question the rated volts and watts that much of a PV panel? Perhaps so?? I don't really give a hoot about Vmin. I assume panel ratings are based on some arbitrary ambient temp. Say 20c/72f -- so they would actually produce LESS when hotter (this is what I think I know).... And since bright sun at the proper angle is practically never going to occur, especially in cooler months, just why do I need to fuss over this detail?????
