Long AC run versus long DC run in gridtie. Which is better?

cdherman
cdherman Solar Expert Posts: 32 ✭✭
I can locate my invertor in several places. One location would make for very compact DC runs back from the strings, but about 40 feet of AC wiring to get over there from the breaker box.

I could also get longer DC cables and locate the invertor right next to the breaker box.

Since I already own plenty of 6 guage wire (which Fronius recomends, I need a 220 40A AC feed) I can make the AC really easy, but the longer DC runs would mean a little more cash outlay.

So I'd rather do the longer AC run, as you can see.

Fronius says max .5 Ohm resistance, but 40ft of 6 at 220v comes to like only 0.015so that ought to be OK, right?

Sorry for all the questions. I am obviously a beginner at solar, but I think I have a decent understanding of electrical stuff. I WILL have an electrition do the final connect, but basically to make the untility happy......

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    AC or DC doesn't much matter. What matters is the Voltage. The higher the voltage, the lower the loss. The lower the voltage the higher the loss. Run the HV further.

    Tony
  • cdherman
    cdherman Solar Expert Posts: 32 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    icarus wrote: »
    AC or DC doesn't much matter. What matters is the Voltage. The higher the voltage, the lower the loss. The lower the voltage the higher the loss. Run the HV further.

    Tony

    OK, please don't be insulted, but isn't the whole reason we use AC is so that we avoid losses over long distances? IIRC, the DC line litterally has to push electrons to the other end of the line, and that builds up surounding fields that eventually really mess with the resistance of the line.

    AC of course cycles, but even more importantly, the two sides of a 220v system"push and pull" from each others electrons and fields. Thus the importance of balancing loads in your breaker box. Also why the utility company wants 3 phase users to balance their loads I think.

    Well, anyhow, I don't know a lot about solar, but I'm trying to learn..... Bear with me please......

    I think I am going with a Fronius IG inverter thas has the built in DC fuses and DC kill. So I can ditch the combiner box, if the DC runs to the inverter are short, right? I mean, why would Fronius design their product to accept up to 6 strings and fuse them all, if I am just going to put fuses and a combiner in upstream.
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    AC or DC doesn't much matter. What matters is the Voltage. The higher the voltage, the lower the loss.

    And the reason Edisons DC system failed was???

    I think it went something like this.. its a lot harder to run DC than AC for medium distances..
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    Mind you, I'm no engineer, and not even an electrical expert. At least in the voltages that we are dealing with, 1-240 and as far as I know, loses go down with voltage.

    Tony
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Long AC run versus long DC run in gridtie. Which is better?

    The cost of DC (at the time) was that is was very hard to Transform between high voltage distribution and low voltage home/consumption.

    AC Transformers are pretty easy devices to make and use.

    For long distance transmission, AC has more losses and some other issues (such as synchronization between major electrical/grid hubs).

    We have, especially in the west, quite a few long distance DC distribution networks that bring Hydro power from the Northwest and Canada (thank you both) and bring it down into California. The cost are the stations at each end that convert from AC to DC and back to AC again.

    As Tony said--for the distances and such you are asking about, which ever is the higher voltage--make that the longer run. What you are trying to minimize is the I^2 * R losses (heating effect in the wiring... with the same wire, 2x the current gives you 4x the heating/losses--1/2 the current is 1/4 the heating losses).

    Grid Tied Solar GT Inverters (no batteries) typically run around 200 to 550 volts DC. Vs the 240 VAC of the typical home split Phase AC circuit.

    For off grid--use a good MPPT solar charger running the Array at 100 VDC for the long distance, and put the controller+batteries close together at the 12-48 Volts of the battery bank.

    Also, a Battery Charger needs to be next to the batteries to keep the cable short, low voltage drop, and usually connect to a battery temperature sensor.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    Someone please let me know if I have this wrong....

    I am of the understanding that the number of volts lost to internal resistance is the same per foot/ meter/ yard/ whatever constant unit you want to use for a given wire size, regardless of the voltage in the wire...

    thus if you lose 1 volt out of 12 you are at 11 volts, or a little more than 91.6% of the starting voltage
    If you lose 1 volt out of 120 you are now just a bit over 99.16%

    the problem is with low voltage vs high voltage, not the type.

    With DC there are significant safety issues the higher the voltage like sustained arcing, not as bad with AC.

    I hope I got this right.

    In this case the OP is going to send 220V AC to the Distr box, no way he can come close to that with DC, so AC wins with this example ;)

    cheers,
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    the problem way back was that as you went farther away from the dc generation point the voltage was progressively smaller so there wasn't any consistency to all of the electric customers along the line. by the time it went a few blocks it was useless and another generating station needed to fill that gap. it was too impractical to have that many generating sites around. to extend distances the wires needed to be huge as well making the dc quite an ordeal to utilize. tesla came along with the help of westinghouse and made it more simple, consistent and cost effective.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Long AC run versus long DC run in gridtie. Which is better?

    Eric,

    Yep, for what we are talking about you have summed it up nicely.

    The one issue that we need to look at is I believe "cdherman" is installing a Grid Tied Inverter... His DC may be in the 240-400 volt range (depending on the particular GT inverter)--so in his case, the DC voltage will probably somewhat higher than his AC.

    For example, my 3kWatt GT system runs between ~290 and 340 VDC... And my house is around 244-247 VAC.

    So, in these cases, the Longer DC runs win-out marginally... Unless having an AC connection near the array/inverter is handy in the back lot.

    For me--I was able to mount my inverter in a cool garage (insulated and tree shaded) vs out on a southern wall exposed to sun and weather... And that was really what defined my trade off (and, at the time, my utility required us to have an Solar AC cutoff near our Utility Meter/Main Box).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    cdherman wrote: »
    OK, please don't be insulted, but isn't the whole reason we use AC is so that we avoid losses over long distances?

    No, because it's really cheap and easy to use transformers to step AC up to 200,000 Volts for long distance transmission. And now there are ultra high voltage DC transmission lines too.

    The highest voltage, has less losses in copper, make your longest run, the highest voltage.
    12V, 40A = 480W
    60V 8A = 480W
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • cdherman
    cdherman Solar Expert Posts: 32 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    OK, I know more now than before. Thanks....

    Yes, my AC at 220v will be less than the DC at about 450v

    So all things being equal, I should keep the AC run short. That bearing in mind that the subpanel is already about 200 ft from the meter. So I don't know how much more losses I am talking with another 40 ft of 6-0 AC.....

    But I have the concept I think.....
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    try the calculator here in post #1. may work for you, assumes batteries are used.
    http://forum.solar-electric.com/showthread.php?t=29&page=6 ;)
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    Remember that these comparisons are dependent on conductor ampacity -- just because one voltage is higher than the other doesn't mean Vdrop is going to be lower with the higher voltage.

    ALWAYS check both with a VD calculator. --keep it family friendly--Bill B. Moderator

    (No, really -- Dad taught me resi wiring when I was a kid.)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Long AC run versus long DC run in gridtie. Which is better?

    TG,

    I am not sure what you are going with your Voltage Drop post... In constant power applications (which MPPT charge controllers are...) Higher voltage will give lower current (when, for example comparing two solar panels in series vs the same two solar panel in parallel).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    tallgirl,
    you are right that higher voltage does not change the voltage drop itself. if one gets for example a .1v drop at 12v then going to 24v with the same wire and same current level will show the same voltage drop. what does change is the % in this case. now some items will lower the current required when using higher voltages as these are by wattage. in these cases it will lower the voltage drop due to less ampacity flowing in the same wire for the same power. this is provable on the voltage drop calculator. the nec does refer to percentages for voltage drops.
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    niel wrote: »
    tallgirl,
    you are right that higher voltage does not change the voltage drop itself. if one gets for example a .1v drop at 12v then going to 24v with the same wire and same current level will show the same voltage drop. what does change is the % in this case. now some items will lower the current required when using higher voltages as these are by wattage. in these cases it will lower the voltage drop due to less ampacity flowing in the same wire for the same power. this is provable on the voltage drop calculator. the nec does refer to percentages for voltage drops.

    Yes, but when comparing two different power systems, you can't just compare volts and declare that System A has less Vdrop than System B. You really have to go back and calculate Vdrop as well as the power lossage. For example, typical panel wiring is #10 AWG USE-2. If the installer went with 200 feet of #10 (which has a fairly high resistance, especially after a 400 foot round trip, Vdrop might well be higher than with a 400 foot round trip of AC wiring sized for the maximum amperage, but seldom used at that amperage.

    One advantage of long AC runs is that the NEC requires they be sized for the maximum load, which is often much higher than the maximum DC load. Remember that the source current for a PV array is limited by the "width" of the array, while the maximum AC current is set at 125% of the maximum CONTINUOUS inverter output. Since the solar array is going to operate much closer to the rated maximum (even with the 156% derating) than the inverters, Vdrop should be much, much lower.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    tallgirl wrote: »
    Remember that these comparisons are dependent on conductor ampacity -- just because one voltage is higher than the other doesn't mean Vdrop is going to be lower with the higher voltage.

    But loss-wise, it will be a lower percentage. We are talking about a fixed amount of watts, and for the same wire, the higher voltage, will always experience less loss.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    mike90045 wrote: »
    But loss-wise, it will be a lower percentage. We are talking about a fixed amount of watts, and for the same wire, the higher voltage, will always experience less loss.

    Well, yeah, but you don't have the same systems. The OP asked about DC conductors for a solar array output, versus AC conductors for an inverter output. Those are two completely different systems. And the reason is that inverter size is quantized by the vendor, while array size is much more flexible, allowing a closer fit between actual power and conductor size.

    NEC requirements are that both AC and DC wiring be sized at 125% of the maximum continuous amperage, with the caveat that DC wires from solar sources be sized at 156% of Isc. Yes, for the higher voltage wire, the amperage will be lower.

    However, AC and DC sources have different characteristics. The maximum AC conductor ampacity is determined by the maximum inverter output -- it's nameplate rating -- while the DC conductor ampacity is determined by the 156% of the short circuit current of the array. The inverter's maximum output power is NOT equal to the array's maximum output power.

    Thus, it is possible that the AC conductors are oversized relative to the short circuit current of the DC source. Since Vdrop is related to conductor size and resistance per foot times current, the AC conductors may be oversized relative to the available DC source power.

    To work an example, let's assume that a 3KW DC array with Vmpp off 300VDC has an Isc of 15A. 15 * 1.56 = 23.4A, which is well within bounds for #10 AWG USE-2. If that is feeding a 6KW inverter producing 240VAC power, Imax is 31.25A and the conductor is #8 AWG. However, the actual output is limited to 3KW minus conversion losses. Let's say it's 95% efficient, so the maximum output is 2850 watts @ 240VAC, or about 12A.

    400 feet (200' x 2) of #10 has a resistance of 0.4072 ohms. 400 feet of #8 has a resistance of 0.256. For the #10 @ 23.4A, Vdrop is 9.53V and Ploss is 9.53 x 23.4 = 223 watts. For the #8 @ 12A, Vdrop is 3.07V and Ploss is 3.07 x 11.9 = 36.5 watts.

    As you can see from the difference, even with a larger array, the requirement that the inverter's feeders be sized to 125% of the nameplate rating (if you can get the AHJ to allow a smaller ampacity feeder, good on you) means that Vdrop is greatly reduced.

    You really have to do the math both ways.
  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    tallgirl wrote: »
    To work an example, let's assume that a 3KW DC array with Vmpp off 300VDC has an Isc of 15A. 15 * 1.56 = 23.4A, which is well within bounds for #10 AWG USE-2. If that is feeding a 6KW inverter producing 240VAC power, Imax is 31.25A and the conductor is #8 AWG. However, the actual output is limited to 3KW minus conversion losses. Let's say it's 95% efficient, so the maximum output is 2850 watts @ 240VAC, or about 12A.

    400 feet (200' x 2) of #10 has a resistance of 0.4072 ohms. 400 feet of #8 has a resistance of 0.256. For the #10 @ 23.4A, Vdrop is 9.53V and Ploss is 9.53 x 23.4 = 223 watts. For the #8 @ 12A, Vdrop is 3.07V and Ploss is 3.07 x 11.9 = 36.5 watts.

    As you can see from the difference, even with a larger array, the requirement that the inverter's feeders be sized to 125% of the nameplate rating (if you can get the AHJ to allow a smaller ampacity feeder, good on you) means that Vdrop is greatly reduced.

    You really have to do the math both ways.

    tallgirl,

    No disrespect but I think your example is misleading. You are using theoretical current limits on the DC side but practical current limits on the AC side and that is leading to an erroneous conclusion in my opinion. Using your example...

    3000W Solar Array
    Vmp = 300V
    Isc = 15 A
    6000W Inverter with 95% efficiency
    Vac = 240V
    Iac = 25A (max)
    Resistance of 400' of #10AWG wire = 0.4072 ohms
    Resistance of 400' of #8AWG wire = 0.256 ohms

    While you have calculated the minimum wire sizes correctly for the DC and AC sides this doesn't directly translate into whether it's better to make the long run on the DC or AC side. And here's why. The name of the game is minimizing power losses. Your 3000W solar array with Vmp = 300V implies that your Imp = 10A. This is the practical amount of current that is flowing on the DC side. Yes the current can be higher at temperatures colder than STC and for solar insolations greater than STC but that merely translates into higher currents on the AC side since the inverter is so oversized. A 300V and 10A DC input to our 95% efficient inverter gives an output of 240V and 11.9A AC. From this practical point of view the power losses to consider should be

    Setup #1 - 400' of #10AWG for DC run versus 400' of #8AWG for AC run.

    Power loss (DC) = (Imp)^2 x R = 10A x 10A x 0.4072 ohms = 40.72 Watts
    Power loss (AC) = (Iout)^2 x R = 11.9A x 11.9A x 0.256 ohms = 36.25 Watts

    Still a win for making the long wire run on the AC side rather than the DC side but not the lopsided win that you were showing. And I'll further argue that if you are willing to spend the extra money to make the long run in #8AWG wire instead of #10AWG then you should also consider the power losses on the DC side with this larger wire.

    Setup #2 - 400' of #8AWG for DC run versus 400' of #8AWG for AC run.

    Power loss (DC) = (Imp)^2 x R = 10A x 10A x 0.256 ohms = 25.6 Watts
    Power loss (AC) = (Iout)^2 x R = 11.9A x 11.9A x 0.256 ohms = 36.25 Watts

    In this case it's better to make the long run on the DC side of the system.

    Another practical argument for keeping the AC wire runs as short as possible is the issue of voltage sag. At locations near the ends of utility transmission lines and in older parts of the utility grid it's very possible to have transient grid voltages that are only marginally within the lower part of the voltage window of most modern inverters. Because of the UL anti-islanding requirements deviations outside of this window will force the inverter to kick off. The greater the deviation the quicker the inverter needs to decide whether to turn off. While the 3V voltage drop on the AC side you calculated above is only a small % drop at 240V, it may be enough to drop out of the grid voltage window. Even with short AC runs, some installations require an additional voltage regulator to keep the utility voltage within the inverter window.

    Others may think differently, but for my grid tied designs, I always try to minimize the AC wire runs and will spend the money to upsize the wire on the DC side if necessary.

    Scott.
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    solartek wrote: »
    tallgirl,

    No disrespect but I think your example is misleading. You are using theoretical current limits on the DC side but practical current limits on the AC side and that is leading to an erroneous conclusion in my opinion. Using your example...

    No, I screwed up and used the NEC required ampacity as the actual current. At least I showed my work ;)
    Setup #2 - 400' of #8AWG for DC run versus 400' of #8AWG for AC run.

    Power loss (DC) = (Imp)^2 x R = 10A x 10A x 0.256 ohms = 25.6 Watts
    Power loss (AC) = (Iout)^2 x R = 11.9A x 11.9A x 0.256 ohms = 36.25 Watts

    In this case it's better to make the long run on the DC side of the system.

    It depends -- the #8 conductors are required by CODE on the AC side, but not on the DC side.
    Another practical argument for keeping the AC wire runs as short as possible is the issue of voltage sag. At locations near the ends of utility transmission lines and in older parts of the utility grid it's very possible to have transient grid voltages that are only marginally within the lower part of the voltage window of most modern inverters.

    That's an argument against using those inverters. In the States the permissible window for 120/240 single phase power is 120 VAC +/- 5%. If the Utility can't maintain that window, they need to retap their transformers. It's not like the voltage isn't under utility control ...
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    tallgirl wrote: »
    That's an argument against using those inverters. In the States the permissible window for 120/240 single phase power is 120 VAC +/- 5%. If the Utility can't maintain that window, they need to retap their transformers. It's not like the voltage isn't under utility control ...

    Pretty bum advise.

    The Voltage tolerances are specified in UL1741 and good luck getting either the utility to change a tap which may not be possible depending on the other loads and distance or getting the written approval to have wider tolerances that must be provided to the manufacture before they can allow tolerances other than UL1741

    As the response states, your better at keeping the AC short to prevent nuisance UL1741 trips, something the owner/install CAN control, instead of trying to battle the power company to change and provided documentation the is REQUIRES before a UL listed device can be modified ... ALL UL1741 LISTED inverter must follow this procedure
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?

    When I lived in The States "acceptable" line voltage fluctuated between 107 and 137 VAC! It was horrible. I doubt today they manage to regulate it at every house to +/- 5%. Or is that just a requirement for grid-tie inverters? On average, inverters put out cleaner, better regulated power than you get from the common household outlet.

    Of course I'm old, senile, and never knew there was a difference between AC wires and DC wires. Thought it was all in the application.
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    Pretty bum advise.

    It's much sounder legal advice than "use heavier wire."

    What the heck are you going to do when the line voltage, even with fat wire, falls out of tolerance? Use even fatter wire with even less voltage drop?
    The Voltage tolerances are specified in UL1741 and good luck getting either the utility to change a tap which may not be possible depending on the other loads and distance or getting the written approval to have wider tolerances that must be provided to the manufacture before they can allow tolerances other than UL1741

    UL1741 doesn't apply to utilities. The +/- 5% and +/- 10% limits DO.
    As the response states, your better at keeping the AC short to prevent nuisance UL1741 trips, something the owner/install CAN control, instead of trying to battle the power company to change and provided documentation the is REQUIRES before a UL listed device can be modified ... ALL UL1741 LISTED inverter must follow this procedure

    I'm not sure that a small reduction in meter-side Vdrop is really going to impact the much longer line-side problems. I mean, it's 200' or so from my inverters to my transformer. If someone is out in the sticks, it can be several kilofeet to the next polepig. On the other hand, I have #2 AWG back to my meter, so I'm likely enjoying one of the benefits of grid-interactive versus grid-tied inverters. I can get a heck of a lot more than 2.8KW AC out of my inverters.

    If the POCO is having troubles keeping their voltage up (or down), call the public utility commission. If that doesn't work, write a letter to FERC.
  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    tallgirl wrote: »
    It depends -- the #8 conductors are required by CODE on the AC side, but not on the DC side.

    I understand. However, the NEC is a minimum requirement. Nothing stops you from upsizing the wire size except cost and hassle factor. My point still stands: If you're willing to run #8AWG for 200' on the AC side of the system, you're better off (power loss wise) running it on the DC side of the system.

    Scott.
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    solartek wrote: »
    I understand. However, the NEC is a minimum requirement. Nothing stops you from upsizing the wire size except cost and hassle factor. My point still stands: If you're willing to run #8AWG for 200' on the AC side of the system, you're better off (power loss wise) running it on the DC side of the system.

    Scott.

    Well, we're definitely talking past each other because you've completely ignored what I wrote.

    You MAY upsize the DC conductors to #8. But you MUST size the AC conductors at #8. That's it -- entire message.

    If you want to run #8 end-to-end, that's your business. No one says you have to, and as I said from the outset, you have to run your voltage drop calculations, you can't just ASSUME that one works one way and the other works the other way. If the NEC were followed, along with your rule of thumb, there'd be more power loss than the other way around. If for some other reason you decided to move the inverters closer to the meter, you'd again have to check Vdrop to resize the DC conductors for lower Ploss. Which was yet another thing I said.
  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Long AC run versus long DC run in gridtie. Which is better?
    tallgirl wrote: »
    Well, we're definitely talking past each other because you've completely ignored what I wrote.

    I apologize if it appears this way.
    You MAY upsize the DC conductors to #8. But you MUST size the AC conductors at #8. That's it -- entire message.

    I understand this message and agree completely.
    If you want to run #8 end-to-end, that's your business. No one says you have to, and as I said from the outset, you have to run your voltage drop calculations, you can't just ASSUME that one works one way and the other works the other way. If the NEC were followed, along with your rule of thumb, there'd be more power loss than the other way around. If for some other reason you decided to move the inverters closer to the meter, you'd again have to check Vdrop to resize the DC conductors for lower Ploss. Which was yet another thing I said.

    And here is where we will have to agree to disagree. The OP wanted to know if one way was better than the other and in my opinion the answer is yes. For grid tied systems employing high DC side voltages, long DC runs are better than long AC runs for the reasons I've given. Your point about this not being true by following the minimum NEC wire size requirements is perfectly valid. However, this is not an optimally designed system in the sense that the DC and AC power losses are minimized. If the vast majority of the wire cost is in the long 200' run, then only one configuration minimizes the power loss of the system for the cost of the long #8 wire run. And that's my message.

    Scott.