Kent, Nice to meet you I've enjoyed your past posts and I like what you wrote below, and I definitely understand that you're just roughing it.
Sometimes I hear people say we should "Add more PV," even when it doesn't necessarily solve our problem. Most often it's not that easy to add more PV, or we likely would've. We are typically either at the clients budget limit, the inverters limits (P,V,A,Strings, etc.), we could be at a Megawatt PTC, so no more rebate on that PV, or we have used up all the unshaded space on the roof, etc. And if that's no trouble, with more PV comes more racking, install time, and with more power at a ceiling voltage (Inv Input/NEC 600V)... we end up with more amps, which means bigger wire anyways. Then of course, the whole thing costs more. For the chart below, would you think that instead of the straight module price of $3.00W or whatever we SELL them for, not bought them for... Would you think a more fair number would be closer to the installed cost of $8.00/W or whatever we sell it for? In which case, #8 would make sense. If one sells a system at the NEC minimum recommendations, with significant voltage drop, I say fine, as long as you gave the client an accurate production estimate and the financial numbers still work for them, and as long as those conductors last as long and perform as you estimated. Once we tell a client, "Your system will produce X, and cost X." We have to ethically and, often contractually, give them that. If you have to pull that wire in a few years, due to module voltage degradation and exacerbated excessive Vdrop, with an inverter that isn't starting up, and maybe after a few inverter replacement trips until the guys figure out that the Vrop is too much with the modules settled or degraded voltage, etc, it should be done for free. If not, that original deal with the client is not being honored and therefore was an unethical sale, and another unsatisfied customer. That is if the inverter repairs/replacements didn't upset them enough. :o) LCOE To really compare this, I would say you would have to do a few designs with the proposed wire sizes and Vdrops, calc the costs of each system, including larger conduit and fittings, etc. Then run production estimates, with PVSyst, don't forget that year 2 should be about .5% less, and on and on, for 20,25 or 30 years, depending on how aggressive you are, and calculate every lifetime cost to the best of your ability, to get the Levelized Cost of Energy (LCOE) and compare the one that yields the lowest generated cost per kWhr. Anything less is Swiss cheese...holes all through it. I do have to wonder though about WHEN that power is made, I'm sure that conductor loading throughout the curve can factor in somewhere. Maybe because actual amp loading will grow, peak and shrink throughout the day, with relatively constant voltage, maybe our Vdrop calculations are too rudimentary using fixed Vmp & Imp numbers. Is Imp variable enough for a conductor with 5.5%Vdrop where it only spends a small amount of time with that high of a drop, and in under STC conditions be within a reasonable Vdrop? Thanks to all who participate on here, the education is greatly appreciated. Look forward to other opinions, comments, etc. Sorry about the length, Ryan LeBlanc Natural Energy Works > Nick, > > Advocating for an economic comparison between the cost of wire and the > energy saved by larger wire is not the same as advocating for high voltage > drops, or low ones either. Even with the present low prices for PV modules > and high prices for copper wire, a 100-ft long 350-volt dc input to a 3-kW > inverter should have around 1% voltage drop. Now consider a 350-volt > 10-amp PV circuit that's 500 feet long. Using 12 AWG copper the dc voltage > drop would be 5.5%. Sounds like that might be a poor wire choice, right? > Look what happens as the wire size is increased: > > Conductor Power $ per > AWG $/ft Cost ---- Loss ---- watt saved > 12 0.62 $620 193W (5.5%) -- > 10 0.95 $950 123W (3.5%) $4.71 > 8 1.54 $1540 77W (2.2%) $12.83 > 6 2.37 $2370 49W (1.4%) $29.64 > 4 3.73 $3730 32W (0.9%) $80.00 > > It would be reasonable to use 10 AWG copper, but before going up to 8 AWG, > I'd consider buying more PV instead. Why buy a watt of power at $12.83 when > it cost less to buy a watt of PV? The conductor price used here, just for > illustration, is from Southwire's price list for THHN/THWN wire dated 7 > April 2010. In the column of conductor costs I only considered the cost of > two current carrying wires. The cost of the equipment ground wire, conduit, > connectors, etc all go up too. That makes the dollars per watt saved look > even worse. > > Kent Osterberg > Blue Mountain Solar, Inc. > > > Nick Soleil wrote: > > I feel that it is best to maintain a 1.5% voltage drop on the AC and DC. > However, I was just sizing conductors for a 400 KW project, with the array > 1000' from the main service panel. With AC modules, I would have needed > 5-Parallel runs of 700MCM at 208VAC (20 wires at 700MCM for 1.5%VD!) The > cost would have been over 100K, which was cost prohibitive. However, by > running DC wiring, and utilzing AL, we were able to maintain 1.5 VDC drop > without being too expensive (yet still expensive.) > > Nick Soleil > Project Manager > Advanced Alternative Energy Solutions, LLC > PO Box 657 > Petaluma, CA 94953 > Cell: 707-321-2937 > Office: 707-789-9537 > Fax: 707-769-9037 > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20100409/204700dc/attachment-0001.htm> ------------------------------ Message: 2 Date: Fri, 9 Apr 2010 11:46:28 -0600 From: R Ray Walters <r...@solarray.com> To: RE-wrenches <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] DC wire sizing Message-ID: <54b514a9-a671-489b-b2e0-d86684a7b...@solarray.com> Content-Type: text/plain; charset="us-ascii" All our economic analysis is based on a 20 to 25 year life. Safety first, but then good design is to spend the customer's money where it does the most good. No matter how big the wire, you will always have losses. It is an exponential curve that never reaches zero. It just costs more and more for each extra watt saved. Nobody would install a 4/0 cable for a 50 watt panel, even though the losses would be lower. As soon as the design wire starts getting big, I start looking at alternatives: raise the array voltage, relocate the array closer, etc. Also, this discussion concerns the DC losses, we've all agreed that AC losses can cause shutdowns that greatly exceed the wire loss itself. R. Walters r...@solarray.com Solar Engineer On Apr 9, 2010, at 11:31 AM, Bill Hoffer wrote: > Ray > > I think that the point is not the cost, but what is good electrical design! > > Voltage Drop in a wire is still undesirable and equates to an unneeded "heat" load on the wire. Are we advocating that if your water pipe is too small just increase the pressure so you get the same output you desired. Sure as long as you are well within the operational limits of what the pipe was designed to do (over the lifetime expectancy of the product). Maybe for a short period of time that is fine, but we are talking about a system that we want to perform for 25+ years. I am sure we have all seen 25+ year old wiring in a house that has become brittle due to operating right at the limit, not enough to pop the breaker, but enough over a long period of time to deteriorate the copper. Heat is not our friend. > > The worse use of solar electricity is for heating, and I really do not want to install extra PV just to heat my wire. Nobody said that good electrical design was cheap! I will continue to design my voltage drop at 1.5% and as always attempt to meet that goal as inexpensively as possible. NEC is a minimum we need to meet and is not necessarily the best electrical design practice! When we are talking about Mega-watt commercial installations, It would be pretty silly to have a system shut down (ie lost production = lost investment money) because of saving some money on wire. > > Bill Hoffer PE > Sunergy Engineering Services PLLC > > On Thu, Apr 8, 2010 at 11:41 PM, R Ray Walters <r...@solarray.com> wrote: > Over the same amount of time a similar investment in PV would save even more money. > > R. Walters > r...@solarray.com > Solar Engineer > > > > > On Apr 8, 2010, at 4:28 PM, Bob-O Schultze wrote: > >> Guys, >> Is it just me being dense or are none of you folks advocating for higher VD looking at the savings over time? >> If we assume that Kent's wire costs are correct (and even assuming a 33% mark-up, he's paying WAY, WAY too much for wire) , the difference in delivered watts between #10 and # 4 wire in this situation is 91W. If I were installing this in Southern Oregon, which is pretty average as far as peak sun hours/day go, we'd be looking at 91 x 4.5 (peak sun hours) x 365days/yr x 25yrs = 3736 KW/H. Even at $0.10/KWH that's about $375 AT TODAY'S POWER RATES. Anyone think those rates are going to stay the same or go down over the next 25 years? Anybody think they won't go up by 5X? 10X? 20X? >> So... for what and for whom are we designing these systems? >> Bob-O >> >> On Apr 8, 2010, at 12:40 PM, Kent Osterberg wrote: >> >> Nick, >> >> Advocating for an economic comparison between the cost of wire and the energy saved by larger wire is not the same as advocating for high voltage drops, or low ones either. Even with the present low prices for PV modules and high prices for copper wire, a 100-ft long 350-volt dc input to a 3-kW inverter should have around 1% voltage drop. Now consider a 350-volt 10-amp PV circuit that's 500 feet long. Using 12 AWG copper the dc voltage drop would be 5.5%. Sounds like that might be a poor wire choice, right? Look what happens as the wire size is increased: >> >> Conductor Power $ per >> AWG $/ft Cost ---- Loss ---- watt saved >> 12 0.62 $620 193W (5.5%) -- >> 10 0.95 $950 123W (3.5%) $4.71 >> 8 1.54 $1540 77W (2.2%) $12.83 >> 6 2.37 $2370 49W (1.4%) $29.64 >> 4 3.73 $3730 32W (0.9%) $80.00 >> >> It would be reasonable to use 10 AWG copper, but before going up to 8 AWG, I'd consider buying more PV instead. Why buy a watt of power at $12.83 when it cost less to buy a watt of PV? The conductor price used here, just for illustration, is from Southwire's price list for THHN/THWN wire dated 7 April 2010. In the column of conductor costs I only considered the cost of two current carrying wires. The cost of the equipment ground wire, conduit, connectors, etc all go up too. That makes the dollars per watt saved look even worse. >> >> Kent Osterberg >> Blue Mountain Solar, Inc. >> >> >> Nick Soleil wrote: >>> >>> I feel that it is best to maintain a 1.5% voltage drop on the AC and DC. However, I was just sizing conductors for a 400 KW project, with the array 1000' from the main service panel. With AC modules, I would have needed 5-Parallel runs of 700MCM at 208VAC (20 wires at 700MCM for 1.5%VD!) The cost would have been over 100K, which was cost prohibitive. However, by running DC wiring, and utilzing AL, we were able to maintain 1.5 VDC drop without being too expensive (yet still expensive.) >>> >>> Nick Soleil >>> Project Manager >>> Advanced Alternative Energy Solutions, LLC >>> PO Box 657 >>> Petaluma, CA 94953 >>> Cell: 707-321-2937 >>> Office: 707-789-9537 >>> Fax: 707-769-9037 >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Options & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > -------------- next part -------------- An HTML attachment was scrubbed... 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