All of us agree our goal is to increase total production and reduce cost of the system over its life. Basically : total lifetime KWH production / total system cost. If I can increase the production for the dollar spent by using bigger wire, I'll do that, and if it makes more power for the money to add a module, I do that. Basically, I think the volt drop issue comes down to the following: Inverter to grid not more than 1.5% drop. Not more than 5% total losses ( code suggested, but not required) So that leaves up to 3.5% losses from the array to inverter. anything under 1% begins to get really expensive, so I'm playing in about +/- 1 either side of 2% for best economics. Its all about designing the most system for the money: I do this same cost benefit math for framing and layout, tracking, module choice, MPPT, inverter choice, wire choice, etc. Surprised everyone's getting so concerned over a few watts on the line, when module, inverter, and layout choices will have a much larger effect on long term production. (ie. I spend the money saved on wire to buy better monocrystalline) Ray
Friends don't let friends buy amorphous...... On Apr 7, 2010, at 5:09 PM, Joel Davidson wrote: > Hello Ray, > I agree with Bob-O. Smaller wires between the inverter and the grid may meet > ampacity requirements and your I2R design goal, but high grid-tie voltage has > been a problem. Also, 1% of lost power over the life of a PV system adds up > especially when grid power continues to go up in price. I agree your > reasoning, but I also agree with Feed-In Tariff PV system operators who want > every kWh possible and are willing to pay the up-front cost. I'm sticking > with less than less than 1% wire loss between the inverter and the grid and > less than 2% overall wire loss unless the customer explicitly states that he > wants more loss. > Joel Davidson > > ----- Original Message ----- From: "Bob-O Schultze" > <[email protected]> > To: "RE-wrenches" <[email protected]> > Sent: Wednesday, April 07, 2010 1:50 PM > Subject: Re: [RE-wrenches] DC wire sizing > > >> Ray, >> Sorry, I'm not buying that argument over a 25+ year design lifespan. Also, >> -and perhaps something many folks don't consider- the NEC requires a MAX >> loss of ≤5% over the ENTIRE circuit. That means all the way to the mains >> panel. Best practices would require no more than 1.5% VD between the >> inverter and the mains so as to avoid any potential of overvoltage >> disconnection by the inverter. As it tries to push the current into the >> grid, it also pushes up the voltage on that line and on it's own sensors. >> Obviously, the larger the current flow and the higher the VD, the worse the >> situation could be. Bear in mind that the grid is not held to the same over >> and under voltage specifications as your inverter is. 3% + 1.5% is pushing >> it just a bit too close for comfort and I still think wasting watts in wire >> losses is bad design. We agree that orienting PVs to an azimuth other than >> 180° and a bit less than latitude elevation is less than ideal, but you "run >> what you brung" in terms of the orientation and roof pitch of a structure. >> Better to take a hit on production if we're talking about the backasswards >> method of incentivizing by the installed watt, than not installing PV at >> all. That said, we DO have control over wire sizing. IMO, throwing away >> watts forever just to cheapen it up a very little bit upfront is poor >> economy and as the price of gird supplied power increases over the years, >> the waste and lost revenue is even more acerbated. >> Best, Bob-O >> >> On Apr 6, 2010, at 6:04 PM, R Ray Walters wrote: >> >> Just run the numbers sometime. Compare the cost difference of #6 vs. #4 wire >> say, and then look at how many more watts you're actually saving, then >> multiply that additional wattage by the installed cost per watt. >> Very simply, once you've satisfied code requirements, there is a point at >> which it is cheaper to add more panels than use bigger wire. >> Also, that point is a moving target that fluctuates with PV and wire costs. >> I've found recently for our projects, that that point is falling at about 3% >> loss. >> I also include a cost factor for oversizing the conduit, extra labor (bigger >> wire is harder to handle), and any connectors needed to land the larger wire. >> I've got very well designed systems working for decades, using this method. >> This is how large commercial systems are designed as well. >> You can't simply pull 1%, and then call us bad designers because we actually >> do an economic analysis for each wire run. >> It used to be unheard of to install PV facing anything but due south at >> latitude tilt, but now we know to add a few more modules. Same concept. >> >> R. Walters >> [email protected] >> Solar Engineer >> >> >> >> >> On Apr 6, 2010, at 1:37 PM, Bob-O Schultze wrote: >> >>> Ray, >>> A 2% wire loss is the generally accepted metric for battery based systems >>> with relatively low PV voltage input (<150Voc). It's just plain bad design >>> to accept more than a 1% VD on higher voltage systems. PVs ain't THAT cheap. >>> Best, Bob-O >>> >>> On Apr 6, 2010, at 11:44 AM, R Ray Walters wrote: >>> >>> >>> Once I have fulfilled NEC min. requirements, I use a spreadsheet to analyze >>> the cost of larger wire vs. the cost of power lost. Going under 2% is >>> usually not worth it, if copper prices are high, and PV cost is low enough >>> (current market). Sizing for under 2% was good economics a few years back, >>> when PV was high, and copper was low, though. >>> >>> For NEC 2011, I agree: while I readily welcome development of DC AFI, >>> implementing code before the technology is ready, is a bad idea. But that >>> may be the only way to get the technology in place..... >>> >>> Ray >>> >>> On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote: >>> >>>> Ray, >>>> >>>> Considering that we design PV wiring to be efficient with voltage (and >>>> power) loss typically less than 2%, the wire size is nearly irrelevant to >>>> arcing issues. Essentially all the energy available from the PV array >>>> can be dissipated in the dc arc. And since the current is limited by the >>>> nature of the IV curve, breakers alone usually won't clear the fault. The >>>> best combiner breakers can do (if you have enough parallel circuits) is >>>> isolate the fault to one string in the PV array. With one string being 1 >>>> or 2 kW in many systems there is still the potential for a lot of heat. >>>> >>>> With the 2011 code just around the corner and no dc arc fault protection >>>> on the horizon, it looks like our industry is again going to have a code >>>> requirement that no one can fulfill. >>>> >>>> Kent Osterberg >>>> Blue Mountain Solar, Inc. >>> >>> _______________________________________________ >>> List sponsored by Home Power magazine >>> >>> List Address: [email protected] >>> >>> 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: [email protected] >> >> 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: [email protected] >> >> 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: [email protected] > > 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: [email protected] 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
