Jerry,
2.5% loss is fine, especially considering the cost of reducing that further. Remember that you are calculating the line loss at the peak power output of the array, at noon on a sunny day. Since the array will only provide peak output for a rather small part of the day (unless you have a tracking mount), your losses will be much less most of the time. At 50% of peak sunlight, you will only have 50% of the voltage drop. Efficiency on a less than fully sunny day will be very high, as will mornings and afternoons on a sunny day. Voltage drop losses in PV source circuits are not nearly the problem that t used to be when PV was very expensive and before we had MPPT charge controllers. Back in the day when we had 36-cell modules, or even 33 or 35-cell modules, charging 12V batteries, and we only had relay or PWM controllers, it was very important to reduce voltage drop as much as we could. First, PV power was very expensive, so it was cost effective to spend more money on wire. Secondly, because the array voltage was only slightly higher than the battery voltage, too much voltage drop in the wiring could drop the charging voltage below the voltage that the battery needed to get fully charged, especially on hot days. Now, with PV “cheap” and MPPT controllers allowing array voltages way above the battery voltage, these are not big problems anymore. Certainly, any lost power is indeed lost power, but it doesn’t affect the actual functioning of the system. I think that if you were to calculate the “cost” of the lost power due to using a smaller gauge wire, it would be below the cost of the larger wire. On a 3 kW array (60A at 50 VDC) a 1.5% loss (the difference between 2.5% and 1% losses) is only 45W. That’s only about $40 worth of PV, and less than a quarter of a kWh/day average in a 5 peak sun-hour location. Your PV sizing for an off-grid system shouldn’t be so tight that this would make any real difference I n system performance. Brian Teitelbaum AEE Solar *From:* RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Jerry Shafer *Sent:* Wednesday, July 22, 2015 10:38 AM *To:* RE-wrenches *Subject:* [RE-wrenches] DC conductor line loss numbers Wrenches Some time back there was discussion on the conductor size and efficiency rating requirement for long DC runs. What I am looking at is this, 400 feet of MCM 400 to keep the line loss at or below 1% per NEC code for an off grid application, cost vs return is not acceptable. 2/0 is less than 2.5% and the cost is far less. Specs are 4 strings of 3, 250 watt modules feeding one Outback FM 80 charge controller. There are lots of things I can do like SMA instread, or 200 VDC charge controller but nothing can be changed except the wire gauge. Does anyone recall a thread with this topic. thoughts ?? Jerry
_______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
