PV array wire size

Re: PV array wire size

Mark,

“High” array voltage has its pros and cons. Pro’s include “low” array current, which leads to reduced voltage drop in the array home run wiring. This is particularly beneficial for long wiring runs. The result is less power loss and/or the ability to specify smaller (cheaper) wire. Since array voltage drops as temperature increases, another pro is that high-ambient temperature array voltage can be set high enough to meet battery charging voltage requirements.

There are also several cons. One is that the controller efficiency drops as the input voltage increases, so a trade study might be required to balance this loss against high-voltage gain in the wiring.

For example, when operating at 500 W input to charge a 24 V nominal battery bank, the MX60’s efficiency drops by ~2-1/2% when the array’s operational voltage is increased from 40 V to 95 V. (ref: User Manual Rev. 7.0, 1/29/07, SW rev 5.10 and 5.11)

Another con is that all of the major consumer MPPT controllers (i.e., Apollo, OutBack, Xantrex) have operational/absolute input voltage limits in the 140 VDC to 150 VDC range. Circuit breakers used between the array and the controller have similar voltage limits. Accordingly, one has to be especially careful in configuring a high-voltage array – particularly in cold environments – for systems with 48 V nominal battery banks.

For example, the NEC temperature correction factor for array Voc in ambient temperatures between 13 F and -4 F is 117%.

We need to know more about your system before we can offer specific configuration suggestions. Please provide the PV modules' brand and model number, and also provide your intended nominal battery voltage. Ambient temperatures can also affect system configuration and performance, so it would also be handy to have an idea about where the system will be installed. Finally, also let us know if you plan to “grow” your system, and by how much.

Note that PV modules can operate at very high temperatures, so wire rated for 90 C conditions is warranted. Check the printed specs on the wire insulation. Elevated temperatures and/or running the wire in conduit will reduce wire ampacity, and we'll deal with that when we have the additional info requested. However, based on the info you've provided so far (i.e., 600 W array size, 30 foot home run), and assuming the wire is rated for 90C or better, your stock of 6 AWG wire will likely be more than adequate.

HTH,
Jim / crewzer

Re: PV array wire size

Mark,

You have two configuration options. One is to wire up the array “4 x 2”, with four modules per series string (“48 V” nominal), and two strings in parallel. This configuration would allow you to use either 12 V or 24 V batteries. The other option is a “2 X 4” array, but this one wouldn’t work for a 24 V battery bank due to low array voltage in the summer heat. I'd probably go the former and not worry about slightly reduced controller efficiency.

FWIW, my present system is a "48 V array" and a "24 V" battery bank.

You’ll need to combine the string wiring and install circuit breakers between the array and the controller. A Midnite Solar MNPV-6 would work for up to six circuits. Midnite also makes a three circuit model, but that might limit reconfiguration and/or expansion options.
See: http://www.midnitesolar.com/MNPV%20Instructions1.pdf

An appropriate 100% duty circuit breaker for each string would be rated at 4.8 A Isc x 125% = 6 A. The Midnite Solar MNEPV6 6A DC breaker would be perfect. See: http://www.midnitesolar.com/Price_sheet_Dec_20071.pdf

The space between the PV array and the metal roof is going to get pretty damn hot in the summer. 90C wire between the array and the combiner box is probably the minimum, and 105 C wire might be warranted. Tilting the south-facing array up at an angle of ~ local latitude plus 15 degrees may help with convection cooling the array as well as with winter energy harvesting.

An appropriate baseline 12 V battery bank size for your 600 W STC array would be ~600 Ah. You’ll need the optional remote battery temperature sensor for the MX60. How long this bank will sustain your loads will depend upon energy usage and local insolation. That’s a separate calculation, and we’d first need to know a lot more about the various loads’ individual energy needs.

A true sine wave inverter is a good idea. My gut feel is that 2000 W may be a bit large. A very good 1100W unit is the Exeltech XP1100. See: http://www.exeltech.com/xpspecs.htm and http://store.solar-electric.com/exxp12vol11w.html and http://www.exeltech.com/downloads/xp600-1100manual.pdf

The Midnite Solar E-Panel “Lite” might be an effective way to integrate the MX60 controller, the inverter, the PV array circuit breakers (omitting the MNPV), other “small” DC breakers, the big DC breaker between the batteries and the inverter, and a couple of AC breakers as well. See: http://www.midnitesolar.com/MidNite-Products.html

HTH,
Jim / crewzer

Re: PV array wire size

Good catch on the pump, Mike. Here's a link to an example of 24 V pumps available from NAWS, this site's sponsor: http://store.solar-electric.com/2088-474-144.html

HTH,
Jim / crewzer