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Specifying PV Wiring
Questions flood into HP Central via mail and phone. High on the hit parade is, "What size wire do I need to hook-up my PV array?" Well, here's the straight info on specifying wire gauge between the photovoltaics and the controller/battery. Electricity produced by photovoltaics is low voltage.
Low power loss in low voltage wiring is essential for effective performance.
A Short History
I feel that the subject of wiring and interconnection is so important and so misunderstood, that we have run many articles about this subject. Here's a list: HP2-pg 11, HP2-pg 33, HP3-pg 40, HP4-pg 33, HP6-pg 35, HP7- pg.36. Wiring articles after HP11 are listed under Basic Electric in the index in this issue, see page 49.
This article doesn't probe the depths of Ohm's Law and the Copper Wire Table. Here the work is done for you. You have only to look up your situation on the tables. You must, however, use the tables properly if you are to get real answers instead of bogus info.
Wiring on the PV Array
A photovoltaic array consists of several PV panels wired in parallel (12 Volt systems) or in series/parallel (24 Volt systems). The interconnects between the individual modules can be made with 10 gauge copper wire. Use stranded, copper wire with a "USE" or "UF" insulation because these outer coatings will resist sunlight degradation. Use of 10 gauge is practical because the current from an individual module is low (≈3 Amps.) and the distances are short (usually <2 feet). These wires connect to the better-quality PV panels with ring connectors. Solder these ring connectors to the wire! If you are using panels that don't take ring connectors, then tin the wire ends with solder before installation. See page 35 of this issue for soldering info.
Wiring between the PV Array and the Controller
This is really what we are interested in here. The distance between the array and the control is often many feet, and is different for each system. Since the PV controller is usually located very near the batteries, the distance here is not a problem. This is not the case with the array to controller circuit. Often we have to mount the PV array some distance from the battery in order to get the array into a better, more sunny, location. The illustration to right shows the PV array to Controller wiring circuit covered here.
On the next page, there are two wiring tables. Both
tables are for copper wire only. The upper table is for 12 Volt systems and the lower for 24 Volt systems. The upper horizontal row of each table contains the PV array current in Amperes. The left most, vertical column contains the round trip wire length in feet. Now, this is ROUND TRIP! If the array is 100 feet from the controller, then 200 feet of wire is required (two conductors).
The information on these tables was computed on the following criteria. The wiring efficiency must be 97.5% efficient. Efficiency was computed as power through the circuit minus the power lost to the voltage drop in the wiring . The wire is also specified by ampacity. Ampacity is the current handling capability of a conductor regardless of its length. Ampacity is directly proportional
Specing PV Wiring
to the cross-sectional area (diameter) of the wire.
Using the Tables
Use the upper table for 12 Volts and the lower table for 24 Volts. Locate your array's current on the upper row. Now locate your round trip wiring length in feet on the left most, vertical column. The correct wire gauge number is located at the intersection of the row and column you have chosen. The wire gauge number is American Wire Gauge (AWG). Please note that the computer designates "00" gauge as -1,"000" gauge as -2,"0000" gauge as -3.
If you want even higher wiring efficiency, then use the next larger wire gauge. If you live in an area that is hot (average daytime temperature >90°F. for at least six months yearly) then use the next larger wire size. Remember, as the wire diameter gets larger the AWG number gets smaller. Since you may want to add more PV panels to your array in the future, specify the wire for the biggest array you think you will ever use. Wire doesn't wear out, so if you buy wire that is large enough, you will never have to replace it.
I bought the BIG Wire, so I'm in, right?
Not quite. Even the largest wire can be rendered completely ineffective by poor electrical connections to the wire. A series electrical circuit is like a chain- only as strong as its weakest link. Each and every element in a series circuit must be low loss in order for the entire circuit to be low loss. It only takes one funky connection to shoot down the whole circuit. Solder all low voltage connections wherever possible. The article on page 35 of this issue gives you all the soldering info you need to make your investment in large wire permanent.
PV Charge Control
I have the original, working spreadsheets that generated these tables. These spreadsheets are in Excel 2.2 for the Macintosh computer. If you want a copy of the spreadsheets, send me a 3.5 inch diskette in a mailer with return postage included. Richard Perez, C/O Home Power Magazine, POB 130, Hornbrook, CA 96044.
Home Power #18 • August/September 1990
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