Low Voltage Wiring Techniques by Richard Perez In many AE systems it is efficient and inexpensive to use the low voltage DC electricity directly from the batteries. Here is all the info you need to get this energy down the line, to the job, with a minimum of loss. Resistance- The BIG Problem Resistance is the impedance to electron flow within any material. All electrical wiring, connections, plugs, and switches have some electrical resistance. This resistance causes losses within the entire low voltage circuit. The idea with low voltage wiring is to minimize this resistance, and thereby the associated losses. The reasons for this are: 1) we don't want to waste power, and 2) 12 VDC from the batteries is already low enough in voltage, we can't afford to lose any more than necessary transferring this energy from the batteries to the load. Low voltage at a load causes substandard performance. It means slow motors, dim lights, and generally poor appliance operation. The Entire Circuit Every electrical appliance in a system must have a complete circuit to the batteries. Consider the lightbulb on the ceiling. The electrons that power this lightbulb follow a very specific path to accomplish their purpose. Every electron originates at the battery's negative pole. From this pole it makes a journey through the wiring, connections, and switch(es) to the lightbulb. After any given electron passes through the lightbulb it makes its way through the wiring, connections, and switch(es) back to the positive pole of the battery. This path is set. Every electron must make this entire journey in order to do work. Every electron must pass through each circuit element (piece of wire, connection, plug and/or switch) in order to complete the circuit. In technical terms, what we have here is a series circuit. A series circuit means that there is only one path available to the electrons. A series circuit is like a chain: it is limited by its weakest element. The total resistance of a series circuit is the sum of all the resistances within that circuit. Each individual element within the circuit introduces losses based on its resistance. The primary lesson to be learned here is that ANY (and it only takes one) high resistance element within the circuit will make the ENTIRE circuit's resistance high enough to be unacceptable. Every element within the circuit must have low resistance for the entire circuit to have low resistance. It only takes one piece of undersized wire, one funky connection, or one wornout switch to make the loss of the entire circuit unacceptable. So, in low voltage circuits we must consider every element in the circuit. It is not good enough to use properly sized wire if it is connected improperly, or if the wire is connected to a switch (or any other single circuit element) with high resistance. Attention to the details of the circuit is essential. Let's look at the individual elements that make up the circuit. Wiring The size of the wire (or gauge) feeding the load is critical. Wire size is specified in any application by considering two factors: 1) the amount of current that the wire transmits, and 2) the total wire length (both conductors) from the battery to the load. Ohm's Law (see Home Power #1 if this is a new idea for you) gives us the relationship between voltage, current, and resistance in an electrical circuit. INSERT OHMS LAW EQUATION Wiring makes up many of the elements in a circuit. Larger sizes of wire have more copper in them, and hence lower resistance. Wire size is specified by a gauge number. The lower the gauge number, the larger the diameter of the copper wire, and thereby the lower its resistance. The actual resistance per 1,000 feet of various copper wire gauges is detailed in Table 1, the Copper Wire Table. We encourage you to use only copper wire in your AE system. Aluminum wire has greater resistance (about twice for the same cross sectional area) and is virtually impossible to interconnect without higher resistance connections. If you don't think so, then try soldering an aluminum wire sometime. INSERT COPPER WIRE TABLE From the Copper Wire Table, we can calculate the resistance of any particular piece of wire. The resistance per foot times the number of feet gives us the total resistance of a length of wire. When estimating the resistance of wiring be sure to include BOTH conductors, i.e. if an appliance is 100 feet from the battery, then the total wiring length is 200 feet (there are two wires actually, each one 100 feet long). If we know the amount of current being consumed, the resistance per foot of any given wire gauge, and the length of the total wire in the circuit, then how do we determine the actual gauge of wire we should use? The answer is determined by exactly how much loss we find acceptable. In general, consider a 5% loss to be the maximum acceptable (2.5% is better). If we are using 12 VDC, then 5% voltage loss is 0.6 volts (2.5% is 0.3 volts). Consider the following equation to specify exactly which wire gauge to use for any given application. INSERT WIRE EQUATION R = Resistance expressed in Ohms (½) per 1000 feet. E = Maximum allowable voltage loss in the wiring, expressed in Volts. I = Amount of current flowing through the circuit, expressed in Amperes. L = The length of wire in the complete circuit, expressed in feet. This equation gives us a value in Ohms per 1,000 feet. Simply find the copper wire gauge size that has LESS than this amount of resistance per 1,000 feet, and you've found your wire gauge size. Consider a PV array that produces 12 amperes. This array is located 100 feet from the batteries. What gauge size of wire should be used to keep the voltage loss in the wiring to less than 0.6 volts? Well, there is 200 feet (two conductors, remember) of wire in the circuit, and a current of 12 amperes flowing. The equation above gives us a maximum resistance of the wire as 0.25½ per 1,000 feet. By consulting the Copper Wire Table, we find that 4 gauge wire has a resistance of 0.2485½ per 1,000 feet. Since this is less than the 0.25½/1,000 ft. the equation generated, 4 gauge wire is the size to use. Get on the Bus In reality houses and systems contain many circuits. Some of these circuits are straight series types as mentioned above. Others are parallel circuits, where two or more loads are supplied electricity by the same piece of wire. The mathematical analysis of all these circuits can become very complex. A way around this complexity is to use a standard wiring technique that is very effective in low voltage systems--The Bus. A bus is a heavy set of wires used to carry current to other smaller wires which eventually feed the loads. The battery's energy can be distributed by two heavy wires (usually 2 or 4 gauge) that run the entire length of a building. Smaller 8 or 12 gauge wires are soldered to this bus to supply the individual loads. This structure is similar to the skeleton of a fish, a heavy spine with smaller bones attached to it. This technique allows low voltage energy to be distributed with a minimum loss. Ideally, each load should have its own individual feeder wires soldered to the bus. All feeder wiring lengths should be as short as possible. This technique also allows the use of standard wiring components like switches, plugs and sockets, which will not accept the huge diameter of 2 or 4 gauge wire. Solder Connections Wherever Possible In standard 120 VAC house wiring, it is very unusual to solder connections. In low voltage systems, soldered connections should be made wherever possible. All wire to wire connections should definitely be soldered. Mechanical connections using wire nuts are OK for higher voltage systems, but these connections have too much loss for low voltage systems. Soldering assures a permanent, low resistance connection. Mechanical connections gradually oxidize over a period of time. While copper is a very good conductor of electricity, copper oxide is not. Gradual oxidation in mechanical connections increases their resistance. Remember, a single high resistance connection within the circuit will make the resistance of the entire circuit high. So get into solder. Once you've made a good solder joint, it's good forever. Switches, Sockets & Plugs The switches, sockets and plugs in a low voltage systems must have low loss (i.e. low resistance) just like every other component in the system. We can assure low loss in these components by two techniques. The first is to purchase specialized low voltage switches, sockets and plugs. These components have more massive contacts, with higher contact pressures, to deliver low resistance. These components are expensive and hard to find. Another technique is to use standard 120 VAC components and to derate them. Derating means that we run only a portion of the rated current through the component. Derate 120 VAC switches, sockets and plugs by at least a factor of three. Consider a plug or a switch that is rated to handle 15 amperes of current at 120 VAC. If we run 5 amperes or less (15/3) through the component, then its losses will be acceptable. Derating allows use of the more commonly available, higher resistance, components by reducing the current we run through them. In any case, keep the use of switches, sockets, and plugs to a minimum in a low voltage system. If an appliance can be soldered to its power wiring, then this should be done. If you are using standard 120 VAC sockets and plugs in low voltage systems, be sure to use the 3 conductor types. The three-prong type of sockets and plugs are polarized. They will only connect in one fashion. If they are wired with proper polarity to start with, it is impossible to plug in a polarized low voltage appliance backwards. This can save electronics, fluorescent lights and other DC appliances from being connected backwards and destroyed. The third conductor on these plugs and sockets can also be used to carry current. Simply wire this third connector (normally used for the ground in AC systems) in parallel with either of the power wires. This even further reduces the overall resistance of the plug and socket combination. Low voltage wiring is not difficult. It only requires that you cozy up to Ohm's Law. If you can work with the concepts of resistance, voltage and current, then you can apply these concepts in your system. Low voltage wiring requires attention to detail. Consider every element in the circuit. If you keep the individual losses within components to a minimum, then the overall system will take care of itself.