Wiring up your home’s electricity is no easy task, especially if you’re installing solar panels. In this chapter we’ll look at the different kinds of wires and what they are used for – like determining an accurate amount to size them with. We also discuss factors that contribute towards wire diameter as well as how exactly one calculates these measurements without any math experience whatsoever (you don’t need any!).
Wire core material
There are many types of wiring you can use. It is recommended to use stranded wire, which consists of multiple wires in one. This has the advantage of being flexible, while solid cables are tough to work with. The stranded core is better for DC applications than solid-core wire because it does not get as hot when current flows through them.
Whether you decide to buy your wires at the store or online, there are three options for purchase:
- Copper-clad aluminum
- Copper wire
- Aluminum wire
The best material for an electrical wire is copper, but it’s also the most expensive. To get around this problem and make your home wiring more affordable while still delivering high performance all you need to do is increase its diameter so that electricity can easily flow through them.
As you may have purchased your copper wires from an online retailer, it is possible that they are actually aluminum. If this happens to be the case then there’s no need for concern as long as we know what kind of wire our installation requires. Copper-clad or not – always make sure when purchasing new cables (or upgrading) only select one with bare solid conductors throughout so nothing else can get mixed in along side them during shipping process which could cause shorting out due intense temperature changes between different parts moisture exposure.
The gauge of a wire is determined by its diameter and the current that it can carry. We will use American Wire Gauge (AWG) throughout this book to describe different types with their varying ampacities, which depend on size selection.
The range of wire sizes we’ll be using for our solar panels covers two different categories: 18AWG up to MCM scale (which will not appear here), and 12 gauge all the way down through 2/0AWG thick cables.
To size your wire for the best possible result, you must consider these factors:
- Ambient temperature
- Current Capacity or Ampacity
- DC voltage drop
What do these terms mean? Let’s take a closer look at what they all entail.
The wire manufacturer makes it easy to find out how much amperage or current their product can handle.
The following table offers a reference from the National Electric Code. In this left-handed column, you see different sizes of wire ranging anywhere between 14 (the smallest) – 2000 (the biggest) amps with each size having its own corresponding wattage requirement for electrical installations.
The wire core material can either be copper or aluminum. It’s recommended that you use the former, as it has a better heat transfer rate than other options and will produce more stable electricity for your home appliances to work with.
The below table shows the different temperature ratings for wire. Higher numbers mean that it can handle higher temperatures, but remember they also cost more money! The three most common types of insulation are 60°C (140 °F), 75 º C(167º F), and 90°C(194º F) which is good if you want your home’s wiring to last longer without getting fried along with everything else in its path when storms hit.
The last is the most important data representation on a table. The ampacity rating tells you how much current can go through it and what kind of insulation must be used to prevent electrical interference with other devices in close proximity.
When selecting wire sizes for an electrical project, it is important to estimate the maximum current that will flow through each section of your installation. This way you can make sure they are strong enough and won’t melt or start a fire if too much electricity flows through them. To do this properly though we need some tools like ironing boards which have spacing on both sides so small traces of voltage don’t escape while being impressed by larger currents happening nearby.
A 1,000 watt inverter that feeds itself from a 12 volt battery has the same current of:
Current = 1000 Watts/12 Volts = 83.3 Amps
Using THWN-2 copper wire with 194°F (90°C) insulation rated, at an ambient temperature of 83°F (30°C) and use the previous table, we will need a 4AWG wire.
The ambient temperature has a big impact on how hot or cold your home will be. If it’s warmer, then you’ll get hotter wires that might start rusting faster than expected; however colder climates produce colder metals which can cause corrosion in the electrical system as well if not handled properly.
The current values in the previous table are based on an 86°F (30C) ambient temperature. This may not be true for your location or even if you’re measuring within battery compartment. The correction factor is necessary to account for the variations in temperature. There are two methods that can be used: equation or predetermined tables, depending on what’s more accurate in your jurisdiction needs.
Remember, the equation method consists of applying the following formula:
- I= Ampacity shown in the previous table.
- I’= Ampacity corrected for ambient temperature.
- Ta’= New ambient temperature (°C).
- Tc= Temperature rating of the conductor (°C).
- Ta= Ambient temperature used in the table (°C).
The I’ value is the new ampere rating that can be placed on a conductor in response to changes of ambient temperature.
The formula looks like something from another world, but it’s actually very easy to understand with this example.
When you need a perfect copper wire for your high- temperature applications, look no further than the THWN-2 #2AWG cable. This Especially made to order at 194°F (90 °C) with an ampacity of 130A per meter.
In this example, the battery temperature is 40 degrees Celsius or 104 Fahrenheit. With the values from NEC table, this would result in a heat index value of approximately 30°C.
The current carrying capacity of the wire will be reduced from 130A to 118A when it’s exposed for longer periods at higher temperatures. This can result in shorter lifespan and more failures over time due too unsafe working conditions caused by extreme weather changes that are not uncommon during summer months in some areas.
The second method is a little faster and easier, because it uses pre-established correction factors by temperature ranges. These can be found in the following table.
In this case, we’ll assume that the ambient temperature reduces to 59°F (15 °C) and your current demand estimation value is 140A which makes you select a THWN 1/0 AWG cable rated for 75 degrees Celsius.
The manufacturer rated this cable at 167°F (75 °C) and 59°F(15 °C), so we can expect a temperature correction factor of 1.15.
Apply the following expression:
Current = Current Load x Correction Factor
150 Amps x 1.15 = 172.5 Amps
The wire’s current carrying capacity increases with colder temperatures. A 1AWG cord can be used to carry 140 Amps.
130 Amps x 1.15 = 149.5 Amps
By using the worst case scenario, you will end up with a much higher number than expected. To get an accurate estimate of how long your wire would last under pressure cookers and other high usage conditions we need to use realistic numbers.
The hot summer months are not all created equal. The hottest time of year, when temperatures can exceed105-113°F (41 – 45C), has a correction factor that’s been carefully considered and researched by scientists who have studied this region for decades.
150 Amps x 0.82 = 123 Amps
It turns out that our wire can only carry 123 amps instead of 140 at the specified temperature. We need larger wires to account for this change, so we’ll have an increase in cost due to increased material costs and labor time spent on installation.
175 Amps x 0.82 = 143.5 Amps
The only way to ensure that your wiring is up for the task of carrying enough current, no matter what size wires we use and where in our house they’re located will be by using a higher temperature rated wire. For example, if you have an outlet with 2/0 AWG rated at 175 Amps then it’s going require 140amps maximum when worst case scenario occurs which means proper protection must also accompany this type of installation so make sure there are sufficient ampacity ratings on all components near any exposed insulation such as lights or power outlets.
DC Voltage Drop
When selecting wire gauge sizes, the other factor that must be considered is how much voltage will drop across it. If you have a long and narrow cable with low resistivity material used for its construction – say an aluminum or copper core covered by PVC insulation- then there’ll only tiny amounts of electricity flowing through each mile of this system which means less heat generated as well.
For example, if you have a solar panel that is wired with an 18 Volt output and has the same 5% voltage drop as your battery then 0.9 volts will be lost in transit from when it’s installed on top of one another until they’re both connected to charge controllers.
18 Volts x (5 / 100) = 0.9 Volts
The ideal voltage for our system is 18 volts. When the initial charge drops to 17 1/2, it’s important that we find a wire with bigger diameter so as not too reduce resistance and therefore improve performance.
voltage drops are the bane of any PV system because they lead to power losses, reduce voltage inputs for various devices and can make certain equipment operate inefficiently.
To avoid voltage drops, you must calculate the wattage of each wire gauge and verify if that amount is allowed or not.
To ensure a safe charge, the voltage drop between modules and controllers should not be lower than 3%. A 1% difference is preferred.
Voltage drop will be measured by applying this expression:
A = (ρ x 2 x l x U) / (v x Vsys)
- A= Transversal section of the cable (mm²)
- ρ= Specific Resistance [Ωmm²/m]
- 0.0171 Ωmm² for copper
- 0.026 Ωmm² for aluminum
- 2= Total travel length for both + and – wire
- l= Length of the cable [m]
- I= Nominal current through the cable [A] (Imp in this case)
- V= Permissible voltage drop in the cable [no unit] (1% is 0.01)
- Vsys= Open circuit voltage [V] (Vmpp of the string)
For example, two solar panels in series with a 82ft (25-meter) cable length to the charge controller can be used for this system.The string has an Impp of 5.8A and Vmpp at 35 volts (17,500 x 2).
The desired voltage drop in the PV system is 1%. Applying the expression:
A = (p x 2 x l x I) / (v x Vsys) = (0.0171 [Ω x (mm² / m)] x 2 x 25 m x 5.8 A) / (0.01 x 35V) = 14.16mm²
You can see that the result was calculated in mm2. This represents how much copper wire must be placed along with its thickness to limit voltage drop at 1%. The following table can be used to calculate mm2/AWG. The following table lists the wire size needed for this application. #4AWG is required, so make sure you have enough of it on hand.
The length of wiring in your solar system can have a significant effect on overall performance. When wires are too long, voltage drops due to resistance and other factors which impacts how much power flows through them.