How to size wires and fuses for a solar electric system

welcome to the solar eclipse video series covering the basics of solar photovoltaics or solar pv my name is drew chavallen and i'm an extension specialist with the university of maryland in previous videos we explored various ways to measure the output from a solar module and we estimated the size of a solar array based on electrical load requirements and solar site assessments now in today's video we'll learn how to size the conductors of the wiring of a solar electric system as well as how to size the fuses that protect the system wiring and other components of the system we'll start by considering this basic string inverter system the national electric code or nec provides a set of guidelines to help minimize the hazards associated with electrical installations solar electric systems are specifically detailed in section 690. with that said each part of this solar electric system will require different types sizes and colors of wire depending on the design parameters of the system with that said wire and fuse sizing is ultimately based on the electrical current that's present or possible within each part of the system we use stranded wire for most solar pv applications with the actual number of strands making up the wire typically seven or more depending on how flexible the wire is whether it's a solid conductor or a stranded conductor wire sizes are usually expressed in american wire gauge awg or aug with solar pv systems typically ranging from 18 aug to 4 aug the awg or aug rating will typically be printed on the wire itself with all gradings the higher the number the smaller the wire a number 18 wires much smaller than a number one wire you may also note that the smaller conductors have more resistance for the same given length of wire we'll touch more on this later but for that reason bigger wires are needed when working with higher electrical currents we'll also take note of the different letter codes that describe the type of insulation that surrounds the conductor these letter codes may be found on the wires themselves or they will be provided in tables that you reference when you're designing an electrical system these codes only specify the insulating material that surrounds the conductor while the conductor itself is the same most cases it's a copper conductor on the inside but these letter designations in general describe the conditions under which the insulated material protects the conductor that it's surrounding for example all of these different types of wire can tolerate different temperatures the insulated material on the 60 degree rated wire is somewhat softer and has a lower melting point for this reason the 60 degree rated wire can only handle a temperature of 60 degrees celsius before it starts to melt the insulated material on the 90 degree rated wire on the other hand is somewhat harder and has a higher melting point this means that you can put more current through the 90 degree rated wire before it melts or potentially starts a fire we can also see that the higher temperature ratings have higher current ratings for example the number 10 wire a uf type wire could handle 30 amps of current while the same size usc 2 wire could handle 40 amps of current now as i mentioned earlier conductors in different parts of your solar electric system will have different requirements based on their particular application rhw2 wiring for example as referenced in this chart can be used in moist environments at elevated temperatures exceeding 90 degrees celsius and requires a conduit this nmb wire on the other hand is intended for interior use with no need for a conduit we may also note that the 12-3 designation on the nmb wire specifies that there are three 12-gauge conductors within the insulated wrap like much of the romex wiring that's used in the walls of your home now again different parts of your solar electric system will require different wiring let's say out of the array we'd want to use moisture resistant wire that's also uv stabilized that means it can withstand exposure to the sun i would also need to be rated for high temperatures and would likely be a single conductor that has an exposed installation now the wiring used in the output circuit would be pulled through conduit but would still need to be moisturized and designated for use under high temperatures we also need to be aware of conductor color codes although we won't explore the details of color coding in this video it's generally important to remember that equipment ground is always green or uninsulated these grounding conductors are for the safety ground all other colors should indicate that the current is flowing through the wire white for instance is used for neutral when you have a circuit consisting of more than one hot wire in this case the white wire is a grounded conductor because it is electrically connected to the grounding conductor somewhere in the system it's important to remember this distinction between the grounded conductor and the grounding conductor since improper handling of the two could lead to serious injury or death now we also need over current protection devices or ocpd's including fuses or circuit breakers fuses and circuit breakers are both designed to interrupt the flow of electricity but they operate through different mechanisms the metallic link in a fuse melts down when it's overheated while a circuit breaker operates a switching mechanism when it detects an overflow of electricity now you'll likely find some single and double pole circuit breakers in your home's electrical panel a single would be used for connecting something like a lighting circuit while a double would be used for larger appliances like air conditioners that use more electricity with double circuit breakers using both phases a single switch is used to operate both of the breakers at the same time you may also see three-phase power and commercial operations in order to run large pumps or other equipment in any sense we might use back fed circuit breakers to feed power from our solar electric system back onto the main service panel using a 2-pole breaker for residential or 3-pole breaker for 3-phase commercial now that we've explored the basics of wires and fuses we'll consider how to size these components for a solar electric system as an example we'll consider this 100 watt solar panel which has a short-circuit current isc of 6.1 amps at standard test conditions but this rated capacity of 6.1 amps could be exceeded if our actual weather conditions deviated from the standard test conditions increased solar irradiance as well as reflections from any snow water glass or clouds could cause additional light to reach the solar panel so to account for any increased electrical electrical current the industry rules to increase the short circuit current by 25 percent so in this example the maximum current that could come out of the panel we'll call this the imax would be equal to the short circuit current of 6.1 amps times 1.25 so in that case we would design the rest of the electrical system based on the maximum voltage of 7.625 amps but now let's consider another example let's suppose we were to combine three strings of solar panels in this combiner box assuming each string has a short circuit current of 6.1 amps the maximum current coming out of the combiner box would be 6.1 amps times 3 to account for each string times 1.25 to account for any potential increase in the electrical current in this case we would use a maximum current of 22.875 amps to size our wire now we'll consider the maximum current that could come out of an inverter we could determine the maximum current by referencing the spec sheet of the inverter itself which for this particular inverter is specified as 25 amps or we could determine the maximum voltage by dividing the rated inverter size by the voltage that we're wiring it to in this example we'll divide the rated sides of the inverter 6000 watts by 240 volts for a standard residential install in this case we verified that the maximum voltage is 25 amps and now we can size our wire based on this number since the inverter is not physically capable of putting out more than 25 amps now let's consider the input currents for a 5500 watt battery based inverter in this case the maximum input current for the inverter increases as the battery bank starts to become depleted this is because the inverter will track right along with the voltage of the battery bank as it drops the current will go up in order to maintain the same power output of 5500 watts the inverter will then shut off once it reaches a minimum voltage which for this particular inverter is 100 volts according to its spec sheet now to determine the maximum current on the dc side of the inverter we'll divide the rated sides of the inverter 5500 watts by the minimum voltage of 100 volts but we'll also need to divide by the inverter's efficiency of 94 percent since some of the power will be lost going through the inverter in this case we'll size our wiring and install over current protection either a fuse or circuit breaker in order to handle the maximum current that we get of 58.51 amps now the amount of current that you can put through a wire before it burns up is called ampacity ampacity is influenced by the conducting material either copper or aluminum also its temperature in the cross-sectional area of the conductor and the length of the wire the electrical resistance of current traveling through a really long wire for instance could cause it to overheat with the voltage level dropping and being somewhat reduced by the time that it reaches the other end so we'll use a voltage drop calculation to ensure that the voltage level at the end of a really long wire run is still acceptable we'll start by multiplying the operating current i op by the resistance of the conductor r and twice the length of the conductor l the operating current or i op is simply the amount of current that we intend to push through the wire we'll assume the operating current is 15 amps for this example the resistance of the wire is based on the conductor that you select in this example we'll select a number 10 aug stranded copper conductor which has a resistance of 0.00124 ohms per foot we'll use 100 feet for the link between the array and the disconnect switch a distance that must be doubled to account for the round-trip distance that electricity travels inside of an electrical circuit then we'll divide the operating voltage or v-op which will assume to be 300 volts in this case we have a voltage drop of 0.0124 or 1.24 percent which is pretty reasonable considering that we typically want less than a two to three percent voltage drop if however we had determined this to be an unacceptable voltage drop then we could shorten the distance to the disconnect switch or we could simply use the next biggest wire size so in that case we would use a number eight wire which is bigger than the number 10 wire that we originally chose so again anytime you push current through a wire the wire will heat up to varying degrees based on how much current you're actually pushing through it as well as the size of the wire you probably won't notice if it's just a little current being pushed through a really thick wire but pushing a great amount of current through a small wire could heat it up to the point where it burns out or it starts a fire now we also need to account for something called continuous duty which describes those systems which operate for more than three hours at a time according to the national electric code section 690.9b if the national electric code specifically states that an overcurrent protection device must have an opacity of not less than 125 percent of the continuous load so this safety factor is prescribed on the basis that a device will have reached some equilibrium after three hours of operation whereby the wiring is heated up as much as it's going to and this guideline applies to solar electric systems which are essentially continuous duty as long as the sun is up so we'll use another 1.25 multiplier in this case as a safety factor in sizing our conductors by doing so we'll we're going to intentionally upsize the wire to make sure that we don't start a fire now to determine the size of the fuse to put in a combiner box we could reference the spec sheets for our solar panels or we could calculate the size of the fuse by multiplying their short circuit current isc by 1.25 to account for any additional solar irradiance that could increase their electrical current as well as another safety factor of 1.25 to account for continuous duty so for this example we'll assume the short-circuit current of each string is 8.34 amps multiplying by 1.25 for increased solar irradiance and another 1.25 for the continuous duty safety factor gives us 13.03 amps in this case we'll have to round up to a 15 amp fuse and make sure that all wiring can handle 15 amps this won't be a problem with number 10 wire which can handle 40 amps i hope this video has provided you with an understanding of how to size your wiring and the fuses within your solar electric system you can subscribe to this channel to stay connected on upcoming episodes of this solar eclipse video series but in the meantime please visit our website for more information on solar photovoltaics and other energy related topics

Channel: EnergyUME

Views: 29,212

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Id: UagM0KDU-aQ

Length: 13min 1sec (781 seconds)

Published: Wed Dec 08 2021