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What Size of Wire to Run to a Shed or Barn?

Outbuildings, sheds, and barns are much more useful when they have available electrical power. The ability to use lights and tools can turn a shed or outbuilding into a workshop, garage, or conditioned storage space.

Successfully running electrical power to an outbuilding will require a few tools, some research, and experience. Working with electricity is safe, but only if you know what you are doing.

Source: pexels.com

An electrician you hire to run power to your building will follow a few simple steps, ensuring that your building will have the power it needs and that it can be used safely. Here we will discuss the process of deciding which electrical wire to run to a typical outbuilding or shed, and the steps the electrician will take to do it safely and effectively.

Why Would I Run Electricity to an Outbuilding?

Source: cdn.cranegardenbuildings.co.uk

A typical storage building is built either from an aluminum kit or constructed from lumber, just like a house. A common storage building will provide about a hundred square feet of space that will keep the contents dry. However, most do not have a power source for lights, heat, tools, or security.

Providing electrical power to an outbuilding or shed can add enormous functionality to the structure. For example, you can add not only interior lights, but exterior flood lights as well to illuminate the home. Contents can be better protected from freezing when an electrical heat source is available, especially if the outbuilding is insulated.

What Should I Consider When Running a Wire to a Shed?

When you are planning to run power to an outbuilding that has never had power, you are in effect creating a circuit. A circuit is a closed loop of wire whose electrical power can be disconnected with a single switch. For example, homes usually have lots of circuits dedicated to individual rooms or zones, which can be disconnected by flipping the switch on a circuit breaker.

Generally, electrical codes require individual circuits for bedrooms, bathrooms, laundry rooms, etc., so that the electricity can be disconnected one circuit at a time in case there is a problem. When installing a circuit for your shed, the circuit will be dedicated to the outbuilding only, so that the power can be disconnected from the outbuilding without affecting the other circuits.

The resistance in a correctly sized outbuilding electrical circuit (or any electrical circuit) will be a combination of material conductivity, distance, and load. Here we will briefly discuss this resistance to electrical flow and why sizing wire correctly is so important.

Distance

Electricity flows through and around a wire based on how much resistance the wire provides to the flow of electricity. For example, the two most common forms of electrical wire are aluminum and copper. Both offer very little resistance to the flow of electricity, so they are considered conductors, meaning they conduct, or allow electricity to flow nearly unimpeded.

However, there is enough resistance in any wire to eventually stop conducting electricity altogether if the wire gets long enough. So, the wire run to an outbuilding must be short enough (or big enough) to conduct all of the voltage being sent to it. If not, the wire will overheat and trip the circuit breaker.

An electrician will measure the distance needed to the building and calculate the “voltage drop”, or amount of electricity the wire’s size is losing from point A to point B. If the electrician’s math indicates that the voltage drop is very small, the wire will be sized based on how many Amps, or amps, the wire will be required to carry.

Wire Size

Wire size can be thought of as a pipe carrying water. If you want to move a large volume of water from one place to another quickly, you need either a large pipe with some pressure, or a small pipe with lots of pressure. Large wire sizes are available to move large amounts of electricity where it is needed, like a range, oven, or clothes dryer.

Small things like lamps, phone chargers, and laptops do not require large amounts of electricity, so the wires can be smaller. As mentioned earlier, this is why circuits are designed for their purpose. 

All electrical wires in the same sheathing (commonly called romex), will be imprinted with the conductivity limits of the wire, how many individual conductors are inside, and if it contains a bare ground wire as well. Wire sizes and types are conveniently color coded to make identification easier for electrical inspectors.

So for example, white colored, 14/2 w/ground romex will contain two (2), number fourteen (#14) conductors, along with a bare copper wire for grounding. Most homes have either 14/2 w/ground, or 12/2 w/ground for the majority of the circuits, which is yellow. So a 12/2 w/ground romex will have two conductors and ground just like 14/2, but it will be slightly larger and yellow.

Circuit Size

The circuit size will determine the other two criteria of distance and wire size, as both are a function of the circuit itself. Circuits are designed for what they need to safely power, so each component in the circuit must meet the criteria. Like a weak link in a strong chain, any undersized component in the circuit will render it unusable.

For example, a typical electrical circuit begins and ends at your breaker box, which is supplied power by your power meter. Each circuit has its own circuit breaker, which determines the functionality of the circuit. The circuit breaker will determine how much electricity the circuit receives, based on what machine the circuit needs to power.

By most electrical codes, a minimum 15 Amp circuit is required for any electrical service. As a result, most residential homes have far more 15 Amp circuits than any others and most home appliances are designed to function with 15 Amp circuits

What Wire Can Be Used For a Shed?

When running power to a detached shed or outbuilding, you essentially have three choices, assuming you are not running separate electrical service to your shed. To be clear, if you are needing significant power for a workshop or garage and you need to separate the bill from your residential bill, have a qualified electrician design the circuit for you from scratch.

The options for running the wire are aerial, direct burial, and conduit. Aerial running refers to suspending the wire from a pole, much like telephone and power poles. Unless you have underground utilities, your electrical power, cable tv, and internet access are attached to your home aerially. However, as this is a rarity in running power to an outbuilding, we will focus on the direct burial and conduit methods.

What is Direct Burial Electrical Wire?

Source: thespruce.com

Direct burial electrical wire is a self contained romex type of electrical cable designed to withstand contact with the ground. Generally, you would never directly bury a normal romex cable because the sheathing cannot withstand the exposure to groundwater and pressure. Direct burial electrical cable is much tougher than regular romex and contains a water repellent inside the cable.

Direct burial cable is usually gray, and it will say “sunlight resistant” or “UF-B”, meaning you can bury the cable without the benefit of conduit. Direct burial cable usually has a better temperature range than regular romex as well, but the larger sizes can be cumbersome to work with due to the additional protective covering it provides the wires.

What Is UF Cable?

Underground feeder cable, or UF, is designed to be installed in appropriate conduit and buried. UF cable has similar water resistant properties to UF-B, but UF cable requires the benefit of conduit to protect the wires. UF cable is commonly used when the electrical requirements might change in the future, as opposed to UF-B cable, which is considered more permanent.

What Are the Pros and Cons of Using UF-B Cable?

Most builders base their decision on cost, speed of installation, and future requirements. UF-B cable is ideal for projects that are unlikely to evolve, like a shed or storage building. However, the power demands of garages and workshops may change over time, so installing conduit is a better option for these projects.

Direct Burial UF-B Pros

  • Speed

UF-B can be installed as quickly as the trench can be prepared, making UF-B the fastest method of running power to your shed.

  • Cost

UF-B cable is usually less expensive to both purchase and install when you consider the time savings and no need for conduit, adhesives, or primers.

  • Durability

UF-B cable is very well designed with multiple layers of material protecting not just the wires, but the separation between them.

Direct Burial UF-B Cons

  • Permanency

UF-B cable is a permanent circuit, so you can’t add additional cables or power capability without starting over. In an attempt to help, many builders and electricians will install one wire size larger than they anticipate the project will require. For example, instead of running a UF-B 14/2 w/ground, the builder might install UF-B 12/2, or even UF-B 10/2 cable.

What this essentially does is allow for upgrading the system up to the limits of the cable, even if the capacity isn’t used today. For example, if a homeowner wanted to run a hand drill, light, and radio a smaller UF-B cable would be more than capable of handling the current demand. 

However, if the homeowner later decides to purchase a table saw, welder, or other tool with large current demands, the smaller cable will become useless and will need to be replaced. By installing a larger UF-B cable on the front end the cost is a bit higher, but the cable is less likely to become obsolete in the future.

Conduit with UF Cable: Pros

  • Flexibility and Modification

Conduit provides a safe channel for wires to travel through. Generally, a conduit is considered full when 50% of the conduit is filled. This may seem odd, but the extra space inside the conduit provides the flexibility for modification later, as well as reducing friction between the wires.

Any electrician will install conduit containing not just the wires, romex, UF or other cable, but a pull string as well. Pull strings are really just cheap insurance against future modifications or repairs. Unlike directly buried UF-B cable, conduit establishes a channel by which other cables can travel later. The pull string is used to pull new cables through the conduit at a later time.

  • Durability

Buried cables, or any cables for that matter, that require the most physical protection will be inside conduit. Conduit offers superior impact protection, as well as limited protection from corrosive agents. Conduit is usually installed in sections, so making repairs and modifications is a simple process.

Conduit with UF Cable: Cons

  • Cost to Purchase and Install

The only real drawback to using conduit with UF cable to power your shed is the upfront expense. The trench is essentially the same as for UF-B cable, so the only con to using conduit is the cost. However, most electricians will tell you that given a choice, and all else being equal, they will choose installing conduit most of the time.

What Size Wire Do I Need for a 150 Foot Run?

If you need a 20 Amp circuit run to your shed and the shed is 150’ from the main panel, you will need #6 gauge wire. 

To determine what size wire you will need for a 150’ UF cable run, you’ll first need to know what the circuit will need to do. For example, you wouldn’t want to design a circuit to power a light and radio, only to decide later you need to use it for a table saw.

Depending on which wire you are using, the voltage drop will vary depending on the wire and the distance. The voltage drop standard of performance is 3%, so these wire gauges will not drop more than 3% in voltage for a given distance. Here are a few common examples for a 150’ copper wire run, based on the wire size:

Wire Size/GaugeCircuit Size
8 Gauge15 Amp
6 Gauge20 Amp
4 Gauge30 Amp
2/0 Gauge100 Amp

To be clear, there is nothing wrong with choosing the next larger wire size when the performance is in doubt. In fact, wires aren’t available in every possible gauge, so even if the device you want to power only slightly goes over the capability of the wire you want to use, you must upgrade the circuit to the next available larger size.

In case you aren’t familiar, 2/0 cable can be purchased inside a sheathing like romex, or as individual wires. These 2/0 cables are common in cars as battery cables for their ability to carry large amounts of electricity without overheating. This cable is mostly used to connect sub panels to main panels that will support an addition or outbuilding.

Pro Tip: Electrical power is not as consistently delivered as we might believe. Although in the US, we use a 120 volt AC system, the actual measured power delivered can be lower or higher. Whenever possible, it’s usually a good idea to solve voltage loss issues by increasing the size of the wire. Larger wires can travel further distances with less voltage loss than small wires.

What Is Voltage Drop?

Voltage drop occurs when the electrical current is impeded by the wire or other components in a circuit. Although electrical components are designed to conduct electricity, the impurities in the materials will reduce the voltage on the other end of the wire. Known as “voltage drop”, this reduction in current must be considered in the design of a circuit to ensure the circuit performs as it should.

What Size Wire Do I Need for a 500 Foot Run?

To illustrate how distance affects the ability of a wire to transfer electricity, let’s change the distance to 500’ but keep the power requirements the same and see how it changes the wire gauge:

Wire Size/GaugeCircuit Size
1/020 Amp

To compensate for the large voltage drop that will occur over a 500’ distance, a 1/0 cable would be required for a 20 Amp circuit. Comparing this to our previous example when the distance was only 150’, the circuit only required a #6 gauge wire to provide 20 Amps.

What Size Wire for 15 Amp Circuit?

The wire size needed for a 15 Amp circuit will depend on how far the circuit travels. For example, if a circuit will only use 25 feet or less, a 14/2 with ground will work just fine. But if that same circuit was closer to 150 feet long, you would need #8 gauge wire to avoid the voltage drop.

What Size Wire for a 20 Amp Circuit?

Just as described before, the wire will have to be matched with the amount of voltage it can safely carry. For a 20 Amp circuit traveling 25’ or less, a 12/2 with ground (yellow) will suffice. If that same 20 Amp circuit must travel more like 150 feet, the cable would have to be #6 gauge or larger. For comparison, a 20 Amp circuit traveling 200’ would require #4 wire. 

What is a 240V Circuit?

A 240V circuit is used for appliances like electric water heaters, ranges, cooktops, ovens, and clothes dryers. This is because these appliances generate large amounts of heat, which requires large amounts of current. As the name implies, a 240V circuit provides double the voltage of a normal outlet.

What Wire Does a 240V Circuit Use?

In contrast to regular 120V circuits, 240V circuits use two (2) power sources, commonly called two ”hots”, as well as a neutral and ground wire. As a result, 240V service requires two “hot” wires (red and black). So, the electrician will either run appropriately sized romex, or one red, one black, one white, and one green (or bare) wire inside the conduit. 

Should I Run 240V Electrical Service to My Shed?

Source: familyhandyman.com

Before electric cars were prevalent, most homeowners really only needed to run 240V service to a shed if they were planning to use large, powerful tools. For example, welders and large air compressors usually require 240V service, so if that’s a possibility, installing 240V service is a good idea.

Installing 240V service also adds to the functionality of the structure because you can choose to heat or cool it as well. Even if you add 240V service, you will also want 110V service for things like lights and small power tools. Most electric cars also offer a 240V quick charge option, so if you have an electric car you might benefit from the faster recharges.

How Far Can You Run Electrical Wire?

To stay within the maximum 3% voltage drop threshold, wire gauges are given maximum distances they can be safely run. If the maximum distance for a wire is exceeded, the circuit must receive additional voltage from another source, like a subpanel.

For example, let’s say there is 100’ between the main panel of the home and the shed, and we want to install a 20 Amp, 120V circuit, which is common. A #6 gauge UF-B cable could be directly buried, or we could use UF cable and put it inside conduit. However, once we need more than 20 Amps of current, we have to replace the cable with a larger one.

A much better option is to install a subpanel in the shed, and power it with much larger cable. In this instance, we would need 1/0 cable to provide 100 Amps of service to the shed, so the initial cost is a little higher, but it allows for a much larger capacity with the least amount of materials.

Consider the consequences of installing a #6 UF cable once, which will provide 20 Amps of service. To get (2) two 20 Amp circuits, the entire process of digging the trench and so forth would have to be repeated every time the shed needed more power.

Installing a subpanel not only ensures the shortest distance to the main panel, but it adds that distance to any circuit connected to it as well. For example, if a subpanel is installed 100’ from the main panel, the circuit would still only require #8 gauge wire to travel another 100’. This way, if the length of a circuit becomes too large for the wire, a subpanel can be installed and solve the problem wherever it is needed.

How Far Can I Run 10/2 Wire?

You can run up to 50’ of 10/2 wire before there is enough voltage loss to matter. If the run is between 50’ -100’, the next wire size up can be used to avoid the voltage drop.

How Far Can I Run 12/2 Wire?

Technically, 12/2 wire can be run up to and including 50’ before a significant voltage drop occurs, as with other wires. Copper wire can travel a little farther than aluminum wire before a significant voltage drop because it is a slightly better conductor.

Do You Need a Subpanel In Your Shed?

If you use your shed for simple storage of non-delicate items, you might only need to run power for lights or an electric trimmer. If you tend to use your shed for lawn mower maintenance and other household projects, adding a subpanel to your shed is a smart move.

As mentioned previously, subpanels tend to solve power problems easily because they offer room for expansion. You may not require 100 Amps of service to power a welder, but you might benefit from using a pneumatic ratchet powered by an air compressor. Subpanels are available in 30A, 60A, 100A, and larger sizes to accommodate essentially any requirements.

What Size Wire to Run to Barn

Electrical power is very useful in barns and other animal shelters. The availability of lights, water pumps for water troughs, and electrical outlets are very useful, but they should always be contained within conduit. Most farmers run 100 Amp service to barns when possible because they might need to operate electrical machinery like automatic feeders for horses.

If the barn is less than 50’ from the main panel, you could use #4 UF wire contained in conduit. If the run was closer to 150’, the circuit would require #00 gauge wire, which is quite large. An electrician would place the wires in 2” or larger conduit to maintain the 50% threshold mentioned earlier.

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