When it comes to choosing the right wire for your electrical needs, understanding the differences between indoor and outdoor wires is crucial. Each type has specific uses to ensure safety and performance. In this article, we'll quickly compare them to help you decide better.

Indoor or Outdoor Wire: Points of Comparison

Indoor and outdoor cables have unique features tailored for specific uses. Each aspect, whether it's the gauge or flexibility, is designed to enhance their performance for particular applications. Let’s review them. 

Applications

The outdoor wire is specifically designed for applications like landscape lighting, outdoor outlets, and underground circuits, where it needs to withstand exposure to such elements as moisture, sunlight, temperature fluctuations, and physical wear. Indoor wire is tailored for internal house wiring, connecting appliances, and powering electronics within controlled indoor environments.

Insulation

The outdoor wire is equipped with sturdy insulation to shield against moisture, temperature fluctuations, and physical stress caused by outdoor conditions. Common examples of these insulating materials include cross-linked polyethylene (XLPE) and thermoplastic elastomers (TPE), chosen for their resilience against environmental challenges. 

Indoor wire generally features lighter insulation, such as polyvinyl chloride (PVC) since it doesn't encounter the same harsh outdoor challenges. PVC is a versatile insulation suitable for both indoor and outdoor uses, depending on its specific type.

Gauge

Exterior wire often employs a thicker gauge to compensate for potential power loss over extended outdoor distances. Indoor wire can have a lighter gauge due to the typically shorter indoor distances.

For instance, Romex, a common type of indoor cable, is available in sizes like 14/2, 12/2, or 10/2, which is enough for the indoor setups. Service entrance cables demonstrate the AWG system's significance in larger applications. These cables are designed for higher power demands and are classified by their gauge sizes. For example, a 4/0 gauge is used for 200-amp services, while 250 MCM is employed for 300-amp services. For even higher demands, cables like 350 MCM or 400 MCM are utilized for 400-amp services. 

Flexibility

Outdoor wire maintains flexibility to facilitate installation in tight spaces and around obstacles inherent to outdoor setups. With indoor wire, flexibility can vary depending on the specific indoor application.

Labeling & Certification

Outdoor wires have certification indicating UV resistance, often conforming to standards like UL, to ensure durability when exposed to sunlight. If the product is listed as “UL Recognized”, it means that it has been tested under specific conditions created by UL standards. This product will be able to withstand UV exposure to a certain extent without being damaged. 

If the products have a marking “Sun Res.” or “UV”, it means they are suitable for sun exposure. Usually, they are made from materials that can resist UV degradation. 

Meanwhile, lack of UV-resistant certification is common for indoor wires, as they are not exposed to direct sunlight.

Visual Identification

In order to determine whether a wire or a cable is suitable for outdoor use, you can rely on specific marking and characteristics. 

Insulation or Jacket Markings

Outdoor wires have the following markings: 

• “UF”, standing for “Underground Feeder”. These cables are designed for outdoor use and direct burial. 
• “Sun Res” or “Sunlight Resistant”. These cables can be exposed to direct sunlight. 
• “Wet” or “W”. These cables are suitable for wet locations. For example, in designations like “THWN”, “W” stands for wet location suitability. 

Indoor wires often have different insulation or jacket markings that are tailored to indoor use:

• "NM" or "NMB": These indicate non-metallic sheathed cables commonly used in residential wiring.
• "CM" or "CMP": These markings denote cables suitable for general indoor use or plenum spaces (areas used for air circulation in buildings).

Physical Characteristics

While outdoor wires have thicker insulation to withstand harsh environmental conditions, indoor wires may have thinner insulation since they don't need to endure the same level of exposure to the elements.

Color

Some of the outdoor wires that are suitable for direct burial may be gray. Indoor wires also come in a variety of colors, each serving a different purpose in electrical installations. For example, black, red, and blue wires are often used for hot conductors, white or gray for neutral conductors, and green or bare for grounding conductors.

Yet, you shouldn’t rely solely on the color of the wire, also consider other markings. 

Safety Considerations for Indoor and Outdoor Wire

Your electrical systems will serve longer, be safe and work properly if you  use the right type of wire, whether for indoor or outdoor applications. Mixing these wires can cause different risks and complications. Let’s explore in more detail why it’s crucial to use the indoor and outdoor wires in the respective environments. 

Outdoor wires are designed with more robust insulation to withstand harsh environmental conditions. Using them indoors can result in unnecessary costs due to over-engineering and might lead to difficulties in routing and installation due to their increased rigidity.

In specific cases, such as conduit-enclosed installations, it might be acceptable to use outdoor-rated wires indoors. However, you should always consult local electrical codes to ensure compliance.

Indoor wires have lighter insulation, making them susceptible to moisture, temperature extremes, and physical damage when exposed to outdoor elements. This can lead to insulation deterioration, short circuits, and potential hazards.

Certain indoor-rated wires can be used outdoors if adequately protected within appropriate conduit or enclosures. However, the wires should not be directly exposed to outdoor conditions.

Several cable types are versatile enough to be used in both indoor and outdoor settings with proper considerations:

• THHN/THWN: These wires have thermoplastic insulation and are suitable for dry and damp locations. They can be used indoors within conduit systems and, in certain cases, outdoors when properly protected.
• XHHW: Designed for wet or dry locations, XHHW cables can be used both indoors and outdoors when installed in conduit or raceway systems.
• SER/SEU: These cables are suitable for both indoor and outdoor applications, often used for service entrance applications and feeder wiring.
• MC (Metal-Clad Cable): MC cables are versatile and can be used indoors and outdoors in various applications. They provide better protection against physical damage and can be used in wet or dry locations.

Types of Outdoor Cables

Outdoor cables come in various types, each engineered to withstand different conditions and serve specific applications.

UF-B (Underground Feeder - Direct Burial)

These cables are resistant to various conditions, such as moisture, sunlight, mechanical damage, etc. They have solid insulation and can be buried directly in the ground. They are the right option for underground wiring applications, such as residential wiring, outdoor lighting, and landscape projects. They typically have a PVC jacket, and their insulation allows for direct burial without conduit.

USE-2 (Underground Service Entrance - Direct Burial)

USE-2 cables are known for their exceptional resistance to sunlight, heat, and moisture. They have a tough outer jacket and can handle direct burial. These cables are commonly used for utility service entrances, solar installations, and other outdoor power distribution needs. USE-2 cables are usually single-conductor and can be buried without conduit.

RHH/RHW (Thermoset Insulated)

RHH/RHW cables feature thermoset insulation that provides excellent resistance to high temperatures, abrasion, and chemicals. They are often used in industrial settings, factories, and outdoor installations where exposure to harsh conditions is likely. These cables can be used in conduit or cable tray systems.

Mobile Home Feeder

Mobile Home Feeder cables are designed to supply power to mobile homes. They have high ampacity and resistance to sunlight, moisture, and heat. Used for supplying power from the main power source to mobile homes and manufactured housing. Typically used for above-ground installations and can be used in conduit or cable trays.

URD (Underground Residential Distribution)

URD cables are designed for underground power distribution, featuring excellent resistance to moisture and mechanical stress. Used in direct-burial applications for distributing power in residential and commercial areas. URD cables often consist of multiple conductors and can be directly buried without conduit.

PV Cable (Photovoltaic Cable)

These cables have great UV resistance and flexibility. They are used for solar power applications to connect photovoltaic arrays to inverters and other components. PV cables are typically used outdoors and can withstand harsh weather conditions. 

Types of Indoor Cables

NM-B Romex (Non-Metallic Building Cable)

These cables have many insulated conductors and a grounding wire, which are all connected by a non-metallic outer jacket. They are flexible and easy to work with. NM-B Romex is widely used for residential wiring, such as switches, lighting fixtures, connecting outlets, etc. Available in different conductor configurations, typically 2-3 conductors with a grounding wire. This cable is suitable for dry indoor locations and rated for various ampacities.

Low-Voltage Wire