Experienced electrical workers know there is a wire or cable for nearly every application. But what happens when applications overlap, and multiple wires fit the job? 

This thinking is especially true for building wire, which stretches far and wide across many applications. Sure, it’s fair to say most types of building wire fit a general role by delivering electrical power to machines and appliances, but it isn’t always clear-cut. 

It’s What’s on the Outside That Counts 

Although there are plenty of conversations we can have about conductors, when people discuss wire applications, it’s generally tied to the wire or cable’s stranding. 

Knowing how each cable and its insulation work and what makes each of them unique can help you easily track down the best wire for the job, resulting in better installations. 


At first glance, XHHW and XHHW-2 look like a series of random letters, but those letters mean a lot. 

XHHW stands for Crosslinked (X) High-Heat Resistant (HH), Water Resistant (W), using a thermoset polyethylene to insulate the wire. The wire is oil and gas resistant, carrying an Oil and Gas Resistance II rating, and is sunlight resistant if black insulation is used. Other colors need an ultraviolet (UV) stabilizing compound added to achieve the same sunlight resistance. 

XHHW carries the FT2 rating, per UL, thanks to its flame and moisture-retardant thermoset insulation. The FT2 flame tests finished cables for flame resistance using a 1,700 BTU/hour flame on a horizontally suspended wire. To earn an FT2 rating, less than 100 mm (slightly longer than the long edge of a credit card) of wire can be charred, and the flammable material under the wire cannot ignite. 

Depending on whether XHHW or XHHW-2 is used, the wire can operate successfully in 75°C or 90°C wet locations. The “-2” designation means the wire is designed to work in higher wet temperatures, per National Electric Code (NEC) and UL standards. Both wires are rated for 90°C in dry locations.  

How is XHHW/XHHW-2 Used? 

XHHW/XHHW-2 is generally a solid or stranded conductor used in 600V systems in the United States. 

It’s often used in residential and commercial wiring situations for general-purpose wiring, including power distribution, lighting, control wiring, and HVAC and motor systems. XHHW-2 has even found a home in renewable energy because of its sunlight resistance and carries a cable tray (CT) rating if it’s #1/0 AWG or bigger. 


RHW-2 stands for Rubber (R) Heat Resistant (H), Water Resistant (W), and is insulated using a moisture-resistant thermoset process. Although it has the rubber moniker, RHW-2 also covers neoprene and crosslinked polyethylene (XLPE) insulation. 

RHW-2 is also called USE-2, which stands for Underground Service Wire. Despite the different names, the wire carries the same ratings and applications. 

What Is RHW-2/USE-2? 

Rubber is a good insulator with a lot of flexibility but struggles against heat, abrasion damage, or chemicals. To fix the problem, some RHW features crosslinked polyethylene (XLPE) insulation, making it more resistant to heat, chemicals, crush and abrasion damage, and flames. 

Like XHHW, RHW carries the “-2” designation, allowing it to operate in wet locations up to 90°C. However, unlike XHHW wire, RHW works well in a wide range of temperatures, from -40°C to 105°C in dry applications, and performs well in bend tests rated at -25°C. Just like XHHW, RHW-2/USE-2 is sunlight-resistant when coated with black insulation. 

Unlike XHHW, RHW-2/USE-2 doesn’t carry a high-heat designation but can be VW-1 rated. Like the FT2 flame test, the VW-1 tests a wire’s flammability but is much stricter. Compared to the FT2 test, where the wire is held horizontally, the VW-1 test suspends the wire vertically. A torch is applied to the wire for 15 seconds and reapplied once the flame goes out. This process happens several times, and if the wire extinguishes itself in less than 60 seconds each time, it passes and receives a UL 1581 certification. 

How is RHW-2/USE-2 Used? 

Like XHHW, RHW-2/USE-2 conductors can be solid or stranded. The wires are typically found in 600V and 1kV systems, though configurations exist for 2kV installations. The wider use case makes RHW more versatile than XHHW. 

RHW is well suited for lighting and power applications and works well in aerial and underground applications where moisture or chemicals like oil and gas are common. It can also work well inside conduits and in raceways where you need to stub out for a direct burial situation. 

SIS Wire 

Stranded Insulated Switchboard (SIS) wire is primarily used for… switchboards.  

Despite its specific naming, SIS wire has applications in control panels, low-voltage applications like machinery and lighting systems, and connecting circuit breakers and fuses as part of a bigger electrical system. You can also find SIS wire connecting panels in utility substations. 

Thanks to its flame-retardant thermoset insulation, SIS is an excellent wire for installations in high-heat locations where there could be potential for fires. 

What is SIS Wire? 

Unlike XHHW and RHW, SIS uses stranded tinned copper wire to run power, though copper-only options are available. 

SIS wire has many stranding options, depending on the flexibility needed. Standard strandings are 16, 26, 41, 65, 105, and 133, though other options include 7, 19, 37, and 259, covering Class B through Class K ratings. Compared to solid copper, stranded conductors are easier to install in tight spaces and conduits because they’re easier to bend than solid copper. 

The tinning used on the copper conductor also plays a critical role by increasing the wire’s flexibility and staving off corrosion after installation. It also makes the wire easier to solder. 

SIS Wire Specs 

The insulation used on SIS wire is like what you’d see on XHHW-2 but carries a VW-1 flammability test rating like RHW-2/USE-2. 

The thermoset cross-linked polyethylene insulation carries an oil and gas resistant II rating up to 90°C in wet locations. It’s also sunlight resistant, flame retardant, and moisture and abrasion resistant, though it cannot be buried. 

Similar Wires, Different Applications

This is a lot of information to digest but illustrates a critical point: even though all three cables are common types of “building wires,” each has subtle distinctions separating them from the others. 

Dozens of variables determine wire needs, including temperature ratings, voltage ratings, chemicals, ozone, and abrasions, so it’s worth knowing how to address every concern effectively. 

Of course, if you ever have questions or concerns, consult your local codebook for specific applications or check the National Electric Code (NEC) for more information.

Related Products