Industrial control panels are the nerve centers for modern machinery, often operating in environments where heat and vibration are constant. The internal wiring must withstand these conditions to ensure long-term reliability. While standard building wire might seem sufficient, its insulation can become brittle and degrade over time, leading to system failures and costly downtime. SIS wire is a specialized conductor engineered specifically to perform in these demanding applications.
What is SIS Wire?
SIS wire is a single-conductor electrical wire designed specifically for the internal wiring of switchboards and control panels. The name “SIS” is an acronym for its construction and primary application: Stranded Insulated Switchboard wire. It is purpose-built to handle the unique challenges within these enclosures, such as high operating temperatures and the need for high flexibility in densely packed spaces.
SIS Wire Construction and Specifications
The Conductor: The core of an SIS wire is its highly flexible, stranded tinned copper conductor.
- Stranding: SIS wire uses Class B or the more flexible Class K stranding, which consists of many fine copper strands twisted together. For example, a 14 AWG SIS wire may have 41 individual strands, while a 12 AWG version can have 65 strands. The construction makes the wire extremely flexible, allowing it to be easily routed through tight corners and into terminal blocks without putting mechanical stress on the connections.
- Tinning: The copper strands are coated with a layer of tin. It provides two main advantages: it protects the copper from corrosion in industrial environments and makes the conductor easier to solder for certain types of terminations.
Insulation: The insulation is the most critical element of SIS wire. It is made from Cross-Linked Polyethylene (XLPE), a thermoset material.
- Thermal Stability: Unlike thermoplastic materials (like PVC on THHN wire) that can melt and drip when exposed to extreme heat, thermoset XLPE insulation does not melt. Instead, it will char while retaining its insulating form, providing a crucial safety advantage in the event of a component failure or short circuit inside a panel.
- Durability: The XLPE insulation offers excellent resistance to moisture, oil, and chemicals.
- Temperature Rating: The standard temperature rating for SIS wire is 90°C. However, many products are dual-rated under different UL styles, which can allow for a higher operating temperature of up to 125°C in certain applications.
Technical Data Table
This table provides a quick reference for the typical SlS wire specification data for common sizes:
| AWG Size AWG | Conductor Stranding (No. of Strands) | Insulation Thickness (mils) | Nominal O.D. (inches) | Approx. Weight (lbs/1000ft) | Ampacity (Amps @ 90°C) |
| 16 | 26 | 30 | 0.117 – 0.130 | 12 – 15 | 8 |
| 14 | 41 | 30 | 0.130 – 0.150 | 18 – 20 | 25 |
| 12 | 65 | 30 | 0.153 – 0.160 | 27 – 30 | 30 |
| 10 | 104 / 105 | 30 | 0.170 – 0.195 | 40 – 42 | 40 |
| 8 | 133 | 45 | 0.238 – 0.266 | 67 – 74 | 55 |
Where are SIS cables used?
Approved Applications✅
As its name implies, SIS wire is intended for the internal wiring of switchboards, panelboards, distribution boards, and industrial control panels. Its high flexibility makes it particularly well-suited for wiring components on hinged panel doors, where repeated movement can cause less flexible wires to fatigue and break. It is designed to be “laid in” to the protected wireways inside an enclosure, where its properties can be used to full advantage.
Critical Limitations❌
The most important limitation of standard Type SIS wire is that it is not permitted for installation in building raceways or conduits. Its soft XLPE insulation lacks the hard, slick outer jacket (like the nylon on THHN) needed to protect it from abrasion and damage when being pulled through long runs of conduit. Using standard SIS wire in this manner is a code violation and can create a safety hazard.
However, there is a common and important exception. Many manufacturers offer a dual-rated SIS/XHHW-2 wire. Because the core construction is very similar, these wires are certified to meet both standards. When a wire is explicitly marked “SIS/XHHW-2” on its insulation, it can be used inside a panel as SIS wire and can also be legally installed in conduit as XHHW-2 wire. It is essential to always check the printed legend on the wire to verify its ratings and approved uses.
Choosing the Right Conductor: SIS Wire vs. THHN and MTW
While SIS, THHN, and MTW are all single-conductor wires used in industrial settings, they are designed for different purposes and are not interchangeable.
SIS Wire vs. THHN Wire
Insulation: SIS uses thermoset XLPE, which does not melt under extreme heat. THHN uses thermoplastic PVC with a nylon jacket, which can soften and melt.
Flexibility: SIS is highly flexible due to its high strand count, making it easy to route inside a panel. THHN is much stiffer, which is a disadvantage for panel wiring but makes it easier to push and pull through conduit.
Abrasion Resistance: THHN’s nylon jacket gives it high abrasion resistance for pulling through raceways. SIS wire lacks this jacket and is not designed for such an installation.
SIS Wire vs. MTW (Machine Tool Wire)
Insulation: The key difference is that most standard MTW uses PVC insulation, while SIS uses XLPE.
Wet/Oily Conditions: This is where the distinction matters most. The temperature rating of many PVC-insulated MTW products drops significantly (often to 60°C) when exposed to oil or coolants. In contrast, SIS wire (especially when dual-rated as XHHW-2) maintains its 90°C rating in wet conditions, making it a far more reliable choice for machinery where fluid exposure is common.
This table provides a quick comparison of the three wire types.
What Should I Choose?
- Choose SIS Wire when: Your primary need is maximum flexibility and superior thermal safety for the internal wiring of a control panel, switchboard, or distribution board.
- Choose THHN Wire when: You are running general-purpose building wiring between panels, junction boxes, and equipment through conduit.
- Choose MTW Wire when: You require a flexible wire for the internal wiring of machinery in a consistently clean and dry environment where cost is a primary consideration.
- Choose Dual-Rated SIS/XHHW-2 when: You need a versatile, single-wire solution for both the intricate internal wiring of a panel and the conduit runs connecting to it, streamlining inventory and installation.
Of course, the quality of a wire is only as good as the manufacturer that stands behind it. For your critical control panel applications, consider a trusted partner like ZW CABLE. Contact us today for a detailed datasheet and quotation.
Frequently Asked Questions (FAQs)
- What does the “SIS” in SIS wire stand for?
SIS stands for Stranded Insulated Switchboard wire. The name directly reflects its highly flexible stranded conductor and its primary, NEC-approved application for the internal wiring of switchboards and control panels.
- Can I use SIS wire in conduit like THHN?
No, standard Type SIS wire should not be installed in conduit. Its XLPE insulation is softer and can be damaged by the friction of pulling. However, if the wire is dual-rated and specifically marked on its legend as “SIS/XHHW-2,” it is approved for use in conduit under its XHHW-2 rating.
- Is SIS wire rated for wet or outdoor locations?
Standard Type SIS wire is rated for dry locations only. It is not suitable for outdoor use or direct burial. The exception is when the wire is dual-rated as Type XHHW-2, which is approved for use in both dry and wet locations at a 90°C temperature rating.
