SIS UL 3173: The High-Heat, "No-Melt" Solution for Control Panels

When you're wiring up a control panel or a piece of switchgear, you aren't just looking for "wire." You’re looking for something that won’t melt when the cabinet gets hot and won't fight you when you’re trying to make tight bends around a relay. That’s where SIS High Temp Wire 125°C (UL 3173) comes in.

1. What is SIS UL 3173 Wire

SIS stands for Switchboard Insulation Strategy. It is a specialized, 600V power and control wire designed specifically for the internal wiring of switchboards, control panels, and transformer secondary leads.

Commonly referred to as "Switchboard Wire" its defining characteristic is its Cross-Linked Polyethylene (XLPE) insulation. Unlike standard PVC wire, XLPE is a "thermoset" material, meaning it won't melt, drip, or deform under extreme heat. Carrying a UL 3173 label, this wire must is rated for 125°C, making it one of the toughest control wires on the market.

2. Why the Pros Make the Switch

Most people think wire is wire, but SIS is built differently for a reason:

• It Handles the Heat: While standard THHN is only good up to 90°C, SIS is rated for 125°C (257°F). In a crowded cabinet where airflow is tight, that extra temperature headroom is the difference between a reliable system and a melted mess.

• The "No-Melt" Insulation: It uses XLPE (Cross-Linked Polyethylene). Unlike the plastic on standard wire, this stuff is "thermoset." If there’s a short or a massive heat spike, it won't drip or run. It stays put.

• Tinned Copper is Standard: The copper strands are tinned, which makes your crimps bite better and keeps corrosion from eating your connections—especially in damp or industrial environments.

3. Where You'll Use it

If it’s inside a box and it’s important, you probably need SIS:

• Control Panels & PLCs: For those tight "S-turns" between terminals.

• Utility Switchgear: The gold standard for power company breakers.

• Transformer Leads: Where things get hot and stay hot.

• Battery Racks: Perfect for backup power systems and UPS setups.

4. Why Choose SIS Over Standard Wire?

• It’s Cheap Insurance: A control panel can cost tens of thousands of dollars. Using wire rated for 125°C means you aren't sweating it when the equipment is running at full load. It handles the "hot spots" that cause standard wire to get brittle and fail.

• No "Bird-Nesting" in the Cabinet: Because SIS uses high strand counts and soft tinned copper, it has "memory." When you bend it into a corner, it stays there. It makes for a cleaner, safer, and more professional-looking cabinet.

• Code Compliance: Many inspectors and engineers specifically look for the SIS or UL 3173 mark on the jacket for switchgear and transformer leads. Using the right tool for the job avoids the headache of a "red tag" and a forced rewire.

5. Why Buy SIS Wire?

• Build it to Last: Using SIS means you are installing a high temp wire (125°C) that won’t get brittle, won't crack under vibration, and won't fail because the cabinet got a little too hot on a summer afternoon.

• Professional Grade: There is a specific "look" to a pro-grade panel. The way SIS bends cleanly and the way the tinned copper catches the light tells your customer that you didn't cut corners.

You don't buy SIS because you have to—you buy it because you’re a pro who wants to do the job once, do it right, and never have to think about it again.

Click Here to Shop Our Switchboard SIS High Temp Wire Selection

At Wire America, we believe in genuine conversation. You'll always talk with real people who understand that every project requires careful product selection. We're here to ensure you feel educated, confident, and great about your final purchase. Feel free to give us a call to discuss your project! You can reach us at: 833-268-8644

⚠️ Important Disclaimer: Safety and Compliance

Wire America provides this information as a guide only. Electrical installations must strictly adhere to the National Electrical Code (NEC) and all state, local, and jurisdictional requirements, which vary widely. Always consult a licensed electrician, electrical engineer, or the local inspecting authority before purchasing material or starting any electrical project. Wire America assumes no liability for the installation, use, or interpretation of these guides.