If you build or service switchboards, panelboards, or switchgear, SIS wire is the conductor the code and the listing agencies expect to see inside the enclosure. This guide covers what SIS is, why it exists, and how to specify it.
What Is SIS Wire?
SIS is a 600V, single-conductor wire with cross-linked polyethylene (XLPE) insulation, listed under UL 44. The name comes from its original designation: Switchboard wire, Insulated, Stranded. Key characteristics:
- Voltage: 600V
- Temperature: 90°C in dry locations
- Insulation: Thermoset XLPE — it chars rather than melts
- Flame rating: VW-1
- Conductor: Stranded copper, commonly tinned, 18 AWG through 4/0 AWG
Why XLPE Matters Inside a Panel
The defining feature of SIS is its thermoset insulation. Thermoplastic wire (like THHN) softens and melts when it gets hot; thermoset XLPE chars in place and keeps its shape. Inside a crowded switchboard — where conductors bundle tightly, terminate on lugs that heat up, and can't dissipate heat like open wiring — that difference is why UL 891 (switchboards) and switchgear standards call for SIS-type wire.
Where the NEC Comes In
NEC Article 408 covers switchboards and panelboards. Equipment listed to UL 891 uses SIS as the standard internal wiring because it survives the temperature rise and bundling conditions inside the gear. When an inspector or a UL 508A panel shop audit looks inside your enclosure, SIS on the load carriers is the expected answer. Many shops also stock UL 3173 dual-rated wire, which satisfies SIS, MTW, and AWM requirements with one part number.
SIS vs MTW vs THHN in Panels
| SIS | MTW | THHN | |
|---|---|---|---|
| Insulation | XLPE (thermoset) | PVC (thermoplastic) | PVC/nylon (thermoplastic) |
| Temp rating | 90°C dry | 60°C–90°C | 90°C dry |
| Typical use | Switchboards, switchgear | Machine tool panels (NFPA 79) | Building branch/feeder in conduit |
| Flexibility | Good (stranded, often tinned) | Very good (fine strand options) | Fair (coarser strand) |
Short version: THHN belongs in the building's raceways, MTW belongs in industrial machine panels, and SIS belongs in switchboards and gear. There's overlap — but using the listed product for each location keeps inspections simple.
Specifying SIS: What to Tell Your Supplier
- Gauge: 18 AWG to 4/0 AWG stocked; sized per your load schedule
- Color: full color range for phase, neutral, ground, and control identification
- Tinned or bare: tinned resists corrosion and solders cleanly
- Put-up: spools or cut lengths to match your build
We stock SIS switchboard wire in a full range of gauges and colors, cut to length and shipped nationwide.
