MI Wire
Specifications
| Property | Value |
|---|---|
| Full name | Mineral-Insulated (Metal-Sheathed) |
| Insulation material | Compressed magnesium oxide (MgO) |
| Outer covering | Seamless copper or stainless steel sheath |
| Temperature rating (dry) | 90°C (194°F) |
| Temperature rating (wet) | 90°C (194°F) |
| Voltage rating | 600VV |
| Approved locations | Dry, Wet, Hazardous, High temperature |
Advantages and limitations
When to use MI wire
Fire-rated circuits, fire alarm systems, emergency power. Inherently noncombustible. Listed in 90C column.
Common applications for MI:
- Specialized applications - MI is used where its specific temperature and environment ratings are required
- See manufacturer specifications for approved installation methods
NEC code references for MI
The NEC defines wire insulation types in Table 310.4(1) (formerly Table 310.104(A)). Key sections that apply:
- Table 310.4(1) - lists MI with its temperature rating, insulation material, and approved locations
- Table 310.16 - ampacity values for conductors in raceway, cable, or earth (the values shown in our ampacity table above)
- Section 110.14(C) - termination temperature limits that determine real-world usable ampacity
- Section 310.15(C)(1) - adjustment factors for more than 3 current-carrying conductors in a raceway
Frequently asked questions about MI wire
Where can I use MI wire?
MI is approved for use in dry, wet, hazardous, high temperature per NEC Table 310.4(1). It has a dry temperature rating of 90°C and a wet rating of 90°C. Always verify that your specific installation meets the approved location requirements.
What is the temperature rating of MI?
MI is rated 90°C in dry locations and 90°C in wet locations. The ampacity values in NEC Table 310.16 use the 90°C column for conductors rated at or above 90°C, but actual usable ampacity is limited by the termination temperature (typically 75°C per 110.14(C)).
Other wire insulation types
NEC 2023 references verified April 2026