Short answer: Type MI cable (Article 332) has solid conductors insulated with highly compressed mineral powder, usually magnesium oxide, inside a continuous liquidtight and gastight metal sheath. It is permitted in a long list of locations, including wet locations, embedded in concrete and many hazardous locations (NEC 332.10), but not where corrosive conditions would damage the sheath unless it is protected (NEC 332.12). The bending radius is at least 5 times the external diameter for cable not more than 3/4 inch across, and at least 10 times for cable over 3/4 inch up to 1 inch (NEC 332.24). It is supported at intervals not over 6 feet (NEC 332.30), and the mineral insulation must be sealed against moisture right after the sheath is stripped (NEC 332.40(A)).
PSI lists cables and cords in the Wiring Methods and Materials subject area of the Texas Journeyman exam: 10 scored NEC Knowledge questions and 2 scored Calculations questions. PSI does not say how many cover MI cable. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Why MI cable is different
The insulation is not plastic. It is compressed mineral powder that does not burn, which is why MI cable shows up where a circuit has to keep working in a fire or where heat and oil would ruin ordinary insulation. The same powder takes in moisture from the air once it is exposed, and wet insulation loses its resistance. That one fact explains the rule people remember: seal the end right after stripping.
Where it may and may not go
| Situation | MI cable |
|---|---|
| Services, feeders and branch circuits; power, lighting, control and signal circuits | Permitted (NEC 332.10) |
| Dry, wet or continuously moist locations; indoors or outdoors; exposed or concealed | Permitted, under the conditions in 332.10 |
| Embedded in plaster, concrete or masonry, above or below grade | Permitted, under the conditions in 332.10 |
| Hazardous locations | Where the hazardous location articles allow it, with fittings listed for the location |
| Underground runs | Not permitted unless protected from physical damage where necessary (NEC 332.12) |
| Where conditions are destructive or corrosive to the metal sheath | Not permitted unless additional protection is provided, as the section describes |
Where the cable runs through or parallel to framing members, the protection rules of 300.4 apply where they are relevant (NEC 332.17).
Bending, support and single conductors
- Bending radius. At least 5 times the external diameter for cable not more than 3/4 inch, and at least 10 times for cable over 3/4 inch up to 1 inch (NEC 332.24). Check your 2026 NEC for anything larger.
- Support. Secured at intervals not over 6 feet by straps, staples, hangers or similar fittings designed so they will not damage the cable, with the exceptions listed in the section, such as where the cable is fished (NEC 332.30).
- Single conductors. Where single-conductor MI cables are used, all phase conductors, and the grounded conductor where used, are grouped together to reduce induced voltage on the sheaths (NEC 332.31). Separating them lets current induce heating in the metal around each conductor.
Fittings and end seals
Fittings that connect MI cable to boxes, cabinets and other equipment are identified for the use (NEC 332.40(A)). The same section requires an end seal fitting installed immediately after stripping, so moisture cannot enter the insulation, with the conductors beyond the sheath individually insulated. On site, follow the manufacturer’s termination instructions; an insulation resistance test is how a crew checks that a termination stayed dry.
The metal sheath can serve as an equipment grounding conductor where it meets the conditions in 250.118. Read that list in your 2026 NEC for the cable type in the question.
Worked example: bending radius
- Cable 0.5 inch in diameter. Not more than 3/4 inch, so the multiplier is 5. 5 × 0.5 = 2.5 inches minimum radius.
- Cable 0.9 inch in diameter. Over 3/4 inch up to 1 inch, so the multiplier is 10. 10 × 0.9 = 9 inches minimum radius.
The jump from 5 to 10 at 3/4 inch is the detail a question tests: a cable only slightly larger than 3/4 inch needs roughly twice the radius.
Check yourself
- What is the minimum bending radius of 0.75 inch MI cable? 5 × 0.75 = 3.75 inches.
- How far apart are MI cable supports in a normal exposed run? Not over 6 feet, with the exceptions in 332.30.
- When is the end seal installed? Immediately after stripping, to keep moisture out of the mineral insulation.
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Find Article 332 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Write “5× up to 3/4 in, 10× over 3/4 to 1 in” next to 332.24, so the size break is the first thing you see on a bending question. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is MI cable?
Type MI, mineral-insulated, metal-sheathed cable: solid conductors in compressed mineral insulation inside a continuous liquidtight and gastight metal sheath.
Can MI cable be used in wet locations?
Yes. Article 332 permits it in dry, wet and continuously moist locations, among others. It is not permitted where corrosive conditions would damage the sheath unless it is protected.
What is the bending radius of MI cable?
At least 5 times the external diameter for cable not more than 3/4 inch, and at least 10 times for cable over 3/4 inch up to 1 inch.
Why must MI cable be sealed right after stripping?
The mineral insulation absorbs moisture once exposed, which lowers its insulation resistance. The end seal keeps moisture out.
How should single-conductor MI cables be run?
Grouped together, all phase conductors and the grounded conductor where used, to reduce induced voltage on the sheaths.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 250, 300 and 332
