Mineral-Insulated Cable (Type MI)

Type MI is unusual: instead of plastic insulation, the copper conductors sit in tightly packed magnesium-oxide powder, all sealed inside a continuous copper (or alloy) tube. Nothing in that construction is flammable, so the cable can operate at high temperature and keep functioning while a fire rages around it.

That fire survivability is its whole reason for existing. MI wires fire-pump feeders, emergency and life-safety circuits, and hazardous locations. It's demanding to terminate — the hygroscopic powder must be sealed against moisture immediately with special end fittings — which limits it to jobs that truly need its resilience.

When it applies

Type MI is the near-fireproof cable, copper conductors buried in compressed magnesium-oxide powder inside a seamless metal tube. That mineral insulation does not burn, so MI shows up on circuits that have to survive a fire, emergency feeders, fire-pump supplies, and other critical circuits, as well as high-temperature spots.

Common mistakes

• Leaving a cut end open, the mag-oxide powder is hygroscopic and pulls in moisture fast, so unsealed ends fail insulation tests.
• Rushing the specialized terminations, MI needs its listed sealing pots and the right tooling, not improvised fittings.
• Kinking the tube, a sharp bend can crack the sheath and ruin the seal.

In the field

The first rule with MI is seal the ends the same shift you cut them. I megger every termination after potting, because moisture that wicked in overnight will read low and force you to cut back and start the pot over.