Intermediate Metal Conduit (IMC)

Intermediate Metal Conduit is the middle child between EMT and rigid. It's threaded steel like RMC and shares the same trade sizes and fittings, but its wall is thinner, so a length of IMC weighs noticeably less while still standing up to physical abuse and corrosion.

Wherever the Code permits rigid metal conduit, it generally permits IMC, including outdoors and in many hazardous or high-abuse locations. It costs and weighs less to install, which is why many contractors default to it — the interior diameter is actually slightly larger than same-size RMC, easing conductor fill.

When it applies

Intermediate metal conduit is a threaded steel raceway lighter than rigid but still tough enough to use anywhere RMC is permitted, including many outdoor, exposed, and hazardous-area runs. You get most of the mechanical protection at less weight and cost.

Common mistakes

• Cutting threads too short or too long so the fitting does not fully engage and the raceway loses electrical continuity.
• Exceeding the allowed bends between pull points and making the wire pull nearly impossible.
• Treating IMC and EMT interchangeably. The threading and fittings are different.

In the field

On an exterior industrial run I chose IMC over rigid to save the crew's backs on a long overhead job. Threaded properly and reamed clean, it gave us the same protection with far less weight to hoist. Just respect the bend limits and cut clean threads; a sloppy die on IMC ruins your continuity and the whole raceway becomes an unreliable ground path.