Does the NEC require sealing conduit between a cold and warm space?

Yes, where a temperature difference and a condensation problem are known. NEC 300.7(A) requires the raceway or sleeve to be filled with an approved material to prevent circulation of warm air into the colder section.

Is duct seal an approved material under 300.7?

In most jurisdictions yes. NEC 300.7(A) uses the word approved rather than listed, so acceptance is up to the authority having jurisdiction, and duct seal is the traditional accepted material.

Can duct seal be used in a hazardous location?

No. NEC 501.15 requires a listed sealing compound in a fitting listed for the purpose. Duct seal putty does not meet that requirement.

Can I use duct seal to seal a conduit between heated and unheated areas?

In most cases yes. Section 300.7(A) is the rule, and it uses the word approved rather than listed, which gives the authority having jurisdiction room to accept duct seal.

What 300.7(A) requires

Section 300.7(A) of the 2023 NEC states that where portions of a raceway or sleeve are known to be subjected to different temperatures, and where condensation is known to be a problem, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve. An explanatory seal, bushing, or other means shall be provided in the raceway to prevent circulation of warm air to a colder section.

Note two things. First, the trigger is a known temperature difference combined with a known condensation problem, not merely crossing a wall. Second, the required material is approved, meaning acceptable to the authority having jurisdiction under Article 100. Duct seal putty is the traditional field material and is accepted almost everywhere. Expanding foam listed for electrical use, moldable putty pads, and listed raceway sealing fittings are also used.

Where this actually bites

The classic failure is a conduit running from a cold attic or an unheated crawl space down into a heated interior panel. Warm moist air travels up the raceway, hits the cold section, and condenses. The water runs back down and collects in the panel. On refrigerated spaces, walk in coolers, and outdoor to indoor transitions this is a routine correction. Section 312.2 separately requires enclosures in damp or wet locations to be weatherproof and to be mounted with a 6 mm air space, and it prohibits moisture from entering or accumulating within the enclosure.

Underground raceways get their own rule. Section 300.5(G) requires conduits or raceways through which moisture may contact live parts to be sealed or plugged at either or both ends. That is the reason for the duct seal in the bottom of an outdoor pull can fed from below grade.

Where duct seal is not acceptable

Do not confuse a 300.7(A) temperature seal with a hazardous location seal. Section 501.15 requires conduit seals in Class I locations to use a listed sealing compound with the manufacturer specified minimum plug thickness and a fitting listed for the purpose. Duct seal putty is not a listed sealing compound and will never pass in a classified area. Likewise, a fire rated wall penetration falls under Section 300.21, which requires the penetration to be firestopped using approved methods to maintain the fire resistance rating, which means a listed firestop system, not duct seal.

Field takeaway

Duct seal on both ends of a raceway crossing a temperature boundary satisfies 300.7(A) in most jurisdictions. Confirm with your inspector before you seal a hundred penetrations. Use a listed sealing compound in classified areas and a listed firestop system through rated assemblies.