Because 6/3 wire has three conductors, not including a grounding one, it requires at least 3/4 inch conduit for all types of conduit, including EMT (electrical metallic tubing), GRC (galvanized rigid conduit), and IMC (intermediate metal conduit). However, the best strategy is to use a 1-inch conduit. It will guarantee easier pulling and better heat dissipation. Depending on the specifications of your electrical project, you can use a larger conduit.

According to the National Electric Code, the maximum conduit fill for three or more wires is 40% of the total available space.

Conduit size depends on the number of conductors, the cross-sectional diameter of the wire, type of conduit, and applications. Let's dive deeper into conduit sizes for 6/3 wire.

What Size Conduit For 6/3 Wire by Cable/Wire Type

The right size of conduit for cable also depends on the exact cable used.

NM-B and UF-B Wire: These cables are often run without conduit in residential construction, except where needed for protection. If a conduit is used, 6/3 NM-B or 6/3 UF-B cable will generally require at least a 1-inch conduit due to the larger cable size and to facilitate pulling the cable. The types of conduit used for these cables are often NM-B or UF-B.

While NM-B wire can be run in conduit, it is not that common. If the NM-B wire is indeed run in conduit, it is rarely for its entire run. This is because NM-B wire is primarily designed for in-wall indoor applications that do not require conduit. There are better alternatives for conduit runs, such as individual THHN/THWN wires. NM-B wire can be used in conduit in short transition points between walls, or to protect the cable from physical damage.

SE Cable and MC Cable: These cables are typically larger than their NM-B or UF-B equivalents due to their construction. For 6/3 SE or 6/3 MC cable, conduit size needs to be larger, 1-1/4 inch at minimum or more. The conduit that is commonly used with service entrance cable is the rigid galvanized conduit. The intermediate metal conduit or a PVC conduit are also possible, though less common.

Service entrance cable is often run in conduit for mechanical protection or underground. Depending on the local electrical codes, certain installations may require conduit. Conduit may also be used at transition points, such as when the service entrance cable enters an electrical panel or meter.

MC cables can be run in conduit, for protection from moisture or the environment because some MC cables are not rated for outdoors, some are not built to sustain a prolonged exposure to moisture. MC cable might be run in conduit where it transitions to a different wiring method or enters a junction box, panel, or device.

Because of the nature of the installations where MC and SE cables are put in conduit, even larger conduit sizes may be required, as there are cases when several cables are simultaneously placed in one conduit. In this case, you can use 2 inches, a 2.5-inch, or even larger conduit, depending on the number of cables run simultaneously.

THHN/THWN Wire: If you're using individual 6-gauge THHN/THWN wires, you could likely use a 3/4' or 1' conduit, depending on the number of wires and local code requirements. You can generally fit 3 THHN wires in 3/4' conduit and 4-5 wires into 1' conduit. THHN/THWN is used with all types of conduit, with PVC being a very common and cost-effective choice.

If you consider running a 6/3 wire in conduit, you may want to reconsider and run individual THHN wires instead. They are cost-effective, easy to pull, and most commonly used with all types of conduit.