For outdoor commercial runs, the “best” conduit depends on what kind of exposure the run will see, but if you want the short version: rigid metal conduit and IMC are usually the strongest choices where physical damage is a concern, while PVC is often the best choice where corrosion, buried sections, or wet locations are the main issue. EMT can work outdoors in some cases, but it is usually not my first pick for long exposed exterior runs unless the environment is mild and the installation is well protected.
If the conduit is going to be on a rooftop, along an exterior wall, or anywhere that forklifts, carts, service equipment, or people could hit it, rigid metal conduit gives the best protection. It is tough, holds up well mechanically, and is a common commercial choice when durability matters. IMC is also used a lot because it is lighter than rigid steel but still offers strong physical protection. The downside is cost, weight, and corrosion protection requirements. Outdoors, especially in damp or coastal areas, you need to think about fittings, threads, and the condition of the coating, because corrosion becomes a real maintenance issue over time.
PVC is a very practical choice for underground runs and other wet or corrosive environments. It does not rust, and it is easy to work with on longer runs. For underground commercial feeder or branch circuits, PVC is often the go-to. The tradeoff is that it has less physical strength than metal conduit, so it is not ideal where the conduit is exposed and vulnerable unless it is specifically protected. On long outdoor runs, PVC also moves more with temperature changes, so expansion fittings matter. People sometimes skip those and end up with bowed or stressed conduit later.
EMT is the lightest and often the cheapest metal option, and it can be perfectly acceptable outdoors if the installation is not in a severe environment and the conduit is not likely to take abuse. The problem is that EMT is thin-walled, and outdoors it is more prone to corrosion and damage than rigid options. In commercial work, I usually see EMT used where the run is under an overhang, on a protected wall, or in a location with less risk of impact.
If you are deciding between the common options, I would think in terms of exposure first. For buried or corrosion-prone sections, PVC. For exposed areas with real physical damage risk, rigid metal or IMC. For lighter-duty protected exterior runs, EMT may be fine if the local code and site conditions allow it. Also, don’t forget that the best choice is the one that matches the local code, the environment, and the long-term maintenance needs, not just the lowest material price.
