Yes, you are generally correct. Let's break down the requirements and reasoning

Romex (NM-B) Wiring:
Requirement

For a 50-amp circuit, you typically need 6 AWG copper conductors (or equivalent aluminum, but we'll stick to copper for this discussion). Therefore, 6/3 NM-B with ground is the standard choice for this application. The '3' indicates two current-carrying conductors (hot legs) and one neutral conductor, plus the separate ground wire.

EMT Conduit with THHN/THWN Conductors



Ampacity and Correction Factors

The ampacity of a wire depends on its insulation type and the ambient temperature. THHN and THWN are common insulation types suitable for conduit. At 75°C rating they have an ampacity of 50 amps.

Grounding Conductor Size

The Equipment Grounding Conductor (EGC) doesn't need to be the same size as the ungrounded conductors. The NEC (National Electrical Code) specifies the EGC size based on the overcurrent protection device (in this case, the 50-amp breaker). For a 50-amp circuit, a #10 AWG copper EGC is generally sufficient, assuming the length of the run and voltage drop considerations don't require a larger size.
Why the Difference? Heat Dissipation

Conductors in conduit have better heat dissipation compared to Romex bundled within walls. Romex is often derated due to the increased temperatures when bundled.

Conductor Rating

THHN/THWN insulation types generally have higher temperature ratings than NM-B cable. This allows them to carry more current for a given wire size.

Flexibility

Using individual conductors in conduit gives you the flexibility to use specific wire types that are rated for the environment and application.

Important Considerations and Recommendations


Local Codes

Always check with your local electrical inspector or permitting office. They have the final say and may have specific requirements or interpretations of the NEC.2.

Voltage Drop

For long runs, voltage drop can become a factor. Calculate the voltage drop to ensure it's within acceptable limits (typically 3% for branch circuits and 5% total). A longer run might require you to upsize the conductors to compensate. Voltage drop calculators are readily available online.3.

Conduit Fill

Ensure that the EMT conduit is sized appropriately to accommodate the conductors. NEC Table 1 in Chapter 9 outlines conduit fill requirements. Overfilling a conduit can make pulling wires difficult and can also lead to overheating.4.

Grounding

Make sure the EMT conduit itself is properly grounded. This is usually achieved by using listed EMT fittings and ensuring good mechanical connections. The #10 ground wire needs to be connected properly at both ends (generator inlet box and main panel).5.

Professional Installation

If you're not comfortable with electrical work or unsure about any aspect of the installation, it's best to hire a qualified electrician. Improper wiring can be dangerous and can lead to fires or electric shock.6.

NEC (National Electrical Code)

All electrical installations must comply with the NEC, as adopted by your local jurisdiction. The NEC is updated every three years, so make sure you are using the most current version.

Summary

Yes, in most cases, using #8 THHN/THWN for the two hots and neutral, and a #10 THHN/THWN for the ground within an EMT conduit, is acceptable for a 50-amp generator inlet circuit. HOWEVER, confirm this with your local electrical inspector and consider the specific run length and potential voltage drop issues. Also make sure to install to all electrical codes. Flag for review