You are correct. According to Table 310.12(A) in the NEC (likely the 2023 edition, based on how people are referencing it), a #4 AWG copper THHN conductor can be used for a 100A single-phase dwelling service or feeder.Therefore, yes, you could use #4 AWG THHN copper conductors to feed a 100A subpanel in a detached residential garage, provided you meet all other requirements of the NEC.Here's why it works and some important considerations: Table 310.12(A) is specific

It offers a permitted ampacity for dwelling services and feeders that can be higher than what you would normally get from Table 310.16 for the same conductor size. This is a long-standing allowance in the NEC to allow for smaller conductors to be used for residential services and feeders, recognizing the specific load characteristics of a dwelling.

Important Considerations



Conductor Termination Temperature

Make sure that the terminations (lugs, breakers, etc.) at both the main panel and the subpanel are rated for at least 75°C. Most are, but it's critical to check. The ampacity used from Table 310.12(A) is predicated on using the 75°C column. If you are using 60°C terminals, you would need to use the 60 degree colum from table 310.16, rendering the #4 unusable for 100amps.

Overcurrent Protection

The overcurrent protection (the 100A breaker) must protect the conductor.

Voltage Drop

Always calculate voltage drop, especially for longer runs. While the #4 AWG may be ampacity adequate, voltage drop might require a larger conductor to ensure proper voltage at the garage.

Grounding/Bonding

A separate equipment grounding conductor (EGC) must be run with the feeder conductors to the garage. Do NOT use the neutral conductor as the grounding conductor. You will also need to have a grounding electrode system in the detached garage, usually driven ground rods.

Conduit/Raceway Fill

Ensure that the conduit or raceway you're using is properly sized to accommodate the conductors and any grounding conductor according to the conduit fill tables in the NEC.

Local Codes

Always check with your local electrical inspector and follow local codes, as they may have amendments or additional requirements beyond the NEC.

Proper Installation

Ensure all work is performed by a qualified electrician and in accordance with all applicable codes and regulations.

In summary: While the NEC permits* the use of #4 AWG copper THHN for a 100A feeder to a detached garage in a residential setting, it's crucial to verify that all other requirements are met, including termination temperature ratings, voltage drop calculations, grounding/bonding, conduit fill, and local codes. Flag for review