Okay, let's break down the wire sizing and grounding requirements for your 100-amp subpanel in a residential garage, considering the 2020 NEC (since that's a very common reference point).1. Wire Sizing for a 100-Amp Subpanel The Key is Termination Temperature Rating

The column you use in Table 310.16 depends on the

temperature rating of the terminals on your breakers and panel. This is critical. Most residential-grade breakers and panels are rated for 60°C or 75°C. You must check the equipment labeling to be sure. Common Scenarios



60°C Terminals

If your breakers and panelboard are specifically marked for 60°C terminations, you must use the 60°C column in Table 310.16.

75°C Terminals

If your breakers and panelboard are marked for 75°C terminations, and the wire you're using is also rated for 75°C (or higher), you can use the 75°C column. Many modern breakers and panels are rated 75°C, but always verify.

No Marking

If the terminations are not marked, you default to the 60°C column according to NEC 110.14(C)(1).

Table 310.16 Values (Simplified)



60°C Column

For a 100-amp circuit, you typically need at least #3 AWG copper wire or #1 AWG aluminum wire.

75°C Column

For a 100-amp circuit, you typically need at least #4 AWG copper wire or #2 AWG aluminum wire.

Why the Difference? Higher temperature-rated conductors can carry more current for a given wire size because they are designed to operate at a higher temperature. Why the Large Wire Size? A 100-amp subpanel provides a significant amount of power, and the conductors must be sized accordingly to safely handle the potential load and prevent overheating.

Important Considerations for Wire Sizing



Voltage Drop

Longer wire runs can experience voltage drop, which can negatively affect the performance of appliances and equipment. It's always a good practice to calculate voltage drop, especially for runs over 100 feet. Online voltage drop calculators are readily available. If voltage drop is excessive, you might need to upsize the conductors.

Conduit Fill

Make sure the conductors you choose will fit properly within the conduit you are using. Table 1 in Chapter 9 of the NEC provides conduit fill information.

Ampacity Correction and Adjustment

If you are running multiple current-carrying conductors in the same conduit or if the ambient temperature is significantly higher than 30°C (86°F), you may need to apply ampacity correction and/or adjustment factors per NEC Table 310.15(C)(1) and 310.15(B)(3)(a). This will potentially require larger conductors. This is more common in commercial or industrial settings, but it's good to be aware of.

2. Grounding Requirements Metal Raceway as Grounding Conductor

Yes, you can use EMT (Electrical Metallic Tubing) or other approved metallic raceway as the equipment grounding conductor, provided that the raceway is installed according to the NEC requirements and provides a continuous, low-impedance ground path back to the main panel. NEC 250.118 lists acceptable equipment grounding conductors, and metal raceways are included.

Supplemental Ground Wire (Typically Required)

While the metal raceway can be used, it is almost universally recommended (and sometimes required) to also run a separate equipment grounding conductor (ground wire) inside the raceway. This provides an additional layer of protection and ensures a reliable ground path, especially in situations where the raceway connections might loosen or corrode over time.

Ground Wire Size

The size of the equipment grounding conductor is based on the overcurrent protection device (the 100-amp breaker) protecting the circuit, according to NEC Table 250.122. For a 100-amp breaker, you'll typically need at least a #8 AWG copper ground wire.

Grounding Electrode System (Ground Rod)

You will also need to have a connection to a grounding electrode system at the subpanel location (in the garage). This is in addition to the equipment grounding conductor that runs back to the main panel. The grounding electrode system typically consists of one or more ground rods, ground plates, or connections to a metallic water pipe (where permitted). NEC Article 250 covers grounding electrode requirements. This is critical for safety.

Isolated Ground Receptacles

If you have any isolated ground receptacles, consult the NEC for specific grounding requirements for those types of receptacles.

In Summary


Determine Termination Temperature

Check the temperature ratings of your breakers and panelboard. This dictates which column of Table 310.16 to use.
2.

Size Conductors

Use the appropriate column to determine the minimum conductor size for 100 amps. Account for voltage drop and any ampacity correction/adjustment factors.3.

Run a Ground Wire

Run an equipment grounding conductor (ground wire) inside the raceway.4.

Grounding Electrode System

Install a grounding electrode system at the subpanel location (typically a ground rod).5.

Bonding

Bond the metal enclosure of the subpanel to the equipment grounding conductor.

Disclaimer: I am an AI chatbot and cannot provide electrical code advice. Electrical work can be dangerous and should only be performed by qualified and licensed electricians. This information is for general educational purposes only, and you should consult with a qualified electrician to ensure that your installation complies with all applicable codes and regulations. Always refer to the latest edition of the National Electrical Code (NEC) and local amendments for specific requirements in your area. Flag for review