You're right, it's a confusing topic and easy to get spun around on! Let's break it down and clarify.

You're mostly correct in your understanding, with a few key adjustments



Table 250.66: Grounding Electrode Conductor (GEC) - This is, as you pointed out, for the conductor that connects the grounding electrode system (ground rods, water pipe, etc.) to the grounded conductor (neutral) at the service equipment. It's not the neutral conductor itself. Think of it as the umbilical cord that connects the neutral to the earth. Table 250.102(C)(1): Grounded Conductors - You're close! This is the table you use for sizing the grounded service conductor (the neutral) in most situations. The table is titled "Grounded Conductors," and it's important to note that the NEC does not use the term "Service" in the title. This table covers sizing the neutral conductor from the service point to the service equipment, as well as the neutral in a feeder or branch circuit.

So where's the confusion? Here's the breakdown


The Key: The "Ungrounded" Conductors (Hots)

The size of your neutral conductor generally depends on the size of your ungrounded service conductors (the hot wires). Table 250.102(C)(1) instructs you to look up the size of your ungrounded service conductors, and then find the corresponding size for your grounded service conductor (neutral) in that table.
2.

Exceptions and Demand Factors

This is where it gets a little more complex. You can reduce the size of the neutral under certain conditions if you can demonstrate that the neutral carries significantly less current than the hot conductors. These situations are typically for dwellings or other situations where you know you have a largely balanced load. This reduction is

permissible by code but requires careful calculation using 220.61. If you are doing a straight neutral sizing calculation, it's usually easier and better to run the full size.

Example

Let's say you have a 200A service with (2) #3/0 AWG copper ungrounded conductors (hots).1. You would go to Table 250.102(C)(1).
2. Find the row that corresponds to your #3/0 copper ungrounded conductors.3. In that row, the table will tell you the minimum size required for your grounded service conductor (neutral). It will tell you that the neutral conductor has to be a minimum of #2 AWG Copper.

In Summary



Table 250.102(C)(1) is your primary table for sizing the grounded service conductor (neutral). It's based on the size of the ungrounded conductors (hots). Table 250.66 is for sizing the Grounding Electrode Conductor (GEC) which connects your grounding electrode system (ground rods, etc.) to the grounded service conductor. It is based on the size of the Service Entrance Conductors (unfrounded). Remember to consider any applicable demand factors (220.61) if you want to reduce the size of the neutral, but be cautious and document your calculations.Important Notes



Always consult the latest edition of the NEC and local codes. When in doubt, upsize the neutral! It's often a small price to pay for peace of mind and future-proofing. A qualified electrician should always perform electrical work to ensure safety and code compliance.* Aluminum vs. Copper: If your service conductors are aluminum, you'll use the aluminum columns in the tables.I hope this clarifies the issue for you. Let me know if you have any further questions! Flag for review