Okay, let's break down your plan and address the NEC concerns for your residential rewire project. This advice is based on common NEC interpretations, but it's crucial to consult your local electrical inspector and follow their specific requirements. They have the final say.

Regarding Shared Neutrals (Multi-Wire Branch Circuits)



Yes, you CAN use a shared neutral, BUT it's limited to Multi-Wire Branch Circuits (MWBCs) and has specific requirements. What you're describing sounds like you're trying to create multiple MWBCs. NEC Article 210.4 covers Multi-Wire Branch Circuits. A MWBC is a circuit consisting of two or more ungrounded (hot) conductors having a voltage between them, and a grounded (neutral) conductor having equal voltage between it and each ungrounded conductor of the circuit and which is connected to the neutral or grounded conductor of the system. Crucial Requirement: Common Disconnect. NEC 210.4(B) requires that all hot conductors of a MWBC originate from the same panel and are disconnected simultaneously by a single device (multi-pole breaker). This is critical for safety. You cannot have the hot conductors on separate single-pole breakers. Neutral Size

The neutral conductor must be sized appropriately for the maximum unbalanced load. In a purely resistive load situation (lights, heaters), the neutral will likely carry the same current as the hot conductors. If you have significant non-linear loads (electronics with switching power supplies, LED lighting), the neutral current can be higher due to harmonics. Consider oversizing the neutral if you anticipate a high percentage of non-linear loads.

Example

You can run a 12/3 cable (12 AWG hot, hot, neutral) from a 2-pole 20A breaker to supply two separate circuits. Each hot would serve its own receptacles/lights, but they share the neutral. The breakers must be a two-pole breaker. If you were to run 12/2 to two separate circuits sharing a neutral, they must be controlled by a 2-pole breaker.

Why This Matters

Without the common disconnect (multi-pole breaker), if one hot conductor shorts to ground, the neutral can become energized on the other circuit(s) that share it. This is a serious safety hazard. Also, without a properly sized neutral, it can be overloaded.

Can I do what was done before?

No. I can guarantee that the existing batch of similar circuits with four breaker fed branches that share one larger neutral, which passed inspection, does not meet NEC, assuming they are controlled by individual breakers. This is not allowed.

Regarding Your Plan to Run All Wires to a Junction Box



Junction Box Fill

You are correct that you need to calculate the box fill based on the number of conductors, devices (splices, etc.), and equipment grounding conductors entering the box. NEC Article 314 covers outlet boxes, junction boxes, and handhole enclosures.

Number of Conductors

NEC 314.16 dictates the maximum number of conductors allowed in a box based on its size (volume).

Clamps

You also need to consider space for clamps or connectors if required.

Derating

If you have many current-carrying conductors in a single conduit or junction box, you may need to derate the ampacity of the conductors (NEC 310.15(C)).

Physical Accessibility

While a large junction box can theoretically handle many circuits based on fill calculations, practical accessibility becomes a concern. Working on circuits crammed into a tight space is difficult and can lead to mistakes.

Practical Recommendation

While the code might allow for a massive junction box with 15+ circuits, it's

strongly discouraged for residential work. It's better practice (and often safer) to use multiple smaller junction boxes or a sub-panel.

Is there a maximum amount of separate circuits that can be run to one junction box regardless if the physical size of the box is large enough for x circuits?

Yes. As mentioned above, the NEC determines the maximum number of conductors allowed in a box based on its size, which ultimately limits the number of circuits.

Switching to a Sub-Panel: The Better Approach

Given your scenario, running a sub-panel is the recommended and safer solution. Here's why: Simplifies Wiring

Reduces the complexity and congestion of a large junction box.

Better Organization

Provides a dedicated space for each circuit, making troubleshooting and future modifications easier.

Code Compliance

More straightforward to comply with NEC requirements for conductor fill, derating, and accessibility.

Future Expansion

Offers capacity for future circuits without having to add more junction boxes.

Safety

Reduces the risk of accidental shorts or miswiring in a crowded junction box.

How to Implement the Sub-Panel


Calculate Load

Determine the total load you expect on the sub-panel. This will dictate the size of the sub-panel (number of breaker spaces) and the size of the feeder conductors (wires running from the main panel to the sub-panel).2.

Feeder Conductor Size

Size the feeder conductors according to the calculated load, ampacity tables in the NEC (Table 310.16), and any derating factors. Use THWN conductors for wet or damp locations.
3.

Conduit Size

Select the appropriate conduit size to accommodate the feeder conductors (based on NEC Chapter 9, Table 4).4.

Grounding

Run a separate equipment grounding conductor (EGC) from the main panel to the sub-panel.

Do not use the neutral conductor as the grounding conductor in the sub-panel. Bond the sub-panel enclosure to the EGC.5. Sub-Panel Installation

Install the sub-panel according to the manufacturer's instructions and NEC Article 408 (Switchboards and Panelboards). Ensure proper bonding of the enclosure.6.

Breaker Installation

Install the appropriate breakers in the sub-panel for each circuit.7.

Wiring

Run individual circuits from the sub-panel to their respective locations using appropriate wiring methods (Romex, conduit, etc.).8.

Labeling

Clearly label all circuits in both the main panel and the sub-panel.

Proposal to Submit to the County

Your proposal should clearly outline your plan, including:1.

Load Calculation

Include a detailed load calculation for the proposed sub-panel.2.

Sub-Panel Specifications

Specify the size (number of breaker spaces) and ampacity rating of the sub-panel.3.

Feeder Conductor Information

Specify the size, type (THWN), and quantity of the feeder conductors.4.

Conduit Specifications

Specify the size and type of conduit.5.

Grounding Method

Describe how the sub-panel will be grounded (separate EGC).6.

Wiring Methods

Specify the wiring methods to be used for branch circuits (Romex, conduit, etc.).7.

Diagram

Include a simple diagram showing the wiring configuration from the main panel to the sub-panel, including the grounding connection.8.

Statement of Compliance

State that the installation will comply with the latest edition of the NEC (specify the edition adopted by your local jurisdiction) and any applicable local amendments.

Example Proposal Snippet

"This proposal outlines the installation of a 125A sub-panel in the basement to distribute power to branch circuits throughout the house. The sub-panel will be fed from the main panel in the garage using 3 #2 AWG THWN conductors (2 hot, 1 neutral) and a #8 AWG THWN equipment grounding conductor, all run in 1 1/4" PVC conduit. The sub-panel will be grounded via a separate equipment grounding conductor back to the main panel. All branch circuits will be wired using NM-B (Romex) cable and will be protected by appropriately sized circuit breakers in the sub-panel. The installation will comply with the 2020 National Electrical Code and all applicable local amendments."

Important Considerations



Local Amendments

Your local jurisdiction may have amendments to the NEC. Make sure you are aware of these and incorporate them into your plan.

Inspection

Be prepared for inspections at various stages of the project (e.g., before covering wiring).*

Safety: Always prioritize safety. Work de-energized whenever possible. Use proper personal protective equipment (PPE). If you are unsure about anything, consult with a qualified electrician.By taking a thorough and well-documented approach, you can increase the likelihood of your proposal being approved and ensure a safe and code-compliant electrical system for your home. Good luck! Flag for review