You've raised a very interesting and nuanced question about grounding requirements when running multiple circuits, especially with different voltages, through a single non-metallic conduit or using NM-B cable. Let's break it down based on the NEC

Key Considerations & NEC Sections

1. Individual Circuit Requirements

Each circuit within the conduit or NM-B still has to independently meet the grounding requirements for that circuit's ampacity. That's fundamental.2.

Simultaneous Fault Consideration (NEC 250.122)

The crux of your question hinges on the simultaneous fault scenario. NEC 250.122 dictates the sizing of equipment grounding conductors (EGCs) based on the overcurrent protective device protecting that specific circuit. This is where things get interesting when circuits are bundled together. There is no requirement to upsize grounding conductors to cover more than the largest OCPD.3.

NM-B Cable (Article 334)

NM-B cable already includes an EGC that is sized appropriately for the circuit ampacity it's carrying.4.

Non-Metallic Conduit (Article 352)

PVC and other non-metallic conduits always require a separate EGC to be installed within the conduit, as the conduit itself is not a grounding means.5.

Identifying Conductors (NEC 200.6, 210.5, 310.8(A))

Proper identification of conductors is critical, especially in multi-wire branch circuits and when using different voltage systems. Using colored tape or other approved means is often necessary.

Analyzing Your Scenarios Scenario 1: NM-B Cable (e.g., 12/4 NM-B)

This is generally not a compliant or safe way to do this, nor would any manufacturer list a 12/4 cable as being suited to this use case.

Issue

You are not compliant and you can not identify the conductors of the 240V circuit.

General comment on 12/4 NM-B

While you can run two 120V circuits on two of the hots, you need to identify the other hot, typically with red or black tape on the white conductor, per 200.7, but you aren't required to use a dual pole breaker.

Scenario 2: Two Separate 12/2 NM-B Cables

This is the common method, although perhaps not the most intuitive.

Issue

There is not an obvious way to tell that there is 240V present in the box. The white conductor needs to be reidentified and a warning label in the box would be advisable.

Pro

Each circuit has its own EGC.

Scenario 3: PVC Conduit with Individual Wires



Minimum Requirement

You must run at least one EGC sized according to the largest overcurrent protection device present in the conduit (250.122). In your case, both circuits are 20A, so a single 12 AWG copper EGC is the minimum.

Better Practice

Running a separate EGC for each circuit is a better practice, even though not strictly required. This provides a more direct path for fault current for each circuit and may reduce impedance.

Important

You must properly identify all conductors within the conduit using color-coding or labeling.

Specific Answers to Your Questions



"Is one 12Ga ground sufficient or does there need to be an EGC return sufficient for a simultaneous fault of both circuits?" Strictly according to the NEC, a single 12 AWG EGC is the minimum if both circuits are protected by 20A breakers. However, for enhanced safety and lower impedance grounding, running a separate 12 AWG EGC for each circuit is a superior practice. The NEC does not require you to size the EGC for a theoretical simultaneous fault current. "Or could you feed this box with 12/4 NM-B?" No. As stated above this violates many portions of the NEC.

Why the NEC Doesn't Explicitly Address "Simultaneous Fault" Sizing

The NEC focuses on protecting each circuit individually. The assumption is that a fault on one circuit is the primary concern. Designing for theoretical simultaneous faults on multiple circuits in a residential/commercial setting would lead to significantly larger EGCs and is generally considered impractical and not cost-effective. The existing EGC sizing rules provide an acceptable level of safety.

In summary

For your scenario, running individual 12/2 NM-B cables or using PVC conduit with individual conductors and at least one 12 AWG EGC is code-compliant. Using PVC conduit with a separate 12 AWG EGC for each circuit is a better practice. Properly identifying the conductors is crucial, especially the hot conductor of the 240V circuit to show that it is not neutral.

Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. Consult with a licensed electrician for all electrical work. They will ensure the installation complies with all applicable codes and regulations in your specific location. Flag for review