You've hit upon a common point of confusion and a situation where understanding the intent of the NEC, along with local regulations, is crucial. Here's a breakdown of what you're likely missing

1. NEC Minimum vs. Engineering Judgment & Local Amendments
NEC as a Minimum

The National Electrical Code (NEC) provides minimum safety standards. It's a baseline. It doesn't guarantee perfect performance in all situations or cover every conceivable scenario. It sets the lowest acceptable standard for safety.

Engineering Judgment

Engineers and designers often exceed the NEC requirements based on specific project needs, risk assessment, or local regulations.

Local Amendments/Specifications

Many jurisdictions (cities, counties, states) adopt the NEC but can also add their own amendments or requirements. A specification like the Houston 154 TC can override the NEC if it is more stringent.

2. The Purpose of the Integral Ground Equipotential Bonding

The integral ground in TC cable serves primarily as an equipotential bonding conductor. Its main purpose is to maintain the metallic cable assembly at approximately the same potential, reducing touch potential for personnel during a fault condition.

Fault Current Return, But Limited

While it can carry fault current, its smaller size means it's not intended to be the primary path for a large ground fault.

Grounding Electrode Conductor vs. Equipment Grounding Conductor

It is NOT a replacement for the equipment grounding conductor required by the NEC.

3. Why the NEC Requires the Larger Equipment Grounding Conductor (EGC) Fault Current Capacity

The EGC (Equipment Grounding Conductor) must be sized to safely carry the available fault current back to the source (usually the transformer) to allow the overcurrent protective device (circuit breaker or fuse) to operate quickly and clear the fault. The larger conductor size minimizes voltage rise on the grounding system during a fault.

Impedance Reduction

The larger EGC lowers the impedance of the ground fault return path. Lower impedance leads to higher fault currents, which is what you want to trip the breaker quickly.

Safety

A faster trip drastically reduces the duration of a ground fault, lowering the risk of electrical shock, arc flash, and equipment damage.

4. You're Correct: You Need the Separate EGC.

Based on your description and the NEC sizing chart (typically 250.122), you're likely required to run a separate, code-compliant equipment grounding conductor (in your case, at least a 1/0 AWG) in addition to the integral ground in the TC cable.5. What You're Missing: The Critical Difference in Function Integral Ground (Smaller)

Primarily equipotential bonding, provides a path for limited fault current, helps with noise reduction. It's usually a smaller gauge.

Equipment Grounding Conductor (Larger)

Primarily a high-capacity fault current return path for safety. It's sized to trip the overcurrent protection quickly. It also contributes to equipotential bonding, of course.

6. Why the TC Spec is Calling for a Specific Wire Size Project Specification

Your specific TC cable specification (Houston 154) likely calls for a 3 AWG integral ground wire. It might be specific to the application or project requirements.

Additional Requirements

The TC spec can be for more than just safety, such as reducing electrical noise, reducing inductive interference in control and instrumentation systems, or meeting industry standards.

In summary

The integral ground in TC cable is a supplemental grounding feature, not a replacement for the NEC-required equipment grounding conductor. The NEC mandates a larger EGC to ensure adequate fault current capacity for safety. Local amendments and project specifications can be more stringent than the NEC.

Recommendations


Consult with an engineer

If there's any doubt, consult with a qualified electrical engineer to ensure you're meeting all applicable codes and specifications.2.

Check local amendments

Always verify if your jurisdiction has any amendments to the NEC that might affect grounding requirements.3.

Clarify the specification

Talk to the TC cable manufacturer or the project specifier to understand the rationale behind the 3 AWG integral ground requirement in the Houston 154 spec. It's possible there's a specific reason beyond just minimum code compliance.4.

Document everything: Keep thorough documentation of your grounding design, including code references, calculations, and any deviations from standard practice. This will be helpful during inspections.You're thinking about this correctly – it's always best to err on the side of safety and compliance when dealing with electrical systems! Good luck. Flag for review