Okay, let's break down the grounding situation for your subpanel and what the NEC says about it. It's crucial to get this right for safety.

Understanding the Situation Old Subpanel, No Ground

This is common in older installations. It presents a safety hazard because it relies on potentially unreliable paths for fault current.

Both Panels in House

This simplifies things compared to having a detached building, but a proper ground is still essential.

FMC Conduit

Flexible metal conduit can be used as a grounding conductor if it meets specific conditions (more on this below).

Desire for Separate Ground

Your instinct to run a separate grounding conductor is correct and the best way to go.

NEC Requirements

The NEC does allow you to run a separate grounding conductor without conduit in your situation, but there are specific requirements you must adhere to.

Here's the breakdown of the relevant NEC sections and considerations


General Grounding Electrode Conductor (GEC) Sizing (NEC 250.66)

This section dictates the size of the grounding electrode conductor based on the size of the ungrounded service entrance conductors (the wires coming from the utility). However, in your case, you're dealing with a subpanel (a separately derived system) not the service entrance. So this section doesn't apply directly to the grounding electrode of the subpanel.2.

Equipment Grounding Conductor (EGC) Sizing for Subpanels (NEC 250.122)



This is the important one. This section covers the sizing of the equipment grounding conductor for your subpanel. The size of the EGC (your ground wire) is determined by the size of the overcurrent protection device (the breaker) protecting the circuit feeding the subpanel. Since you have a 50-amp breaker feeding the subpanel, you must reference 250.122(B) and Table 250.122 to find the minimum size of the equipment grounding conductor. Looking at the table, a #10 AWG copper conductor is sufficient for a 50-amp overcurrent device. So your 10 AWG THHN is sized correctly.3. Installation of Equipment Grounding Conductors (NEC 250.130)

If a grounding means exists within the enclosure, the EGC must be connected to this means. 250.130(C) says that the Grounding conductor connection shall be made at any point on the grounding electrode system or on the grounding conductor or at the equipment grounding terminal bar within the enclosure.
4.

Protection of Conductors (NEC 300.4)

300.4(D) requires protection where subject to physical damage. In an unfinished basement, it might be prudent to protect it by running it along a wall or otherwise where it is less susceptible to getting bumped. If running through bored holes in wood, you need to be a minimum of 1-1/4" from the nearest edge of the wood or it must be protected by a steel plate at least 1/16" thick.5.

Type THHN Wire and Installation (NEC 310.10(C))

THHN wire is a common and acceptable choice for this application. It's rated for 600 volts and 90°C in dry locations. You can run THHN loosely through joist spaces (unless subject to physical damage in which case it must be protected.) Securely fasten it to the joists with appropriate staples or cable ties rated for the application, ensuring you don't damage the insulation.

Key Considerations and Best Practices Separate Grounding Bar

Ensure the subpanel has a dedicated grounding bar separate from the neutral bar. The neutral and ground should only be bonded at the main panel.

FMC Conduit as a Ground? While FMC can be used as a grounding conductor under specific circumstances, it's generally not recommended as the sole grounding path, especially for longer runs. Reliance on FMC alone can be problematic due to potential loose connections or corrosion over time. Adding the #10 AWG EGC is much better. Terminations

Use appropriate connectors and ensure all connections are tight and clean. Use anti-oxidant paste on aluminum lugs.

Continuity

After installation, test the continuity of the grounding conductor with a multimeter to ensure a solid, low-resistance path.

Local Codes

Always check with your local building department or a licensed electrician to ensure compliance with local codes and interpretations. Local codes can sometimes be stricter than the NEC.

Licensed Electrician

If you're not completely comfortable with electrical work, it's best to hire a qualified, licensed electrician. Electrical work can be dangerous, and mistakes can have serious consequences.

In Summary

Yes, you can run a 10 AWG THHN wire as a grounding conductor from your main panel to your 50-amp subpanel without conduit, as long as you follow the NEC guidelines, especially NEC 250.122. Ensure it's properly connected, protected from physical damage where necessary, and that you check your local codes. This is a significant safety upgrade for your electrical system.

Disclaimer: I am an AI chatbot and cannot provide electrical advice. This information is for educational purposes only and should not be substituted for the advice of a qualified, licensed electrician. Electrical work is dangerous, and you should always consult with a professional. Flag for review