You're on the right track, but there are a few more things to consider besides just adding the resistance of the wire to the ground ring resistance, especially for sensitive electronic devices. While your simple series resistance model is a reasonable starting point for 60Hz, it's important to understand its limitations.Here's a breakdown

1. The Basic Series Resistance Model (and its limitations):
You are correct in principle

For low frequencies like 60 Hz, the impedance of the wire and ground ring will be mostly resistive. In theory, you could add the calculated resistance of the wire to the estimated resistance of the ground ring.

Wire Resistance Calculation

You're using NEC tables for wire resistivity which is good. Remember that resistance is calculated as: `R = (ρ L) / A` where: `R` is resistance (in ohms) `ρ` (rho) is resistivity (ohm-meters, ohm-circular mils/foot, etc. Make sure your units match!) `L` is length (meters, feet, etc.) `A` is cross-sectional area (square meters, circular mils, etc.)

Ground Ring Resistance Estimation

IEEE Std 142 (Green Book) provides methods for estimating the resistance of ground rings. This is usually based on the soil resistivity, the depth of burial, and the geometry of the ring. Be realistic about your soil resistivity value! Testing the soil is the best way to get this number, however, you can use the typical soil resistivity values in IEEE STD 142 (Green Book) if testing is not possible.

Why the simple model might be sufficient

If the wire connecting your ground bar to the ground ring is relatively short and a large gauge (low resistance), and the ground ring is already very low resistance (significantly below 10 ohms), then the added resistance of the wire might be negligible.

2. Factors You Must Consider for Sensitive Electronics



Impedance (Not Just Resistance)

While at 60 Hz the reactance might be small, it's not zero. For sensitive electronics, you need to think about impedance (Z), which includes both resistance (R) and reactance (X): `Z = R + jX` where `j` is the imaginary unit (√-1). Reactance arises from inductance (L) and capacitance (C) in the wiring and the ground system.

Inductance is Key

The inductance of the wire run is the most likely culprit for problems with sensitive electronics. A long, straight wire has significant inductance. Changes in current through this inductance will generate voltage drops, even with a low resistance. This is because voltage drop across an inductor is given by V = L di/dt.

Ground Loops

This is a major concern. Creating multiple ground paths can lead to circulating currents (ground loops) that inject noise into your sensitive electronic equipment.

High-Frequency Noise

Sensitive electronics are often susceptible to high-frequency noise (MHz and GHz ranges). At these frequencies, inductance and capacitance become much more significant, and the simple resistance model is useless. The ground wire can act as an antenna, picking up noise.

Skin Effect

At higher frequencies, current tends to flow on the surface of the conductor (skin effect), effectively reducing the cross-sectional area and increasing resistance.

Transient Events

Lightning strikes and other transient events generate very high-frequency currents. The inductance of the ground wire will impede these currents, potentially causing voltage spikes.

Electromagnetic Interference (EMI)

The grounding system plays a role in managing EMI. Proper grounding helps to prevent equipment from radiating or being susceptible to EMI.

3. Recommendations for a Good Ground for Sensitive Electronics



Short, Wide Conductors

Minimize the inductance of the ground wire by using:

Short Lengths

Keep the wire run as short as possible.

Wide Conductors

Use a large gauge wire or, even better, a grounding strap or braid. These have much lower inductance than round wire. The increased surface area and geometry minimize skin effect and inductance.

Avoid Sharp Bends

Sharp bends in the wire increase inductance.

Single-Point Grounding (Star Grounding)

Connect all sensitive electronic devices to the same ground bar. This minimizes the potential for ground loops.

Dedicated Ground Wire

Run a dedicated insulated ground wire from the ground bar to the ground ring. Do not rely on conduit or building steel for the ground path.

Bonding

Bond all metal enclosures and chassis of equipment to the ground bar. Use short, wide bonding straps.

Grounding Bar Location

Position the ground bar as close as possible to the sensitive equipment.

Signal Reference Grids (If Necessary)

For extremely sensitive equipment, you might need to create a signal reference grid (SRG) or a mesh bonding network. This is a more complex approach that involves creating a grid of interconnected conductors under the equipment. Only do this if your equipment documentation specifically calls for it and you understand the principles. Improperly implemented SRGs can make things worse.

Soil Resistivity Testing

If possible, conduct soil resistivity testing to get an accurate reading for your location. Don't rely solely on generic values.

Measure Impedance, Not Just Resistance

Use an impedance meter to measure the impedance from your grounding bar to the ground ring at various frequencies. This will give you a better understanding of the ground system's performance.

Consider Surge Protection

Install surge protection devices (SPDs) at the service entrance and at the equipment level to protect against transient events.

Consult with an Expert

If you're dealing with very sensitive equipment or have complex grounding requirements, consult with a qualified electrical engineer who specializes in grounding and bonding.

In summary:

While your initial approach of calculating the series resistance of the wire and ground ring is a basic starting point, it's insufficient for ensuring a reliable and noise-free ground for sensitive electronic devices. You must* focus on minimizing inductance, avoiding ground loops, and considering the effects of high-frequency noise. Prioritize short, wide conductors, single-point grounding, and proper bonding. And don't hesitate to seek expert help if needed. Flag for review