Okay, let's break down how to determine the correct EGC size in your pole light scenario, considering the code rule and the exception you mentioned, and avoiding overly complex calculations.

Understanding the Core Concepts Purpose of the EGC

The primary goal of the EGC is to provide a low-impedance path for fault current to return to the source, causing the overcurrent protective device (OCPD - your 20A breaker) to trip quickly. This is crucial for safety, minimizing shock hazards, and preventing equipment damage.

Fault Current Magnitude

The higher the fault current, the faster the OCPD trips. A "weak" EGC (high impedance) limits fault current, delaying the trip and increasing the risk of harm.

Voltage Drop & EGC Sizing

Your scenario involves up-sizing the ungrounded conductors for voltage drop. The general rule requires you to proportionally increase the EGC size, BUT the exception allows for engineering judgment to determine if a smaller EGC is still sufficient to clear a fault.

Step-by-Step Approach1. Determine the Minimum Required EGC Size (Without Upsizing)

Refer to NEC Table 250.122. For a 20A circuit, the minimum required EGC size is typically 12 AWG copper. (Assuming conductors are copper).2.

Calculate the Proportional Increase (If You Were Following the General Rule)

Original Conductor Size: Assuming 12 AWG would normally be used for a 20A circuit. 12 AWG has a circular mil area of approximately 6530 CM (circular mils). Upsized Conductor Size: 8 AWG has a circular mil area of approximately 16,510 CM. Calculate the Ratio: (Upsized CM) / (Original CM) = 16,510 / 6530 = ~2.53 Proportionally Upsized EGC: If you were following the general rule, you'd multiply the CM area of the original EGC by this ratio. Starting with a 12 AWG EGC, you would need to install at least a 6 AWG EGC.3.

Apply the Exception: Evaluating Fault Current and Impedance

This is where the "qualified person" comes in (ideally a licensed electrician or electrical engineer). Here's the thought process: Fault Current Goal

The general rule, as you mentioned, aims for a fault current that is at least 5 times the OCPD rating to ensure a fast trip. In your case, 5 20A = 100A. This is a good rule of thumb, but remember code doesn't state this directly.



Impedance Matters

The key is to ensure the total impedance of the fault current path (source impedance + conductor impedance + EGC impedance) is low enough to allow that level of fault current.

Worst Case Scenario

The longest circuit run (500') is your worst-case for voltage drop and impedance.4.

Simplified Calculation ApproachWe will take a simplified approach and make an assumption on what the voltage will be at the equipment in question when a fault occurs.

Desired Fault Current

100 Amps (5 times the breaker rating)

Assume available Voltage

10 Volts (Voltage will drop dramatically when fault occurs)

Calculate Impedance

R=E/I (10 Volts / 100 Amps = 0.1 Ohms)

Conductor Resistance

We will now calculate resistance of the current carrying and ground conductor based on their material, length, and AWG

8 AWG Copper

500 Feet = .0007785 Ohms per foot (NEC Chapter 9, Table 8) .0007785 Ohms x 1000 Feet (There and back) = 0.7785 Ohms

12 AWG Copper

500 Feet = 0.001982 Ohms per foot x 1000 Feet = 1.982 Ohms

Total Ohms

.7785 + 1.982 = 2.7605

Assessment

The total resistance with a 12 AWG is 2.7605 Ohms. That will leave very little voltage at the equipment in order for the breaker to trip. In order for us to effectively achieve our goal of 0.1 Ohms, a much larger ground than 12 AWG is needed.
5.

Considerations and Potential Solutions



Reduce the Impedance of the EGC

Upsizing the EGC is the most direct way. But other ways to reduce impedance are available.

Utilize Metal Conduit

If your circuit is run in metallic conduit (RMC, IMC, or even properly bonded EMT), the conduit itself serves as a supplementary EGC. This can significantly reduce the overall impedance. The metal conduit provides a parallel path for fault current. You MUST properly bond the conduit at each end to create a reliable path.

Consider an Equipment Grounding Conductor

If you utilize metal conduit, it will serve as the EGC.

Example Solution

In this scenario, the BEST and SAFEST approach would be to install a metallic conduit. The conduit itself acts as the EGC, in parallel with the equipment ground wire, and can handle most of the fault current.The next best solution would be to increase the EGC to a 6 AWG to comply with the general rule.

Important Considerations



Source Impedance

This calculation assumes you know the impedance of the power source (transformer, panel). In reality, this impedance is a factor in the total fault current and needs to be considered for precise engineering analysis.

Coordination Study

For larger or more critical systems, a full coordination study (performed by an electrical engineer) is recommended. This analyzes the entire electrical system to ensure proper OCPD selection and coordination for optimal safety and reliability.

Local Requirements

Always check with your local electrical inspector for specific requirements or interpretations.

Disclaimer: This information is intended for educational purposes only and should not be considered professional engineering advice. A qualified electrician or electrical engineer should always be consulted for the design and installation of electrical systems. Improper grounding can be dangerous and could violate electrical codes. Flag for review