Okay, let's break down the electrical calculations and the GFCI issue.1. Wire Size Calculation

Yes, #2 AWG Aluminum for a 320-foot underground run at 20 amps and 120 volts with a 4% voltage drop sounds about right. Voltage drop is significant over long distances. Here's why: Voltage Drop Matters

A 4% voltage drop on 120V is 4.8 volts. This means you'd be getting approximately 115.2 volts at the end of the run. While your devices might still operate, they won't be running as efficiently, and some sensitive electronics might have issues. Exceeding the acceptable voltage drop can also damage motors over time.

Aluminum vs. Copper

Aluminum has a higher resistance than copper, so for the same amperage and voltage drop, you need a larger gauge of aluminum wire. This is why #2 AWG aluminum is needed, while a smaller gauge copper wire would suffice.

Online Calculators are Helpful

Online voltage drop calculators are excellent for getting a quick estimate, but they are only a tool. They depend on the numbers that you input and can't know the particulars of your situation.

Important Considerations for your calculation



Temperature

The temperature of the wire affects its resistance. Underground wire is generally more stable than in-air but may vary between summer and winter. If you can set the temp in the calculator, do so.

Load Type

Some calculators allow you to choose an inductive load (motor, etc.). Motors have an in-rush current that affects the voltage drop.

Continuous Load

20 amps 80% derate = 16 amps, is that still a good number?

Future Use

Consider whether you might need more power in the future. Over-sizing the wire now can save you trouble later.

Local Codes

Always, always consult with your local electrical codes and a qualified electrician. They will have the final say on what is safe and compliant in your area.

Calculator Accuracy

If you have concerns about the accuracy of one calculator, try a few different ones and compare the results.

2. GFCI Tripping Issues on Long Runs

The issue of GFCI outlets tripping on long runs is a real phenomenon, and here's why: Capacitance

Long wires, especially underground, act like capacitors. Capacitance is the ability to store an electrical charge.

Leakage Current

The capacitance between the hot (ungrounded) and neutral (grounded) conductors in the cable can cause a small leakage current to flow to ground.

GFCI Sensitivity

GFCIs are extremely sensitive. They are designed to trip if they detect a difference of as little as 5 milliamps (0.005 amps) between the current flowing in the hot and neutral wires.

Cumulative Effect

Over a long run, even a tiny amount of leakage current per foot of cable can add up. This cumulative leakage current might be enough to exceed the GFCI's trip threshold, causing it to trip even when there isn't a true ground fault.

GFCI Location: Beginning vs. End GFCI at the Beginning (Feeding the Run)

This is the most common and often the most convenient approach. However, it's also the most likely to cause nuisance tripping on long runs due to the cumulative leakage current described above.

GFCI at the End (Protecting the Receptacle/Device)

This is often the recommended solution for long underground runs to minimize nuisance tripping. By placing the GFCI at the end, you are only protecting the device or receptacle connected to it, and not the entire length of the cable. This reduces the cumulative leakage current that the GFCI "sees."

Trenching Depth and GFCI Location

You mentioned wanting to trench more shallowly if you feed the GFCI from the beginning. Trenching depth has nothing to do with GFCI location. It is determined by code, and must be followed regardless of GFCI choice or location. NEC Requirements

The National Electrical Code (NEC) specifies minimum burial depths for underground wiring. For example, direct-burial cable for 120V circuits might require 12 inches of cover in residential areas. If you are running in conduit, you may be able to get away with a shallower trench, but it depends on the conduit type and local codes.

Protection

The purpose of the burial depth is to protect the cable from physical damage (shovels, digging, etc.).

Solutions to Reduce Nuisance Tripping

If you really want to feed the long run from a GFCI breaker or outlet at the source, here are some strategies to try:1. Use a GFCI Breaker with a Higher Trip Threshold

Some GFCI breakers have a slightly higher trip threshold (e.g., 10mA instead of 5mA). These are sometimes called "Equipment Ground Fault Protection" (EGFP) breakers. However, check local codes to ensure that they are allowed in your specific application. Also, keep in mind that they offer slightly less sensitive protection.2.

High Quality Cable

Use high-quality underground cable with good insulation and shielding. This can minimize leakage current.3.

Separate Grounding Conductor

Make sure you have a dedicated grounding conductor that is properly connected at both ends.4.

Reduce the Run Length

If possible, try to shorten the overall run length to reduce capacitance.5.

Test Before Burying

Once you have the wiring run, test the GFCI before burying the cable. This will allow you to troubleshoot any tripping issues more easily.6.

Use a Line Reactor/Filter

In some cases, a line reactor or filter installed at the beginning of the run can help to reduce noise and leakage current. Consult with an electrical professional for recommendations.

Recommendation1. Consult with a Licensed Electrician

This is the most important step. They can assess your specific situation, local codes, and recommend the best solution.2.

Consider GFCI at the End

For long underground runs, placing the GFCI at the end is often the most reliable way to avoid nuisance tripping.3.

Adhere to Trenching Depth: Always follow local electrical codes regarding minimum burial depths for underground wiring.I hope this comprehensive explanation helps! Let me know if you have any other questions. Flag for review