Okay, let's break down this situation and address your concerns. You're right to be concerned about voltage drop, especially with that long run.

Understanding the Issues Voltage Drop

Voltage drop is the reduction in voltage that occurs along a conductor due to its resistance. Excessive voltage drop can cause appliances to malfunction, lights to dim, and motors to overheat. The longer the wire run and the higher the current, the greater the voltage drop.

Neutral Conductor Sizing

The neutral conductor carries the unbalanced load back to the source. The NEC (National Electrical Code) has rules about neutral sizing, and it's generally permitted to be smaller than the ungrounded conductors in certain situations (especially with ranges, ovens, and dryers), BUT

this doesn't necessarily apply here. Mobile Home Feeder

While mobile home feeder can be used for other applications (like your barn subpanel), it's crucial to ensure it meets all the requirements for the specific installation. You can't just assume it's okay because it has the right size conductors for some of the load.

Analysis of Your Specific Situation1. Individual Circuit Considerations



20A Lighting/Receptacle Circuit (L1)

This circuit's contribution to the overall voltage drop will be relatively small compared to the 30A camper circuit.

30A Camper Circuit (L2)

This is the critical circuit that drives the large conductor requirement. You've correctly identified that a 2/0 conductor might be needed to keep the voltage drop under 5% at 620 feet.2.

Mobile Home Feeder & Neutral



The Problem

Your concern about the #1 neutral is valid. While the #1 conductor can physically carry 30A, the issue is whether it will contribute to unacceptable voltage drop under unbalanced load conditions.

Why it matters: When the camper is consuming 30 amps on one leg, that current MUST RETURN on the neutral. The neutral conductor will experience its own voltage drop. This voltage drop gets ADDED to the voltage drop of the line conductor. The total voltage drop experienced by the camper will be the SUM of the voltage drop from the LINE (hot wire) and the NEUTRAL.

Can You Use the Mobile Home Feeder?Probably not without carefully analyzing all the factors. Here's why:1. Voltage Drop on the Neutral

With a 30A load solely on one leg, the #1 neutral will contribute significantly to the total voltage drop. Southwire's calculator likely only accounts for the line conductor voltage drop, not the total which includes neutral voltage drop.2.

Code Compliance

NEC 220.61 allows for a reduced neutral size under certain conditions,

but it is typically related to ranges, ovens, and dryers due to the predictable nature of those loads. It's very unlikely to apply to your camper circuit. The code generally requires the neutral to be sized adequately for the maximum unbalanced load. Your situation is a worst-case scenario for an unbalanced load, so it is likely that the neutral will need to be sized to the maximum ampacity.3. Calculations & Documentation

You need to perform accurate voltage drop calculations that include the neutral conductor's contribution. You must be prepared to show these calculations to the electrical inspector.

Recommendations1. Accurate Voltage Drop Calculations (INCLUDING NEUTRAL)

Use a voltage drop calculator, but manually calculate the voltage drop in both the line (hot) and neutral conductors. Then, add the two voltage drops together to get the total voltage drop at the camper. Consider using a more sophisticated voltage drop calculator that allows for unbalanced loads and separate conductor entries (some professional software does this).2.

Consult the NEC (National Electrical Code)

Review NEC Article 220 (Branch-Circuit, Feeder, and Service Calculations), specifically 220.61 (Permitted Reduction of Neutral Conductor) to see if any exceptions might apply (unlikely). Also review Article 310.15 for ampacity requirements. Consult with a qualified electrician who is familiar with NEC requirements in your area. They can provide accurate interpretations of the code and help you ensure compliance.3.

Consider Alternatives



Upsize the Entire Feeder

The simplest solution is often to use a larger feeder cable where all conductors are sized appropriately. This might mean moving to a 3/0-3/0-3/0-6 or larger. You might need to upgrade the breakers in the main and sub panel.

Run Separate Conductors

Instead of using a pre-assembled cable, run individual THHN/THWN conductors in conduit. This gives you maximum flexibility to select the correct conductor sizes for each phase and the neutral.

Increase Voltage, then step it back down

Use a transformer in the barn that will take 240v and step it down to 120. This will help decrease the voltage drop and allows for a smaller wire size.
4.

Document Everything

Keep detailed records of your calculations, code references, and any consultations with electricians. This will be invaluable during inspection.

Example Calculation (Simplified - You Need to Do a More Thorough One)

Let's assume (for example) that using a #1 neutral on your Southwire calculator results in a 4% voltage drop on the Line (hot) conductor. Because the neutral is a #1, and the current is the same, then it will ALSO have 4% voltage drop.Total voltage drop = Line VD + Neutral VD = 4% + 4% = 8%.8% is higher than the recommended 5%.

Important Safety Note: Working with electrical systems can be dangerous. If you are not qualified to perform this work, hire a licensed electrician. Improper wiring can lead to fire, electric shock, and equipment damage. Flag for review