Okay, let's break this down and see if we can find where the discrepancy lies. It's good you're double-checking – voltage drop is critical, and using the Southwire app is a smart move.Here's a structured approach to verifying the calculation, and some areas where differences might arise

1. Your Calculation (General Breakdown):
200A Load

Assuming you've confirmed this is the maximum calculated load based on the dwelling's size and appliances (Article 220). This is crucial, because if your actual load is significantly less than 200A, your voltage drop will be less.

345' Run

Confirmed distance. This is the actual length of the cable run, not just a straight-line distance. Underground runs often have bends and curves.

Aluminum Conductors

350 kcmil Aluminum is a common choice for this type of service.

Conduit Size (2" PVC Sch 40)

You mentioned tight tolerances. This is important to check against the NEC conduit fill requirements (Chapter 9, Table 4). Remember to account for all conductors and their insulation types. If you're even slightly over, you'll need to upsize the conduit.

Neutral

3/0 Aluminum. This seems reasonable, given the phase conductors.

Voltage Drop Formula

`Vd = (2 K I L) / CM` Where: Vd = Voltage drop K = Constant for conductor material (Aluminum) I = Current (Amps) L = One-way length of run (Feet) CM = Circular Mils

2. Key Parameters and Potential Discrepancies



"K" Value (Aluminum)

This is crucial! Use the correct "K" value for aluminum.

The NEC 2020 doesn't explicitly list a "K" value. Usually, a value of 17 is used for Aluminum. Verify the "K" value you used. Current (I)

Did you use the full 200A? If you are using a demand factor, remember to use the demand load value.

Circular Mils (CM) for 350 kcmil Aluminum

This is a fixed value. You should find this in Chapter 9, Table 8 in the NEC. 350 kcmil Aluminum =

350,000 CM Voltage Drop Limit

What voltage drop are you allowing? The NEC has recommendations (not strict requirements) in informational notes in 210.19(A)(1) and 215.2(A)(3). Generally, a maximum of 3% voltage drop for branch circuits and 5% overall (feeder + branch circuit) is the commonly accepted practice. If your voltage drop is close to this limit, even small calculation errors can be significant.

Source Voltage

You didn't mention the system voltage. Split-phase residential is typically

240V. This is necessary to calculate the percentage voltage drop.3. Southwire App Discrepancies



Input Values

Carefully double-check every single input you entered into the Southwire app. Typos happen! Make sure you selected the correct conductor type (aluminum), conductor size (350 kcmil), distance (345'), current (likely 200A), and voltage (240V).

App Settings

Some voltage drop calculators have settings for ambient temperature, conductor temperature, and other factors that can subtly influence the results. Make sure these are set appropriately, or at least to reasonable default values.

App Versions

It's unlikely, but possible there's a bug in the app. If you're still getting wildly different numbers after double-checking everything, consider trying a different online voltage drop calculator as a third point of comparison.

4. Suggested Calculation (Using Typical Values)

Let's assume: K = 17 (Aluminum) I = 200A L = 345 ft CM = 350,000Vd = (2 17 200 345) / 350,000 = 6.72 VoltsPercentage Voltage Drop = (6.72 V / 240 V) 100% = 2.8%

5. Troubleshooting Steps


Share Your Exact Calculation

Post the exact numbers you plugged into your formula, including the "K" value.2.

Screenshot the Southwire App

If possible, provide a screenshot of the Southwire app's input screen and output screen. This makes it much easier to spot errors.3.

Double-Check Conduit Fill

Re-verify your conduit fill calculation using NEC Chapter 9, Table 4.4.

Demand Factors

Are you using a demand factor in your calculation? If so, what percentage/calculated value?5.

Verify Load Calculation: Did you perform a proper load calculation based on Article 220 of the NEC?By working through these steps systematically, we should be able to pinpoint the cause of the discrepancy and ensure your calculations are accurate. Good luck! Flag for review