You are absolutely correct. You do need to consider the voltage drop all the way to the pump motor, not just to the control box.Here's a breakdown of why and what you need to consider: Voltage Drop is Cumulative

Voltage drop is the reduction in voltage along a conductor due to its resistance. It's a function of the wire's resistance, the current flowing through it, and the length of the conductor. The total voltage drop is the sum of the voltage drop in each segment of the circuit. If you only consider the distance to the vault, you're ignoring a significant portion of the circuit and, therefore, a significant portion of the voltage drop.

Pump Motor Sensitivity

Submersible pump motors are particularly sensitive to voltage drop. Undervoltage can lead to: Reduced pump performance (lower flow rate and pressure). Overheating of the motor windings, leading to premature failure. Increased current draw, which can further exacerbate voltage drop and potentially trip the breaker. Shortened lifespan of the motor.

Complete Circuit Path

Think of the circuit as a loop: from the breaker in the house, to the vault, through the control box, down the well to the pump motor, and back up to the control box, and back to the breaker. All of that needs to be considered.

NEC Requirements

The National Electrical Code (NEC) recommends limiting voltage drop on branch circuits to 3% to the farthest outlet and 5% overall (to the farthest load). Ignoring the well depth portion of the circuit will likely result in exceeding these limits.

Why the Misunderstanding? The person who told you to only worry about the distance to the vault might have been simplifying things, or perhaps they assumed that the voltage drop down the well would be negligible. With #10 wire for 300' on a 3 HP motor it will be more than negligible. It's a dangerous assumption to make.

Here's what you need to do


Calculate the Total Distance

The total distance to consider is the distance from the house to the vault (280 ft) plus the depth of the well (300 ft) plus the return trip (300ft) = 880 ft.2.

Determine the Motor's Full Load Amps (FLA)

A 3 HP, 240V single-phase motor will draw somewhere around 15 amps at full load. Check the motor's nameplate for the actual FLA.3.

Calculate Voltage Drop

Use a voltage drop calculator to determine the appropriate wire size. You'll need to input the distance (880 ft), the current (FLA), the voltage (240V), and the type of wire and its insulation temperature rating.* Many online calculators are available (search for "voltage drop calculator"). Or consult a licensed electrician.4.

Choose the Correct Wire Size

The calculator will tell you the voltage drop for a given wire size. You want to choose a wire size large enough so that the voltage drop is less than 3% (ideally) or, at a maximum, no more than 5% total for the entire run. If you need to upsize the wire to reduce the voltage drop, do it.5.

Consider Wire Type

Copper is a better conductor than aluminum and will result in less voltage drop for the same wire size.6.

Consider the existing #10 wire in the well If you must keep the #10, consider using a VFD to boost the voltage before the pump.

Recommendation

Consult with a qualified, licensed electrician. They can perform the voltage drop calculations accurately, ensure that the wiring meets local codes, and ensure that the pump motor receives the correct voltage for reliable operation and longevity. It will be money well spent to avoid costly problems down the road.

In short: You are right to be concerned about the total distance. Ignoring the well depth would be a mistake. Flag for review