Okay, let's break down this voltage drop problem. You're right to question the engineer's design. It's highly likely that 1/0 is undersized for that distance and load, especially with the 2% voltage drop requirement.Here's a step-by-step approach to analyzing the situation and determining the correct wire size

1. Understanding the Circuit and Load Distribution:
Source

300A breaker at 120/208V (3-phase or single-phase? This is crucial information!).

Feeders

(2) sets of parallel conductors from the breaker to the wiring trough.

Distance

300 feet.

Load 1

60A panel.

Load 2

225A panel (MDP).

Voltage Drop Limit

2% (very strict).

2. Key Assumptions and Missing Information



System Type

Is this a 120/208V 3-phase or single-phase system? This massively impacts the voltage drop calculation.

You MUST know this. Load Distribution in the Trough

Where in the trough do the 60A and 225A panels connect? Are they at the far end of the trough from the breaker? This affects current distribution and the overall voltage drop.

Load Types

Are these mostly resistive loads (like heating) or inductive loads (like motors)? Power factor significantly affects voltage drop. If you don't know, assume a power factor of 0.8.

Conductor Material

Assume copper unless otherwise specified.

Conduit Material

This affects the impedance calculation, but usually has a smaller impact than conductor size.

3. Why Your Initial Calculation is Problematic



Entering 300A as the Current at the End of the Run

This is the maximum current the breaker is rated for, but it's not necessarily the actual current flowing in the conductors. The actual current is the sum of the loads on the 60A and 225A panels after they branch off from the trough.

Ignoring Load Distribution

The 60A and 225A loads aren't necessarily pulling current equally through the entire 300 feet of conductor. The conductor sections closer to the breaker will carry more current.

4. The Correct Calculation Approach (Simplified, assuming 3-phase)



Step 1: Determine Total Load Current

Assume both panels are fully loaded (the worst case). Total load is approximately 60A + 225A = 285A.

Step 2: Account for Parallel Conductors

Each set of conductors carries half the current. So, each 1/0 conductor is intended to carry 285A / 2 = 142.5A.

Step 3: Calculate Voltage Drop (Using a Simplified Formula for 3-Phase)

Voltage Drop = (1.732 K I D) / (CM Number of conductors) 1.732 = Square root of 3 (for 3-phase) K = Material constant (12.9 for copper) I = Current in amps (142.5A per conductor) D = One-way distance in feet (300 ft) CM = Circular mils of the conductor (1/0 copper = 105,500 CM) Number of Conductors = 1

Step 4: Calculate Percent Voltage Drop

% Voltage Drop = (Voltage Drop / Source Voltage) 100 Source Voltage = 208V

Example Calculation (with approximations, assuming 3-Phase, and 285A continuous load) Voltage Drop = (1.732 12.9 142.5 300) / 105,500 = 9.61V % Voltage Drop = (9.61 / 208) 100 = 4.62%5. Why 1/0 Fails (Based on the above 3-phase approximation) A voltage drop of 4.62% is WAY over your limit of 2%. This is highly simplified and may be inaccurate given the missing information. If your actual load is lower, the voltage drop will be lower. If your actual voltage is higher, the voltage drop will be lower.6. Refined Calculation Needed

You need to perform a more accurate calculation considering:

3-Phase vs. Single-Phase

This is the most important factor.

Actual Load Currents

Don't assume full load on the 60A and 225A panels. Get realistic load estimations.

Load Distribution Along the Trough

Account for the fact that the entire 285A doesn't flow through the entire 300 feet.

Power Factor

If you know the power factor of the loads, use that in your voltage drop calculations.

7. How to Calculate Voltage Drop with Distributed Loads

This is a more complex calculation, but here's the idea: Segment the Run

Imagine the 300-foot run as segments (e.g., 10-foot segments).

Calculate Current in Each Segment

Determine the current flowing through each segment based on where the 60A and 225A panels are connected.

Calculate Voltage Drop in Each Segment

Use the voltage drop formula for each segment, with the appropriate current and distance.

Sum the Voltage Drops

Add up the voltage drops in each segment to get the total voltage drop.

8. Software Assistance

Using electrical design software or more sophisticated online voltage drop calculators is HIGHLY recommended for complex scenarios like this. These tools can handle parallel conductors, distributed loads, and power factor more accurately.9. Conduit Sizing

If you need to upsize the conductors to 500 kcmil (or larger), you will almost certainly need to upsize the conduit as well. Use the NEC (National Electrical Code) tables (Chapter 9) to determine the correct conduit size based on the number and size of the conductors.

10. Your Next Steps


Find out if the system is 3-phase or single-phase. This is crucial.2. Get Realistic Load Data

Determine the actual loads on the 60A and 225A panels.3.

Clarify Load Placement

Know where the loads connect to the trough.4.

Perform Accurate Calculations

Use appropriate voltage drop formulas (or software) that consider parallel conductors, distributed loads, and power factor.5.

Document Everything

Show your calculations to the engineer and explain why the 1/0 is inadequate. Get written confirmation of any changes.6.

Follow the NEC

Ensure that your final design complies with all applicable NEC requirements.

Important Considerations Continuous Load

If the load is considered "continuous" (operates for 3 hours or more), you must derate the breaker and conductors to 80% of their rating. This may require further upsizing.

Safety

Voltage drop can cause equipment malfunction, overheating, and even fires. It's critical to get this right.

In conclusion, the engineer's specification of 1/0 seems highly suspect. You're right to question it. A thorough voltage drop calculation is essential, taking into account all the relevant factors. Document your findings and communicate them clearly to the engineer. Good luck! Flag for review