Okay, let's break down this wiring scenario and address your questions about voltage drop calculations. This is a substantial run, so accurate calculations are crucial.

Understanding the System



Voltage

277/480V system. This means you have 480V between phases (A-B, B-C, C-A) and 277V from each phase to neutral (A-N, B-N, C-N).

Load

800A breaker feeding a 800A panel.

Distance

700ft total: 500ft underground PVC, 200ft overhead RMC.

Conduit

One 2" conduit used, one 2" conduit spare.

Answering Your Questions


Voltage for Voltage Drop Calculation



Yes, you should use 480V for voltage drop calculations. You're feeding a 480V panel. The voltage drop calculation will determine the percentage of that 480V that is lost along the conductor run. Using 277V wouldn't give you an accurate representation of the voltage drop on the 480V system.2. Current (Amperage) for Voltage Drop Calculation



Using 800A is a very conservative, but safe, approach. Using 80% of the breaker rating (640A) is a more realistic approach IF you have a reasonable expectation of the maximum continuous load the panel will actually see. Here's a more detailed thought process: NEC Requirement

The NEC states to calculate the load on the equipment.

Conservative Approach (800A)

This assumes the panel COULD theoretically be loaded to its full 800A capacity for an extended period. This will result in a larger wire size. This is a safe bet.

Realistic Approach (640A or less)

If you know, based on the load served by the panel (e.g., lighting, motors, specific equipment), that the continuous load will not exceed, say, 400A, then you could use 400A in your voltage drop calculation. Continuous load is defined as a load that is expected to operate at its maximum current for 3 hours or more. If your continuous load is less than the rating of the breaker then you are able to reduce the size of the conductors as long as you adhere to the NEC(National Electrical Code).

Important Considerations



Future Load Growth

Account for future expansion. Under-sizing the conductors now can lead to problems later.

Documentation

If you use a lower amperage than the breaker rating for your voltage drop calculation, document your reasoning and the basis for your load calculation. This is important for future reference and potential inspections.3.

Conduit Size and Wire Size (600 MCM)



600 MCM and Two Conduits

You're likely correct that for 640A (or especially 800A) and a 700ft run, 600 MCM conductors will be needed to meet acceptable voltage drop limits. This requires parallel conductors.

Parallel Conductors NEC Guidelines (NEC 310.10(H)): You MUST follow the NEC guidelines if you are going to install parallel conductors. ALL of the following applies All conductors must be the same length Same conductor material Same size in circular mil area Same insulation type Terminated in the same manner Conduit Fill Calculations

You must calculate conduit fill according to NEC Chapter 9, Table 1 and Table 4. 600 MCM conductors are large. You'll need to ensure that two 600 MCM conductors plus a ground will fit within the 40% fill limit of a 2" conduit. You will need to use the dimensions from the manufacture and refer to the NEC for the fill capacities.

Steps to Take (Recommended)


Accurate Load Calculation

Perform a thorough load calculation for the 800A panel, considering existing and future loads. Determine the maximum continuous current the panel is likely to draw.2.

Voltage Drop Calculation

Use a voltage drop calculator or formula. Most online calculators are fine. Input: Voltage: 480V Distance: 700 ft (total) Current: Use either 800A (most conservative) or the value from your load calculation. Conductor Material: Copper or Aluminum Conductor Size: Start with 600 MCM and adjust based on the results. Power Factor: Assume 0.8 or 0.85 unless you have specific data for the load. Aim for a voltage drop of no more than 3% on the feeder (between the source and the panel). A lower voltage drop is always better.3.

Conductor Ampacity

Once you determine the conductor size from the voltage drop calculation, check its ampacity. Make sure the conductor's ampacity (NEC Table 310.16, adjusted for temperature) is equal to or greater than the breaker size (800A) if that's what you used for your voltage drop calculation.
4.

Conduit Fill

Verify that the conductors you choose will fit in the 2" conduits, adhering to NEC conduit fill requirements. Use the dimensions from the wire manufacturer to verify the conduit fill.5.

Grounding Conductor

Don't forget to size the grounding conductor according to NEC Table 250.122, based on the size of the overcurrent protective device (800A).
6.

Temperature Correction

Account for ambient temperature. The 500 feet of underground will most likely be warmer, and the higher the ambient temperature, the lower the conductors rated capacity.7.

Consider Aluminum Conductors

Aluminum conductors can be significantly cheaper than copper for large sizes. However, they have lower ampacity for the same size, and voltage drop is generally worse. If you consider using aluminum, make sure it is not a prohibited installation.

Example Scenario (Illustrative)

Let's say your load calculation determines that the maximum continuous load on the panel will be 500A.1. You perform a voltage drop calculation using 480V, 700 ft, and 500A. You find that 600 MCM copper conductors result in a voltage drop of approximately 2.5%. This is acceptable.
2. You check the ampacity of 600 MCM copper conductors. In typical conditions, it will likely be above 500A, but you need to account for the temperature rating.3. You verify that two 600 MCM copper conductors and a ground conductor will fit in your two 2" conduits according to NEC fill rules.

Important Notes



NEC is the Law

Always consult the latest edition of the NEC for specific requirements and interpretations.

Engineering Judgment

These calculations require professional electrical engineering judgment.

Local Codes

Be aware of any local codes or regulations that may be stricter than the NEC.

Safety First: Working with electrical systems is dangerous. Use proper safety procedures and equipment. If you are not a qualified electrician, hire one.By following these steps and carefully considering the factors involved, you can accurately size the conductors for this critical power feeder. Good luck! Flag for review