Okay, let's break down this situation and see what is permissible according to the NEC (National Electrical Code) and industry best practices.

Summary of your setup



Existing

800A, 480V panel fed with 2 parallel runs of 500 kcmil conductors per phase, a reduced neutral, and a #3 ground.

Proposed Change

Refeed the 800A panel from a different service, using an 800A breaker with triple lugs (max 500 kcmil). The plan is to use 3 parallel runs of 400 kcmil initially, junction them to 2 runs of 600 kcmil, then feed the panel. 480V 3-wire (no neutral needed). Typical load 400-550A.

Analysis and NEC Considerations


Parallel Conductors (NEC 310.10(H)) Parallel conductors are permitted, but they must meet strict requirements: Size

Conductors must be 1/0 AWG or larger. Your 400 kcmil and 600 kcmil meet this.

Material

All parallel conductors must be of the same material (e.g., all copper or all aluminum).

Insulation Type

All parallel conductors must have the same insulation type (e.g., THHN, XHHW).

Termination

All parallel conductors must terminate in the same manner.

Equal Length

This is critical. All parallel conductors in a phase must be the same length to ensure equal current sharing. You acknowledged this is a potential problem.

Equal Conductor Properties

Must have the same impedance, ampacity, insulation, etc.2.

Ampacity and Overcurrent Protection (NEC 240.4) Your 800A breaker must be protected by conductors with sufficient ampacity. The ampacity of each parallel set must be at least 800/number of sets. 400 kcmil

According to NEC Table 310.16, 400 kcmil THHN copper, rated for 75°C, has an ampacity of 335 amps. 3 parallel runs of 400 kcmil provide 3 x 335 = 1005 amps.

600 kcmil

According to NEC Table 310.16, 600 kcmil THHN copper, rated for 75°C, has an ampacity of 420 amps. 2 parallel runs of 600 kcmil provide 2 x 420 = 840 amps.Note ampacity calculation may need to be derated based on ambient temperature, number of conductors in a raceway, and terminal temperature rating.3.

Junction Boxes (NEC 314) You can use a junction box to transition from the 3 runs of 400 kcmil to the 2 runs of 600 kcmil. The junction box must be sized appropriately based on the number and size of conductors entering and leaving it (see NEC 314.28). All connections within the junction box must be made with approved connectors rated for the conductor sizes. You cannot simply twist the conductors together and tape them. Use listed and labeled split bolt connectors, insulated mechanical connectors, or similar.4. Equipment Grounding Conductor (NEC 250) Parallel equipment grounding conductors (EGCs) are required when parallel conductors are used for the phase conductors. (NEC 250.122(C)). The size of each parallel EGC must be based on the overcurrent protective device protecting the circuit (800A breaker). From NEC 250.122, for an 800A overcurrent device, you need a minimum of a 1/0 AWG copper EGC. Therefore, you need three parallel runs of 1/0 AWG copper as equipment grounding conductors.5. Neutral Conductor (NEC 220, 250) Since this is a 480V, 3-wire system, you don't need a neutral conductor. Your original setup used a "reduced neutral." This is typically only allowed when the load is primarily non-linear (like lighting). However, you are not including a neutral so this does not apply.

Problems and Concerns



Unequal Lengths of Parallel Conductors

This is the biggest issue. If the 3 runs of 400 kcmil are not the same length, and the 2 runs of 600 kcmil are not the same length, current will not distribute evenly. This can lead to overheating and conductor failure. It is very difficult to achieve equal length when you are only using parallel conductors for a portion of the run.

Triple Lugs and Conductor Size

You said the 800A breaker has triple lugs that only accept up to 500 kcmil. This is a problem. You are proposing to use 600 kcmil for part of the run, which cannot directly connect to the breaker. You would have to find a way to get 3-500 kcmil to connect to the breaker, and then junction that into the two 600 kcmil conductors.

Ampacity Margins

While the ampacity of your conductors exceeds the breaker rating, you're not providing a significant safety margin, especially considering the fluctuating load and potential for future load growth.

Recommendations


Eliminate the Mixed Parallel Conductor Sizes

The best solution is to avoid mixing conductor sizes. Find a way to run either 3 sets of 400 kcmil or 2 sets of 600 kcmil (or larger) for the entire run. This eliminates the equal length problem. This may require additional conduit.2.

Breaker Compatibility

You must ensure that the conductors connecting to the breaker are compatible with the breaker's lugs. This means using conductors that are 500 kcmil or smaller.3.

Equipment Grounding

Ensure you run three 1/0 AWG EGC.4.

Consider Load Growth

Think about future load growth when selecting conductors. Oversizing slightly now can save money and headaches later.5.

Consult with an Electrical Engineer

Given the complexity and importance of this installation, it is highly recommended to consult with a qualified electrical engineer to ensure the design meets all NEC requirements and is safe and reliable.6.

Get a Permit and Inspection

This work definitely requires a permit and inspection by your local electrical authority.

In summary, your proposed plan presents significant challenges and potential code violations, especially with the unequal lengths and mismatched conductor sizes. The best course of action is to simplify the design, ensure conductor compatibility with the breaker, and consult with a qualified professional. Flag for review