Okay, I understand what you're asking. You have an 800A panelboard and are exploring alternatives to 3 sets of 500 kcmil Aluminum conductors in 3 raceways. You want to use only 2 raceways, with each raceway containing 2 sets of parallel 250 kcmil Aluminum conductors, leaving one raceway unused for future expansion.Here's a breakdown of the key considerations and a preliminary assessment based on common electrical code principles (remembering that I don't have the specific code version you're working under, so you must verify all calculations with your local code and a qualified electrician)

Key Considerations and Code Requirements:

1. Ampacity of Conductors

The total ampacity of the conductors in each raceway must equal or exceed the required current. Two sets of 250 kcmil AL: Based on typical code tables (e.g., NEC 310.16), 250 kcmil Aluminum conductors (THHN/THWN-2) are typically rated for about 205 amps (at 75°C). Two sets in parallel would give you 2 205 amps = 410 amps per raceway.2.
Parallel Conductor Requirements (NEC 310.10(H) as a starting point)

When using parallel conductors, specific rules must be followed. These are some of the most important:

Minimum Size

Conductors in parallel must generally be at least 1/0 AWG (this requirement is met with 250 kcmil).

Same Length

All parallel conductors must be the same length.

Same Conductor Material

All parallel conductors must be of the same material (aluminum in your case).

Same Size

All parallel conductors must be the same size (250 kcmil).

Same Insulation Type

All parallel conductors must have the same insulation type (e.g., THHN/THWN-2).

Terminate in the Same Manner

All parallel conductors must be terminated in the same manner.

Adjustment Factors

Be aware that adjustment factors for ambient temperature and the number of current-carrying conductors in a raceway might apply, further reducing the allowable ampacity.3.

Raceway Fill

The raceway fill cannot exceed code limits. This is based on the cross-sectional area of the conductors and the type of raceway. You'll need to calculate the total cross-sectional area of the conductors in each raceway (including the neutral and ground) and compare it to the allowable fill for the chosen raceway type.4.

Neutral Conductor Sizing

The neutral conductor must be sized according to code requirements. In many cases, it can be smaller than the phase conductors, but it must still be adequate for the calculated neutral load. If a significant portion of the load is non-linear (e.g., electronic equipment), the neutral conductor may need to be the same size as the phase conductors or even larger.5.

Grounding/Bonding

The equipment grounding conductor (EGC) must be properly sized. When using parallel conductors, the EGC must also be run in parallel. The size of each parralled EGC must be based on the size of the overcurrent protective device protecting the circuit. Table 250.122 in the NEC is used to find the size of the EGC. In addition, a supply-side bonding jumper needs to be sized accordingly to the requirements in the NEC 250.28(D).
6.

Overcurrent Protection

The overcurrent protection (e.g., the breaker feeding the panelboard) must be sized to protect the conductors. It cannot exceed the ampacity of the conductors after any adjustment or correction factors are applied.7.

Voltage Drop

Voltage drop must be within acceptable limits. Using smaller conductors (250 kcmil instead of 500 kcmil) will increase voltage drop. You'll need to calculate the voltage drop for the expected load and ensure it's within the limits specified by code and good engineering practice (typically 3% for feeders and 5% total for feeder and branch circuits).8.

Raceway Size

You need to ensure that the raceway is large enough to accommodate all the conductors, including the neutral and ground, while still meeting raceway fill requirements.

Preliminary Assessment



Ampacity

Two sets of 250 kcmil AL conductors (410A per raceway) in two raceways provides a total ampacity of 820A which exceeds your 800 amp panelboard, so the ampacity of conductors is satisfactory.

Cost Savings

Using smaller conductors (250 kcmil instead of 500 kcmil) may indeed reduce the cost of the conductors. However, you have to account for additional costs like more connectors, the larger raceways, and the labor to install the additional conductors.

Steps to Take


Consult the Applicable Electrical Code

This is the most important step. Get your hands on the code book that applies in your jurisdiction (NEC or other).2.

Calculate the Load

Determine the actual load that the 800A panelboard will serve. Don't just assume it will be 800A. Proper load calculations can often reduce the required conductor size.3.

Calculate Ampacity with Adjustments

Determine the ampacity of the 250 kcmil Aluminum conductors, considering any adjustment factors for ambient temperature and the number of conductors in the raceway.4.

Size the Neutral and Ground

Properly size the neutral conductor and equipment grounding conductor.5.

Calculate Raceway Fill

Determine the size of the raceway needed to accommodate all the conductors while meeting code requirements.6.

Calculate Voltage Drop

Calculate the voltage drop for the proposed configuration.7.

Compare Costs

Get accurate pricing for the different conductor sizes, raceways, and associated hardware. Factor in labor costs.8.

Consult with a Qualified Electrician/Engineer

This is essential. Have a qualified professional review your calculations and ensure that your design complies with all applicable codes and standards. They can also help you optimize the design for cost-effectiveness and safety.

In summary: Your idea might* be feasible, but it requires careful analysis and adherence to code requirements. The cost savings may not be as significant as you expect once you factor in all the associated costs and code compliance requirements. Always prioritize safety and code compliance. Flag for review