You're right, trying to run a 2500A service with multi-conductor MC cable can quickly become unwieldy and expensive in a cable tray. Let's break down the options and consider the most economical and practical approach

Why 750 MCM 4-Conductor MC is Impractical
Size and Weight

As you pointed out, the sheer size and weight of multiple runs of 750 MCM 4-conductor MC cable make installation extremely difficult, especially in a cable tray. The bend radius is significant, and the weight will require robust tray supports.

Fill Capacity

Cable trays have fill capacity limitations. Multiple runs of large MC cable will quickly exceed those limits.

Cost

MC cable is generally more expensive than single-conductor wiring, especially in these large sizes.

Heat Dissipation

Concentrating multiple insulated conductors within an MC cable can hinder heat dissipation.

Why Single Conductors are Generally the Preferred Choice for High Amperage Services in Cable Trays Flexibility

Single conductors are much easier to maneuver and train within a cable tray.

Spacing and Airflow

Single conductors allow for better spacing, which is crucial for heat dissipation. This can lead to higher ampacity ratings (based on NEC Table 310.15(C)(1)).

Easier Installation

Individual conductors are lighter and easier to pull, especially when dealing with long runs.

Cost

Typically, single conductor cables, specifically THHN/THWN-2 or XHHW-2, will be a more economical option.

Code Compliance

Allows for easier compliance with spacing requirements, bundling rules, and ampacity adjustments.

Single Conductor Options for Cable Tray

You're correct that you need to ensure the single conductors are specifically tray-rated. The most common and economical options are: THHN/THWN-2

This is a very common and versatile building wire. Ensure it's marked "THHN/THWN-2 CT" This "CT" designation means it's suitable for cable tray use. THHN/THWN-2 is generally rated for 90°C in dry locations and 75°C in wet locations.

XHHW-2

Also a very popular choice. Ensure it's marked "XHHW-2 CT." XHHW-2 is rated for 90°C in both wet and dry locations. This can be beneficial in terms of ampacity derating in certain high-temperature environments.

RHH/RHW-2

This is another common option, again needing to be marked for cable tray use (if not marked otherwise it might not be rated for trays).

Key Considerations for Specifying the System1. Ampacity Calculation

Start with the service requirement of 2500 amps. Derate for ambient temperature if necessary (NEC Table 310.15(C)(1) for over 3 current carrying conductors in a raceway or cable). In a cable tray, "raceway" is not necessarily the same as enclosed conduit. The NEC allows for more favorable ampacity adjustments when conductors are installed in a cable tray with sufficient spacing. Table 310.15(C)(1) Note 5 specifies free air ampacities.
2.

Number of Conductors Per Phase

Determine how many conductors you need per phase to achieve the required ampacity after derating. For example, with a derated ampacity of 500A per conductor, you'd need 5 conductors per phase (2500A / 500A = 5).3.

Conductor Size

Select the appropriate conductor size (e.g., 500 MCM, 600 MCM, or 750 MCM) based on the ampacity required after derating and the allowable ampacity of the conductor per NEC Table 310.16. Make sure to use the 75°C or 90°C column depending on your equipment termination temperature rating (usually 75°C).
4.

Neutral Conductor

Determine the neutral conductor sizing. The NEC has specific requirements for neutral conductors, especially in systems with significant non-linear loads (harmonics). In general, you will want to run a neutral of equivalent size.5.

Grounding Conductor

Size the grounding conductor according to NEC Table 250.122. This table is based on the size of the ungrounded conductors.
6.

Cable Tray Selection

Choose a cable tray that is appropriately sized for the number of conductors, conductor size, and required spacing. Consider the tray's load-bearing capacity and any derating factors for conductor bundling. Ladder type cable tray is great for larger installations because it allows for the greatest amount of free air and heat to escape. Ensure the cable tray is properly grounded and bonded. Consider cable tray material (steel, aluminum, etc.) based on environmental factors and cost.7.

Terminations

Select appropriately sized and rated lugs and connectors for the conductors. Ensure compatibility with the equipment to which you're connecting (panelboards, switchgear, etc.). Use torque wrenches to ensure proper connections.

Economical Considerations Aluminum vs. Copper

Aluminum conductors are typically less expensive than copper. For large feeders like this, aluminum is often a viable and economical choice. You'll need to upsize the aluminum conductors to achieve the same ampacity as copper. Make sure your equipment is rated for aluminum conductors.

Minimize Runs

Optimize the layout to minimize the length of the cable run.

Competitive Bidding

Get quotes from multiple suppliers for both the conductors and the cable tray.

Consider Paralleling

Paralleling is almost always a better solution than increasing conductor size.

Engineering Judgment

The NEC is a minimum* standard. Use your engineering judgment and consult with experienced electricians and engineers to ensure a safe and reliable installation.

Example (Simplified)

Let's say, after accounting for ambient temperature and other factors, you need each conductor to carry at least 500 amps continuously.1.

Conductor Choice

Using NEC Table 310.16, a 750 MCM THHN/THWN-2 (rated 75°C for termination requirements) has an ampacity of 475 amps. A 750 MCM Aluminum conductor in the 75 degree column is only good for 420 amps, so we can already see the impact of aluminum.
2.

Paralleling

To achieve 2500 amps with a 480/277V system you would need 5 -750 MCM conductors in parallel. This would be a lot of conductors to install in parallel and might require a larger conduit system.

In summary:

For a 2500 amp service in a cable tray, using properly rated single conductors (THHN/THWN-2 CT, XHHW-2 CT, or RHH/RHW-2 CT) is generally the most practical and economical approach. Carefully calculate ampacity requirements, derating factors, and select the appropriate conductor size and cable tray to ensure a safe and code-compliant installation. Consider aluminum conductors as a potential cost-saving measure, but ensure your equipment is rated for them.Always consult with a qualified electrical engineer and electrician for the final design and installation. This information is for general guidance only and is not a substitute for professional engineering advice. Flag for review