Okay, let's break this down. You're right to be cautious; DC systems have some nuances compared to AC. Your initial thoughts on the THHN conductors are generally sound.

Regarding your initial THHN Calculation



Voltage

575VDC is high. Make sure all components, including connectors, lugs, and conduit fittings, are rated for at least this voltage (and preferably a safety margin above that).

Current

511A is your key figure.

Conduit

Using two conduits is a good practice for separating polarities and simplifying installation.

Conductor Size

You are thinking correctly with the 300 kcmil THHN.

Ampacity

300 kcmil THHN is rated 285A in NEC Table 310.16. Since you are installing 2 sets of conductors, the ampacity is 2 285 = 570 amps. So you are covered.

75°C Rating

You're using the 75°C column because that is the standard used for termination temperature, unless equipment is specifically marked otherwise. Be sure that both the UPS and battery system terminals are rated for 75°C or higher.

Derating (Not Needed Here)

You've already accounted for the number of conductors in the conduit by using the 75 degree column for a single conductor.

Important Considerations for THHN



NEC Compliance

Ensure that all wiring and installation methods comply with the National Electrical Code (NEC), particularly Article 310 (Conductors for General Wiring), Article 300 (Wiring Methods), and any applicable sections for battery systems (Article 480) or energy storage systems (Article 706).

DC-Specific Considerations

DC currents don't have the skin effect issues of AC, meaning the entire conductor is utilized, but voltage drop is a critical factor due to the constant current flow.

Overcurrent Protection

Ensure appropriate overcurrent protection (fuses or circuit breakers) is in place as close to the battery as possible. Coordinate the overcurrent protection with the conductor ampacity.

"Welding Cable" Considerations (OMNI CABLE B225001 250 KCMIL)



Flexibility

Welding cable's primary advantage is its flexibility due to the very fine stranding of the copper conductors. This makes it much easier to maneuver in tight spaces.

Insulation

The EPDM insulation is the potential issue. EPDM (Ethylene Propylene Diene Monomer) is a rubber insulation. Welding cable is not specifically covered under the NEC, and typically is NOT listed as a NEC wiring method.

This is where it gets tricky and why you need to be very careful. Ampacity Determination (The Big Question)

This is the crucial point. You cannot just assume the ampacity of welding cable based on its size alone.

Listing and Standards

The biggest issue is whether the cable is listed by a recognized testing laboratory (like UL, ETL, etc.) for general wiring use, and whether the listing covers use in a raceway. Welding cable is typically not listed for this type of application.

NEC 310.15(B)(3)

The NEC allows for using ampacities not specifically in the tables IF the cable is specifically marked with the ampacity by a qualified testing agency.

Manufacturer's Data

Even if the cable has a UL listing, the ampacity might be specific to certain applications. Contact OMNI directly and get a letter stating the ampacity of the cable when used in a raceway with the specific parameters you are proposing (temperature, number of conductors in conduit, etc.).

Voltage Rating

Verify that the welding cable's voltage rating is sufficient for 575VDC. Many welding cables are rated for only 600V AC. DC voltage can stress insulation differently than AC.

Recommendations and Cautions


Verify OMNI Cable Listing and Ampacity

Before using any welding cable, get written documentation from OMNI (or any manufacturer) stating the specific ampacity of the cable when installed in a conduit and its suitability for your voltage (575VDC). Look for a UL listing that covers the intended use. If you cannot get this documentation, you cannot use it.2.

NEC Compliance

You must be able to demonstrate that your wiring method complies with the NEC. If the welding cable isn't explicitly listed for use in your application, it is a violation.3.

Alternative High-Flexibility Conductors

Explore other options for high-flexibility conductors that are listed for NEC-compliant installations. Some manufacturers offer extra-flexible THHN/THWN conductors specifically designed for tight spaces. These often have a higher strand count for increased flexibility.4.

Consult a Qualified Electrical Engineer

Given the high DC voltage and current levels, it's highly recommended to consult with a licensed electrical engineer experienced in DC power systems and NEC compliance. They can perform calculations, review your design, and ensure a safe and code-compliant installation.5.

Voltage Drop Calculations

As mentioned earlier, voltage drop is critical in DC systems. Calculate the voltage drop for your chosen conductor size and run length to ensure it stays within acceptable limits (typically no more than 3%).

In Summary:

Your initial THHN approach is generally correct if you can work with the bend radius and space constraints. Welding cable might be an option, but only if you can get documented proof of its suitability for your application and NEC compliance. If you are unsure, consult a qualified electrical engineer to ensure the design is safe and meets all applicable codes and standards. Don't cut corners on safety or compliance, especially with high-voltage DC systems. Flag for review