You're right to be skeptical and to question the ampacity based on the existing installation. Let's break down the situation and address your concerns

Why the Belief in Higher Ampacity for Fine Stranded Cable (DLO/Welding Cable)?

The belief stems from two potential (but limited) advantages:1. Flexibility

Fine stranding allows for much greater flexibility, reducing stress on terminations and making installation easier in tight spaces. This is primarily a mechanical advantage, not an electrical one.2.

Surface Area (Theoretical)

A fine-stranded conductor does have a larger surface area than a coarse-stranded conductor of the same gauge. The theory is that this larger surface area could lead to slightly better heat dissipation, potentially increasing ampacity.

The Reality: Code Rules and Standards 310.16 and Ampacity Tables

As you noted, NEC Table 310.16 is the primary source for ampacity. This table is based on standardized testing and provides safe ampacity values for conductors installed in various conditions (temperature rating of the wire, ambient temperature, number of conductors in a raceway or cable, etc.).

DLO and Welding Cable

While DLO (Diesel Locomotive) cable and welding cable are fine-stranded, their ampacity is

NOT determined by Table 310.16 in a general application like feeding a load panel. Their ampacities are often found in manufacturer's specifications and are typically lower than what you might expect from a similarly sized THHN conductor in free air or a raceway. Manufacturer Specifications are Key for Special Cables

DLO and Welding Cable ampacities are based on their intended use and the specific insulation temperature rating. Some DLO or welding cables may be rated at 90°C, but that doesn't automatically mean you can use the 90°C column in Table 310.16. You must consult the manufacturer's datasheet.

Limited Duty Cycle

Welding cables, in particular, often have ampacity ratings based on a duty cycle (e.g., 60% duty cycle at a certain amperage). This means they are designed to carry that current for only a portion of the time. You cannot use duty-cycle-based ampacities for continuous loads.

Your Specific Situation: Parallel 4/0 and Cable Tray1. 4/0 Ampacity

You are correct. According to Table 310.16 (assuming 75°C insulation and not more than three current-carrying conductors in a raceway or cable), 4/0 copper is typically rated for 230 amps. (Remember to check the temperature rating of the wire; 90°C rated conductors allow higher ampacity, but terminations also need to be rated at 90°C.)2.

Parallel Conductors

Since you have parallel 4/0 conductors, you would add the ampacity of each conductor together. So, 230 amps + 230 amps = 460 amps.3.

Cable Tray Considerations



Is it Actually a Cable Tray Installation? You mentioned it's "cable tray type" but not tray-rated. If it's not a listed cable tray (and the cable is not rated for cable tray use) then the installation doesn't meet code requirements for a cable tray installation. Cable Tray Fill

Even if it is a proper cable tray installation, you need to consider the cable tray fill limitations (NEC 392.22). Too many cables in the tray can restrict heat dissipation and reduce the ampacity of the conductors.

Adjustment Factors

You might need to apply ampacity adjustment factors based on the number of conductors in the cable tray (NEC 310.15(C)).
4.

Cable Type

What specific type of cable is it? The cable insulation and its temperature rating are crucial. Is it THHN, XHHW, RHH, or something else? This determines which column in Table 310.16 you can use (assuming it's a type covered by the table).
5.

Terminations

Equally important, are the terminals rated to handle the increased ampacity of a 90 degC conductor? If not, then the 75 degC column must be used.

Why Increasing to 500 Amp Fuses is Likely a Violation Overcurrent Protection

The purpose of overcurrent protection (fuses or circuit breakers) is to protect the conductors from overheating and potential fire.

NEC 240.4(B) Round Up Rule

You cannot simply "round up" the fuse size above the ampacity of the conductors, unless there is a specific exception permitting it (which is unlikely in this situation). The conductors are only protected by the fuse up to their ampacity.

Code Violation

Replacing 400-amp fuses with 500-amp fuses when the conductors are only rated for 460 amps (maximum) is a clear violation of the NEC and creates a serious safety hazard. You are potentially overloading the conductors, which could lead to insulation damage, overheating, and fire.

What You Need to Do (Critical)


Identify the Cable

Get the exact cable type and manufacturer. Look for markings printed on the cable jacket. Get the cable's temperature rating.2.

Consult Manufacturer's Data

Obtain the manufacturer's specifications for the cable. This is crucial to determine its ampacity.3.

Verify Cable Tray Installation

Is the cable tray properly installed and rated? Is the cable rated for cable tray use?4.

Calculate Ampacity

Determine the correct ampacity based on the cable type, temperature rating, installation method (cable tray or not), and any adjustment factors.5.

Check Terminations

Are the terminations rated to handle the ampacity of the conductor?6.

Correct the Overcurrent Protection

The fuse size must be appropriate for the ampacity of the conductors. If the load truly requires more than 400 amps, you need to increase the conductor size to handle the load safely, not just increase the fuse size.7.

Document Everything

Document all your findings and calculations. Take pictures of the cable markings and the installation.

In Conclusion:

It's highly likely that increasing the fuse size is a code violation and a safety hazard. The "super fine stranded" aspect of the conductors does not automatically grant them higher ampacity. You must follow the NEC and the manufacturer's specifications to ensure a safe and compliant installation. It is your responsibility to ensure the installation is safe and complies with the NEC. Do not allow pressure from a customer to override your professional judgment and put people and property at risk.I highly recommend consulting with a qualified electrical engineer or inspector if you are unsure about any aspect of this installation. Flag for review