Okay, let's break down the calculation of the circuit capacity for your described setup: 6 sets of 4#600 KCMIL THHN cables in free air, in parallel.1. Base Ampacity of the Conductor

Look up the ampacity of a single 600 KCMIL THHN conductor in

Table 310.16 or Table 310.17 of the NEC (National Electrical Code). Table 310.17 is used for free air. Important

The allowable ampacity depends on the temperature rating of the terminals and equipment you are connecting the cables to. Typical ratings are 60°C, 75°C, and 90°C. You must use the lowest rated termination to determine the correct conductor ampacity. For example, if you're using 75°C rated terminals, the ampacity of 600 KCMIL THHN in free air (Table 310.17) is 545 amps. If you're using 90°C terminations, the ampacity is 620 amps.

We will use 545 amps for this example.2. Adjustment for Ambient Temperature



Table 310.16 and Table 310.17 ampacities are based on a specific ambient temperature (usually 30°C or 86°F for Table 310.16 and 40°C or 104°F for Table 310.17). If the actual ambient temperature is higher, you must apply a temperature correction factor. This is found at the bottom of Table 310.16 or Table 310.17. Example: If the ambient temperature is 45°C (113°F), and the table is based on 40°C and the conductor is THHN rated, the correction factor is typically around 0.94. Calculation: Ampacity adjusted for temperature = Base Ampacity Temperature Correction Factor. Example with 45C ambient: 545 amps 0.94 = 512.3 amps.3. Adjustment for Number of Conductors in a Raceway or Cable



Article 310.15(C)(1)

Since the cables are installed in free air and NOT in a raceway/conduit, this section does not apply. This does NOT mean you don't need to derate. You are still required to derate for the number of current-carrying conductors per 310.15(C)(1).

4. Derating for Number of Current-Carrying Conductors in Free Air



310.15(C)(1) Adjustment Factors

This is where things get tricky. Even though the cables are in free air, you still need to apply an adjustment factor based on the number of current-carrying conductors. This is because of the combined heating effect of multiple conductors in close proximity.

Key Consideration: Grouping

How closely grouped are these sets of cables? The NEC doesn't give hard and fast distances for what constitutes "grouping" in free air, so this requires some judgment and typically the strictest reasonable interpretation.

Generally accepted practice

Treat all the conductors in the same immediate area of installation as a group. For instance, if you install the 6 sets of cables next to each other on a cable tray or ladder rack, they are grouped.

Number of Current-Carrying Conductors (CCCs)

In your case, you have 6 sets of 4 conductors. Assuming a 3-phase system, you have 3 current-carrying conductors per set (3 phases and a neutral, or 3 phases without a neutral if it's a purely 3-phase load). This means you have 6 sets 3 CCCs/set = 18 current-carrying conductors. If the neutral is also carrying current, then you have 6 sets 4 CCCs/set = 24 current-carrying conductors.

Adjustment Factor from Table 310.15(C)(1)

Find the adjustment factor based on the number of CCCs. In the 16-20 CCCs range, the adjustment factor is 0.5. In the 21-30 CCCs range, the adjustment factor is 0.45. Calculation: Ampacity adjusted for number of CCCs = Ampacity adjusted for temperature Adjustment Factor. Example (18 CCCs): 512.3 amps 0.5 = 256.15 amps per conductor. Example (24 CCCs): 512.3 amps 0.45 = 230.535 amps per conductor.

5. Calculate Total Circuit Capacity

Multiply the derated ampacity of a single conductor by the number of conductors in parallel per phase. In your case, you have 6 sets of conductors, meaning you have 6 conductors in parallel per phase. Total Circuit Capacity = Derated Ampacity per Conductor Number of Conductors in Parallel Example (18 CCCs): 256.15 amps/conductor 6 conductors = 1536.9 amps Example (24 CCCs): 230.535 amps/conductor 6 conductors = 1383.21 amps

Summary



Example (18 CCCs)

With 6 sets of 4#600 KCMIL THHN conductors in free air, an ambient temperature of 45°C, 75°C termination rating, and 18 current-carrying conductors, the total circuit capacity is approximately

1536.9 amps. Example (24 CCCs)

With 6 sets of 4#600 KCMIL THHN conductors in free air, an ambient temperature of 45°C, 75°C termination rating, and 24 current-carrying conductors, the total circuit capacity is approximately

1383.21 amps.Important Considerations and Caveats



Overcurrent Protection

The overcurrent protection (circuit breaker or fuses) must be sized appropriately based on the calculated ampacity and must comply with NEC Article 240. You cannot simply install a breaker or fuse that matches the calculated ampacity. Article 240 provides specific rules for selecting overcurrent protection, and you will typically need to round down to the next standard size.

Terminal Ratings

Crucially important! You must use the lowest rated termination temperature rating when determining the initial conductor ampacity. Using a 90°C ampacity for a 75°C termination is a code violation and could lead to overheating and failure.

Voltage Drop

Parallel conductors help reduce voltage drop. You should still calculate the voltage drop to ensure it's within acceptable limits (typically 3% for feeders and 5% total for feeder plus branch circuit).

Maintenance and Accessibility

Ensure the installation provides adequate access for inspection, maintenance, and future modifications.*

Engineering Judgment

The NEC provides a framework, but some interpretation is required. Engage a qualified electrical engineer for complex installations. The AHJ (Authority Having Jurisdiction) has the final say on code compliance.

Disclaimer: I am an AI Chatbot and cannot provide professional electrical engineering advice. This information is for educational purposes only. Always consult with a qualified and licensed electrical engineer for any electrical installations. Electrical work can be dangerous, and it's critical to follow all applicable codes and safety practices. Flag for review