Okay, let's break down your plan and address the conduit fill and derating concerns. This is a complex situation, and accuracy is key.

Understanding the Setup



400A Service

Full 400A service, not split bus.

26kW Generator

Supplying power through ATSs.

200A Subpanel

Being backed up by a 200A ATS. Feeds the majority of backed-up circuits.

8 Additional Circuits

Need to be backed up, currently on the 400A bus.

Proposed Solution

A 100A ATS with a distribution panel to back up the 8 additional circuits.

Conduit

3" PVC

Conduit Fill and Derating Analysis

You're proposing running the following conductors in a 3" PVC conduit:1.

200A ATS Line/Load (From 400A Bus to 200A ATS)

(3) 4/0 AWG THHN Aluminum (Line to ATS) (2) 4/0 AWG THHN Aluminum (Load from ATS to 200A Subpanel) - Note: I am assuming 2 hots, due to being single phase.2.

100A ATS Line (From 400A Bus to 100A ATS)

(2) #4 AWG THHN Aluminum (Line to ATS)3.

8 Branch Circuits (From 100A ATS Distr. Panel to 400A Bus)

(8) #12 AWG THHN Copper (Line to ATS)4.

Ground

(1) #2 AWG Copper (Ground)

Let's analyze this



1. Conduit Fill Calculation

First, we need the area of each conductor. You'll need to consult Chapter 9, Table 5A of the NEC (National Electrical Code) for conductor dimensions. Here's a rough estimate based on common values, but

you MUST use the actual values from the conductor manufacturer and NEC table

4/0 AWG Aluminum THHN: Approximately 0.3224 sq. in. (use actual value) #4 AWG Aluminum THHN: Approximately 0.0819 sq. in. (use actual value) #12 AWG Copper THHN: Approximately 0.0133 sq. in. (use actual value) #2 AWG Copper: Approximately 0.0330 sq. in. (use actual value) Now, calculate the total area of all conductors: (5) 4/0 AWG AL: 5 0.3224 sq. in. = 1.612 sq. in. (2) #4 AWG AL: 2 0.0819 sq. in. = 0.1638 sq. in. (8) #12 AWG Cu: 8 0.0133 sq. in. = 0.1064 sq. in. (1) #2 AWG Cu: 1 0.0330 sq. in. = 0.0330 sq. in.

Total Conductor Area: 1.9152 sq. in. Next, determine the allowable fill area for a 3" PVC conduit. NEC Chapter 9, Table 1 specifies conduit fill percentages. For more than two conductors, the maximum fill is 40%. From Chapter 9, Table 4, the area of a 3" PVC conduit is approximately 8.04 sq. in. Allowable fill area: 8.04 sq. in. 40% = 3.216 sq. in. Conclusion (Based on Estimated Values)

1.9152 sq. in. (total conductor area) < 3.216 sq. in. (allowable fill area). Based on these estimations, you are within the conduit fill limit, HOWEVER you MUST verify with the actual conductor dimensions from the manufacturer to make sure this is correct.

2. Derating for Number of Conductors

NEC 310.15(C)(1) states that you need to adjust the ampacity of conductors when there are more than three current-carrying conductors in a raceway. Grounding conductors do not count as current-carrying conductors.

Counting Current-Carrying Conductors

(3) 4/0 AL (for the 200A ATS line) (2) 4/0 AL (for the 200A ATS load) (2) #4 AL (for the 100A ATS line) (8) #12 CU (for the branch circuits)

Total Current-Carrying Conductors: 15 Derating Factor

Consult NEC Table 310.15(C)(1). For 10-20 current-carrying conductors, the ampacity derating factor is

50%. Applying Derating



4/0 AL

4/0 Aluminum THHN typically has an ampacity of 180A (at 75°C termination rating - check your equipment). After derating by 50%, the adjusted ampacity is 180A 0.50 = 90A. This is way below the 200A load it needs to carry.

THIS IS A PROBLEM! #4 AL

#4 Aluminum THHN typically has an ampacity of 60A (at 75°C). After derating by 50%, the adjusted ampacity is 60A 0.50 = 30A. This needs to carry a 100A service.

THIS IS A PROBLEM! #12 CU

#12 Copper THHN typically has an ampacity of 30A (at 75°C). After derating by 50%, the adjusted ampacity is 30A 0.50 = 15A. This is acceptable for a 20A circuit (after overcurrent protection)

Conclusion

The derating factor significantly reduces the ampacity of your conductors. The 4/0 and #4 aluminum will be undersized after derating and need to be upsized.

Recommendations and Alternatives


Separate Conduits

The best solution is to use separate conduits. This eliminates the derating issue and simplifies the installation. One conduit for the 200A ATS line and load. A second conduit for the 100A ATS line and branch circuits.2.

Upsize Conductors

If you must use a single conduit, you'll need to significantly upsize the conductors to compensate for the 50% derating factor. This will likely make conduit fill an even bigger issue. Very expensive and potentially impractical.3.

Reduce Number of Current-Carrying Conductors

If possible, see if you can remove some of the branch circuits from the 100A ATS setup to reduce the number of current-carrying conductors. This would allow for a less severe derating. However, your ampacity derating factor for 4-6 current-carrying conductors is 80%. and 7-9 is 70%. Very unlikely to get the ampacity to be correct.4.

High Temperature Rated Conductors

Using THHN with a higher temperature rating (e.g., 90°C) might give you a slightly higher starting ampacity, but this still requires verification and depends on the termination temperature ratings of your equipment. Very unlikely to get the ampacity to be correct.5.

Evaluate Existing Panel Space

Revisit the possibility of working within the existing 400A panel. Even if it's tight, could you possibly use a larger lug kit or a different type of connection that could free up just enough space? This might allow you to consolidate the backup system and avoid the complexities of the dual ATS setup. If you can utilize a subfeed lug, they make models that do not require cutting the existing conductors.

Important Considerations



NEC Compliance

Everything must comply with the latest edition of the NEC adopted in your jurisdiction.

Local Codes and Inspections

Be sure to check with your local electrical inspector regarding any specific requirements or interpretations.

Equipment Ratings

Verify the temperature ratings of all equipment (ATSs, panels, breakers, lugs) and choose conductors and terminations accordingly.

Professional Consultation

Given the complexity of this installation, I strongly recommend consulting with a licensed electrical engineer or master electrician.

In summary, while your conduit fill may be close to acceptable, the conductor derating requirements will likely make your proposed plan unworkable without significant modifications. Using separate conduits is the most straightforward and reliable solution.Disclaimer: This information is for general guidance only and should not be considered a substitute for professional electrical engineering advice. Always consult with qualified professionals and follow all applicable codes and regulations.* Flag for review