Okay, let's break down this scenario and how it relates to NEC 240.21 and 445.13(B). This is a common point of confusion, especially with generator installations and tap rules.

Understanding the Setup Source

26,000 kW (26 MW) Standby Generator (This is a VERY large generator. Confirm this value, as it seems unusually large for a residential application).

Generator OCPD

125A breaker at the generator.

Tap #1

From generator to a 200A transfer switch (Transfer Switch #1) located 20 feet away, conductors in conduit.

Tap #2

From Transfer Switch #1 to another 200A transfer switch (Transfer Switch #2) located 80 feet away, conductors in conduit.

Question

Does this configuration violate NEC 240.21 and 445.13(B), and if so, can an OCPD at the tap point solve the problem?

Analysis Based on NEC and Assumptions1. NEC 445.13(B) and 240.21 Relationship

NEC 445.13(B) directly sends us back to NEC 240.21 for overcurrent protection of generator conductors. This means the tap rules of 240.21 must be followed.2.

The Core Problem: Tap Rules Violated Your scenario as described most likely violates the tap rules of 240.21. Here's why: 240.21(B) Tap Rules Generally

These rules allow taps from larger conductors to smaller conductors under very specific conditions. The goal is to protect the smaller conductors. Key requirements generally include: The tap conductor's ampacity must be at least 1/3 of the overcurrent protection device (OCPD) protecting the larger feeder it's tapped from. The tap conductor must terminate in a single circuit breaker or set of fuses rated to protect the tap conductor's ampacity. Specific length limitations (10-foot tap rule, 25-foot tap rule).

Why it Likely Fails

In your case: The 125A breaker at the generator is protecting the feeder to Transfer Switch #1. You are using a 200A transfer switch and likely 200A-rated conductors from Generator to Transfer Switch #1. Then another run to Transfer Switch #2 with 200A-rated conductors. In order for this installation to pass the tap rule, the conductors to each transfer switch, need to be protected by a fuse / breaker, that is within the ampacity value of the conductors.3.

Installing OCPD at the Tap Point



Yes, installing overcurrent protection at the tap point can potentially solve the problem, BUT it needs to be done correctly. Why it Works (Potentially)

By placing a properly sized OCPD at the tap immediately after Transfer Switch #1 (where the conductors run to Transfer Switch #2), you can protect the conductors running to Transfer Switch #2.

Key Considerations



OCPD Sizing

The OCPD must be sized to protect the conductors running to Transfer Switch #2, based on their ampacity (determined by conductor size, insulation type, and installation method per NEC 310.16). If you are using 200A conductors you need 200A protection, but you should verify the rating based on the tables and installation.

Location, Location, Location

The OCPD must be located at the point of the tap from the feeder conductors. This means right after Transfer Switch #1 for the conductors going to Transfer Switch #2.

Transfer Switch Compatibility

Ensure the OCPD installation doesn't interfere with the transfer switch's operation or create any safety hazards.

Load Calculation

Confirm the load on each transfer switch doesn't exceed the conductor ampacity and OCPD rating.

Revised Scenario & Example

Let's assume the 200A conductors from Transfer Switch #1 to Transfer Switch #2 are rated for 200 amps. To comply with NEC 240.21 and 445.13(B), you could do the following:1. Generator OCPD

125A breaker at the generator (as you have it).2.

Conductors to Transfer Switch #1

Use conductors rated for at least 125A from the generator to Transfer Switch #1.3.

Transfer Switch #1

200A transfer switch.4.

OCPD at Tap Point

Install a 200A breaker immediately after Transfer Switch #1, where the conductors are tapped to run to Transfer Switch #2. This breaker protects the 200A conductors running to Transfer Switch #2.
5.

Conductors to Transfer Switch #2

Use 200A-rated conductors from the OCPD after Transfer Switch #1 to Transfer Switch #2.6.

Transfer Switch #2

200A transfer switch.

Feasibility

Whether this is feasible depends on your physical space, the existing installation, and the specific requirements of the equipment you're using. Things to consider: Space

Do you have room to install a properly sized and rated breaker enclosure immediately adjacent to Transfer Switch #1?

Cost

Adding OCPD and potentially upgrading conductor sizes will add to the cost of the installation.

Alternatives

If the load is not high consider 100A breakers for the generator and both transfer switches with appropriate sized conductors.

Important Considerations Local Codes and AHJ

Always check with your local electrical inspector (Authority Having Jurisdiction - AHJ) before making any modifications. They have the final say on whether an installation meets code.

Engineering Review

For a generator of this size, it's highly advisable to have a qualified electrical engineer review the entire system design to ensure it meets all applicable codes and safety standards.

Generator Sizing

Double-check that 26,000 kW rating. It's exceptionally large for a residential standby generator. An oversized generator is inefficient and can lead to other problems.

Conductor Ampacity

Be certain to accurately determine the ampacity of your conductors based on the NEC tables (310.16 and associated correction/adjustment factors). Don't assume a 200A transfer switch means you must use 200A conductors.

In Summary

The tap rules in 240.21, as enforced by 445.13(B) for generators, are crucial for safety. Installing an OCPD at the tap point can* be a solution, but it requires careful attention to OCPD sizing, location, and conductor ampacity. It is imperative to confirm all conductor sizes, load values and confirm the 26,000 kW generator rating and consult with a qualified electrical engineer and local electrical inspector to ensure compliance and safety. Flag for review