Okay, let's break down this distribution scheme and address your concerns, referencing the NEC where applicable. Your understanding is generally correct, but there are nuances and specific calculations required.

Your Proposed Scheme: Pad-Mounted Transformer Supplying Two Switchboards

This arrangement is quite common and perfectly acceptable if it meets specific conditions regarding overcurrent protection and conductor sizing. Here's the breakdown

1. NEC 240.21(C) - Outside Transformer Taps

You are correct in referencing 240.21(C). This section allows for tap conductors supplied by an outside transformer without overcurrent protection at the tap point if the following conditions are met (and you've already identified them, but let's reiterate for clarity): 240.21(C)(1) - Protection of the Tap Conductors

The tap conductors are suitably protected from overcurrent at the point where they receive their supply. This is usually handled by the primary overcurrent protection device of the transformer and the transformer's inherent impedance.

240.21(C)(2) - Ampacity of the Tap Conductors

The ampacity of the tap conductors is not less than the calculated load. This is CRITICAL and often where engineers run into issues.

240.21(C)(3) - Length of the Tap Conductors

The tap conductors must not exceed 25 feet.

240.21(C)(4) - Overcurrent Protection for the Feeder

The tap conductors are installed in a raceway or otherwise protected from physical damage.

240.21(C)(5) - Location

The tap conductors terminate in a single circuit breaker or a single set of fuses that will limit the load to no more than the ampacity of the conductors.

Why Your Engineer Suggests a Downstream Switchboard (and why it's likely unnecessary)

The engineer's recommendation for a downstream switchboard with a main breaker and two branch breakers stems from the concern about

overcurrent protection at the termination of the tap conductors meeting 240.21(C)(5) and/or the desire to comply with 450.3(B) which calls for secondary overcurrent protection Misinterpretation of Termination Requirement

The key is the single circuit breaker or single set of fuses. The intention of the code is to ensure that each conductor is protected by a circuit breaker. If you have two switchboard feeders from the transformer, you need to be careful how you connect them at the switchboard and ensure each feeder is protected by its own overcurrent device.

Meeting Code WITHOUT an Additional Switchboard

You can often meet the code by bringing each conductor pair from the transformer to separate overcurrent devices located in a panel at the switchboard. This satisfies the "single" requirement. The main breaker in each existing switchboard effectively acts as this overcurrent protection if it is properly sized, and if each switchboard feeder goes directly to this main breaker. If the existing switchboard is not properly rated for the transformer full load ampacity, then you will likely need to add another panel.

Secondary Overcurrent Protection

If the primary side protection doesn't meet the ratings of Table 450.3(A) AND the location is not constantly attended by qualified people, then you may need to install secondary overcurrent protection as indicated by 450.3(B). This overcurrent protection can be provided by a switchboard with a main breaker.

"Tapping a Tap" - The Validity of This Statement

The statement "you can't tap a tap conductor" is generally accurate,

but it's being misapplied here. Correct Application

The spirit of the rule is to prevent cascading tap conductors. You can't have a conductor that's already considered a tap (protected by upstream overcurrent devices allowing for a reduced ampacity) then tap that conductor to feed another load without adequate protection. This creates an unprotected, and potentially dangerous, situation.

Your Case

Your conductors from the transformer are not inherently taps in the cascading sense. They are conductors from a transformer secondary. The whole system is being considered under the tap rules because there is no overcurrent protection on the secondary side of the transformer. Therefore, the conductor must terminate into a single overcurrent device.

Feeder Sizing and Overcurrent Protection: The Critical Calculations

This is where the rubber meets the road.1. Transformer Secondary Current

Calculate the full-load secondary current of the transformer using the standard formula: `I = (Transformer kVA x 1000) / (√3 x Voltage)` For a 12kV/208-120V transformer: `I = (kVA x 1000) / (1.732 x 208)` (Remember to convert kVA to VA).
2.

Demand Factor

Determine the actual load on each switchboard. This is NOT the panelboard rating. Perform a load calculation in accordance with NEC Article 220. This is the most important piece of information.

In summary

Your plan of running two sets of conductors is acceptable if the proper steps are followed. Proper protection of the secondary conductors must be verified. If the current system is acceptable for the transformer secondary output, no changes need to be made. If the existing system is not acceptable, a panelboard must be installed downstream of the transformer.

Recommendations



Hire a Qualified Electrical Engineer

This is strongly recommended. This analysis requires thorough knowledge of the NEC and electrical engineering principles. Don't rely solely on online advice for a critical design.

Coordinate with the Utility

Discuss your plans with the utility company. They may have specific requirements for transformer installations and overcurrent protection schemes on their systems.

Documentation: Document everything*. Load calculations, conductor sizing, overcurrent protection coordination, and a single-line diagram are essential.By following the NEC guidelines and these recommendations, you can design a safe and compliant distribution system for your client. Good luck! Flag for review