Okay, let's break down the NEC 2017 compliance of your setup, focusing on Article 225.30 (Number of Supplies).

Understanding the Concerns NEC 225.30 (Number of Supplies)

This article limits the number of supplies permitted to a building or structure. The primary goal is safety and coordination during emergencies or maintenance. The rule is generally ONE supply, but there are several exceptions.

Your Situation

You have a feeder (#4) going to a shed (Panel P2), and then feeders from the shed (Panel P2) to a portable classroom (Panel C). The portable classroom (Panel C) is the concern.

Analysis of NEC 2017 Compliance

Your feeder #4 to Panel P2 likely IS compliant with NEC 2017, Article 225.30, provided Panel P2 in the shed is considered a distribution point and the portable classroom is considered a single building or structure.Here's the reasoning:1. Shed (Panel P2) as a Distribution Point

The electrical shed housing Panel P2 is acting as a distribution point. The shed is not the ultimate destination for the power; it's simply a place where the power is sub-divided and sent onward.2.

Portable Classroom (Panel C) as a Single Building/Structure

The most critical aspect is whether the portable classroom is considered a "building or structure" under the NEC. Typically, portable classrooms are considered separate buildings or structures.3.

Exception No. 4 of 225.30 allows additional supplies to "supply the different buildings or structures of a multi-building or industrial complex under single management." So feeder #4 would supply shed Panel P2. Then the feeders from Panel P2 to Panel C(portable) would be the other supply.

Important Considerations and Recommendations1. Single Management

The multi-building complex must be under single management. This means a single entity responsible for the operation and maintenance of all the buildings and the electrical system.2.

Review Exceptions

Read NEC 2017, Article 225.30 carefully to see if any other exceptions apply that might be more directly relevant to your specific situation.3.

Local AHJ (Authority Having Jurisdiction)

The most important step is to discuss this design with your local electrical inspector (AHJ). They have the final say in what is considered compliant in your area. They may have specific interpretations or local amendments to the NEC.4.

Panel P2 Design

Ensure Panel P2 is properly sized to handle the total load of all feeders going to the portables, with appropriate overcurrent protection for each feeder. Coordination of overcurrent protection is important.5.

Grounding and Bonding

Pay close attention to the grounding and bonding requirements in Article 250, especially regarding separately derived systems and grounding electrodes. Since you have multiple buildings, proper grounding is crucial.6.

Feeder Calculations

Perform accurate feeder calculations (Article 220) to ensure the feeders are sized appropriately for the load, considering demand factors and voltage drop.

In summary

Assuming the portable is considered a separate building/structure, and Panel P2 is only acting as a distribution point. This arrangement is likely compliant with NEC 2017, especially if Exception No. 4 of 225.30 is applicable. However, a discussion with your local AHJ is essential to confirm compliance and address any local interpretations or amendments.

Disclaimer: This information is for general guidance only and should not be substituted for the advice of a qualified electrical engineer and the approval of your local AHJ. Always consult with qualified professionals for code-compliant electrical design and installation. Flag for review