You're right to be concerned. While not a clear-cut violation in every single case, drawing power from one tenant space's service to supply another tenant space's equipment is generally problematic under the NEC, especially with the distinct service equipment and fire rating between the spaces. Here's a breakdown of the relevant code sections and potential issues, along with possible (but unlikely) scenarios for acceptance

Primary Concerns & Relevant NEC Sections:

1. Article 220 - Load Calculations

This is the foundation. Each service is sized based on the calculated loads within that space. Taking 1000A from the 2500A service to supply the other space significantly alters the load calculation basis. Even if the existing service appears oversized now, future tenant additions could easily overload it if this 1000A is not accounted for.

Violation

Failure to properly calculate the total load served by the 2500A service after this 1000A feed.2.

Article 225 - Outside Branch Circuits and Feeders

This article applies because you are running a feeder outside the confines of the original tenant space where the service is. You're essentially feeding one building (or part of a building) from another.

225.30 Number of Supplies

"A building or other structure shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E)." This is a very strong limiting factor. The exceptions are quite specific and unlikely to apply to your situation. They mainly relate to:

(A) Special Conditions

Where reliable power is crucial (e.g., fire pumps, emergency systems, hospitals). This doesn't sound applicable to your situation.

(B) Capacity Requirements

Where the load requirements exceed the capacity of a single feeder. Possible but unlikely given your situation and the presence of an existing adequate service.

(C) Different Characteristics

Different voltages, frequencies, or phases are required. Not relevant.

(D) Documented Switching Procedures

This applies to installations under single management where a written procedure prevents overload. This may not be what you are considering since the landlord is ok with it, meaning it might not be a single management.

(E) Integrated Electrical Systems

Coordinated electrical system with a zone equipment distribution system under a single management. Requires a written coordinated procedure, so if this is a consideration it may be a lot more work.

Violation

Highly likely violation of 225.30 unless one of the very specific exceptions can be demonstrated.
3.

Article 230 - Services

This reinforces the idea of a single service for a building or structure, and the service equipment must be designed for the entire load served.

230.2 Number of Services

Similar restriction to 225.30. "A building or other structure shall be supplied by only one service unless permitted in 230.2(A) through (D)." The exceptions are similar to 225.30, but even less likely to apply in your case because you already have two services.

Violation

Likely violation of 230.2, further solidifying the single-service rule.
4.

Article 300 - Wiring Methods

You will have to ensure all the wiring methods from space 1 to space 2 are in compliance with Article 300.

Why it's Problematic



Safety

The biggest concern is safety. The 2500A service may appear oversized now, but future load additions could overload it. The existing overcurrent protection scheme for that service isn't designed to protect the wiring within another tenant space with its own potential hazards. Coordination issues could arise.

Future Changes

What happens when Tenant #1's needs change and they need more power? Now you have a complex situation where Tenant #2 is dependent on Tenant #1's service. This complicates future lease agreements and building expansions.

Liability

If a problem arises in Tenant Space #2 that's related to the power feed from Tenant Space #1, who is liable? This creates a significant liability issue for the building owner.

AHJ Approval

This kind of installation is highly unlikely to be approved by the Authority Having Jurisdiction (AHJ). They will likely cite the above NEC violations and concerns.

Possible (But Unlikely) Scenarios for Acceptance

These are very difficult to achieve and will require extensive documentation and AHJ approval:1.

Completely Redo Load Calculations

You would need to perform very detailed load calculations for both tenant spaces, taking into account: The existing loads. The 1000A feed to the other space. Maximum potential future loads for both spaces. Prove, beyond any doubt, that the 2500A service is adequately sized even with the 1000A feed and even with future load growth in both spaces. This would require sealed calculations by a qualified professional electrical engineer.2.

Dedicated Overcurrent Protection & Coordination Study

The 1000A feeder would need dedicated overcurrent protection at the 2500A service. A full coordination study would be required to ensure that the new breaker and the downstream overcurrent protection devices in Tenant Space #2 are properly coordinated to prevent nuisance tripping and ensure selective coordination. This is expensive and requires specialized expertise.3.

Emergency or Standby Power System Transfer

700.12(I) permits feeders from a standby or emergency system to extend to a remote building, so as long as it complies with all of Article 700 or 701.
4.

Extensive Documentation & Agreement

A legally binding agreement between the building owner, both tenants, and the utility company outlining: Responsibility for the 1000A feed. Metering arrangements. Liability in case of problems. Procedures for future load changes. Termination clauses for the feed.5.

AHJ Approval

Even with all of the above, the AHJ has the final say. They may still reject the proposal based on safety concerns or code interpretations.

Recommendation

Strongly advise your client against this approach. The potential code violations, safety risks, liability issues, and the difficulty in obtaining AHJ approval make it a very risky proposition.The proper solution is to either:

Upgrade the 1000A service in Tenant Space #2 if it's inadequate.* The landlord could add capacity to the transformer feeding tenant space #2.

While the initial cost might be higher, it's a far safer, more reliable, and code-compliant solution that avoids future headaches. Flag for review