This is a great question and a common scenario when splitting commercial buildings. Here's a breakdown of the code requirements and best practices

NEC Requirements:
300.3(B) Conductors of Different Systems

"Conductors of wiring systems of 600 volts, nominal, or less shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway."

Interpretation and Practical Application



Legally Permissible Sharing (in most cases)

The NEC does allow circuits from different systems (i.e., different meters/panels) to share conduits and junction boxes, provided that all conductors have insulation rated for the highest voltage present. In a typical 120/208V or 277/480V system, this usually isn't an issue, as most modern THHN/THWN conductors are rated for 600V.

However, Consider This

While code technically permits sharing, it's often not the best practice from a practical and future maintenance perspective.

Arguments AGAINST Sharing


Troubleshooting Nightmare

Imagine someone working on Tenant A's circuit years from now and inadvertently disconnecting Tenant B's circuit because they're all bundled together in a common junction box. This can lead to significant disruption and wasted time. It also increases the chance of accidental short circuits between systems.2.

Tenant Disputes

Even if legal, sharing can lead to disputes between tenants regarding maintenance, upgrades, or even blaming each other for electrical problems. Clear separation minimizes these potential conflicts.3.

Future Modifications

What happens if one tenant wants to add a circuit that requires a different size wire or conduit? It becomes much more complicated if you're sharing the existing infrastructure.4.

Potential for Misidentification

Even with careful labeling, it's possible for future electricians to misidentify which circuit belongs to which tenant if they are intermingled in shared raceways and boxes.5.

Liability

If a problem arises and it's difficult to determine which tenant's wiring caused it, you could be opening yourself up to liability.

Arguments FOR Sharing (Cost Savings)



Reduced Labor and Material Costs

Rerunning all the circuits is significantly more expensive in terms of materials (conduit, wire) and labor.

Minimizing Disruption

Rerunning circuits can be disruptive to existing tenants, especially if walls or ceilings need to be opened up.

Recommendations and Best Practices


Consult with the AHJ (Authority Having Jurisdiction)

This is critical. Some local jurisdictions have stricter interpretations of the NEC or have local amendments that prohibit or discourage sharing circuits between different tenants. Get their explicit approval before proceeding.2.

Thorough Documentation and Labeling (if sharing)

Create detailed "as-built" drawings showing exactly which circuits are shared and which meter/panel they are connected to. These drawings should be readily available for future maintenance. Use clear and permanent labels inside every junction box indicating which circuits belong to Tenant A and Tenant B. Use different colors for the labels (e.g., blue for A, red for B). Label the circuit breakers in each panel very clearly with the corresponding circuit number and tenant name. Consider using wire markers to identify the circuits within the conduits.3.

Consider Using Sub-Metering (If possible)

If the loads on the circuits are fairly consistent and predictable, you might be able to install sub-meters for each tenant even if the circuits originate from the same panel. This would require careful engineering and would need AHJ approval. This only works if you don't have to create a separate electrical service for the second tenant (i.e., if you're just allocating a portion of the existing service). This is usually not the case when you split the building and are required to install a separate service and main disconnect.4.

Weigh the Costs and Benefits

Get quotes for both options: Sharing the existing conduit and boxes (with careful documentation and labeling). Rerunning all the circuits separately.Present the costs, benefits, and risks of each option to the building owner. Make sure they understand the potential long-term consequences of sharing circuits.5.

If Possible, Segregate

If budget and logistics allow, the best solution is to rerun the branch circuits separately for each tenant. This avoids all the potential problems described above and provides the most future-proof and maintainable electrical system.

In Summary

While the NEC generally permits sharing circuits from different systems, it's often not* the most prudent approach when splitting a commercial building due to potential long-term problems with troubleshooting, tenant disputes, and future modifications.

Always consult with the AHJ and document everything meticulously if you choose to share circuits. Carefully weigh the costs and benefits and consider the long-term implications.

Disclaimer: Always consult with a licensed and qualified electrician for specific project requirements and code compliance. Flag for review