Okay, you've described a scenario that, while potentially code-compliant on the surface, raises some serious practical concerns about future capacity and potential overloading. Here's a breakdown

The Situation as You Described It:
400A Meter

The primary service entrance is rated for 400 amps.

400A Rated Cable

The cable feeding the bus from the meter is rated for 400 amps.

Bus

A bus (typically a set of conductive bars) that distributes power.

350A Fused Disconnect

Feeds one tenant suite.

150A Fused Disconnect

Feeds another tenant suite.

Location

Everything is on the exterior of the building.

Why It Might Be Code Compliant (Initially)



Calculated Load

You're correct that, under NEC (National Electrical Code) rules, the service size is based on the calculated load, not the sum of the breaker sizes. If a load calculation, done according to NEC Article 220, demonstrates that the actual expected load for both tenant suites is 400 amps or less, then the existing setup could be considered initially code-compliant. The load calculation would take into account things like demand factors (the likelihood of everything running at 100% simultaneously) and diversity (the likelihood of different loads peaking at different times).

However, Here Are The Significant Issues and Reasons to Be Concerned (and why it's likely a bad design/long term plan)


Overcurrent Protection vs. Overload Protection

The fuses in the 350A and 150A disconnects primarily provide overcurrent (short circuit and ground fault) protection. They will trip if there's a major fault in either suite. However, they won't necessarily trip immediately if the combined load gradually creeps above 400 amps. The 400A cable and the meter are at risk of overheating and potential damage long before the fuses blow. This is because fuses have an inverse time characteristic - they take longer to trip with a slightly overcurrent condition than a severe overcurrent condition.2.

Potential for Overloading and Damage

As you pointed out, it is entirely possible that the actual combined load could exceed 400 amps, even if the initially calculated load was less. Tenants change, their equipment changes, and their usage patterns change. A new business might move in with more energy-intensive equipment. A constant overload on the 400A cable will cause it to overheat. Over time, this degrades the insulation, increasing the risk of a short circuit, fire, and premature cable failure. The meter itself can also be damaged by prolonged overloading.3.

Lack of Headroom for Future Expansion

The biggest problem is the lack of any capacity for future growth. The electrical system is already maxed out. Any new electrical equipment or increased usage by either tenant will put the system at risk. This makes the building less attractive to potential tenants, as they will be concerned about the limited electrical capacity.4.

Code Considerations About Continuous Loads

The NEC requires that overcurrent devices and conductors be sized to carry 125% of the continuous load. A continuous load is defined as a load that operates at its maximum current for 3 hours or more. If either suite has significant continuous loads (like HVAC systems, refrigeration, or lighting in certain businesses), then the 400A service may not be code compliant, even if the initial load calculation seems okay.5.

Safety Concerns

Overloading conductors can lead to overheating and potentially fires. This situation creates an unnecessary safety risk for the building and its occupants.

Recommendations

You are correct in your assessment!

Upgrading the service entrance equipment is highly recommended. Here's what should be considered:1. Comprehensive Load Calculation

Do a fresh, thorough load calculation for both suites, taking into account potential future needs. Be conservative. Err on the side of overestimating the potential load.2.

Upgrade the Meter

Replace the 400A meter with a 600A or even 800A meter (depending on the load calculation results).3.

Upgrade the Cable

Replace the 400A cable with a cable rated for at least 600A or 800A to match the new meter rating.4.

Consider a Larger Bus

The bus itself might need to be upgraded to handle the increased current.5.

Coordination Study (Optional, But Recommended)

For a building with multiple tenants, it's wise to perform a coordination study. This ensures that in the event of a fault, the closest overcurrent device to the fault trips first, minimizing disruption to the other tenants.6.

Local Utility Coordination

Before making any changes, contact the local utility company. They may have specific requirements for service entrance upgrades and meter installations.

In Conclusion:

While the original setup might* squeak by a code inspection if the initial load calculations are low enough, it's a recipe for problems down the road. The lack of headroom, the potential for overloading, and the safety concerns all make upgrading the service a worthwhile investment in the long-term reliability and value of the building. Your instincts are spot-on: it's much better to have more capacity than you need than to be constantly worried about overloading the system. Flag for review