Let's break this down according to the NEC (National Electrical Code) and consider best practices

Understanding the Setup
400A Meter Can

This is where the power company's service connects.

400A Disconnect (Fused or Breaker)

This is the main disconnect for the house. Its function is to cut off all power to the home. You say it could have 300A fuses; determining the fuse size is CRITICAL.

Two 150A Main Breaker Subpanels

These are fed from the 400A disconnect and distribute power to individual circuits within the house.

Is it Permitted? Key Considerations


Overcurrent Protection

The NEC requires overcurrent protection (fuses or circuit breakers) to protect conductors at their ampacity (carrying capacity).2.

Service Entrance Conductors

The wire size feeding the 400A disconnect from the meter can must be rated for at least 400 amps. If it's not, you already have a violation.3.

Conductor Ampacity to Subpanels

The wires between the 400A disconnect and each 150A subpanel must be rated for the load they will carry. This is the crux of your question.4.

Load Calculation

A proper load calculation (NEC Article 220) is essential to determine if the 400A service is adequate for the total load connected. You CANNOT simply add up the breaker sizes of the subpanels. This is a common misconception.5.

Tap Rules (NEC 240.21)

The NEC has specific rules about tapping conductors downstream of an overcurrent device. In general, a tap conductor must terminate in an overcurrent device that is rated to protect that tap conductor. There are length restrictions on taps, as well.

Possible Scenarios & Considerations Scenario 1: Disconnect Actually Has 400A Fuses/Breaker, and Load Calculation Justifies It Possible, but Requires Careful Evaluation

Theoretically, if your load calculation for the entire house is well below 400A, and the conductors from the 400A disconnect to the 150A subpanels are sized appropriately (based on load calculation, considering demand factors), then it might be permitted. The 150A main breakers in the subpanels would act as the overcurrent protection for those feeders. The 400A disconnect would protect the service entrance conductors. However, it's still a design choice that needs to be justified and is not necessarily the best or most common approach.

Scenario 2: Disconnect Has 300A Fuses, and Load Calculation Justifies It This is better because the service entrance conductors will be adequately protected and the conductors from the 300A disconnect to the 150A panels can be adequately protected. Scenario 3: Load Calculation Exceeds 400A (or the Actual Fuse Size) Not Permitted! This is a serious safety hazard. You must reduce the load or upgrade the service. Conductor Sizing Issues Critical

Even if the load calculation is okay, if the wires feeding the 150A panels are not sized according to their ampacity and the applicable overcurrent protection, you have a violation.

Why This Setup is Questionable Lack of Fused Protection Between the 400A Disconnect and the 150A Panels

In a typical installation, you would expect to see smaller fuses or breakers between the 400A main disconnect and the subpanels to protect the conductors feeding those subpanels. This is the most common practice and generally the safest. The 400A disconnect is primarily intended to protect the main service conductors, not necessarily the smaller branch circuit conductors.

Potential for Overload

While the 150A breakers in the subpanels will trip if those individual panels are overloaded, there's a theoretical risk of overloading the conductors between the 400A disconnect and the subpanels without tripping the subpanel breakers, if the load is distributed in a certain way.

Future Expansion

The 400A service might be adequate now, but what about future additions (e.g., electric vehicle charger, hot tub, etc.)? A more typical design with appropriately sized overcurrent protection for each feeder allows for more flexibility.

Recommendations1. Get the Key! You must determine the actual overcurrent protection (fuse or breaker size) in the 400A disconnect. This is non-negotiable. If the customer can't provide it, you may need to call a locksmith or have them contact the utility.2. Perform a Load Calculation

This is absolutely essential. Use NEC Article 220 and take accurate measurements of the actual load.3.

Inspect Conductor Sizes

Verify the sizes of all conductors involved.4.

Consult a Qualified Electrician or Electrical Engineer

This situation warrants professional assessment. They can perform the load calculation, verify conductor sizes, and ensure the installation complies with the NEC and local codes. They can also provide options for improvement.5.

Consider Adding Overcurrent Protection

The best solution might be to install appropriately sized fused disconnects or circuit breakers between the 400A main disconnect and the 150A subpanels. This would provide better protection for those feeders and allow for more flexibility.6.

Documentation

Document everything. Keep records of your load calculations, conductor sizes, and any changes made to the electrical system.

In Summary

While technically it might be permitted if the load calculation justifies it and the conductors are sized correctly, the configuration you describe is not ideal and warrants a very careful and thorough evaluation by a qualified professional. Safety and code compliance are paramount. Do not proceed without proper assessment and documentation. Flag for review