You've got a good eye for detail, and your confusion is understandable! Here's a breakdown of why the answer likely uses a 400A disconnect, even though the calculated load is 322A, and why your observations about 350A are important

Key Considerations:

1. Rounding Up for Standard Sizes

The NEC (National Electrical Code) generally allows you to round up to the next standard overcurrent device size if the calculated load doesn't perfectly match a standard size. However, there are specific rules and limitations on this rounding up.2.

Future Expansion

While not explicitly stated in the example, the choice of a 400A disconnect suggests the possibility of future load expansion. The electrical contractor or designer likely considered that the homeowner might add significant electrical loads (e.g., a hot tub, electric vehicle charger, large HVAC system) in the future. Therefore, they sized the service equipment to accommodate those potential additions without requiring a complete service upgrade later.3.

Demand Factors and Calculated Load

The calculated load of 322A is based on demand factors. These factors acknowledge that not all appliances and circuits will be operating at their maximum capacity simultaneously. The actual connected load (the sum of all the circuit breakers) is likely much higher than 322A. However, because the 400A disconnect is selected, the loads cannot exceed the disconnect's maximum capacity

Why Not a 350A Breaker?

You're right that 350A is a standard breaker size. However, there are a few reasons why it might not be the best choice in this scenario: Small Margin

A 350A breaker would only provide a very small buffer (28A) above the calculated load of 322A. If the homeowner did add a significant load, even a relatively modest addition, the load could exceed 350A, causing nuisance tripping.

Cost

In most cases, the additional cost of upgrading to a 400-amp service is relatively minor compared to the potential cost of upgrading the service later.

Specific NEC Considerations



215.3 Feeders

This section states that feeder conductors shall have an ampacity not less than required to supply the load as calculated by Parts III, IV, and V of Article 220. If the feeder conductors supplied were rated for more than 322 amps (but less than 400 amps), the overcurrent protective device should be set to allow the conductors' ampacity to be fully realized.

230.42 Minimum Size and Rating

(Service Entrance Conductors): This section states that service entrance conductors shall have an ampacity not less than required by the calculated load according to Article 220.

230.79 Rating of Disconnect

This section specifies the minimum ampere rating or interrupting capacity of the service disconnecting means.

240.4 Protection of Conductors This section of the NEC covers the protection of conductors with overcurrent protection devices (like breakers). It generally states that conductors must be protected at their ampacity rating (with certain exceptions). 240.6 Standard Ampere Ratings

As you mentioned, 240.6 lists standard ampere ratings for fuses and inverse time circuit breakers.

Important Notes



Local Codes and AHJ (Authority Having Jurisdiction)

Local electrical codes or the preferences of the local AHJ can influence service sizing. In some areas, the AHJ might prefer or even require a certain level of "future-proofing" when sizing service equipment.

Professional Judgment

Service sizing involves professional judgment. A qualified electrician or electrical engineer will consider the calculated load, potential future loads, cost, and local regulations when determining the appropriate size of the service equipment.

In Summary:

While a 350A breaker could* theoretically work based on the calculated load, the 400A disconnect is likely chosen to provide a more comfortable margin for future expansion and to align with common practices or local requirements. The electrician is anticipating potential future loads that the homeowner may add. If that is the case, it's a good idea to inform the homeowner of the reasoning behind the larger service disconnect. Flag for review