You're likely encountering a common point of confusion around residential electric service sizing, particularly with larger services like 400 amp. Here's a breakdown of why you're likely hearing that a "400 amp service is really only 320 amps" and what's going on

The Core Issue: Meter Socket and Continuous Load Ratings
Continuous Load

Electrical codes (like the National Electrical Code, or NEC in the US) require that electrical components be sized to handle continuous loads with a safety margin. A continuous load is defined as a load that operates for 3 hours or more. For most applications, components are rated at 80% of their nameplate rating to prevent overloads from continuous operation.

Meter Sockets and Parallel Feed

400-amp residential services often use a meter socket that is designed to accept two separate conductors per phase (parallel conductors). Because of this design, it might use two 200 amp breakers, which is how you get to the 400 amp service in total.

Why 320 Amps? The meter socket that is typically used for 400 amp services may only be rated for 320 amps of continuous load. If the meter is only rated for 320 continuous amps, the POCO (Power Company) is only going to allow you to use 320 amps, because that is all the meter can handle.

Important Considerations Local Codes and POCO Rules

Ultimately, the rules and interpretations of those rules can vary. The POCO has the final say on what they will allow to be connected to their grid. You should always consult with your POCO and a licensed electrical contractor in your specific area.

Peak vs. Continuous

The 400 amp rating doesn't mean you can never exceed 320 amps. It means that your sustained (continuous) draw should not exceed that amount. Short-term peaks above 320 amps are usually acceptable, as long as they are within the design limits of the equipment and don't cause voltage sag or other issues.

In Summary:

The statement "400 amp service is really only 320 amps" is a simplification, but it reflects the practical limitations imposed by the meter socket's continuous load rating. While the overall service capacity is 400 amps, the sustained* load should not typically exceed 320 amps in most residential situations. It's best to verify the rules for your area by contacting a local, qualified electrical contractor or your power provider. Flag for review