Okay, let's break this down to ensure a safe and code-compliant installation.

Understanding the Situation Load Calculation

You've calculated a connected load of 191 amps for the house.

Service Capacity

You have a meter cabinet rated for 320 amps, supplying two 200 amp main breaker panels.

Question

How do you size the feeder conductors from the meter to the two panels?

The Correct Approach

The correct approach is to size the feeder conductors based on the calculated load, considering the "125% rule" (or 80% derating). However, there's a critical detail: Diversity. You're feeding two panels. You do NOT apply the 125% to the total connected load. You must determine the maximum load that will likely be drawn through the meter at any given time. Then apply the 125% rule.

Here's a step-by-step breakdown:1. Verify Load Calculation

First, double-check your load calculation to ensure it accurately reflects the actual expected load. Make sure you've properly applied demand factors per Article 220 of the NEC (National Electrical Code). Load calculations have many demand factors that can lower that 191 amps considerably. For instance, the NEC does not require the A/C and heat to be calculated at 100 % so the numbers can be much smaller then you think.2.

Consider Demand Factors Again

Even with a careful load calculation, real-world use will almost certainly involve diversity. Not everything will run at once. Analyze the anticipated usage patterns. This is where professional judgment comes in. Is the home likely to have two electric vehicle chargers running simultaneously for extended periods? Does it have a hot tub or other high-demand appliances that will significantly add to the load? Or will the occupants normally use only one or two large appliances? Without knowing more about the specific appliances you have that are contributing to that load calculation, it is hard to apply any diversity to the numbers.3.

Apply 125% (or 80% Derating)

Once you've determined a realistic maximum load after considering diversity, apply the 125% rule. Example: Let's say you determine that, even with the diversity, the peak load you expect to see through the meter cabinet is 180 amps after all demand factors. 180 amps x 1.25 = 225 amps.
4.

Size Conductors

Based on the result from Step 3, choose conductors that are rated to carry that amperage, using NEC Table 310.16 (or applicable table based on your wiring method and installation conditions). Example (continued): In our example, you would need conductors rated for at least 225 amps. 300 kcmil copper conductor, in most common installation methods, would be sufficient.5.

Overcurrent Protection

The overcurrent protection at the source (in this case, the utility-provided connection) protects the feeder conductors. However, since you have two 200-amp main breakers at each panel, you've effectively segmented the overall protection. This is where the 320 amp meter can needs to be considered. The conductors must be protected according to its' amp rating. The 320 amp meter can is not a listed assembly, it just allows you to put 2 panels under one meter.

Why NOT Size Solely Based on the Meter Rating Cost

Sizing the conductors to the full 320-amp meter rating (500 kcmil) would be significantly more expensive and likely unnecessary.

Code Compliance

The NEC requires conductors to be sized based on the load, not simply the rating of the meter can.

Diversity is Key

Electrical systems are designed around the likelihood of load, not the theoretical maximum.

Important Considerations Local Amendments

Always check with your local electrical inspector and authority having jurisdiction (AHJ) for any local amendments to the NEC. They may have specific requirements.

Conductor Material

Your load calculation assumed a particular material (usually copper or aluminum). Ensure your conductor choice matches the basis of the load calculation.

Temperature Correction/Adjustment

Remember to apply temperature correction and adjustment factors per NEC 310.15 if conductors are installed in conditions that affect their ampacity.

Neutral Conductor

Don't forget to size the neutral conductor per NEC Article 220. Often, it can be smaller than the ungrounded conductors due to unbalanced load considerations.

In summary

While the meter cabinet is rated for 320 amps, size the feeder conductors based on the calculated load after applying realistic demand factors and the 125% rule. This will likely result in a smaller conductor size than simply using the meter rating.

Disclaimer: I am an AI chatbot and cannot provide definitive electrical design advice. Consult with a qualified and licensed electrical contractor or engineer for a code-compliant and safe installation. They can assess your specific situation and ensure compliance with all applicable codes and regulations. Flag for review