You're asking the right questions, and understanding the basics is crucial. Here's a breakdown of how to approach this, considering the limitations of not having the specific riser diagram and needing to make assumptions

Key Concepts and Considerations:
Available Fault Current

This is the maximum current that the system could deliver to a fault location. It's dependent on the source impedance (transformer), conductor impedance, and equipment impedance.

AIC Rating (Ampere Interrupting Capacity)

This is the maximum fault current an overcurrent protective device (circuit breaker, fuse) can safely interrupt without failing catastrophically. Your equipment (panelboards, disconnects) must have an AIC rating at least as high as the available fault current at their location.

Arc Flash Hazard

While you're focused on AIC ratings, remember that high fault currents also create a significant arc flash hazard. Personal Protective Equipment (PPE) requirements are directly tied to the available fault current and clearing time of the overcurrent device. This should be a consideration in your design as well.

Coordination Study

A thorough coordination study is highly recommended. This ensures that overcurrent protective devices trip in a coordinated manner, minimizing the extent of an outage during a fault. A qualified electrical engineer performs this.

Steps and Considerations for Your Situation


Don't Assume 150.4 kA at Your Equipment

While the main switchboard has 150.4 kA available, you cannot assume that value at your equipment. The 200 feet of cable, the CT cabinet, meter, main disconnect, and any other intervening equipment will introduce impedance and reduce the available fault current.2.

Detailed Fault Current Calculation is Required

To determine the available fault current at your 400A panel, you must perform a fault current calculation. This calculation involves:

Source Impedance

Obtain the transformer's impedance (%Z) from the building's owner or engineer. This is critical.

Cable Impedance

Calculate the impedance of the 200 feet of cable based on its size, type (e.g., copper, aluminum), and installation method (e.g., in conduit, cable tray). Cable impedance is usually expressed in ohms per 1000 feet, so you'll need to adjust for your 200-foot length.

Equipment Impedance

While often small, the impedances of the CT cabinet, meter, and disconnect can be included for a more accurate calculation. Manufacturers often provide this data.

Panelboard Impedance

Panelboards themselves have a small impedance, but it's typically included in fault current calculation software.

Point-to-Point Calculation

The standard method for this is a "point-to-point" calculation where you start at the source (the transformer) and add the impedance of each component in the circuit. The available fault current decreases as impedance increases.3.

Software Tools

Using fault current calculation software (e.g., SKM PowerTools, ETAP, EasyPower) is highly recommended. These tools can handle the complex calculations and automatically generate reports.4.

Minimum AIC Rating

Once you've calculated the available fault current at your 400A panel, select a panelboard and main disconnect with an AIC rating that meets or exceeds that calculated value. Never use equipment with an AIC rating lower than the available fault current.5.

Selective Coordination

After determining the fault currents and AIC rating for your equipment, you'll also want to consider the selective coordination between all of your devices. For example, you want your 400A breaker in your tenant space to trip before the main breaker at the main switchboard. Selective coordination will ensure the proper devices trip under overcurrent conditions and minimize any facility outages.

Practical Implications and Things to Discuss with the Building Owner/Engineer



Obtain Data

The most critical step is getting the transformer impedance data. The building owner or engineer must provide this.

Review the Riser Diagram

Carefully review the existing arc fault riser diagram for any relevant information about upstream protective devices and impedances. While it may not give you exact values, it can provide valuable context.

Coordination Study

Inquire if a coordination study has already been performed for the building. If so, get a copy. This study will provide valuable insight into available fault currents and device settings. If one hasn't been performed, consider recommending that the building owner performs one for the entire building, including your tenant space.

Why 22kA-65kA AIC Panels are Common, But Potentially Insufficient

You're right that many standard panelboards have AIC ratings in the 22kA to 65kA range. However, in a situation like yours, where you're tapping into a high-capacity main switchboard, it's very possible that the available fault current exceeds these standard ratings. You cannot simply assume a standard rating will be sufficient.

Example (Simplified - for Illustration Only)

Let's say after your calculation, you determine the available fault current at your 400A panel is 45kA. In this case, a 400A panel with a 65kA AIC rating would be acceptable. However, if the calculation showed 70kA, you would need to select a panel with a higher AIC rating (e.g., 100kA, which might require using a series-rated system - see below).

Series Ratings

If the calculated fault current is significantly higher than the AIC rating of the panelboard's main breaker, you might be able to use a "series-rated" system. This involves using a specific combination of upstream and downstream breakers that have been tested to interrupt a fault current exceeding the downstream breaker's AIC rating. However, series ratings are very specific to the manufacturer and type of breakers, and must* be verified by the manufacturer's literature. It's best to consult with an electrical engineer about this. Series ratings are complex and require careful application.

In Summary


Get the transformer impedance data from the building owner/engineer.2. Perform a detailed fault current calculation.3. Select equipment (panelboard, disconnects) with AIC ratings that meet or exceed the calculated available fault current.4. Consider arc flash hazard and PPE requirements.5. Strongly consider a coordination study.6. Consult with a qualified electrical engineer.

By following these steps, you can ensure a safe and compliant electrical installation for your restaurant. Remember that electrical design involves liabilities, so it's always best to consult with a qualified professional. Flag for review