Okay, let's break this down. You're right to ask clarifying questions. Here's what you need to know and why

Yes, you absolutely need to know the size of the existing feeder and its overcurrent protection.

It matters tremendously.Here's the reasoning:1. Feeder Ampacity Cannot Be Reduced

You cannot legally or safely reduce the ampacity of a feeder. If the existing feeder is smaller than 225A, you're limited to that existing feeder's ampacity. Splicing doesn't magically increase capacity. You can't splice a 150A feeder to a new 225A panel and call it a 225A feeder.2.

Overcurrent Protection Coordination

The overcurrent protection device (OCPD - breaker or fuses) protecting the existing feeder is the upstream protection for your entire extended circuit (existing feeder + splice + new feeder to the new panel + new panel itself).3.

Load Calculation is required You need to perform a load calculation based on the new panel to see if the existing feeder is sufficient.

Here's what you need to determine



Existing Feeder Size



Visually Inspect

Locate the existing feeder cable. The conductor size should be printed on the cable jacket (e.g., "3/0 AWG Cu," "4/0 AWG Al"). Note the material (copper or aluminum).

Check the Existing Panel

The breaker feeding the existing panel is the OCPD for the feeder. Find the amperage rating on that breaker (e.g., "100A," "150A," "200A").

Existing OCPD Size



Find the OCPD

Go back to the panel board supplying the feeder. Determine the OCPD that protects the existing feeder. It is critical to know the size of the OCPD.

New Panel Load Calculation

Perform a load calculation on the

new panel. What are you powering with this new panel? It would be best practice to get this value, and size the feeder accordingly.

Why 225A May Be Insufficient, and What to Do



Assuming 225A is FINE is Dangerous

You can't just assume 225A is fine. If the new panel requires more than the existing feeder's capacity, even if it's under 225A, you'll overload the existing feeder and potentially cause a fire.

If the existing feeder is say 150A, you must protect your spliced feeder to the new panel with a 150A overcurrent protection deviceSteps to Take


Gather Information

Get the existing feeder size, material, and OCPD rating.2.

Perform a Load Calculation

Calculate the total load of the new panel. Include everything that will be connected to it (lighting, receptacles, equipment, motors, etc.). Use the NEC (National Electrical Code) to determine the appropriate demand factors for the types of loads.3.

Compare and Determine Ampacity

If the existing feeder is equal to or greater than the new panel load calculation AND the feeder is rated at least 225A, then yes, splicing to a 225A-rated feeder and a 225A panel may be acceptable (with a 225A OCPD protecting the new feeder at the point of the splice in the existing panel). If the existing feeder is less than the new panel load calculation, you CANNOT use it. You must install a new feeder from the source with sufficient ampacity to handle the new load.4.

Overcurrent Protection

The OCPD in the panel feeding the existing feeder provides the primary overcurrent protection. You should generally add a OCPD to protect the new feeder cable, at the point of the splice.

Example Scenario

Let's say: Existing Feeder: #3/0 AWG Copper (rated 200A) Existing OCPD: 200A New Panel Load Calculation: 180AIn this case:1. You could reuse the existing feeder because its capacity (200A) exceeds the new panel's calculated load (180A).2. You should install a OCPD in the panel feeding the existing feeder, at the point of the splice, to protect the new feeder. The size of the breaker cannot exceed the feeder size.

Important Considerations



NEC Compliance

All work must comply with the National Electrical Code (NEC) and local codes.

Permitting and Inspection

This work will likely require a permit and inspection. The inspector will verify your calculations and compliance with the NEC.

Qualified Electrician

This type of work should only be performed by a qualified and licensed electrician. It involves potentially dangerous electrical work.

In summary, you MUST know the existing feeder size and the OCPD protecting it. Perform a load calculation on the new panel. Then, ensure that the existing feeder capacity is sufficient for the new load before you proceed with any splicing or wiring. Flag for review