Okay, let's break down the wire sizing for your panel upgrade situation. This is a common question, and it's important to get it right for safety and code compliance.

Key Principles



The Main Breaker Governs

The primary consideration for your service entrance conductors (the wires feeding your panel from the meter) is the ampacity of the main breaker. You're upgrading to a 200 amp panel, therefore, the main breaker is 200 amps.

Buss Bar Capacity is a Rating, Not a Requirement

The 225 amp buss bar rating means the panelboard itself can handle up to 225 amps continuously. It's a safety margin built into the equipment. It does not dictate the size of your service entrance conductors unless you have a specific load calculation demanding more than 200 amps in the future.

Applying the NEC and the 83% Rule (Continuous Loads)


NEC Table 310.16 (Ampacity of Conductors)

This is your starting point for determining wire size.2.

The 83% Rule (Continuous Load Consideration)

The 83% rule comes into play when you have a load that is considered "continuous." A continuous load is defined as a load that operates at its maximum current for 3 hours or more. If you anticipate more than 200 amps being drawn for more than 3 hours, then you have to follow the 83% rule. In order to abide by this rule, you would take the current of the over current protection device, and multiply it by .80. (200.80 = 160) With this new number, you would look for a wire size capable of handling at least 160 amps. If you were using copper, this would still be a 2/0 awg. If you were using aluminum, this would have to be 4/0, as the 3/0 only covers 150 amps.

Therefore, based on your situation (200 amp main breaker and not exceeding 200 amps)



You need conductors rated for at least 200 amps (after considering the 83% rule if necessary). 2/0 Copper would be sufficient to carry the 200 amps. 4/0 Aluminum would be sufficient to carry the 200 amps.Important Considerations and Clarifications



Load Calculation is Key

While the 200 amp main breaker is the immediate determining factor, you must perform a proper load calculation according to NEC Article 220. This calculation determines the actual total load your service will be expected to carry. If your calculated load significantly exceeds 200 amps, you'd need to revisit the main breaker size and wire accordingly.

Conductor Material

Copper and aluminum have different ampacity ratings for the same wire size. Use the correct column in NEC Table 310.16 for the conductor material you're using.

Termination Temperature Rating

Ensure your conductors are rated for the temperature of the terminations in your panel and meter enclosure (typically 75°C). Most modern panels require 75°C rated conductors. Use the ampacity column in NEC 310.16 that corresponds to the termination temperature rating.

Parallel Conductors

Using two smaller conductors in parallel is an option in some situations, but it requires very careful application of the NEC rules and may not be practical for a residential service entrance.

Grounding and Bonding

Don't forget the importance of proper grounding and bonding. The grounding electrode conductor and the bonding jumpers need to be sized according to NEC Table 250.66.

In summary

Size your service entrance conductors based on the 200 amp main breaker rating, confirming that your load calculation doesn't require a larger service. The 225 amp buss bar rating is a specification of the panel's capacity, not a requirement for the wire size in this scenario.

Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. Always consult with a licensed electrician and follow all applicable codes and regulations.* Flag for review