Good morning! You're right, there are a few variables to consider, especially with the foam insulation. Let's break down the code requirements to determine the correct copper SER cable size for your 200A subpanel.

Understanding the Key Factors

Ampacity

The subpanel is rated for 200 amps, so the feeder cable needs to be rated for at least that.2.

SER Cable Ampacity

SER cable has its own ampacity table that is lower than other cable types.3.

Termination Temperature Rating

Your breakers and panel lugs are almost certainly rated for 75°C. This is crucial because you need to use the 75°C column in the ampacity tables.4.

Thermal Insulation

The foam insulation significantly affects heat dissipation. This definitely impacts ampacity, and might require derating (reducing) the allowable ampacity.5.

California Amendments

California might have stricter requirements or amendments to the NEC that you need to consider. Check with your local building department.

NEC Sections to Consider



NEC 310.15 (Ampacity Tables)

This section provides the ampacity tables for conductors, including SER cable. Specifically, look at Table 310.16 and 310.17, but also 310.15(B)

NEC 338 (Service-Entrance Cable: Type SE and USE)

This article covers the requirements for SER cable, including where and how it can be used.

NEC 310.15(B) (Adjustment Factors)

This is the critical section for dealing with thermal insulation and potential derating.

NEC 110.14(C)(1) (Termination Temperature)

As mentioned earlier, this covers the temperature rating of terminations.

Determining the Cable Size

Here's the breakdown of the typical procedure, but remember this is for informational purposes, and a licensed electrician should confirm these calculations based on the specifics of your installation:1.

Start with the 200A Requirement

You need a cable that can handle at least 200 amps.2.

Check SER Cable Ampacity

The most common SER cable is copper. Find the 75°C column in Table 310.16. If not installed per 310.15(B)(7) (freely space for 3" or more) it is probably 310.15(B)(1) that is the applicable section.

Without Insulation (Potentially Incorrect)

If we ignore the insulation for now, a 2/0 AWG copper SER cable is typically rated for 175 amps in the 75°C column. A 3/0 is normally 200A.3.

Account for Foam Insulation (CRITICAL STEP)

Because the foam insulation will completely enclose the SER cable, you must consider the impact on heat dissipation.

NEC 310.15(B)

This section outlines how to apply adjustment factors for conductors in thermal insulation. The installation is basically equivalent to being in a conduit in insulation. A prudent approach is to treat it as if is. This will lead to having to use a larger size cable than the 3/0.

Apply Derating

Without knowing the specific type and R-value of the foam insulation, it's impossible to give you an exact derating factor. However, be prepared to reduce the ampacity significantly. Consult with the insulation manufacturer for their recommendations.

Conservative Recommendation (Based on the limited information provided)



4/0 AWG Copper SER Cable

Given the potential for significant derating due to the foam insulation, a 4/0 AWG copper SER cable might be needed. However this size is based on the idea you have to derate the cable in insulation and is a guess.

Ground Wire

You will also need the correct sized ground wire. Refer to NEC 250.122. For a 200 amp circuit you will need a #6 copper wire

Important Considerations & Steps to Take



Local Building Department

Contact your local building department or inspector and ask them specifically about their requirements for SER cable encased in foam insulation. They may have local amendments that you must follow. This is the most important step.



Foam Insulation Manufacturer

Ask the insulation manufacturer for any specific guidelines or recommendations regarding electrical wiring in contact with their product.

Licensed Electrician

Hire a qualified, licensed electrician to perform the installation. They will be able to properly size the conductors and ensure that the installation meets all applicable codes.

Documentation

Keep records of all calculations, insulation manufacturer recommendations, and building department approvals for future reference.

Disclaimer:* I am an AI chatbot and cannot provide professional electrical advice. This information is for educational purposes only and should not be substituted for the advice of a licensed electrician. Incorrectly sized conductors can lead to overheating, fire hazards, and code violations. Always consult with a qualified professional before performing any electrical work. Flag for review