Okay, let's break down the wire sizing requirements for your feed-through panel scenario.

Understanding the Situation Existing

320A service split into two 200A panels. One feeds the house, the other feeds the shed (small load).

New

You're essentially moving the entire 200A capacity from the shed panel to a new garage panel. The shed will now be fed from the feed-through panel in the garage.

Key Considerations and Code References (Primarily based on NEC)1. Feeder Capacity and Ampacity

Yes, the feeders from the feed-through panel to the garage must be sized to handle the load. A 200-amp rated panel would typically require 250 kcmil Aluminum conductors for the hots as you mentioned.

NEC 310.16 (Ampacity Tables) and NEC 310.15(B)(7) (for dwelling unit services and feeders under certain conditions) are relevant here. Also, see NEC 215.2(A)(1) for minimum feeder conductor size based on the load.2. Voltage Drop

Crucially, you must calculate voltage drop for the 250-foot run to the garage. A 200A load over 250 feet is likely to result in unacceptable voltage drop if you used the conductors you mentioned. You're correct in recommending upsizing. I would suggest that the only way to ensure the proper voltage drop to the garage is to perform a voltage drop calculation, and that is what I will use for my answer.

NEC 215.2(A)(3) addresses voltage drop on feeders. A common rule of thumb is to keep voltage drop under 3% for feeders and 5% overall (feeder + branch circuits). Many electricians aim for even tighter tolerances, especially with sensitive electronic equipment. Use a voltage drop calculator (online or within electrical design software). You'll need to input: Distance: 250 feet. Load: 200 amps (or better, calculate the actual calculated load per Article 220). Voltage: 240V Conductor Material: Aluminum Conductor Size: Start with 250 kcmil and increase until voltage drop is acceptable. Voltage drop calculations are absolutely essential in this situation.3. Neutral Sizing

This is an important point. You generally do not need to size the neutral conductor the same size as the ungrounded (hot) conductors unless specific conditions exist.

NEC 220.61 addresses sizing the neutral conductor. Key considerations for neutral size



Harmonic Currents

If you expect significant nonlinear loads in the garage (e.g., electronic equipment, LED lighting with poor power factor, VFDs), the neutral may carry significant harmonic currents (especially the 3rd harmonic). In this case, you may need to upsize the neutral to 100% of the ungrounded conductors or even use a separately derived system in cases of severe harmonic distortion.

Calculated Imbalance

Calculate the maximum possible imbalance between the two legs of the 240V circuit. The neutral needs to be able to carry that unbalanced current. For a purely resistive load with balanced current, it may not carry any neutral current.

Electric Vehicle (EV) Charging

If you are planning on installing a level 2 EV charger, you should check with the EV charger specifications to ensure they do not require a full size neutral conductor.

Minimum

The neutral must be large enough to carry the maximum unbalanced load, but at a minimum, it cannot be smaller than the grounding electrode conductor required by

250.66.4. Grounding Electrode Conductor (GEC)

This is the wire that connects the new garage panel to a grounding electrode (ground rod, water pipe, etc.). Size this according to

NEC 250.66. For a 200A service fed with 4/0 Al, it is likely to be a #4 Al.5. Equipment Grounding Conductor (EGC)

This runs with the circuit conductors to provide a low-impedance fault current path back to the source. Size according to

NEC 250.122. For a 200A circuit, and after upsizing the ungrounded conductors, you will need to upsize the EGC as well according to NEC 250.122(B).6. Feed-Through Panel Selection

Make sure the feed-through panel is rated for 200A and has enough physical space and knockouts for all the conductors. Ensure the panel has appropriate terminals for the conductor sizes you'll be using. Consider the number of branch circuit spaces needed in both the garage and the shed.7.

Local Codes and AHJ (Authority Having Jurisdiction)

Always check with your local building department or electrical inspector for any specific requirements in your area. They may have amendments to the NEC or additional rules you need to follow.

In summary



Hot Conductors

Size based on the load and voltage drop calculations. 250 kcmil Al is likely not sufficient for 200A at 250 feet.

Neutral Conductor

Calculate the maximum unbalanced load and consider harmonic currents. You may be able to use a smaller neutral than the hots.

Grounding Electrode Conductor (GEC)

Size according to NEC 250.66 (likely #4 Al).

Equipment Grounding Conductor (EGC)

Size according to NEC 250.122, and adjust if the hots are upsized.*

Voltage Drop Calculation

Absolutely essential for this 250-foot run.

Recommendation


Perform a detailed load calculation for the garage. Don't just assume 200A. Calculate the actual anticipated load based on outlets, lighting, appliances, and any specialized equipment.2. Perform a voltage drop calculation. This will dictate the required conductor size for the hots.3. Evaluate neutral load. Determine if harmonics or significant unbalanced loads are expected.4. Consult with the AHJ. Get their input on the design and any local requirements.By following these steps, you can ensure a safe, code-compliant, and reliable electrical installation for your customer. Flag for review