Okay, let's break this down. The key is understanding the definition of a service versus a subpanel. Service Panel (Main Panel)

This is where the power enters the building from the utility. It's the first point of disconnect.

Subpanel (Distribution Panel)

This is fed from the service panel and distributes power to various circuits.

You are correct about these key points



Main Service Panel Bonding

In your 400A main service panel, the neutral and ground are bonded. This is where the connection to the grounding electrode system (ground rods, water pipe, etc.) is made.

Subpanel Isolation

In any subpanel downstream from the main service, the neutral and ground must be kept separate. There should be a dedicated ground bus and a separate neutral bus. The only connection between the two is back at the main service panel where they are bonded.

Now, let's address your specific question about the 100A subpanel on the same mounting platform

The fact that the 100A subpanel is on the same mounting platform as the 400A main service makes no difference. The crucial factor is that the 100A panel is a

subpanel fed from the 400A service.

Therefore



The neutral and ground in the 100A subpanel MUST be kept separate. Do not bond them together in the 100A subpanel.

Why is this important?

Bonding the neutral and ground in a subpanel creates parallel paths for neutral current. This is dangerous because: Increased risk of shock

Current can flow on the ground wires, potentially electrifying metal enclosures and other grounded objects.

Overheating

Ground wires are not designed to carry significant current. This can lead to overheating and fire hazards.

Faulty Equipment Operation

Stray currents can interfere with sensitive electronic equipment.

In Summary

Even though the 100A subpanel is physically close to your 400A main service, it's still a subpanel and must be wired accordingly:1.

Dedicated Ground Bus

Connect all ground wires from the branch circuits to this bus. This bus should be bonded to the subpanel enclosure.2.

Dedicated Neutral Bus

Connect all neutral wires from the branch circuits to this bus.3.

Isolated Neutral and Ground



DO NOT bond the neutral bus and the ground bus together in the subpanel.

Important Considerations



4-Wire Feed

The 100A subpanel must be fed from the 400A service with a 4-wire feeder (hot, hot, neutral, ground). The ground wire provides the safety ground path back to the main service.

Local Codes

Always verify your wiring practices with your local electrical codes and a qualified electrician. Code requirements can vary.*

Grounding Electrode Conductor

Ensure that your 400A main service is properly connected to the grounding electrode system (ground rods, water pipe, etc.).

Disclaimer: I am an AI chatbot and cannot provide electrical advice. This information is for general understanding only. Always consult a qualified and licensed electrician for any electrical work. Improper wiring can be dangerous and potentially life-threatening. Flag for review