Okay, this is a tricky one, but let's break it down. The fact that it's happening on every circuit points away from individual circuit wiring problems and towards a problem affecting the entire electrical system. Here's a systematic approach to troubleshooting

1. Reiterate the Symptoms:
No Load

~120V at both breaker and receptacle.

Under Load

~120V at the breaker, ~99-109V at the receptacle, significant voltage drop.

Distance Matters

Voltage drop increases with distance from the panel.

Shared Neutrals

Some circuits have shared neutrals (multi-wire branch circuits - MWBC).

SE Cable Resistance

0 ohms from SE conductors at main breaker to neutral/ground bus.

2. Ruling Out the Obvious (Again)



Panel Neutral

You said you checked this, but double, triple check it. A slightly loose connection can have 0 resistance with no current but become a problem under load. Tighten every neutral connection in the panel. Use a torque screwdriver to ensure proper tightness according to the panel manufacturer's specifications. Overtightening is just as bad as undertightening.

Panel Ground

Verify the grounding electrode conductor (GEC) is properly connected to a grounding electrode (ground rod, water pipe, etc.). Check the connections at the grounding electrode.

Neutral-Ground Bond

Verify that the neutral is bonded to ground only at the main panel. If there's a neutral-ground bond elsewhere (subpanel or appliance), it can cause circulating currents and voltage drops. This is critical.

3. Focus on What's Different Under Load

The key here is that the voltage at the breaker stays relatively stable, but drops at the receptacle under load. This strongly suggests a series resistance somewhere in the circuit, meaning resistance in the path that current has to flow through. Under no load, current flow is minimal, so the resistance isn't noticeable. Under load, the current increases, and the voltage drop across that resistance becomes significant.

4. The Most Likely Culprits (Based on Your Symptoms)



Service Entrance Conductors (SE Cables)

This is becoming the primary suspect. You measured 0 ohms without load. However, under load, if the connection at the meter base or the connection at the weatherhead/service drop has even a slight oxidation or corrosion, it will create resistance.

Action

Yes, you need to check the meter base connections. This is dangerous work –

CALL THE POWER COMPANY. They have the equipment and expertise to disconnect and reconnect the service safely. Explain your situation. They've seen this before. Corrosion, loose connections, and failing lugs in the meter base are a very common cause of this type of problem. Before the power company comes, visually inspect the meter base for signs of overheating (discoloration, melting, etc.). Also, check the service drop conductors from the utility pole for any damage or signs of wear. Important

Don't attempt to disconnect the service yourself unless you are a qualified electrician and have proper PPE.

Main Breaker Connection

While less likely than the meter base, a poor connection between the SE conductors and the main breaker lugs is still a possibility.

Action

With the power OFF (after the utility disconnects), carefully inspect the connection between the SE conductors and the main breaker lugs. Look for signs of overheating, corrosion, or loose connections. Torque the lug connections to the manufacturer's specifications.

Shared Neutrals (MWBCs)

While you said non-shared circuits also have the issue, MWBCs can exacerbate voltage drop problems, especially if they are heavily loaded.

Action

Make sure the breakers feeding the MWBC are on opposite phases. This will minimize current on the shared neutral. If they are on the same phase, move one to the opposite phase. Also, be absolutely certain the neutral connection for the MWBC is solid. A loose neutral on an MWBC can cause dangerous voltage imbalances.

5. Testing for Voltage Drop Under Load



Procedure

Use a known load (a space heater, hairdryer, etc.) on a circuit. Measure the voltage at the breaker and at the receptacle simultaneously with the load running. Use two multimeters if possible. This will give you the most accurate picture of the voltage drop.

Interpretation

A significant voltage difference between the breaker and the receptacle confirms there is a voltage drop somewhere along that circuit.

6. Advanced Troubleshooting (If the Above Doesn't Reveal the Problem)



Megger Test

A megohmmeter (Megger) can be used to test the insulation resistance of the conductors. This can reveal insulation breakdown that might be contributing to voltage drop under load. This test must be done with the power off and all loads disconnected.

Load Calculations

Verify that the electrical service (150A) is adequate for the house's load. If the house has added significant loads (e.g., electric car charger, hot tub) without upgrading the service, it could be overloading the system.

Important Safety Considerations



Electricity is Dangerous

Work on electrical systems only if you are qualified and understand the risks. If you are not comfortable working with electricity, hire a qualified electrician.

De-Energize Circuits

Always turn off the breaker before working on any electrical circuit. Verify the circuit is de-energized with a non-contact voltage tester.

Lockout/Tagout

Use proper lockout/tagout procedures to prevent accidental re-energization of the circuit while you are working on it.*

Personal Protective Equipment (PPE)

Wear appropriate PPE, including safety glasses, insulated gloves, and arc-rated clothing when working on electrical systems.

In summary, your primary focus should now be on the service entrance conductors and their connections at the meter base and the main breaker. The power company must be involved in inspecting and repairing the meter base. Double and triple-check the neutral connections throughout the panel, especially those associated with MWBCs.

Good luck! Let me know what you find. Flag for review