You're right to question the 3-wire garage feed. It's almost certainly not up to current code, and potentially unsafe. Here's a breakdown of why and what might have happened

Why the 3-wire garage feed is likely incorrect:
Lack of a Dedicated Ground

Modern electrical codes (NEC in the US) require a separate equipment grounding conductor (EGC) to run with the circuit conductors. This is the bare copper or green wire. It provides a low-impedance path for fault current to flow back to the source, allowing the breaker to trip quickly in the event of a short circuit. A 3-wire setup relies on bonding at the garage panel, which isn't sufficient.

Safety Concerns

Without a dedicated ground, a fault in the garage (e.g., a tool with a short to its metal casing) could energize the metal parts. If someone touched the energized metal and ground at the same time, they'd become part of the circuit, leading to a potentially lethal shock.

Neutral as Ground (Historically Allowed, No Longer)

In older installations, it was sometimes permitted to use the neutral conductor as both the neutral and the grounding conductor for outbuildings. This is called a "multi-wire branch circuit" or "shared neutral" . However, this practice is now heavily restricted (and often prohibited) because if the neutral becomes disconnected, metal parts of the equipment in the garage can become energized. You should never bond the ground and neutral in a subpanel, as this will create a "parallel path" for neutral current and cause safety hazards.

Possible Explanations (But Doesn't Make It Correct)



Grandfathered

If the garage wiring was installed a very long time ago, it might have been code-compliant at the time. However, even if it was, it doesn't excuse the safety risk, and any modifications to the circuit must bring it up to current code.

The Electrician Made a Mistake

Even the best electricians can make mistakes. It's possible the ground wire was overlooked, disconnected, or improperly terminated.

It Was Intended to be Temporary

Maybe the electrician intended to upgrade the garage feed later and never did.

Unpermitted Work

The garage wiring might have been done without a permit or inspection, so it never went through a code review.

Deceptive Appearance

Are you absolutely certain there aren't four wires? It's possible the ground wire is very small, bundled tightly, or hidden behind other wires. Look very closely.

Misunderstanding of "Lugs"

You say the garage is fed from lugs on the bottom of the bus bar. This is unusual for a subpanel. Typically, a subpanel is fed from a breaker. Lugs are more often used for things like generators or inverters. Are you sure this is actually feeding a subpanel?

What to Do


Verify the Wiring

Double-check everything. Trace the wires from the main panel all the way to the garage panel. Make sure you understand exactly how it's wired.2.

Don't Proceed with Your RV Panel Installation Until the Garage Issue is Resolved. Adding more electrical load to an already potentially unsafe system could make things worse.3. Consult with a Licensed Electrician

This is the most important step. A qualified electrician can: Properly assess the existing wiring to the garage. Determine if it's truly a 3-wire setup and if it's safe. Bring the garage wiring up to code by installing a proper 4-wire feed with a dedicated ground. Advise you on the best way to install your RV panel, ensuring it's done safely and in compliance with code.4.

Permit and Inspection

Make sure any electrical work, including the garage wiring upgrade and the RV panel installation, is done under a permit and is inspected by your local building department.

Regarding Your RV Panel Installation



4-Wire Feed is Correct

You're right that you need a 4-wire feed to your RV panel (two hots, a neutral, and a ground).

Size the Wiring Appropriately

The wire size will depend on the breaker size you plan to use for the RV panel (e.g., #6 AWG copper for a 50-amp breaker).

Proper Grounding and Bonding: Ensure the RV panel is properly grounded and bonded according to code.In summary, the 3-wire garage feed is a red flag. Don't ignore it. Get it checked out by a licensed electrician before proceeding with any other electrical work. Safety first! Flag for review