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I own a home with a main panel on the corner of the house and a subpanel in the garage that is sort of separate and sort of attached in that the two structures are connected by an outside wall, like this cartoon I drew up:

 (EGC

Edit: The connecting wall is about 8' tall, built from 2x4 studs on a sill plate bolted to a concrete base that runs between the house and the garage. The flexible metal conduit was guided through holes in the studs. Both sides are covered with vertical tongue and groove siding like other exterior walls on this old house, and the top is capped. From the front door side, it appears like the garage is attached to the house and the two are one structure. From the courtyard side, the garage feels decidedly detached, except for the wall

I am looking to add a new circuit to the subpanel to power a 12-volt transformer for some low voltage lighting. While looking into this, I investigated the subpanel wiring and discovered that the white neutrals and green equipment grounding conductors (EGCs) are visibly bonded together.

At first, this seemed contrary to what I keep reading, i.e. that neutral current carrying conductors and EGCs should ONLY be bonded together in the main service panel and never in a subpanel. Then I found folks explaining that prior to 2008, the NEC allowed/required neutrals and EGCs to be bonded together in a subpanel if the subpanel is powered by a 3-wire feed (white, black, and red, but no EGC wire), AND if there are no metallic paths between the two structures.

Well, from what I can tell, I have a 3-wire feed, but my concern is that I also have a very solid metallic path between the two structures, i.e. the flexible metallic conduit that brings the 3-wire feed to the subpanel.

With the main breaker off at the main panel, I checked electrical continuity between the neutral/ground bus in the subpanel and the exterior of the metal conduit and between a grounded outlet within the house. Both checks showed continuity.

I’m going to have a professional electrician look at it in a couple of weeks, but wanted to understand it more first.

My concern is that anytime we use something in the garage, lights, tablesaw, or whatever, then some of the return current traveling on the neutral wire while out in the in the garage finds a nice, juicy additional path back to the main service panel when it reaches the bonding point in the subpanel, i.e. via the entire grounding system of the house. It’s been like this for many years with no ill effect, but it still seems potentially (pun intended) unsafe.

I don't want to make it compliant with current NEC as that seems like overkill. But, I'm concerned that the subpanel wasn't even compliant with the "old NEC" and, if so, would simply like to get it improved to that limited point.

I am specifically concerned about "objectionable current" being injected onto the EGC conductors between the subpanel and the service entrance panel.

Am I being silly or outright wrong to worry about this? If not, is the fix to isolate the metal conduit from the subpanel, say by replacing the conductive connector that binds it to the subpanel with an insulating connector? Or perhaps the fix is to put the neutrals and EGC wires on separate bars in the subpanel and rely on the FMC as the EGC path back to the service entrance panel?

Thanks for your thoughts and explanations! Just trying to learn…

Update Info: The flexible metal conduit appears to be "half inch" FMC. I measured it's OD min as 0.844" and OD max as 0.926".

The gauge of the 3-wire feed is unclear to me. It consists of seven copper conductors. Including the insulation jacket, it has an OD of about 0.315". Here's a photo in case you can tell:

Photo of Subpanel

More Info

The apparent purpose of the connecting wall is to provide privacy for the courtyard (the house has a bedroom and bathroom that face the courtyard) and, perhaps, to provide a non-buried way to carry power over to the subpanel. There is no cover over the courtyard.

The home was built in 1921, making it a century old this year which is pretty old by California standards. It was extensively remodeled in the 1960s when, I believe, among other things, the garage was created by enclosing a covered carport, the connecting wall was built, and the subpanel was added.

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  • Can you tell us more about the exact nature of the connecting wall? Connecting structures will change whether the building is an outbuilding or not. – Harper - Reinstate Monica Apr 6 at 8:38
  • Sure. I'll try. It's a wall about 8' tall, built from 2x4 studs on a sill plate bolted to a concrete base that runs between the house and the garage. The flex conduit was guided through holes in the studs. Both sides are covered with vertical tongue and groove siding like other exterior walls on this old house, and the top is capped. From the front door side, it appears like the garage is attached to the house and the two are one structure. From the courtyard side, the garage feels decidedly detached, except for the wall – JoeA Apr 6 at 9:01
  • What diameter is the conduit, and what size are the wires inside? – ThreePhaseEel Apr 6 at 11:44
  • Your sub was 100% code compliant prior to the 99 code. Could there be current on the conduit sure. The return path of least resistance is how the power returns to the source. The 3 wire system was code for almost 100 years so why now do you think it is not safe? Code changed to make things further foolproof. if you called me and asked how you are asking here I might say sure I will accept ~3k+ to fix your sub panel if you feel the wall is not sufficient attachment to the house I can add grounding electrodes and that adds to the cost but not much. For a sanity check measure the conduit to Gnd. – Ed Beal Apr 6 at 14:38
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    Joe if you have a concern use your volt meter and measure for voltage on the conduit to ground, if you want to get wild run a conductor outside a window and stick it in the earth/ ground and measure the voltage. This is the largest potential that will be on your conduit that might be able to shock you. To make things worse turn on loads that are all on even or odd breakers this will create the largest amount of neutral return. Is there a common roof over the courtyard? – Ed Beal Apr 6 at 18:55
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You are in pretty good shape here. The metal conduit can serve as the grounding connection to the main panel, but having the neutrals and ground bonded in the sub-panel is a no-no. It's not a hard fix. Just move all the grounds to a bus bar attached to the panel (You may have to buy one and install it), and remove any bonding from the neutral to ground.

I'm not sure if the wall connecting the garage and house would make it be considered "one structure". If it is "one structure" you don't need additional grounding. If not and you really wanted to play it safe, install a couple of ground rods for the garage and connect them to your ground bus in the sub panel.

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  • FMC is currently only good for 20A and 6 feet as an EGC, can't be used to bring up to current NEC. – NoSparksPlease Apr 6 at 14:57
  • I don't want to make it compliant with current NEC as that seems like overkill. But, I'm concerned that the subpanel wasn't even compliant with the "old NEC" and, if so, would simply like to get it improved to that limited point. – JoeA Apr 6 at 17:23
  • Joe what year was the home built ? I have been doing this for a long time and pre 99 I believe it was code legal. – Ed Beal Apr 6 at 19:03
  • @JoeA At the time this answer and my response were given the year of construction was not known or what local exceptions my have been in effect, so I can't answer what was compliant at that time. You may have a legal grandfather install, but any modification to the feeder must meet current code. My comment only addressed that the change GA has recommended would not be code compliant. – NoSparksPlease Apr 6 at 19:19
  • @NoSparksPlease While I agree that it may not be current NEC, probably 90% of the country isn't current NEC. I divide this subject into 3 categories: 1) Not current code legal and not safe, 2) Not current code legal (grandfathered), or not code legal when installed, but safe, and 3) Current code legal and safe. Of course we all here prefer current code legal....but there are times when we have to accept #2 (not current code legal, but safe) and just take a practical approach. – George Anderson Apr 7 at 1:31

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