Help with a Sub-Sub panel in a Detached Garage.

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  #1  
Old 08-27-19, 11:50 AM
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Help with a Sub-Sub panel in a Detached Garage.

New member here and I could use some help! I'll try to explain this as well as I can. Family member's home has a detached garage with wiring run via a two wire underground UF Wire hookup. ( UF-BOne black One white Bare One Ground). Wire runs buried about in a 75-100 feet long path to garage without conduit.

From what I remember the garage feed USED to be run as a simple three wire branch circuit from a single 120V 30A single pole breaker in the main panel to the garage. The single pole breaker in the main panel was used as the "garage disconnect". This worked fine for years except sometimes when the air compressor in the garage would turn on or two things at once it would trip the breaker in the main panel. Not too often but when it did happen that meant heading to the house to reset the breaker.

About 20-25 years ago they had central installed in the home. The HVAC Guys installed a 240 V subpanel for the A/C unit. They installed the subpanel directly next to the main panel. A/C subpanel is fed by a 240V 60Amp breaker in the main panel. Goes to a 240V 30 amp breaker for the A/C in the subpanel.

At some point later, another friend/family member decided to change the detached garage feed from the simple branch circuit setup to a subpanel setup, without pulling new feeder wire.

This setup has been in place a good 15-20 years without issue except the last couple years they've noticed people can get shocked by the detached subpanel cover/box. When getting ready to make some changes to existing fluorescent light fixtures in the ceiling for them the issues at hand become readily apparent. I cut power to the breakers in the garage's sub panel. The lights and outlets won't power anything with garage subpanel breakers off. But I'm smart/careful enough to still check for power.

To be sure I always check with a non contact voltage meter. It light up bright red pretty much everywhere. Odd because turning off the breakers should kill everything in the garage. Thinking it was some weird residual voltage issue I tested wires and outlets on the furthest end of the circuit. The voltage tester still indicated voltage. I tested voltage at all outlets with a digital Multimeter and see around 95 volts everywhere, even with the breakers in the garage subpanel off. A three prong outlet tester lights up "hot and neutral reversed" when plugged in and the breakers off.

Long story short I opened the garage sub panel up and discovered it was wired with hots and neutrals reversed. I know I need to correct that but before changing anything I decided to backtrack and figure out what was done further upstream at the main panel etc.

They apparently moved the garage's 120V 30 amp single pole breaker and a room addition's 120V 20A tandem breaker from the main panel to the A/C subpanel. The A/C subpanel's neutral bus bar is wired back to the main panel and apparently bonded with ground there. The UF wire feed going to the garage still eexits at the main panel. The garage breaker (now located in the A/C subpanel) is pigtailed to the black of the UF wire feeding the garage. (My attempt at a red wire nut is in the diagram). The hot load wire comes off the 120V 30A breaker for the garage mounted in the subpanel. It travels to the main panel in the flex conduit connecting the two panels and then is wire nutted/pigtailed to the feeder wire black. The feeder's white neutral and bare ground still go to the main panel's neutral/ground bus bar. Obviously they all exit to the garage.

In the garage's subpanel the feeder wire's hot and neutral are reversed but connected to the hot and neutral bus bars. The bare ground from the feeder is tied to the other grounds for the two circuits in the garage. There is no ground bus bar in the garage panel at this time.

I've attached a crude diagram to try to show how it is wired up currently:




Details on detached garage are below:
  • Detached garage has no plumbing but does have a gas furnace with no separate meter so I'm 99% sure it is tied in to the home's gas lines.
  • No ground rods present at garage.
  • No ground bus bar in the garage subpanel
I should close by saying I'm definitely no expert. I've learned a lot in the last few days but I could definitely use some insight on this from pros or more experienced.

Is the right way to do this running a new feed out to the garage? I think I could move the room addition 110V 20 amp tandem breaker back out of the A/C subpanel and back to the main panel. Then install a 2nd double pole 240V 30amp breaker in the A/C subpanel. I may also have space to add a 240 breaker in the main panel. Then I'd need to run a 4 wire feed out to the garage subpanel?

Home owner is thrilled with the thought of running new wire to garage. Can I save this existing setup? I'm sure I could convert it back to just a simple branch circuit feeding the garage but that's my last resort.

Of course I want this to be code compliant but this a home built in the 70s. I'm most concerned with noone being electrocuted or dying because of this garage subpanel. Once I get the feed and circuits squared away I'll be adding GFCI protection at least at the first outlets in the garage circuits.
 
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  #2  
Old 08-27-19, 12:35 PM
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The way to fix this without replacing the underground feeder:

1) buy an add-on ground bar kit for the garage panel, and removing the bonding screw from the existing bar. The ground bar should bond securely to the metal box, the neutral bar should be offset on plastic stands and not connected to the metal. Relocate the garage ground and neutral wires to the correct new bars.

2) drive a ground rod at the garage, and bond it back to the new ground bar using a brass acorn clamp and #6 bare copper.

3) the hot, ground and neutral on the house-side of the underground feeder should all originate from the same panel. Either move the breaker to the main panel or add extensions to the ground and neutral to the subpanel bars.
 
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Old 08-27-19, 01:26 PM
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Thanks so much Ben! I had a couple of follow up questions. The easiest for me would be to keep the garage breaker in the A/C subpanel.

1. For your suggestion #3 I just remove the feeder neutral and ground from the main panel bars where they are currently bonded and connect them to the subpanel on separate ground and bus bars via wire nutted extensions if needed? (After performing your suggestions in #1 and #2 of course)

2. The A/C Subpanel doesn't have a ground bar and I see no grounds in the panel anywhere. It's connected to the main panel via flexible conduit. I'm assuming the conduit is serving as ground back to main panel? I need to purchase and install a ground bar for the A/C subpanel as well. Once I have the A/C subpanel ground bar installed do I need to/should I also run a ground wire from the A/C subpanel to the main panel ground bar? Or is the conduit ground sufficient?

3. Regarding the grounding rods.. I recall reading where sometimes 2 ground rods are needed at a detached garage. I'm not clear on when/why you need two at an outbuilding. This is a detached garage with an underground feeder wire obviously and only a metal natural gas line running to the garage from the house.

Sorry for the dumb questions but just want to make sure I have it straight before I go buy what I need to square this away!
 
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Old 08-27-19, 01:56 PM
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1. correct, although depending on the age of the subpanel it may not necessarily have separate ground and neutral bars. There could be one shared bar or there could only be a neutral bar if the circuits are grounded through conduit.

2. Flexible metal conduit can be used as a ground -- it's not my favorite, but code allows it.

3. The code says you need one earth grounding method of 25 ohms or less; or a second method. One rod should be sufficient to meet the standard, but the meter to prove it is expensive and testing complicated. A picky inspector will say that since you can't prove 25 ohms or less, a second rod is required. Personally I usually only install one rod at outbuildings and the inspectors don't complain about it, but you can install two if you want to avoid the possibility of a red flag.
 
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Old 08-27-19, 02:52 PM
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I think I'm almost set but just want to verify these final few things.

A. The A/C subpanel appears to have only a neutral bar on an elevated plastic piece with bonding strap and screw installed. I'll need to remove this bonding screw and the tab connecting to the panel. Since there is no ground bar I'll need to purchase an add on grounding bar and install one as per your instructions for the garage subpanel so neutral and ground are only bonded at the main panel. Correct?

B. I'm not too worried about the integrity of the metal conduit but I know it can break or come disconnected. (It is only 12"-18" of flexible conduit going between the main panel into the immediately adjacent a/c subpanel.) Is there anything wrong with running a 2nd redundant ground wire as an actual ground wire between the main panel neutral/ground bus and the subpanel ground bar? Or is it safer to just have one or the other? What size ground wire would you recommend if I do run one?

C. The other breaker that I'm concerned with in the A/C subpanel is a tandem 20 Amp breaker for a room addition. I believe that has the hot load wire coming out of the breaker in the a/c subpanel. The neutral and ground on those circuits are wired to the ground/neutral bus bar in the main panel if I recall correctly. For those circuits I'm assuming I need to move the neutral and ground from the main panel to the subpanel ground and neutral bars so they all originate in the same panel? Like I do for the garage feeder?
 

Last edited by LetMeOverThink; 08-27-19 at 04:48 PM.
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Old 08-28-19, 05:09 AM
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A. correct. Also move any existing ground wires that are currently connected to the shared ground/neutral bar to the new ground bar.

B. I prefer to run a ground wire through flex conduit. #10 copper is the proper size for a 60A panel.

C. That would be best, yes.
 
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Old 08-28-19, 06:51 AM
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Thanks so much. You've been an immense help and I can't thank you enough. Have a great day!
 
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