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I have a 200amp main panel in my home, and my garage has a subpanel fed by a two pole 50amp breaker by approximately 85' of 6/3 plus ground cable. I use my garage as a workshop, greenhouse, and office. I'm planning to install a ventilation system and add more power for my workshop on the lower level. I'm also going to install a ductless minisplit system on both levels of the building; hence, I'd like to update the panel.

When I first opened the existing subpanel, I noticed that the ground and neutral wires were bonded, so I purchased a ground bar and separated everything. All looks good now. There's a #6 ground wire attached to a grounding rod a foot or so away from the building.

My goal has been to replace the 50amp panel with a 100amp panel, running 2-2-2-4cu thhn inside 1.5" pvc conduit, buried 18" deep, from the main panel, 85' to the new 100amp panel in the garage. That said, I'd like to utilize the existing 6/3 cable and grounding rod. My question is, how would I do so appropriately and most gainfully? Does it make sense to have a second panel in the garage despite the redundancy? Honestly, I'd like to be able to utilize close to 90 or so amps in addition to the existing 50 amps in my garage, if possible. Currently, I figure I should play it safe since I spend most of my time out there, and I seem to like to use energy sucking devices for work and play.

Perhaps the old subpanel can be used for the minisplit and 3 fan ventilation system while the new panel can take over the duties of the existing circuits plus new ones I plan on running? I know that the minisplit will need a disconnect, so I'm wondering the best way to do this. Does the new grounding rod need to be 6' away from the existing one? Any tips would be greatly appreciated.

Thanks in advance, Chris

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    You are only allowed one feeder to a building, so your #6 will become useless. NEC 225.30 – NoSparksPlease May 23 '20 at 2:23
  • Have you considered how much power you actually need? Also, what size is that existing grounding electrode wire? – ThreePhaseEel May 23 '20 at 4:19
  • There's a possibility of getting up to 22000 watts. The existing electrode wire is #6cu. – Chris S May 23 '20 at 5:35
  • Are you OK with doing some shopping around locally for parts that you won't ever find at HD/Lowes/...? Also, why do you want to reuse the existing cable? – ThreePhaseEel May 23 '20 at 18:00
  • I am, and I've begun looking. I dont plan on using it. I guess it felt like a waste or buried cable, but I'm planning on doing it right. – Chris S May 25 '20 at 4:27
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First, the 6/3+gnd cable can be pushed to 60A. If it is certain types (NOT NM or UF), it can be pushed to 70A.

You need 2 grounding rods unless your grounding rod passes a particular impedance test which requires some expensive equipment to do. For most people a second grounding rod is cheaper. Those are called ground rods. The wire from the panel to the ground rods is the grounding electrode.

You're not allowed to send 2 sets of the same kind of power to the same outbuilding. Except, your installation was approved and therefore is grandfathered. The approval was not an error because AHJs have the right to waive requirements like this.

And #1/0 Al or #2 Cu wire is good for 125A. (technically 115A but you get to round up to the nearest breaker. You would be better off using Aluminum wire for this type of feeder, especially this much of it at this size. You generally go 2 sizes larger when selecting aluminum instead of copper.

Honestly given that you are grandfathered as-is, I'd be maxing out the existing (free) 6/3 w/g cables before sweating any upgrades. There are 2 legs to this:

  • Upgrade the subpanel(s). Your first priority in panel selection is plenty of spaces. Then make sure both its busing and its main breaker are large enough for your feeder. I recommend main-breaker panels because you must have disconnect switches, and you can't have more than 6 so you can't force people to snap off individual breakers. (or that would limit you to 6 breakers total). The main breaker can be >= the feeder breaker size.
  • Upgrade the feeder breaker to 60A, or if cable rating allows, 70A.

Now you have 120-140A of power in that building, which is what you are after. This will take a little bit of thought to provision power across 2 panels, but the savings are considerable over a trench-and-conduit job.

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  • Thank you for clarifying the terminology. I'll look into getting the second grounding rod; since they only used one, should I be concerned about how long it is? Would the second need to be 6' away from the first, like the rods that ground my meter pedestal? Lastly, and thanks for your time, where does one buy 2/0 wire or cable. My local big box stores don't carry it by the foot. Does everyone use electrical supply stores online? – Chris S May 23 '20 at 5:43
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    @ChrisS If "they" were licensed electricians with the special measuring equipment to confirm it, then 1 may be enough. 6' at least, the farther the better. Nobody buys electrical online, it is too expensive. Newbs buy electrical at the big-box, experienced people buy from local electrical supply houses. – Harper - Reinstate Monica May 23 '20 at 12:55
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    Ground rods need to be not less than 8 foot. You may occasionally see 4' ground rods available, their use is only permitted for unusual telecom type installations. – NoSparksPlease May 23 '20 at 13:50
  • Yeah, they need to be no less than 8' long and 6' apart – ThreePhaseEel May 23 '20 at 17:59
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    There's only one run of 6/3 cable to the garage as it stands, and I need more then 60-70 amps. The secondary run was my idea prior to understanding that you can only have 1 set of the same kind of power run from the main breaker to the same outbuilding. The 115 amps would be good, so just trying to sort out which wire to purchase. – Chris S May 26 '20 at 18:27

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