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How should a sub-panel be grounded if next to the main breaker? Do the ground and neutral need to be isolated (separated) from each other in the sub? Sub-panel will be about 6 - 8 feet away from the main and I will use 8/3 or 6/3 wire to give me additional breaker spots.

1.) Can I use the existing (main) wire to the grounding rod to ground the sub-panel? Note: Keep in mind the ground and neutral wire are tied together in the main breaker box.

2.) Do I need to run a separate grounding rod for the sub-panel? 2a.) What size wire needs to go to the grounding rod from the sub?

Photos Below: Main Breaker Panels (Two 200A Panels)

Main Breaker Panels Open

Main Breaker Panel Left Zoomed

Main Breaker Panel Right Zoomed

Main Breaker Left

Main Breaker Right

Service Feed

Main Breaker Panel Right - 20A Breaker Zoomed

proposed New Sub-Panel Location (To the right of Main Breaker Panel Right

Start of the feed from the main breaker box

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  • When you say "main breaker" is it a box that only has the main breaker in it and nothing else, or is it a full main panel with branch circuit breakers? Is the neutral and ground bonded (connected) at the main breaker?
    – JPhi1618
    Feb 5, 2020 at 21:37
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    Are you entirely sure one is a subpanel of the other? Sometimes houses are built with 400A service using dual 200A main panels. This mainly applies to all electric houses, particularly ones with heat pumps and emergency heat. If you're comfortable with the task, it'd be nice to see the innards of the other panel too. Also as I mention in my answer, close-ups of the breakers and any labeling on the panel interiors would be good too.
    – Harper - Reinstate Monica
    Feb 6, 2020 at 19:10
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    Can you get me close-ups of the branch breakers in the left-hand panel, and of any labeling in the panel cabinet itself, or on the inside of the door please for that matter?
    – ThreePhaseEel
    Feb 7, 2020 at 12:46
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    @Roberts2600, if you can take close up pics of the labels for every kind of breaker in that panel, that would be good. I'm trying to figure out if you have any fire starters in there. See the discussion below NoSparksPlease's answer for details.
    – Nate S.
    Feb 7, 2020 at 18:56
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    If you're able to get a clear pictures of the labels on the back of the panel next to the main breaker, that might also be helpful. I can see half of the labels in the main breaker closeup, but there's enough shadow falling on it that I can't really read much.
    – Nate S.
    Feb 7, 2020 at 19:00

2 Answers 2

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Our first rule of subpanels is Think Big. Really Big.

A 6-space panel might be dirt cheap and cure the itch today, but for a couple of pizzas, you can get a 30-space and cure the itch forever. We really want you to do that. There are plenty of stupid and useless ways to waste money in electrical work, but the one here is going too small and getting in the situatuon again after a few years.

It,s perfectly fine to feed a 200A subpanel from a 60A supply. You do not need a main breaker in this sub.

I want to see 30-40% of the spaces unused even with no use of double-stuff breakers, and between both panels about 48 spaces available for the typical house. That may seem absurd now, but believe me, if you have 48 spaces, it's a game changer. Stuff you've suffered with is now easily fixed.

I recommend using 6 AWG copper cable instead of 8. Thqt will let you run 60A instead of 40A. For such a short run, I would not fool around with aluminum. Wouldn't hurt, the lugs are aluminum, but copper is more flexible for the same ampacity. That may seem undersized for a 30-space sub, but the cable is easy to change when you need to upgrade.

If the existing panel is obsolete (Zinsco, FPE, Pushmatic) then I recommend a full 40-space for the sub, because then you're in a good position to phase out that old panel. That's more than you want to bite off today, but you'll thank me on the day you decide to tackle it.

Neutral and ground are rigidly separated at the sub. Any sub.

So you need separate neutral and ground bars, and you need to pull the neutral-ground bond (green screw or strap). You already have separate neutral and ground in your /3 cable, simply put it to good use. Provided neutral and ground bars should be a criterion in your panel shopping. Also consider "bonus breakers", some panel kits will toss in 3 or as many as 15 breakers.

Grounding rods unneeded

Since you are in the same building, you get to exploit the same, grandfathered ground rods. Your subpanel will get ground through the /3 cable.

Just remember, grandfathering only works if the ground rods are still up to the standard of when they were installed. If they're broken, you definitely want to fix them.

For any of the sizes you are comsidering (up to 60A), #10 is the ground size, but that is already in your /3 cable.

But my /3 cable is obsolete and has no ground

First, if you actually mean /2 and you plan to use the bare wire for neutral-ground, that is a no-go. Cannot do it.

However, if your cable's neutral wire is insulated, then you can do a cheat: run the cable inside metal conduit. The conduit inside diameter must be at least 138% of the widest point on the cable. Since the metal conduit provides a damage shield, there is no need to run up to the ceiling and across; you can simply make a straight beeline across the wall. The metal conduit, properly fastened through fittings, is a valid grounding path. Don't even try to bend conduit that large, just go straight across and stick spacers behind it where you strap it to the wall.

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  • First off thank you for all this information! I do have a few questions from the above if you don't mind. 1.) How would one tell if the panel is obsolete (Zinsco, FPE, Pushmatic)? I don't see a name on this 40-year-old panel. Also, a siemens QT breaker fits in the panel and is one of the breakers I can read. 2.) Being that I have not looked at the grounding rod themselves how would one tell if they are up to standard? (i.e. not broken and/or a #10 grounding rod size) 3.) What is the difference between copper and galvanized grounding rods?
    – Roberts2600
    Feb 6, 2020 at 18:23
  • Also, I added a photo of the 2 main panels I have coming into the house. I think the previous owner or electricians must have done some odd things and that is in part why I would like to move to a clean sub-panel to add some light and plugs. I will see if I can find a new photo and without all the lady bugs later today.
    – Roberts2600
    Feb 6, 2020 at 18:25
  • One last question is there a benefit to go to a larger gauge wire and high amp breaker to feed the sub? Say 2 gauge wire and 75 amp breaker. How much will a 60 breaker run in general?
    – Roberts2600
    Feb 6, 2020 at 18:31
  • @Roberts2600 How do you tell if obsolete -> post close-in high-res photos of the breaker areas of both panels with LOTS of light (so we can read the writing on the breakers, and remove a couple of those stickers so we can get examples) and we'll tell you. That a Siemens QT breaker "seems to fit" is a good sign that your panel is not obsolete; however QT may be the wrong/dangerous breaker for it. On grounding rods, physical inspection is the key. It is part of the grounding electrode system which should be larger than #10. The ground on a 60A subpanel is #10.
    – Harper - Reinstate Monica
    Feb 6, 2020 at 19:11
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    @Roberts2600 The red breakers in particular, you need to peel one of the stickers and let us see the writing on them. I
    – Harper - Reinstate Monica
    Feb 6, 2020 at 19:19
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Edit From your comments and the progression of added pictures it sounds like this is a 400A (Class 320) meter. The last time I did one I had do the ground/neutral bonding in the meter cabinet, run 4 wires from the meter cabinet to each panel, and all the grounding electrodes (rods, metal piping, ufers) had to be run back to the meter cabinet. It looks like your jurisdiction allowed running three wire from the meters, bonding in both, then the wires from your electrodes (ground rods, metal piping) through one panel to the next, bonding to both panels. You should check with the local Authority Having Jurisdiction for their current acceptable methods.

I would also be cautious about adding any loads to the left panel. It doesn't appear to be the typical problematic Zinsco product line, but I can't find any information that indicates anybody makes breakers that are listed for those old panels, including the Siemens breaker that is mounted in the panel. If that panel was in my house I wouldn't even hesitate replacing that panel

A subpanel in the same building just needs to be bonded back to the panel feeding it, typically two hots, a neutral, and a ground path. The neutral needs to be isolated from ground. The ground path could be a metal conduit or a ground conductor. You do not need to make any more connections to grounding electrodes if in the same building. If it is in a detached building then you need another ground rod and to attach to all electrodes.

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    Some jurisdictions have gotten pretty aggressive about "subject to physical damage" for exposed cable. You should seriously consider using some type of raceway and THHN conductors instead of a cable assembly, 1" should be good for (3) #6 and (1) #8 ground. (Some raceway types 3/4" would be large enough).
    – NoSparksPlease
    Feb 6, 2020 at 0:31
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    Also the shelf violates 30" wide and 36" deep clear working space required in NEC 110.26.
    – NoSparksPlease
    Feb 6, 2020 at 20:26
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    All Type BR breakers are cross-listed as Type C as well. So modern high quality replacements are available (and recommended -- the original Type C were Challenger brand, and they have problems).
    – Nate S.
    Feb 7, 2020 at 18:32
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    It's essentially a Challenger panelboard, though it may be branded Sylvania or Bryant, since that design changed hands several times. Challenger breakers were a bad dangerous design, but Bryant fixed them with their compatible Type BR breakers which are still sold new by Eaton, so the panel itself is safe to use, but the breakers require careful inspection. Any that might be a Challenger era design should be replaced with a type BR, likely including the ones that are labeled Zinsco.
    – Nate S.
    Feb 7, 2020 at 18:37
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    @NateS. -- Zinsco and FPE are cases where both the breakers and the busbars were ill-designed
    – ThreePhaseEel
    Feb 11, 2020 at 0:13

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