Can I correct 2 wire supply by adding a bare copper wire buried to sub-panels?

I have two out-buildings with sub-panels fed by 6/2 wire w/ground.

The main is 100 amp The sub-panels each has a 50 amp breaker in the main panel and uses two hots and the copper attached to the bonded ground bar in the sub-panels.

Both panels have a bonded ground bar and one 6' ground rod with a #6 wire from ground bar to a 6' ground rod. This wire does not go back to the main

Thanks to Stack Exchange I know I need to add another ground rod 6' away at each sub-panel and incorporate. I know I need to add another ground bar to use for the ground rods and to float/disconnect the other ground bar to use for the neutrals.

Question: Is this true? I was told I did not need to have a ground wire running back to the main panel because a lightning strike would use the ground rod at the sub-panel and it might not be good to have the lightning run back to the main panel.

If I do have to have another wire back to the main (I want 240 at the sub-panels) can I just bury a #6? copper wire a few inches in the soil above the buried conduit to use as the neutral or ground and if so which should I use it for.

Thank you

  • 1
    What size conduit are these underground runs in? – ThreePhaseEel Jun 30 '16 at 22:19
  • @ThreePhaseEel that's an important question. Putting the wire in the conduit may make the most sense if it fits. – Harper Jun 30 '16 at 22:32
  • 2
    OP You need the copper run back to the main panel a great deal more than you need the ground rods. In fact without the copper wire, the protection they "offer" is largely an illusion. As things are, if you have a fault hot to ground, it will energize all your grounds at 120V and potentially kill someone. The "earth" cannot carry this current back to the main panel, because earth isn't a very good conductor most of the time. – Harper Jun 30 '16 at 22:34
  • The conduit is small and fairly crowded - I don't think I can add to it. I could put the single copper wire in a conduit and bury that. – go-be Jun 30 '16 at 22:49
  • Is the conduit plastic or metal? – ThreePhaseEel Jun 30 '16 at 23:57

You'll need to replace the cable in the conduit with 3 6AWG THHN/THWN dual rated wires + a 10AWG bare copper ground. Using the tables from chapter 9 and assuming concentric stranded conductors and a Schedule 80 conduit, this yields 32*3 + 5 = 101mm^2 of conduit area used out of a maximum of 105mm^2 for a 3/4" conduit. (Trying to use an insulated wire for the ground will lead to an overstuffed conduit.)

The easiest way to do this is to pre-twist the wires (black/white/red/bare) together and then attach them to one end of the existing (installed-by-a-monkey-who-doesn't-know-jack-about-Code) NM cable, then start pulling the NM cable out from the other end.

Also, as Ed points out, the ground rod's too short -- drive a 10' ground rod properly at each outbuilding and connect it to that outbuilding's subpanel ground bar. While bare wire works for this purpose, it must be 8AWG or 6AWG -- 10AWG is too small for a grounding electrode conductor. HOWEVER, I would use an 8AWG or 6AWG Bare Armored Ground (BAG) cable instead of bare wire to make it crystal clear that this isn't some random wire flapping around in the breeze (it also provides damage protection for the wire, which is especially necessary for 8AWG.)

  • You don't need dual-rated wire; THWN will suffice. Although a lot of wire sold today is dual-rated, because it's easy and so retailers don't have to stock two kinds of wire. Also the hot wires can be the same color (any color but green). – Harper Jul 1 '16 at 8:53
  • @Harper -- I called out dual-rated wire because that's what's made these days. I haven't seen THHN-only or THWN-only on any manufacturer linecards, as manufacturers don't want to make two kinds of wire any more than retailers want to stock two kinds of wire. – ThreePhaseEel Jul 1 '16 at 11:42
  • Yup, exactly. I do see it occasionally in the smaller sizes. – Harper Jul 1 '16 at 11:45

Hooboy. This may be prohibitive to bring to full code, so let me discuss some options to get the most safety increase for the least buck.

Romex NM-B is not legal for direct-burial or for running in conduit. But what really has me spooked is bootlegging a neutral off that ground wire. If anything breaks in that ground wire, it will energize every ground in your electrical system at 120VAC - touching a conduit, the panel, light switch screws, all the things that are supposed to be safe will be dangerous! Far away from the house you could be dead before they find you! Do not leave this situation to fester merely because bringing it all the way to current code would be hard: Don't let "perfect" be the enemy of the "good". Every option here restores the proper function of the safety ground and separates neutral from it.

Ed Beal discusses an issue with the ground rod.

I am willing to assume some defects may be grandfathered or permitted via local exemption.

Option 1: Go 120V-only

If you can bear having only 120V, then remove all markings from the white wire. In the main panel, black goes to a 120V breaker (or one side of a 240V breaker), white to neutral bar, and bare to ground bar (they may be the same in the main panel). In the sub-panel, black wire goes to one phase, white to the neutral bar, bare to the ground bar which are isolated. Every other row of breakers will not work - just move your needed breakers to rows that do work.

Cost: $30ish per building, for a separate neutral bar for the sub-panel

Option 2: Go 240V-only - temporary transformers for 120V

If your primary loads are 240V-only and/or multi-voltage loads (lighting, some gadgets), and your 120V loads can be hooked up temporarily as needed, easy - just reconfigure for 240V-only. Stop using neutral; the bare wire is ground-only.

Mark the white wire with tape on both ends to designate it a "hot". In the main panel, black and remarked-white to a 240V breaker. Bare to the ground bus. In the sub-panel, black and remarked-white to opposite "hots", bare to the ground bar. No neutral bar - no neutrals will exist in this panel! No exceptions! Every white wire in wiring will be marked with tape as it's not a neutral.

To power 240V loads, done. Many machines will run on 120-240V. Lighting particularly - check your fluorescent lights, the newest ballasts are 120-277V automatic switching, if yours isn't, just change ballast, they're $10-20. Good time to upgrade to T8 bulbs while you're at it. Many LED bulbs are 120-277V, I've been replacing 175W barn lights with 15W LED lights that are $10 and multivoltage. Old mercury, sodium and halide lights have transformers, and they are often jumpable for 240V. For plug-connected devices that can accept 240V, change your wired receptacles and plugs to NEMA 6-15 or 6-20.

To temporarily power loads that are 120V, use a common step-up/step-down transformer.

Cost: est. $250 per building for new bulbs, ballasts, outlets, plugs, power supplies and a step-up/down transformer.

Option 3: Go 240V, install a 120V transformer permanently

This is like Option 2, except with a permanent 120V transformer feeding a third panel.

I often find 5KVA transformers around "used" for about $100. These have 240V on the primary, and 20 amps of 120/240V split-phase on the secondary (jumpable for 40 amps of 120V only). Do the sub-panel per option 2, and add a 20A breaker to feed this transformer. The third panel gets its hots and neutral from the secondary of the transformer. It gets its ground from the ground rods feeding the sub-panel. Because the third panel is fed from a transformer, it's actually not a sub-panel at all - it's a main panel, and you do bond neutral to ground here. The transformer makes this safe.

You only have 20A per leg (or 40A at 120V-only) - so keep the loads here to a minimum.

Cost: est. $300 per building for used transformer, enclosure, additional mini-panel, breakers and wiring.

Option 4: Use a transformer for all loads.

Here, we use a larger transformer (12 KVA or larger, single phase) to feed the entire sub-panel. Like above, this makes it a main panel.

In the house's main panel, we mark the white wire with tape, and punch it down into a 240V breaker. Ground goes to the ground bar. At the transformer, we jumper the transformer's primary for 240V and attach the black and marked-white wire. Ground goes to the transformer chassis or enclosure.

We jumper the transformer's secondary for 120/240 split-phase. Its hots and neutral go to the 2 hots and neutral bar. Being fed by a transformer makes it a main panel, so ground and neutral are bonded together and the ground must go to the rods and also to the transformer chassis. The now-main panel is hooked up in the normal way. This gives you best of all worlds at higher cost.

Cost: est. $500 per building for a used transformer and wiring; $1000 for a new one.

Option 5: Pull 4 wires (in existing conduit??)

ThreePhaseEel addresses this well in his answer. You're pushing the limits of what that conduit can handle, out and in, and it's going to be one miserable pull. I prefer to install conduit much larger than the wires need, mainly to make the pull easier (and provide room for expansion). I would bring in an electrician for this - simply because they'll have all the right pulling tools on the truck, and the skill to do the pull without tearing up the wire. The right tools make all the difference in the world.

Keep in mind if there's any damage to this conduit, the pull will fail and you will end up digging up the yard. Water in the conduit is no big deal; that's why you use THWN wire.

And don't presume it's in conduit. If it's direct-burial cable, Code requires the cable exit the ground in conduit. That would explain using barely-big-enough conduit that he couldn't possibly pull 200 feet through. If it's direct-buried, you'll have to re-trench it.

Cost: est. $700-1200 per building: $400 for wire; $300 for tools or $800 to hire an electrician for the pull. If the conduit is not pullable, more.

  • Good way of putting the separately derived service rules by the way. – ThreePhaseEel Jul 1 '16 at 11:48
  • WOW! Amazing! Thank you Harper. Those options totally answer my questions with abundant options I never even imagined. I think I will go with option 1 in one location and option 3 in the 2nd location (I may not do this right away but use option 1 and then option 3 down the road). Thank you very much! – go-be Jul 1 '16 at 22:11
  • One more item: I am turning off the breakers until it is rectified. Thank you for the frank discussion of the risk involved. – go-be Jul 1 '16 at 22:15
  • @go-be Good, that'll give you time to look out for a decent used transformer... the longer you look, the more likely you are to find one at a bargain. Eventually, plan to replace that wire and ground rods, unless you are sure you are grandfathered or locally waived. – Harper Jul 1 '16 at 23:02

I can only say this a 6' ground rod is not legal and has not been in my years. A ground rod is 8-10' and if more than one is needed they must be more than 6' apart. 200' from the main I am sure the buildings are not connected and with a new rod at the sub it would be leagal if the ground and neutral are isolated.

  • I apologize... I looked at some pictures of the project - they were 8' copper clad. Thank you for the details. – go-be Jul 1 '16 at 22:13

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