2

I have a subpanel that was installed probably back in the 80s. It has a ground wire running to a local ground rod, not back to the main panel. The ground bar in the panel is not bonded to the common (white) bar, it's just connected directly to the box and then the ground bar has a heavy gauge wire running to the grounding rod outside.

Should the common and the ground be bonded? Or is this correct, that the common goes back to the main panel, and the ground goes to the local ground? If they should be bonded, does the bonding screw bond the box to the common?

Thanks

5
  • What wiring method was used for the feeder to this subpanel? Apr 3, 2022 at 21:44
  • Metal conduit with three large wires in it-- but the conduit ends at the structure, it doesn't go all the way to the panel on either end
    – user101289
    Apr 4, 2022 at 20:48
  • Is it continuous through the entire underground length though? That's the hard part to fix Apr 5, 2022 at 2:56
  • I would assume so, but I'm not sure how to verify
    – user101289
    Apr 5, 2022 at 12:24
  • Try using a shopvac to suck on one end and seeing if you can perceive a vacuum at the other? Not sure how well that'll work for a conduit with wires in it though Apr 5, 2022 at 23:11

2 Answers 2

2

In a sub panel the white or neutral is required to be isolated from ground in the 80’s it was ok to be connected to ground and 3 wire was ok, code changed in the 99 version of the NEC. The ground rod was a requirement but back then this was code.

Today a 4 wire connection to the main is required. And last cycle code has changed allowing a single ground to be added for systems that did not have them.

What you have is still code compliant today because that is how it was built.

The best practice is to add a ground wire from the main panel or ground rod for the main (if connected by metal conduit this is the ground) Conduit used for grounding the sub is still code compliant on new jobs today.

5
  • While this is excellent info, it doesn't seem to answer the asked question of "Should the common and the ground be bonded (in the sub panel)?"
    – FreeMan
    Apr 3, 2022 at 16:46
  • @FreeMan Isn't this the answer: "In a sub panel the white or neutral is required to be isolated from ground" ?
    – Armand
    Apr 3, 2022 at 17:40
  • @freeman Actually there are 2 proper answers to this question. The first is it was code compliant in the 80’s , 2nd now 4 wire or 3 and metallic conduit are required with the neutral isolated. But both were addressed “granthfathered” code and current code. Both are completely legitimate.
    – Ed Beal
    Apr 3, 2022 at 19:26
  • @EdBeal I can't add a dedicated ground wire back to the main panel, since it's about 80' through conduit and I don't think I could fish a wire that far. The main panel's ground rod is accessible, but is it legitimate to just bury a ground wire from the subpanel and run it all the way back to the main panel's ground rod? Just making sure I understand what you meant.
    – user101289
    Apr 4, 2022 at 14:50
  • 1
    A vacuum and light weight /test fishing line works well with a vacuum cleaner or an air compressor we call it (insert term) a mouse. A piece of bread wrapper cellophane tied to the end provides the resistance and can be sucked or blown through conduit, then pull in a heavy string or heavy fishing line to pull the copper. Pvc usually is easy with a mouse as the sides are smooth compared to galvanized but if you had galvanized you would not need the wire as mentioned in the answer, if you make your mouse of plastic 3 to 6 strips tied on a loop of fishing line works well. Yes you can bury it also
    – Ed Beal
    Apr 4, 2022 at 15:35
2

It's called "neutral", not "common". It is not like "Vss/common/negative" in vehicles or electronics. Every circuit must have its own neutral that serves only that circuit.

First, take a look at the wire routing. Is it cable, or is it individual wires (or cable) in conduit? Is it metal conduit that is not made to flex? If so, the metal conduit is a valid grounding path. In that case, you already "have" modern "4-wire" wiring, it just takes the form of 3 wires plus the metal pipe.

That said, see what Ed Beal has to say.

If the panel was installed prior to 2008 and lacks separate neutral and ground, then you need to bond neutral and ground in the subpanel.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.