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So, I just found out on my first question that I am gonna need a 4th (ground wire) out to my sub panel. Was told by my buddy I only needed three, but evidently that is incorrect. So here goes.

panel is 100 amps currently have 3 - 4/0 wires running out to it.

1 - Sub panel is 230 ft away. what size ground wire do i need?

2 - So, the neutral and ground wires are hooked up to separate bars in the sub panel (unlike the main house panel), but where do the neutral and ground wires going back to the main panel hook up? if they both hook up to the neutral/ground bars in the main panel then what exactly is the purpose of having two wires going back instead of one?

3 - with the addition of the ground wire going back to the main panel, do I still need to have a ground stake outside the shop at the sub panel.

Thanks in advance. You guys are great!

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  • Yes you still need the rod but the size of the ground wire can be smaller is this the 100a sub I think I remember and 4/0 aluminum? The size depends on the breaker or OCPD size. If this is the case you need to up vote Harper's answer I thought about this but did not have time to look up the size but give @harper an up vote for the answer and you will get the correct size needed.
    – Ed Beal
    Dec 21, 2018 at 1:51
  • @EdBeal and of course Harper have the detailed answers. But just a few notes until one of them gives the rest of the info: 1 - Distance isn't the issue, capacity (e.g., breaker size) determines wire size, including ground, though ground can often be smaller than hot & neutral; 2 - The ground serves (as I understand it) multiple functions, some of which are based on exactly one bonding point - it gets complicated, but it is the rule; 3 - that depends on a number of factors - as I understand it, if this is a separate building then yes, otherwise usually no - but Harper knows all!
    – manassehkatz-Moving 2 Codidact
    Dec 21, 2018 at 1:54
  • The size of the conductor is based on the OCPD. The distance has a factor because of code issues it is a separate building since the 99 code 4 wire has been has been required and the AHJ may have different requirements. If the feed is 200 or a thousand feet away are you saying the same size wire is ok? I hope not.
    – Ed Beal
    Dec 21, 2018 at 2:10
  • Are you OK with digging up the cable and replacing it with something else, or is that not an option for you?
    – ThreePhaseEel
    Dec 21, 2018 at 2:15
  • it is not buried. The only thing stopping me from getting something else is money. lol. I don't have a very big budget and I already spent $500 on this wire. Wish I would have know about this site before starting. ugh.
    – Todd Stricklin
    Dec 21, 2018 at 2:17

2 Answers 2

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Conduit is your friend here

As a general rule, using direct bury cable for an outdoor feeder is penny-wise and pound-foolish due to the cost of digging it up and replacing it when an upgrade is desired (a trencher rental + labor) compared to the cost of installing conduit (Schedule 80 PVC, or sometimes rigid metal conduit even) to begin with. A 2" Schedule 80 PVC conduit between the buildings provides sufficient space at 742mm2 for 3 4/0 Al XHHW-2 conductors (at about 525mm2 total) and a 6AWG bare copper ground (which adds another 13mm2, to make for a total of 538mm2), while if you really want to go for broke, you could use 3" for easy pulling and extra expansion room (not that you'd need more than about 100-125A to an outbuilding, that is).

If you can't afford to do anything but stick with the cable...

If your only option is sticking with the cable you have, though, then what you can do is wrap a ground wire in with the cable as you lay it, as the NEC 300.3(B) requirements allow multiple individual conductors or cables in the same trench to be used as part of the same circuit:

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1)through (B)(4).

In particular, you'll need a 6 AWG bare copper ground wire for this, instead of the 8 AWG normally called for by a 100A circuit, due to the 250.122(B) size increase (which works out to a a size increase of about 1.6x over the base size from the table):

(B) Increased in Size. Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately, according to the circular mil area of the ungrounded conductors.

You will need separate ground and neutral bars in the subpanel, no matter what

As a result of running a separate ground wire back to the main panel, you will need to have separate ground and neutral bars in the subpanel, with all the ground wires landing on the ground bars and all the neutral wires landing on the neutral bars. Note that unless your panel shipped with extra ground bars factory installed, you will need to go to a local electrical supply house and buy a set that is appropriate for the make of your panel, or order the correct manufacturer's part online.

Furthermore, you will need to pull the (green) bonding screw or strap out of the subpanel and toss it, as it will not be needed -- neutral only meets ground at the main panel, as long as a separately derived systems is not sticking its nose into the tent, which only happens if you have a distribution transformer somewhere downstream of the main panel, or with certain generator configurations.

And you will also need that ground stake at the shed, as well

You will need some more of that 6AWG bare copper to connect the ground stakes (two of them, 8' deep and 8' apart) at the shed to the ground bar at the shed panel. This provides a path for stray natural electricity to get back to the earth, while the ground wire between the main panel and the subpanel provides a path for wayward utility electricity to get back to the utility.

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  • Thank you sir. Got a couple of clarifications if you would please. First, I do have a transformer on my pole (I live out in the boonies). How do I make sure I don't have a separately derived system? Second, do both the neutral and ground wires going back to the main panel get lugged to the neutral/ground bar in said panel? Thank you!
    – Todd Stricklin
    Dec 21, 2018 at 13:34
  • @ToddStricklin -- the whole "separately derived system" business only comes into play when the transformer's downstream of your main breaker. As to the main panel? The neutral and ground wires land on the neutral/ground bars there.
    – ThreePhaseEel
    Dec 21, 2018 at 22:56
  • So... maybe a the last couple questions?? lol. The ground wire, 6 awg bare copper wire..... does it need to be solid or wound and lastly, can I bury it with the direct burial wire without a conduit? Thank for your help.
    – Todd Stricklin
    Dec 23, 2018 at 2:58
  • @ToddStricklin -- solid vs. stranded is a "don't care" on the ground wire, and yes, since it is copper (instead of aluminum), it can be direct buried
    – ThreePhaseEel
    Dec 23, 2018 at 4:53
1

In addenda to TPE's explanation here's a small illustration that might help. enter image description here

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