Big Problem: buried feed to outbuilding sub-panel. Not really buried

by tpcolson   Last Updated June 25, 2017 21:21 PM

So my shop foundation drainage project (3 weekends max) has now turned into a 6 weekend disaster. As far as I can tell, the previous HO, using bits and pieces of PVC he found here and there, pieced them together, ran 3 10 ga and 1 14(?) ga ground through it, punched a hole in the side of the shop, connected the sub panel, and covered the PVC with some driveway gravel. I'm just discovering this as I attempt to solve a water-entering-the-shop-at-grade problem. Obviously I have to fix this. The dilemma I'm faced with now, what size wire to use, do I need a service disconnect, where to bond the neutral in the sub. The only problem I've solved so far, is yes, I know, it needs to be two feet deep. I'll probably go 3...

So the breaker at the main is a 40 amp double pole. There are 5 20 amps and a double 40 in the sub. I'm reasonably confident that each 20 in the shop feeds a pair of double-gangs (makes sense, lots of power tools), one 20 feeds the shop lights, and one....nothing. All the wiring in the shop is EMT, metal gangs, pigtailed with ground wire screwed to the gang, as well as a ground wire to the sub. I DO tend to run a lot of tools at once, chop/table saw with shop vac, compressor, grinder, stereo, chargers, 240 V planer and jointer. I've never noticed any lights dimming or wires getting hot. So clearly I need to get this line buried deeper, and through the side of building using the appropriate hardware. Trenching is not going to be a problem, and I have no problem with using the biggest PVC called for.


In the sub, the neutral is bonded to the bus bar in what I think is a (hole?) that will only take a 10 ga wire. There are 2 neutral bus bars (NOT BONDED TO GROUND!), and the other bus has what appears to be an empty lug which I think is where the neutral should go. Is my assumption correct?

The ground looks like a 12 ga to me, bonded directly to the panel. I'm assuming I need a bigger ground wire, should I bond the same way? Wouldn't I need to tap a bigger screw?

Given the load I described, I'm thinking a 40 amp in the main isn't enough, but I really can't afford the size/type wire needed for a 100 amp. Would a 60 amp double in the main be appropriate here? Let's say with the shop vac, planer, stereo, 2 shop fans, and lights, I'm pulling 40 amps +/- 5......(yes, my neighbors hate me. If they'd take their damned dogs in night, I'd consider turning the stereo off when I plane 1000 ft of wood).

Would 6, or even 8 awg copper for the 2 hots and neutral be appropriate? run is 30 ft. 10 awg for the ground? I'm trying to keep this in the realm of what I can get at BIG BOX STORE. My preference is copper.

Speaking of ground, in the main, ground and neutral are bonded, there's a single ground rod under the meter, another ground rod bonded to the propane pipes 8 ft away. Let's assume they're 8 ft down. the ground from the sub is bonded to the bus bar in the main with all of the other grounds. Reading all the opinions about needing a separate ground rod(s) for the sub is making my head come untwisted. Do need I need two rods at the sub? Do I bond them directly to the panel? While I have the ground open as part of the drainage project, wondering if I just sink two (will require a roto-hammer, soil is not soil it is weathered bed rock), and if it's an issue down the road, I just clamp on the wire. Or, is that just a waste of two copper rods?

All construction was "sometime in the mid 90's".

I've never had an issue with GFCI's in the sub tripping, even while standing in an inch of water in the shop (hence the drainage project), and running the wet/dry vac. I push the button every few months, and they test fine.

By my count, there are 6 breakers in the sub. Does that meet the 6-and-you-don't need-a-service-disconnect rule? Does the double-pole count twice and put me at 7? If not, would the standard disconnect used for an AC unit work here? Where to put it? Under the sub? Flush in the wall? Fused or unfused?

Yes, I know, I should call an electrician. But...this is burying a darned wire. Now if (when) I need Arc Faults installed, or a disconnect installed for my water heater, yes, I will call the nice electrician. Problem is, in my area, we had a little problem that resulted in the instant "removal" of about 2500 homes from the inventory. Rebuilding is in full swing, electricians even from 75 miles away are saying they can come give a quote in 2018 (I do have another project that I'm trying to get done, that does need one: I'm hoping to get a main panel replacement so I can run Arc Faults and BB heating, can't do that with the current panel, it appears to have been wired in the same hokey way as the shop feeder, and I want to keep my HO insurance). So, again, while I got the ground open and the jackhammer rented, would love to get correctly sized wires in to solve my immediate problem, as well as support my planned upgrade which realistically will occur in 18 months. Don't want to have to open my driveway again....

Answers 1

Fat aluminum and fat conduit are your friends here.

We'll start with the conduit -- Schedule 80 PVC won't go anywhere when you put it in the ground, is plenty rugged enough to survive what a homeowner can toss at it when brought aboveground, and is available at the orange borg, in sizes up to 3". Since the electrician won't be there to bail you out with the truck o' pulling tools anytime soon, go with the 3" size even though we're only at about 10% fill here. Bury the conduit to 24" -- that will guarantee your 18" of topcover. As to the terminations -- it will come up the outside of the buildings in question into LBs that then go into the building sides.

Then, we pull out the jumbo aluminum wire. For the price of your proposed 6AWG THHN/THWN-2 copper, you can get 1/0-AWG(!!!) RHH/RHW-2 AA-8000 series aluminum, and since this is a feeder, it's going from aluminum breaker/panel lug to aluminum breaker/panel lug anyhow -- if your insurance complains, tell them to go re-read NEC 310.106(B). That aluminum, by the way, will carry 120(!)A -- 100A is a more practical size, though, so we'll use that instead. With that, you can use an 8AWG bare copper ground wire.

Sorting out your subpanel

You'll need a NL20 lug to land the ground wire on (instead of that kludge-screw) and a BR2100 breaker with a BREQS125 hold-down kit to serve as a submain. The orange borg can order all these in if they aren't already in stock there. You'll also need a torque screwdriver for this.

The hot wires, then, will land on the terminals of the BR2100, which is your service disconnect (you can backfeed any breaker that does not have LINE/LOAD markings on it). The neutral lands on one of the big neutral bar lugs, and the existing kludge-screw is replaced with the NL20, which is where the ground wire is landed. Make sure to torque all connections to spec and use anti-oxidant on all aluminum wire terminations so they don't come loose and become a sparky mess later!

Landing the feeder in the main panel

If you want to bring the new feeder up to its full 100A capacity, you can put another BR2100 breaker and NL20 neutral lug in the main panel in place of the BR240 that is currently protecting your feeder, terminating the ground on the existing bars. Of course, if you have a big lug spare on your main's existing neutral bars, feel free to use it, and you can run the 1/0 at 40A by pigtailing it with Al7Cu or Al9Cu lugs to smaller wire if you wish to postpone the main panel rework until later. Again, remember to torque all terminals to spec and use anti-oxidant with the aluminum wire!

Getting grounded

You'll need a ground electrode at the shop since it's a detached structure. Contact your local electrical inspectors (aka Authority Having Jurisdiction or AHJ for short) and they'll be able to guide you on what to do given your atrocious-sounding soil conditions. (For instance, a concrete-encased or "Ufer" ground would work if you had reinforcements handy to ground to, but not every pour has such.) Simply shoving copper rods 8' into the ground may be a waste of copper, though, in that it may not yield acceptable results in your situation.

No matter what you put in for a ground electrode, it can be connected via more of that 8AWG copper to the shop subpanel provided you protect it from mechanical damage. The neutral and ground being kept separate at the subpanel, though, is absolutely correct despite your seeming bewilderment at this discovery -- neutral is grounded at the service entrance and nowhere else.

June 26, 2017 01:17 AM

Related Questions

Updated June 22, 2017 19:21 PM

Updated June 09, 2015 00:09 AM

Updated July 29, 2018 14:21 PM

Updated July 01, 2016 08:09 AM

Updated September 07, 2018 17:21 PM