Requesting Evaluation of Power Plan at Rural Cabin

Question

What I am looking for is guidance and honest evaluation. I'm not afraid of being wrong. I want to save some bucks and learn some things, without blowing myself up. So please tell me if I'm doing something dangerous, and preferably please tell the section of the code I'm violating.

OK, so I have a cabin in the boonies. Right now there is a pole with a transformer on it that is connected to a meter. Below that is an old box that is going to come out. I met with the rep from the POCO, and here is the culmination of what I remember discussing with him, and my own independent research.

Here is my attempt at explaining my cheesy diagram.

A) So the POCO comes in with 14.4 kV. The transformer steps this down to split phase 240V, delivering two 120V hots and a neutral to the meter base. The transformer neutral is bonded to a ground wire running to the base of the pole. Out of that meter base we come into my wiring. I provide a 100A main lug panel.

B) On that main lug panel are two breakers, a 100A and a 20A. The 100A goes to the house. The 20A goes to a rarely used outdoor GFCI receptacle. The main lug panel has the neutral bonded to the POCO's ground wire. This is the ONLY place in MY wiring where the neutral is bonded to ground. After this point the ground never touches the neutral.

C) Out of the main lug panel we go through underground triplex 1/0 cable to my house panel. I've calculated the voltage drop to about 4.91V at full 100A (which I'll never use), and the POCO rep told me this was perfectly acceptable, and that this was the right cable to use.

D) In the house we come into a 100A main breaker panel, with various breakers etc. The neutral is NOT bonded to ground here, and the ground goes to two ground rods 6 ft apart.

E) From here we run 10/3+ground out to the shed which we treat simply as a multiwire branch circuit, since there's no panel out there. Note that one neutral is run, and both breakers are mechanically connected to disengage if either one disengages

NOTES:

• Inside the house I run EMT conduit, and at every opportunity I attach the green wire to the conduit or metal box.
• In all circuits I plan to use AFCI breakers.
• I figure that two 100A breakers in series is fine (there's still no way that I can exceed 100A. Am I wrong?
• I worried that I screwed up at the pole. Do I need to lose that 20A breaker & receptacle? Or use a 70 and a 20?
• Is my grounding correct at the house? Why or why not?

Thank you very kindly for any help and guidance.

EDIT I've added some corrections:

• number 6 bare copper ground wire added to underground run from pole and terminated in ground bar of house and ground bar of pole (which is bonded to neutral at pole)
• changed shed circuit to 10/2 plus ground, and run the neutral through the GFCI

ALSO: I can lose the 20A GFCI at the pole. If I need power up there I can just haul the generator up there.

Answer 1

The bare ground + triplex is a reasonable solution, assuming we're talking type USE triplex here

Underground direct bury multiplex cables, such as the USE triplex your power company is trying to sell you on, have all their conductors (hots + neutral) insulated. This means that you can use it along with the separate ground wire in the same trench, as per NEC 300.5(I):

(I) Conductors of the Same Circuit All conductors of the same circuit, and where used, the grounded conductor and all equipment grounding conductors shall be installed in the same raceway or cable or shall be installed in close proximity in the same trench.

TORQUE ALL LUGS TO SPEC WITH A TORQUE WRENCH

Aluminum wire on aluminum lugs (as in a typical panel) works great -- no problems with dissimilar metals, expansion rates, or any other things that could invoke aluminum ooga-booga. However, the setscrew-type lugs in a panelboard are sensitive to torque -- too loose is no good, obviously, but too tight can lead to problems with smashed conductors and damaged panel lugs as well, and aluminum is not as forgiving of this as copper is.

As a result (and this is a Code requirement as of 2017, see NEC 110.14(D) for details), you will need to use a torque wrench (preferably calibrated in inch-pounds, since a foot-pound unit will likely be sloppy at the relatively low torques called for on panel lugs) to torque the lugs to manufacturer specified torque in order to achieve proper reliability.

This is especially important for you as the power company can legally sell you a type USE only triplex here (instead of tri-rated USE/RHH-2/RHW-2), and that cable can legally, as per 310.106(B), be made from the unforgiving EC-grade (AA-1350) aluminum that did such a miserable job at being building wiring back in the day. (The assumption is that the power company knows what they're doing when terminating cables.)

Expansion is a concern here

My other concern with this setup, as amended to correct the grounding issue between the pole-panel and the house-panel, is the lack of expandability. You'll be severely constricted in the pole-panel by the "rule of six" limiting how many breaker throws you can put in it, and actual 100A panels don't come in upwards of 24 slots or so, which limits the house panel. Furthermore, having to replace both panels in addition to re-trenching wire and such just aggravates any potential upgrades rather needlessly, and 200A breakers take up lots of panel space.

So, going with Siemens hardware here (although equivalent parts are available in Eaton's BR line (OK) or Square-D Homeline (cough) as fallback options if you can't get the Siemens stuff from your friendly local electrical supply house), this gives us an amended plan as to the panels themselves:

• Starting at the pole, we throw out the idea of using a main lug panel here altogether. Instead, we use a convertible, 200A panel, field fitted with a 100A main breaker but factory fitted with a second pair of lugs, called feed-through lugs -- the Siemens part number for this is PW0816L1200TC, and you'll need a MBK100A kit for the field fitted main breaker. These are normally used as service entrances to mobile/trailer homes, but provide a good slot economy in a pole-mount application like yours as well. In the Square-D world, you're talking about a HOM816M150PFTRB fitted with a QOM2100VH main breaker instead of the stock 150A main, as there are no QO mobile home loadcenters, and putting a NQ panelboard on your pole is a mighty expensive proposition.

• In the pole panel, once you have the main breaker in, the service conductors land on it, and the house feeder is taken off the feed-through lugs instead of off a breaker. This leaves plenty of room for your GFCI (even though you won't be able to use a plug-on neutral type here, even if you go Square-D, as PoN is not available on these mobile home type panels) as well as other outdoor circuits (to other outbuildings and so forth) you wish to add later on.

• Going to the house panel, you want lots of slots, and that means using a 200 or 225A panel chassis. In the Siemens lineup, this'd be a P5470B1200CU. If you are sticking to the Square-D catalog, you can go to 60 slots using a QO160M200PC -- Homeline has an equivalent loadcenter, but I wouldn't bother with it for your main interior panel unless you didn't have the choice.

• Now that you have the house panel taken care of, your plan to run 10/2 to the shed is fine -- you might as well use 10/2 UF for the other outdoor receptacle as well, as that lets you get a straight-forward 100' of 10/2 UF instead of 75' of 10/2 and 25' of 12/2. One problem with putting a GFCI breaker in for the shed circuit, though, is that your lights will go out if you trip it on account of a ground fault. You'll also need a disconnecting means at the shed -- this can be a molded-case disconnect switch similar to what's used as a service disconnect for an air conditioner, though, or even a subpanel, which'd let you put a 20A GFCI breaker in for the receptacles and an ordinary 15 or 20A breaker in for the lighting, along with a main breaker to serve as the disconnecting means.

Answer 2

It looks really good. Few things.

• The house is a subpanel because the neutral-ground bond is not there. This is a common setup, though usually not with such distance between the separate main breaker and the "main" subpanel.

• The 100A breaker in the "main" subpanel in the house is meaningless. Its only purpose is to be a shutoff switch. As such, its amp rating doesn't matter as long as it >= 100A. That means, buy the largest panel (number of spaces) you can possibly bear to buy, and if its main breaker is 225A, don't worry about that. Don't count up 19 spaces and decide a 24 space panel will do. Buy big - 42 is NOT excessive. It's only a few dollars now, but it prevents expensive woe later.

• GFCIs need neutral to go through them. If a second or third hot is involved, those hots must also go through the GFCI.

• I'm wigged out by the main panel being a "main lug" panel with no one main breaker. That would rely on the "Rule of Six" and I thought that was banned in main panels. It feels like you're bending a rule, but it's a reasonable bend under the circumstances - but please check with your PoCo and AHJ to make sure they're signed off on it.

• on the "rarely used" GFCI+receptacle, if that's outdoors, they will weather poorly and also require the more expensive outdoor-rated GFCI+receptacle. Consider instead using a GFCI breaker in the tiny mainpanel. This will also protect the wiring to the receptacle.

• I hope the 275' of 1/0 cable is aluminum. I wouldn't want to pay for that much copper. Some people get spooked by the troubles with aluminum branch-circuit wiring in the 70s, but the real problem was the terminations at switches and receptacles etc. which were never properly vetted/tested for use with aluminum. In this case your terminations are made of aluminum so there's no question of metal compatibility. Aluminum is exactly the right stuff for this application.

• I cringe at having to drag a ground wire 275'. I wish there was another way to do that - wait -

YOU'RE NOT BRINGING THE GROUND WIRE OVER???

You can't do that. WTH. Everything else about this was so right, this is like Lindsay Vonn missing a gate and DNFing when she had the gold locked.

No, just no. Everywhere past the neutral-ground bond must have a wired ground. Grounds do two things: sink natural electricity (which is tens of thousands of volts) - the ground rod does that. And return fault current in artificial electricity (dozens of amps at 120V) - only a wired ground can do that.

If you are hellbound to not run a ground (and I certainly would be), then get permission from the PoCo/AHJ to label the 275' run a service lateral which would make it 3-wire, but also make the house a main panel with N-G bond*. I don't know what you'd call the at-pole box with its little 20A tap. That might not be allowed, in which case you'll have to decide whether to go 4-wire 1/0 or double back with 275' of #10. shrug price it both ways?

• other than that... most people here recommend conduit, but I like burying cable in places wire theft is a risk.

Your edits

• You removed the multi-wire branch circuit to the shed. That's a shame, because it was perfect before (except for the nitpick of not drawing the neutral through the GFCI)... the way you had the whole thing behind a 2-pole GFCI was genius. Good way to provision 4800W of service out there with headroom for 7200W (only 75' doesn't warrant a wire size bump). Your plan implied the use of a GFCI+circuit breaker combination device, which not everyone knows is a thing.

• I believe your plan for a 275' bare ground wire would eliminate the need for ground rods. However I'm not sure the ground wire is large enough.

• The big mess with the 275' is because this section is subpanel feeder because you've put it past the main panel. The PoCo is accustomed to provisioning a service lateral which is a completely different thing with different rules. Though the size is adequate in Cu or Al, they got that right. If you would lose the requirement for a receptacle at the PoCo pole, then you would have a chance to get everyone to agree it's a service lateral (or should be treated like one) and boy, does life get simpler.