After reading the April issue of Fine Homebuilding I had a some questions I was hoping to get some input on.
1: Is a bare ground in PVC conduit going to be subject to degradation due to the natural condensation that collects in the conduit?
2: When installing a subpanel in an outbuilding approx. 120ft from the main, should I drive a ground rod near the subpanel in case of failure of the ground between the sub and main?
3: What size wire and PVC conduit would be required to supply a 100a sub at 120 ft from the main?
Any help would be very welcomed..thanks!
Replies
Buy a code book. Even the 1&2 family dwelling code will have that information. They will also have all of the details that you will not ask about.
Thought I'd toss a response your way regarding part 2 of your question:(I'm not an electrician but have done some research about subpanels when I built and wired my workshop which is about 150' from the main at the house.)
You will want to talk to an electrician and confer with your local building dept. in any case, but be advised that there is often confusion about the ground to a sub-panel. My bldg. inspector was not sure how to address the issue of the ground, in fact was not aware that you MUST NOT use a separate ground rod at the subpanel EXCEPT as an "equipment ground". Also many 'do-it-yourself' books on wiring don't cover the practice of installing a subpanel well; they mention some principles about 'continuous grounds' and discuss 'grounded v.s. grounding wires or conductors, but stop shy of an actual installation of a subpanel, which is not the same as a service entrance panel.
You should bring in your 2 supply wires, plus a shielded neutral (white) wire of the proper size. Plus a copper ground wire (check local code as to bare v.s. insulated) that is connected to the neutral buss back at the MAIN panel. Looking at your main panel you will note that the neutral buss has bare grounds and white neutrals screwed into it.
I think this is where the errors begin in wiring subpanels; a lot of good work is done by those of us who are very good at "monkey-see, monkey-do", but the problem is in knowing when mere copying what we see is the right thing to do.
At your subpanel, you will NOT have bare grounds joined to the same multi-holed buss bar along with the white neutrals. Instead, you'll need a separate buss bar for the bare grounds which is 'bonded' (making contact via a screw) to the subpanel's metal enclosure. The NEUTRAL (white wire) buss bar, distinct from the other in a subpanel, must NOT be bonded to the enclosure. Usually this means backing-out and removing a bonding screw that is occupying one of the holes, extending through and biting into the metal case. Removing this screw leaves you with a bar that is held between 2 plastic legs, and thus isolated from the enclosure. (and from bare grounds that do touch the enclosure when attached to their separate, 'equipment ground' bar.)
Almost always, you will have to buy and attach an equipment-ground buss-barr yourself; most subpanels with more than a mere disconnect are installed using panel boxes that are originally intended as smaller service entrance panels, and not specifically prepared with that additional bar. In any case you must examine the subpanel you are using and modify it to isolate the neutral wire(s). Buy a longer bar for the equip. gnds. (bare) than you think you'll need, don't get an itty bitty 4 holer, keeping in mind that you will use up 2 holes just to attach it to the enclosure.
As for the ground wire coming to your subpanel, it is connected to the ground rod back at your main via the main's white/bare buss bar which is itself tied to the rod with a ground wire. This makes your entire installation (main plus subpanel) unified with a continuous ground. (get an electrician interested in theory to explain more just for fun.) Then, and this finally homes in on your original question, an additional ground rod can be driven near your subpanel, but ONLY to be attached to the bare-wire equipment ground buss bar. I believe the important thing to remember is NOT to attach this 2nd ground rod in any way with the white neutrals in the subpanel. Because of this, some bldg. dept's aren't in favor of the addt'l rod, 'cause it is at risk of being connected this way in the future by a well-meaning tinkerer who doesn't realize they are messing with a sub-panel. But I believe that it does afford an extra measure of protection for your tools, equipment, life-safety, etc. in the event that the original bare ground loses it's continuity to ground/earth. Your current project is a good time, by the way, to check and tighten all of the screws in your main panel, & check that the original ground is good.
Well, hope this helps, but be prepared for varying opinions on this from the bldg. dept., and others.
> 2: When installing a subpanel in an outbuilding approx. 120ft from the main, should I drive a ground rod near the subpanel in case of failure of the ground between the sub and main?
What code says is that if the outbuilding is more than 5 ft. from the main building, a separate ground rod is required (250-24a). This has nothing to do with "failure" of the ground between the buildings. If you lose that, you won't trip a breaker if a hot contacts a box is the outbuilding. The resistance through the earth from one building to another is way too high for that.
That resistance in the path through the earth is the reason for the outbuilding ground rod. What it actually does is it pulls the true earth potential near the outbuilding to the system ground level so you don't get shocks between metal parts of the electrical system and things like damp concrete floors.
-- J.S.
Also if there are no other metalic paths (water line, phone line) between the buildings you have the option of not running a ground and treating pannel at the outbuilding as a main entrance.
Paul,
Good questions. I wrote the article, and wanted to cover subpanels in detached building, but we didn't have the space. So, I'll respond here:
1. Water in the conduit. Water in an underground conduit won't degrade a bare copper grounding conductor enough to worry about. Copper is very corrosion resistant; unless there's unusually acidic water in the pipe, there won't be anything more than a little surface oxidation on the bare wire. It wouldn't hurt, though, given the length of your pull, to run an insulated equipment grounding conductor (THHW insulation is very slick and easier to pull--but use liberal amounts of yellow 77 anyway).
2. Grounding electrodes for a sub at a detached building. You HAVE to have a grounding electrode system for the subpanel in a detached building. It's the main drain for major foreign energy impulses (lightning or high voltage powerline cross) on the building wiring or feeders.
There are two scenarios for grounding a detached building subpanel:
A. If there is no metallic path (copper or iron water pipe, phone line, CATV coax, etc), other than the subpanel feeder, back to the building where the main panel/service is located, you MAY omit the equipment grounding conductor with the feeder, ground the subpanel to it's own grounding electrode system (a concrete-encased electrode, or underground metal water pipe/well casing, and two driven ground rods), AND bond (connect) the ground and neutral busbars in the subpanel. In this sense, you treat the subpanel sort of like a service entrance panel. The grounds and neutrals go on the same terminal bar or connected bars.
B. If there is ANY metallic path between buildings, you have to run an equipment grounding conductor with the feeder (this makes the feeder four wires--two hots, a neutral, and the equipment ground).
You still have to have a grounding electrode system at the subpanel (connected to the ground bus), AND you have to keep the groudning and the neutral buses separate (i.e., have a floating neutral bus). I always set up detached building subpanels this way.
It's not much more cost or effort, and it means that if in the future a phone line or whatever is run to the outbuilding, there will be no possibility of leakage current or major fault current returning to the main building on the phone (or other) wire. This would happen only if the neutral in a three-wire set up was compromised--but this does happen. Phones work on 30 mA of current, and it doesn't take much fault leakage current to blow the brains of all your phones in the main building! Ditto for TV cable, intercom, security systems...
3. Wire gage and conduit size for 120' run. Well, my article gives wire gage and conduit size for a 100 amp sub (sidebar, pg 101). For the 120' distance, I'd go up a size to reduce voltage drop. So #2 copper THHW for the current-carrying conductors, and #6 copper THHW for the equipment grounding conductor (EGC). On the ground, Code does not require larger than a #8, but I strongly suggest upsizing it as well--you want that EGC to have low impedance so it'll clear a fault, and quickly. Go up two sizes in the conduit--one size for the larger conductors, and one for ease of installation. So 1 1/2" schedule 40 would do it.
Other thoughts:
--bury a 1" or bigger schedule 40 conduit in the trench while you've got it open, for phone wires, etc. The low-voltage conduit ought to be separated from the power conduit by at least a few inches (though this isn't required).
--bury a yellow warning tape "CAUTION BURIED ELECTRICAL LINE" about 6" above the conduit. You might be able to talk a local utility crew out of a short roll of this warning tape. Walking up to the foreman with a couple of 6-packs of something would help...
--use a sub with a main breaker, even if it has 6 or fewer branch circuit breakers. If there's a fault in the outbuilding, it's better to clear it with a main breaker at the sub, than at the main building. Also, being able to cut power at the outbuilding to work on the sub is real convenient.
--read Traister's book, "Residential Electrical Wiring" for more essential information before tackling this job. There are many critical details in electrical wiring, and you need to go to several sources to get them all. And realize that there are some things (particularly techniques) that no one covers in a book.
--if your area has a building dept and requires permits and inspections, please get with the program. The inspector can be your friend; most will be very helpful if you've done your homework and are making an effort to do a good job.
Good luck--
Cliff
Cap, I have another one for you.
When my folks had the garage wired for a single light, it came off a subpanel with some other stuff for the (then) new extention, in the basement about 30' away.
Since then we removed all the sub panels (by a licenced electrician) and changed our old 75amp service with fuses to a 125 with breakers.
I have also, at various times, changed the lighting to 3 - 4' flourescent fixtures, and added 2 - 4 outlet plug, and a douplex appliance for the outside 'fridge & freezer.
Should I run a subpanel in the garage and use some curcut breakers to split the load? Although I use power tools in the garage, with 13 amp draw, they are never used at the same time, and the total draw for the freezer, fridge & lamps are less than 15.Quality repairs for your home.
Aaron the HandymanVancouver, Canada
Hello Aaron,
Well, you could add a sub if you want, but if the existing single branch circuit seems to be handling the load, why not go sailing instead?
It's possible that some day the motors in the fridge, freezer, and your power tool will all start up at precisely the same instant, and trip the breaker...if you want to preclude this possibility, you could add another 20A branch circuit (for the power tools) a lot easier and at less expense than running a feeder cable and subpanel.
On the other hand, installing a subpanel is a nice rainy weekend project...
Cliff
p.s. I sure like Vancouer and surrounding area. I've got some land north of Sequim, Washington--just south and east of Vancouver, across the Straight...Next time I'm up there, maybe we could get together for lunch. And I'll have a look at your folks wiring! CP
My wiring, but lunch is always a pleasure.
I better warn you, though. I'm not a high roller like piffin - he likes Merlot.
I'm a dago red man, myself <G>Quality repairs for your home.
Aaron the HandymanVancouver, Canada
IIRC, code allows one lighting circuit running from the main house to a detached garage, but if you want that second circuit, a sub panel in the garage is required.
-- J.S.
I don't live in IIRC country. Quality repairs for your home.
Aaron the HandymanVancouver, Canada
Aaron,
I'd assumed that the garage was attached. If it's a separate structure, then the Code (NEC and I assume CEC also) requires a subpanel.
Cliff