If I am wiring a 100 amp subpanel in my detached garage with 240 using
2-2-2-4 AL, I am carrying the ground from my house's main panel (60 amp breaker). Does NEC require that I have a grounding rod at my detached garage? If that is the case, should I only be running 2-2-2AL? thanks. Any arguments for using copper? The AL seems like it may be overrated, but cheaper in the end.According to :
It is a trifle confusing. In our codes, not all outbuildings need their own grounding system. But if it does, the feed should only be 3-wire, not four. This is probably something you need to ask your local code authority/inspectors.
#2 is a bit of overkill for 60A. Why aren't you breakering it for 100A at the main?
Yeah, you seem to be a bit large for the AL. My 100A subfeed to the garage (buried) is #3 (I think) Al. In copper, it'd be #4. The Al was about a third of the price of it in copper. With 60A, you could probably go a size smaller.
While copper is vastly perferred for ordinary circuits, for high amperage subpanel feeds Al is perfectly fine (if the proper attention is paid to connectors, anti-oxidant grease etc).
Grounding is done at the electrical service. Any installed panel after the service (meter location) is considered a sub panel. Sub panels must have seperate hots neutral and ground conductors. Using the earth as return path can be dangerous. ( ya I know lots of places do it and are installing it as I type, still wrong).
A supplemental ground rod at the garage is confusing and complicated. I encourage you to go to the library and find the "Soars book on Grounding" Simplified explanations for a complex subject. Lots of good pictures and illustrations.
2-2-2-4 is correct, better check your 60 amp breaker, I will bet you have trouble landing #2.As long as you understand AL and work to its needs there is nothing wrong with the wire. Every problem I have seen in 20 plus years of electrical work with AL can be attributed to the workmanship. Utilities seldom use copper any more. Cost and weight is their issues.
Some jurisdictions require grounding at remote buildings. Best check anonymously before you go to far astray.
Read article 250.32(A). Run the four conductors from your main service. Install a ground rod or two for your detached garage. Install a grounding terminal bar in the garage subpanel to connect the #4 aluminum from your main feed and the grounding electrode conductor (#6 or #4 copper) from your ground rods together. All grounding conductors for your branch circuits should also terminate on this bar. Keep the neutral conductor isolated from the grounding conductors. The #2 aluminum is good for 75 or 90 amps depending on your terminal ratings. Have your work inspected.
It is okay to use aluminum, but I don't recommend its usage underground and especially for direct burial. Moisture and aluminum don't go well together.
John Grabowski
Here on the farm, I ALWAYS put grounding rods at every building. Lightning tends to be worse on the electrical systems in rural areas, so I surely do not want a lightning zap that hits by my shed, come surging back thru the house or barn.
I am not sure if you were to use the 4 conductor cable if you can still connect a ground rod or not (according to the NEC). I am aware of stray voltage and ground loops, but not sure what is better. Personally I just use 3 wire cable and install grounds. The more grounds the better is the way I see it.
A couple years ago, lightning hit my main pole or hit downline and surged thru my transformer. I was noticing flickering lights for awhile and was trying to determine if I had a fault in my wiring before calling the electric company. One night my lights got real dim. I went outside to look at my main breakers (on the pole), when I saw a shower of sparks coming down from the pole next to the transformer. I called the electric co. immediately. They found an insulator was cracked and literally blowing itself apart. They replaced it and the problem was solved. This occurred about 2 weeks after the storm.
PS. Look up "Stray Voltage" on the web, This applies more to livestock, but does explain a lot about grounding and is interesting reading.
Mark
According to John Grabowski :
Are you _sure_ about the four wire with ground interconnected to a new ground rod in the outbuilding?
As I understand it, it's either 3 wire plus new ground electrode, or 4 wire with no new ground electrode. In both cases with the ground/neutral lug _removed_ in the subpanel. Not a mixture of both. Ground loops.
Copper and moisture don't go well together in the presence of electricity _either_. Appropriately rated underground Al cable will be no worse than the equivalent Cu. The difference is in the care needed with terminations, not the cable itself.
If you don't run the ground with the feed conductors, you have to meet certain conditions like nothing conductive and grounded can be installed between the two buildings. At least where direct burial cable is concerned, you get one little nick in it and it disintegrates
According to :
I've seen such.
You should read up the grounding sections in the Canadian Electrical Code about "milking parlors". That's where they get _real_ fussy about grounding. Ie: grounding mesh in concrete.
If you run 3 wires (assuming 2 hot wires and a grounded neutral) you most definitely do leave the bonding lug (screw) in the panel -- the panel is a "service entrance".
If you run a separate ground wire to an outbuilding, the rules get kind of fuzzy. You would remove the bonding lug (screw) from the panel, and it's now a subpanel rather than a service entrance, but I think a supplemental grounding electrode would still be a *good* thing.
Note that if the buildings are interconnected by a common water or gas pipes, telephone lines, CATV, etc, you are *required* to run a separate ground wire.
Bob
According to RBM :
Which is what John was proposing - ground conductor with feed (4 wire) and grounding electrodes at _both_ ends. Ground loop. The electrodes themselves will disintegrate.
According to zxcvbob :
We're talking subpanels (downstreams from main panels) here exclusively, not separate service entrances.
If you leave the bonding lug in the panel, the subpanel's grounding goes through the subpanel feed's neutral. Distinctly bad. Lose that neutral and all hell breaks loose.
Subpanels always have their bonding lugs out. Either they get their grounding from the fourth wire in the feed, or they get their grounding from grounding electrode[s]. In neither case is it connected to the neutral.
From the main panel's perspective, we are talking about a subpanel.
But from the outbuilding's perspective, the box is a service entrance and must be listed as such if it is fed without a separate grounding conductor (which is what I think you were proposing.)
Maybe the rules are different up there in Canada, but I installed a
3-wire 240/120V feeder to an outbuilding 1.5 year ago and had it inspected. The incoming grounded neutral wire is bonded to the panel, and I had to add a grounding electrode.Bob
I did one just two months ago in NY. A detached garage with an existing three wire feed in pvc to the house. I had to run two ground rods and did have to bond the neutral. Like Bob said, I was given specific instructions about not having any other conductive, grounded paths. i.e.. telephone, catv, gas, water
Dan
According to zxcvbob :
Think logically about it for a moment - pull full amps on one side of the subpanel. You'll pull the neutral 3V or so way from ground potential. Yet, you have a grounding electrode at either end of it. Means current flow between grounding electrode - in some cases several amps in normal operation. Corrosion of grounding electrodes.
Worse, sever the subpanel neutral. Now the neutral return from the whole subpanel is trying to go through the grounding electrodes, and depending on electrode-to-dirt resistances, every grounded device connected to the subpanel could present frame voltages relative to ground as much as 120V.
Ouch.
From 1999 NEC:
250-32.Two or more Buildings or Structures Supplied from a Common Service.(a) Grounding Electrode. Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), the grounding electrode(s) required in Part C of this article at each building or structure shall be connected in the manner specified in (b) or (c). Where there are no existing grounding electrodes, the grounding electrode(s) required in Part C of this article shall be installed.
Exception. A grounding electrode at separate buildings or structures shall not be required where only one branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the noncurrent-carrying parts of all equipment.
(b) Grounded Systems. For a grounded system at the separate building or structure, the connection to the grounding electrode and grounding or bonding of equipment, structures, or frames required to be grounded or bonded shall comply with either (1) or (2).
(1) Equipment Grounding Conductor. An equipment grounding conductor as described in Section 250-118 shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).
(2) Grounded conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, and (2) there are no continuous metallic paths bonded to the grounding systems in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded.
Part C, Section 250-50 covers the connection of metal underground pipe, metal frame of the building, concrete encased electrodes, and ground rings to form the grounding electrode system. Interior metal water pipe located more than 5 feet from the point of entrance to the building shall not be used as part of the grounding electrode system. A metal underground pipe shall be supplemented by an additional electrode.
Section 250-52. Made and Other Electrodes. Where none of the electrodes specified in Section 250-50 is available, one or more of the electrodes specified in (b) through (d) shall be used.
(b) other local metal underground systems or structures (c) rod a pipe electrodes (d) plate electrodes
My reading of 2005NEC 250.32 is:
If only a single branch ckt (or multiwire branch ckt) is run to the building, and the circuit includes a ground wire, a ground rod is not required at the building. In all other cases (including the feeder described) ground rod(s) or other grounding electrode is required at the building.
There are then 2 options :
(I have seen electricians argue over this section.)
bud--
snipped-for-privacy@mail.com wrote:
Can't you also just run 240V through two conductors, and drive an isolation transformer with it's own local ground?
According to Chris Lewis :
Then I realized, if you _don't_ bond the grounding electrode to the neutral in the outbuilding with a three wire feed, a hot ground short in the outbuilding has to push the fault current thru the dirt. Which in most cases would NOT trip a breaker.
Which is probably the worse of the above evils.
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