Garage feed

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  #1  
Old 10-11-01, 02:49 PM
Able Sashweight
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Currently have 200 amp service to a 1000 sq foot house. Planning to put a shoplet in the detached garage, 50 feet as the wire bends.There would be a subpanel in the garage, rated for 100 amps, but with the feeder limited upstream to 80 amps. the feeder would be underground.

I understand that the feeder needs to be #4 for 80 amp (or 100 amp).

What kind of cabling do people ordinarily run in this situation? A 3 conductor USE type cable w/ ground and buried in earth? Or 3 separate conductors plus ground in a conduit? If so, does the conduit need to be water tight, and is PVC the material of choice?

How large must the equipment ground wire be?

[I'm not going to ask if I need a separate ground rod - I can't take that much excitement today - I'll read old posts and ask the AHJ!!]
 
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Old 10-11-01, 04:53 PM
Wgoodrich
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Most often a #2 URD al or a #4 USE cu are installed direct buried a minimum of 18" deep. Be sure to protect the wire where passing through an area that contains rocks and such that may damage that wire's insulation.

YOu may run sch 80 PVC above ground where subject to physical damage and sch 40 PVC below ground using #4 THWN wire. Just make sure you have a W in the initials showing it to be approved for a wet location.

The conduits must be sealed yet commonly they will eventually fill with water anyway. The W in the initials of the wires in that conduit make it approved for this condition.

You can run two hots and a neutral and install a ground rod at the detached structure, or you can run two hots a neutral and an equipment grounding conductor along with the feeder and still install your ground rod. Just remember that if you run three wires without an equipment grounding conductor marry the whites and bares or greens in the panel together on the combined neutral and grounding bars. If you run four wire feeder with an equipment grounding conductor ran with the feeder from the main building to that detached structure you must separate the neutral bar from the panel box and the grounding bar same as a sub panel.

Good Luck

Wg
 
  #3  
Old 10-12-01, 09:04 AM
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Grounding your detached garage.

You will have to install two ground rods at the garage that are driven at least six feet apart; unless you can demonstrate to the satisfaction of the Electrical inspector, or other authority Having Jurisdiction (AHJ), that the first rod has an impedance of 25 ohms or less; and that presents an opportunity to improve the grounding of both buildings. What I am about to suggest is not standard practice so it would be good idea to clear it with your AHJ in advance. What you will be installing if you follow this suggestion is a single conductor that exceeds the requirements for both the Equipment Grounding Conductor (EGC) and a Grounding Electrode Conductor. By installing the EGC in this way you will have added an additional grounding electrode to both buildings that; even though it does not meet the code requirements for a ground ring; will be electrically superior in performance to the two ground rods that you install to comply with the code.

In order to install this triple function conductor follow these steps. Excavate the trench to a depth of two and one half feet. Drive a ground rod into the bottom of the trench the rods own length away from each building. Run a continuous length of #2 AWG bare copper between the neutral bar of the service equipment, through an acorn clamp on each rod, and terminate it to the ground bar of the disconnecting means enclosure for the garage. It is important that you reread and understand the post by WGoodrich which outlines the need to keep the EGCs and the Grounded Conductors separate at the second building. This is the second approach he outlined.

This will provide a code compliant grounding electrode system for the garage and also provide the functional equivalent of a ground ring to both structures. You then run three number four UF listed conductors in the trench to supply the second building. You only install conduit from the point were the conductors enter each building into the bottom of the trench. UF conductors are listed for direct burial in the earth. The trench depth is only one foot deeper then the depth required for direct buried conductors and non metallic conduit on residential properties. It is the minimum depth that would be permissible for a ground ring and it will allow the conductor to function as a grounding electrode. The number two bare copper serves as your equipment grounding conductor, a grounding electrode conductor, and the electrical equivalent of a ground ring electrode all at the same time. There are some inspectors who will not be able to accept this approach so check with your inspector before doing it that way.
--
Tom

[Edited by hornetd on 10-13-01 at 07:28]
 
  #4  
Old 10-12-01, 09:28 AM
Wgoodrich
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horntd, would you provide the code reference numbers where it is required to design this grounding ring connecting #2 wire to it from the panel and connecting both structures to is with only three conductors in the sheathed feeder. Also would like to know the article number requiring USE cable to be deeper than other type direct buried cable or PVC.

As far as I understand what you are saying you are creating Code violations instead of meeting Code requirements.

Would like to know the article numbers you are coming from before your suggestions are followed.

Wg
 
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Old 10-12-01, 04:42 PM
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Wgoodrich
I did not say that the arrangement I suggested was required. I said that it met all the requirements for the grounding electrode system for the garage and the equipment grounding conductor in the garage feeder.

I also said that it would provide the equivalent of a ground ring electrode to both buildings. I am aware that since the fifty foot of conductor that would be buried is run between the buildings rather than circling either one it does not meet the code requirements for a ground ring. I did not suggest running the remainder of the feeder as cable but rather as individual USE or UF conductors. The single conductors manufactured for that purpose are usually dual listed as UF & USE but using the USE designation was careless and could cause confusion. It is true that in that application they would be feeder conductors so individual UF conductors would be required.

Conductor Constructions and Applications.
Insulated conductors shall comply with the applicable provisions of one or more of the following: Tables 310-13, 310-61, 310-62, 310-63, and 310-64.
These conductors shall be permitted for use in any of the wiring methods recognized in Chapter 3 and as specified in their respective tables.

Only aluminum conductors are required to be free from direct contact with the earth. Bare copper equipment grounding conductors can be directly buried in non agricultural buildings. That is the reason that the provisions on agricultural buildings separately forbid the use of bare conductors less they be exposed to the corrosive effect of animal waste. The language below is the language describing equipment grounding conductors that are run as a separate conductor rather than as raceway, cable jacket, cable tray and so forth. You will notice that the section specifically states that bare conductors are permitted.

"With Circuit Conductors. By an equipment grounding conductor contained within the same raceway, cable, or cord or otherwise run with the circuit conductors. Bare, covered, or insulated equipment grounding conductors shall be permitted. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes."

You were correct in pointing out that raceway and direct buried conductors have the same minimum depth of bury under the property on which a single family home sits. I had forgotten that. As you know direct buried conductors must be two feet down in most installations including farm and ranch work. As long as the detached garage does not contain a business 18" would be the correct minimum depth of bury for both wiring means.

It was never my intent to mislead or confuse the issues. I have installed the system I suggested at a number of rural properties in different jurisdictions. They were all accepted by the AHJs involved; although sometimes after considerable discussion. My justification for having the second ground rod near the other end of the trench is the FPN on the paralleling efficiency of multiple rods.
--
Tom
 
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