Wire type for underground service to garage

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  #1  
Old 10-10-05, 07:01 PM
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Wire type for underground service to garage

Hey all,

I'm running a 220V line to my detached garage. I want to use Underground Service Entrance cable to a 100A panel in the garage. I'm not sure of the exact type of wire to run. I went and looked at the local stores and saw 2-2-2-4 SER. Is this adequate? It says service entrance and is rated at 100A..but what I'm not sure of is whether it can be direct burried. Thanks for the help, and any recommendations are always welcomed.

Thanks
 
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  #2  
Old 10-10-05, 07:15 PM
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SER is not rated for underground use, but USE is.

2-2-2-4 aluminum is allowed for 100 amp feeders in many areas. It depends on whether or not the use of Table 310.15(B)(6) is allowed by your inspector for this application. Check with your inspector.
 
  #3  
Old 10-10-05, 07:38 PM
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Thumbs down 310.15(b)(6)

Originally Posted by John Nelson
SER is not rated for underground use, but USE is.

2-2-2-4 aluminum is allowed for 100 amp feeders in many areas. It depends on whether or not the use of Table 310.15(B)(6) is allowed by your inspector for this application. Check with your inspector.
That table is titled Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. It is only applicable to service entry conductors and feeders that "serve as the main power feeder to a dwelling unit." "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)." There is no way that a feeder to a detached garage fits that description. The ampacity for all feeders that do not "serve as the main power feeder to a dwelling unit." should be taken from Table 310.16 Allowable Ampacities of Insulated Conductors. The ampacities given in Table 310.15(B)(6) are dependent on the diversity of the loads in entire dwelling units. Diversity assures that it is unlikely that the entire dwelling load would be applied to the service for a period of time long enough to cause heat degradation of the insulation. When the load is not the entire load of a dwelling unit then that level of diversity of use of the supplied loads will not be likely and overloading of the feeder may result.
 

Last edited by hornetd; 10-10-05 at 07:39 PM. Reason: To eliminate double signature
  #4  
Old 10-10-05, 07:52 PM
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OK, I'm going to try to interpret what you typed. You are saying that in a "diverse" dwelling a lower rated supply can be used because it is unlikely that all of the power would be used long enough to cause heat degradation. HOWEVER, in an application such as a garage, where tools/compressor will constantly be used and drawing current through the lines, a higher rated supply needs to be used to prevent heat degradation. Am I interpreting this correctly?

My next question is.....so what SHOULD I use??

Thanks again - Drew
 
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Old 10-10-05, 09:05 PM
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Yes you have read my posing correctly. The ampacity of the feeder to your garage must be taken from Table 310.16 Allowable Ampacities of Insulated Conductors...

The size of wire you would need to run to the garage should be no smaller than #3 Copper or #1 Aluminum. If the circuit is a long one you may need even larger conductors! Please advise the length of the wire run to the garage in wire feet from panel to panel. This will allow me to check the voltage drop. What form those wires take will depend on the wire that is available in your area. You can often get individual Type UF (underground feeder) conductors in those sizes. They are run in rigid conduit until they are a full two feet underground and then they are laid beside each other in the same trench until they go up through the rigid conduit at the other end of the the trench. You have to keep large stones and rock out of the back fill until the backfill is one foot over the wires. There may well be other types of of direct burial cable available in these sizes such as Type USE (underground Service Entry). You will need four conductors for this circuit. If you cannot find cable that is suitable for direct burial then you can consider running Rigid Nonmetallic Conduit all the way from building to building. You only need to bury that eighteen inches down rather than two feet. Once the PVC conduit is built you use a fish tape to pull the wires through the conduit. You can run type THWN (Thermoplastic, High temperature [75 degrees centigrade], Wet location, Nylon coated) which cost a lot less than type UF cable conductors.

You will need to construct a grounding electrode system at the garage that consist of at least two driven rods, eight foot in length, spaced six feet apart, if another electrode is not already available at the garage. An example of another electrode would be a piece of rebar turned up out of the footer that is tied to at least twenty feet of reinforcing bars in the footer.

If the panel in the garage will have more than six circuit breaker handles in it than it will need a main breaker to serve as a building disconnecting means. If a clip on breaker is used as the main then you will need to install a tie down kit that prevents the main breaker from being removed from the buss bars without the use of tools. The tie down is intended to make it unlikely that the main breaker would be removed from the buss bar while still energized.



If you have more questions please ask.
 
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Old 10-10-05, 09:31 PM
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I sincerely thank you for your information. I seem to get different instructions from different people. I have an acquaintenance that is an electrician. His instructions were to run #6 copper in 1" conduit off of a 60A breaker in my main panel to a 100A main breaker box in the garage. The run is about 100' or so. He said I'll need a 4-wire cable, 2 hots, neutral, and ground.

Is #3 Cu/#1 Alu still what I should use for a 60A run to a 100A panel in the garage?

I'll head to Home Depot/Lowes tomorrow and check out what they have available. If I can find the USE cable, that would likely be more cost effective. I'd like to get a plan together so I can contact an inspector and see if it flies.

Thanks again...
 
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Old 10-10-05, 09:45 PM
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If you only need sixty amperes then number six copper is fine. The sizes I gave were based on the one hundred ampere rating of the panel board you were supplying. You only run what you will actually use rather than the rating of the panel you are running it to.

Price the cost of the one inch PVC conduit and the four THWN #6 conductors. It may be cheaper. Multi conductor Type UF cable can be pretty pricey. You have to protect the UF cable with rigid conduit were it comes up out of the ground. The conduit must extend from two feet down to eight feet up or were the cable enters the building whichever is lower.

If you feel you may want more power later such as for an auxiliary apartment or larger shop then consider using a larger PVC conduit such as two inch that you can pull larger conductors into later without having to dig. If you will need telephone, intercom, cable tv, or alarm circuits to the garage now or later than add a one inch or larger PVC conduit now that is separated from the power circuit by six or more inches. Those wires do not need to be buried as deep but one foot would be the minimum practical burial depth.
 

Last edited by hornetd; 10-10-05 at 09:56 PM. Reason: Add additional information.
  #8  
Old 10-11-05, 05:47 AM
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I just went to the local Home Depot/Lowes and checked out what they have. They do not have USE cable, just 2-2-2-4 SER ALU cable. I'm not sure if this is rated for underground use though. Looks like either 6-3UF or #6 THHN in conduit. The UF cable is pretty expensive and putting the THHN in conduit is cheaper. If I put the THHN in conduit, does the ground have to be #6 THHN also, or can I put in something smaller? If so, what should I use for that?

Thanks - Drew
 
  #9  
Old 10-11-05, 07:29 AM
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Although #2 aluminum is not allowed by the NEC for a 100-amp feeder, a large number of inspectors allow it nevertheless. So it's always a good idea to talk with your inspector.

As has already been stated, SER is not approved for underground installation.

A 60-amp feeder using #6 conductors can use a grounding wire as small as #10.
 
  #10  
Old 10-11-05, 08:37 AM
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Sorry bout that, I had a brain hiccup for a moment there about the SER cable. I think I'll end up going with the #6 THHN(for black/red/white) and #10 THHN(green) in 1" conduit. This is all from a 60A breaker in my main box heading to a 100A main box in the detached garage. I'll also bury the grounding stakes, etc. I need to read up more on connecting the 100A box correctly, but I have a good start with the information in this and other threads.

Hopefully I'll be able to use my compressor soon!!! Thanks for your help and I always welcome advice, so feel free.
 
  #11  
Old 10-11-05, 08:55 AM
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THHN isn't approved for underground use either. However, almost all THHN sold today is dual rated THWN, which is approved for underground use. To be safe, read the fine print and make sure you find the THWN rating.
 
  #12  
Old 10-11-05, 07:56 PM
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Before I go out and buy the wire, I just want to make sure whether I should be running 2-wire plus ground or 3-wire plus ground? I've read that a detached structure should be wired like the main panel, which has 2 hots and ground but no neutral. I was under the impression that I would run 3-wire plus ground. Red and Black hots, green to the ground bus and white to the neutral bus...but now I'm a little confused.

Sorry for dragging this out. I realize that there are different ways to do things, but I just want to do it right and do it once. Thanks again for your help.

- Drew
 
  #13  
Old 10-11-05, 08:08 PM
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It is theoretically possible to run only two current-carrying conductors (if you only want 120-volt circuits, or only 240-volt circuits, but not both), but this would be extremely ill-advised. So let's not discuss this further. We'll just assume that you will be running two hots plus a neutral.

So now the question is not whether or not you need a neutral, but whether or not you need a ground.

Code says that if there are no metallic paths between the buildings (no phone line, no metal pipes, no coax cable, no ducts, no clothesline, no nothing), then you are allowed to skip the grounding wire (officially called an Equipment Grounding Conductor). However, even if technically allowed to omit the EGC, almost everybody would advise you to run one anyway.

So my strong advice would be to run four wires: two hots, one neutral, one grounding wire. This allows both 120-volt and 240-volt circuits, but it is what I would recommend even if you will never want any 240-volt circuits. Splitting the 120-volt circuits between the two hots reduces voltage drop, and provides a lot more power for a little more money.

The other possible options allowed by code, all of which I strongly discourage, are:
  • One hot, one neutral. This would provide only 120-volt circuits and only allowed if there is no metallic path.
  • One hot, one neutral, one ground. This is the same as above with a grounding wire.
  • Two hots. This would provide only 240-volt circuits and only allowed if there is no metallic path.
  • Two hots, one ground. Same as above with a grounding wire.
  • Two hots, one neutral. This would provide both 120-volt and 240-volt circuits, and only allowed if there is no metallic path.
 
  #14  
Old 10-16-05, 03:40 PM
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Hey all,

Just wanted to say thanks for all the help. I was able to get my electrical feed to the detached garage successfully this past week and all checks out ok.
 
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