Wiring a 100A Subpanel

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  #1  
Old 09-17-18, 10:49 AM
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Wiring a 100A Subpanel

I am looking to run a 100 amp subpanel to a new detached garage off of a 200amp main service in my basement. The planned load will be 15A fluorescent lights, 20A receptacles for power tools, 15A overhead door motors, 15A lights in the storage loft, 15A outdoor lights, and 20A GFI outlets. I will have 6 or 7 breakers in total (15/20A). I may potentially add a 30A double pole breaker for a ductless heating/cooling unit in the future. The install will require two ground rods outside the garage. Ground and neutral will not be bonded at the subpanel. The location of my property is in Southeastern CT.

The total wire run will be approximately 125'. The run through my basement will be about 50' and then out the back of the house to the garage will be about 75', including up the outside wall and through LB to panel. I am reading online and getting electrical advice that underground conduit runs must be individual conductors of type THNN/THWN for wet and dry locations. I ideally wanted to run a single cable type without having to splice in a junction box. I planned to use 2-2-2-4 Aluminum SER which has type THNN/THWN inside the Romex. This would allow me to run cable in the basement with conduit or a raceway. The basement is unfinished. I have not seen any good explanation of why this wire would not be correct for this installation. Further complicating this for me is that I had a 25+ year licensed electrician come and state that this is exactly the wire that he would use. Even said that schedule 80 above grade is not needed, inspectors never check that it is 80 vs. 40. He is reputable and has been in business in the area for a long time.

I plan to use 2" schedule 40 PVC below grade and schedule 80 above grade right to the LB. I can understand if PVC sizing is too small and posing a challenge or friction for the wire O.D. on the pull. Heat dissipation could then also be a problem, but if one uses adequate sized conduit I can not understand the reason to have to change wire types and complicate this. Interested in comments, thoughts, or suggestions.
 
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Old 09-17-18, 11:01 AM
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First thing 2-2-2-4 Al is good for up to 90A, not 100A, feeding breaker can be 90A max providing the SER is not buried/run within insulation. SER is only allowed above ground the simple reason being is the bare ground conductor is only protected by the outer jacket. To have one continuous feeder is to use conduit and run individual wire of THHN/THWN or XHHW or RHH/RHW/USE other known as Mobile Home Feeder (MHF). You can use SER inside to a junction box and switch to conduit with individual wires to the garage. You will see cable that is direct bury listed as URD and is only USE-2 rated and cannot be used inside, it must be used and terminated outside. Sch80 is not required above ground unless it is subject to being damaged. 2" PVC conduit is plenty big enough for the wire you are planning to use.
 
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Old 09-17-18, 11:18 AM
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Thanks pattenp. That is the first explanation that I have been given for this which actually explains why. I forgot to mention that I do plan on using an 80 or 90A breaker in the main panel. If I go with USE2/RHH/RHW-2 (or Mobile Home Feeder) what's an appropriate raceway to use indoors in the basement? Would really love to keep this a single/continuous feed, if possible. If I have to splice and go from SER to USE2/RHH/RHW-2 what are the codes that I need to adhere. For example, does the junction box have to be a certain size, within certain feet of the egress of the basement, etc..?
 
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Old 09-17-18, 11:25 AM
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The MHF is required to be in conduit where above ground, that is both inside and outside. If you do a transition from SER to MHF using 2" conduit the junction box needs to be a 12"X12"X6", conduit entry size determine box size. Box just needs to remain accessible. If using conduit for the underground part you can use XHHW. MHF is direct bury rated so you don't really need that if using conduit.

Oh.. to add. If using #2Al XHHW you can get by with 1.25" conduit and use a 8"X8"X4" box.
You can use #1AL XHHW for the full 100A and use 1.5" or 2" conduit. If using a box it will be the 12X12X6. SER will be Al 1-1-1-3.
 

Last edited by pattenp; 09-17-18 at 11:48 AM.
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Old 09-17-18, 12:58 PM
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We only use schedule 80 pipe where there is obvious damage potential, like a driveway or parking spot right up against the wall of the house where the pipe is mounted. Otherwise sch 40 is OK.
 
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Old 09-17-18, 01:48 PM
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If I use 1/5 PVC for this installation would that change my box size?
I would like to have a smooth pull and pvc pricing difference is minimal.

#2Al XHHW and 1.5" conduit
 
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Old 09-17-18, 02:13 PM
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Basically you use a box 6 times the size of the conduit. So 1.5 X 6 = 9" and a 10X10 box is a hard box to find without having to order one.. You can get 6X6, 8x8 and 12X12 at most big box home stores. A 8X8 will work but doesn't meet the letter of the NEC if being inspected.
 
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Old 09-18-18, 06:58 AM
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Thanks pattenp, very helpful. One last question. From what I understand I need two ground rods at the detached garage. What size rods would I need? They appear to come in various lengths from 4 - 8 '. Also, how far apart should the be from one another?

I have not looked at the big box stores yet but wonder the best place to get a ground rod kit with all parts you need, including cable?
 
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Old 09-18-18, 07:17 AM
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Drive two copper or copper clad 8' rods at least 6' apart and use a continuous #6 bare copper wire for connection. The rods and clamps and wire usually can be gotten at the big box stores. Be sure to drive the rods a few inches below the soil surface.
 
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Old 09-18-18, 07:58 AM
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So #6 bare cooper (stranded/solid?) continuous from panel to rod1/ground clamp and then rod2/ground clamp (at least 6' apart).

On my 200A service panel I see bare cooper running into wall but is in PVC out into the ground. I imagine this a standard good practice to follow here as well. If ground ever were energized I would think you would not want any exposure though.
 
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Old 09-18-18, 11:19 AM
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Bare solid copper is what's typically used. PVC conduit may be used for protecting the wire from physical damage.
 
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