Requesting advice for feeder to 100A subpanel

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Old 06-21-17, 10:42 AM
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Requesting advice for feeder to 100A subpanel

Hoping to get some advice on best choice of feeder wiring to supply a 100A subpanel in a detached shop. My best description of what I want to do follows...

Existing
225A main disconnect box next to meter box on exterior of house
Main disconnect feeds distribution panel on interior wall of garage, this panel has no main breaker

Proposed
Install 100A breaker in the distribution panel to feed the shop sub panel
Run approximately 40' through open attic space to exterior wall of garage where it will enter 2" PVC conduit
Run inside 2" PVC conduit down exterior wall, underground and up interior wall of shop approx. 40' to subpanel. Interior portion in shop is about 5' after exiting slab.

My choice would be to use copper but am open to aluminum as well. Nearest municipality has adopted 2011 NEC code, but I am not subject to their jurisdiction, only county/state. My research has led me to believe 3-3-3 would be the proper gauge for the mains and neutral (for copper), but unsure of size for ground. Also unsure about the proper way to setup the neutral and ground bars in the subpanel and is another ground rod necessary at the shop? Also I am confused on if there is an appropriate wire/cable to do a single run in open attic and then transition to conduit, or does it all need to be conduit?

Many thanks for any guidance!
 
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Old 06-21-17, 12:19 PM
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Take a look at Ray's diagrams here:
http://www.doityourself.com/forum/el...-diagrams.html

As a detached structure, the neutrals and grounds need to be separated and a single ground rod is required.

It's difficult and not advised to run cable in conduit, and you're not allowed to run individual conductors outside of conduit. For a ~90' run, you could really go either way, UF cable the whole way, or NM cable inside, and transition to conduit & THWN outside. Whatever is easiest.

I'll
 
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Old 06-21-17, 01:01 PM
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Industry standard for this type of run would be to use aluminum cables. Use #1 (or larger) Aluminum SER cable through the attic portion where there is no conduit, put a large junction box at the transition point to conduit, then use #1 (or larger) aluminum MHF (mobile home feeder) cable through the conduit to the garage panel. The feeder must be 4 wires -- you'll probably encounter a cable like #1-1-3-5. Depends on what your local supplier carries.

If you are able to run conduit the entire path, then I might consider some other options like XHHW or THHN conductors, and you could replace the transition junction box with an LB fitting instead. That may or may not be possible depending on the configuration of the run.

In the junction box, you will probably want to use insulated splice block connectors. Polaris and Ilsco are common brands, just make sure it's rated for aluminum of the correct gauge. Always use no-ox compound on the aluminum wires prior to termination.
 
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Old 06-22-17, 09:44 AM
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@Zorfdt
What I am understanding from those diagrams is, if you have a neutral and ground buss bar in the distribution panel, you bond (if that is the right word) them to their counterparts in the subpanel. If no ground buss in the 1st panel, then bond neutral and ground busses to the neutral of the 1st.

In my case I believe I would use the 2nd diagram, but I need to verify that ground for the ground buss comes from the main disconnect out by the meter.

Also, if I were to use UF cable the whole way, would it be permissible to enclose that in conduit just for the 8' or so down the exterior wall of the garage, just for protection? I could direct bury it from there to the shop building, and then about 5' of conduit through the shop slab up to the subpanel? And regarding UF cable, web searches don't seem to turn up UF bigger than 6awg, but guessing a local supply house could get it larger?

@ibpooks
If standard is #1 al, should I be looking at #2 for copper instead of #3? I would prefer to have a direct run between panel with no junctions. Would XHHW or THHN in conduit be the best way to do that?

Thank you both for your answers!
 
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Old 06-22-17, 11:25 AM
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If no ground buss in the 1st panel, then bond neutral and ground busses to the neutral of the 1st.
That is correct. If the disconnect on the pole is the first place you have a breaker then that is the only place that should have neutral and ground connected. All others breaker panels they should be separate with the neutral bar isolated and the ground bar bonded to the metal case.

In older systems that may not be the case. You may have 3-wire subpanels. If coming off an older grandfathered 3-wire subpanel you treat it like you would a first panel and you still run four wires to your new subpanel.
if I were to use UF cable the whole way, would it be permissible to enclose that in conduit just for the 8' or so down the exterior wall of the garage, just for protection?
Yes, that is required and standard procedure.

However copper UF that large is very expensive and at that size most will go to aluminum mobile home cable. 2-2-2-4 or 2-2-4-6 aluminum is common at places like Home Depot and technically requires a 90 amp breaker but if they don't make a 90 amp breaker for your panel code allows you to go to 100. (Assumes each panel has 75 rated lugs which they normally do.)
 

Last edited by ray2047; 06-22-17 at 11:56 AM.
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Old 06-22-17, 03:04 PM
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A direct run between panels with no splices would best be accomplished by installing a continuous PVC conduit and pulling four THHN/THWN-2 or XHHW individual wires through it. The conduit can be run through any space in the building as long as you have no more than 360 degrees of bends between accessible pull boxes or LB fittings. The conduit can be buried at 18" depth.

If you go this route you can use #3 copper for the hots and neutral with a #8 ground; or #1 aluminum hots and neutral with a #6 ground. Minimum wire sizes are different for each wiring method due to different maximum allowed temperatures. Cabled wiring methods generally require larger wires than individual wires in conduit.

If you want an all-cable solution, you can order copper UF-B cable in a #2-2-2-8 configuration which is good for 100A. This can run through the house without conduit similar to any other romex-style cable, can be direct buried at 24" depth with no conduit, but does require protective conduit sleeves for the vertical runs to the bottom of the trench.
 
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Old 06-28-17, 09:21 AM
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Thank you all for your responses. I will price out the options and go with make sense.
 
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