100 amp subpanel new garage build

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Old 08-01-17, 12:27 AM
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100 amp subpanel new garage build

Building a 750 sq foot detached garage and looking for some guidance and confirmation of research for the subpanel.

Home service is 200amps.
I want the detached garage to be 100 amp subpanel. The run is about 110' in total; 30' under the house in crawl space followed by 75' of 2" conduit buried 2' down. There a few bends approximately 300 degrees.

The crawl space gets very narrow (belly crawling) I would like to use UF aluminium #2-2-4-6. Transition to a junction box using insulated splice ($13) a pop.
Would like to use individual aluminum wire #2-2-4 with #6 stranded copper for the conduit section.

I have unbounded the neutral and ground bars on the Subpanel, which was a main 100 amp 20 space panel. Have a ufer ground or rebar stick from my foundation to attach #6 copper to.

My understanding is to tie 3 knots with the rope with an good amount of electrical tape and lube. I pull while a friend pushes. Is no-Ox on connections.

Questions would be:
does that all sound right?
Does any of it sound crazy?
Is the wire size appropriate?
Anything that I am not thinking of?

Thank you in advance for your input and time.

Scott
 
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Old 08-01-17, 01:30 AM
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Is the subpanel a main breaker panel? It should be.

Why do you need a 100 amp feed. You can feed a 100 amp subpanel with less than 100 amps. 60 amps is usually enough for a garage. That saves you on wire cost.

Have you considered just using aluminum mobile home feeder cable all the way? Save you the cost of splicing.
 
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Old 08-01-17, 07:56 AM
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If you don't want to run conduit under house then use for that portion 2-2-2-4 Al SER cable which doesn't require conduit unless needed for protection. As Ray mentioned, use 2-2-4-6 or 2-2-2-4 Mobile Home Feeder (MHF) for run to garage that's underground. The MHF can go panel to panel but does need to be in conduit where exposed above ground and installed inside the structure. MHF is direct bury wire so a little cheaper option is to use individual wires of the sizes needed in aluminum XHHW-2. If using XHHW-2 for the whole run panel-to-panel conduit is needed. If you get URD for the outside run it has to be terminated outside of the structures, it is not to be installed inside because it does not have fire rated insulation. It's best to just not use URD. Also #2 aluminum is rated 90A max as a branch feeder.
 

Last edited by pattenp; 08-01-17 at 08:11 AM.
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Old 08-01-17, 08:00 AM
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The MHF can go panel to panel but does need to be in conduit the whole way end-to-end.
Pat, it is my understanding it only needs to be in conduit to where it goes under ground and where it comes up from underground.
 
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Old 08-01-17, 08:04 AM
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Pat, it is my understanding it only needs to be in conduit to where it goes under ground and where it comes up from underground.
Oh.. you're right. MHF only needs to be in conduit where exposed above ground and where inside the structure. I had a brain freeze, will correct my post.
 
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Old 08-01-17, 08:08 AM
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No problem, Pat. I have had a lot worse brain freezes.
 
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Old 08-01-17, 05:31 PM
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Thank you for your great input so far. Some clarification

The sub panel is a main panel that has manipulated to have unbonded neutral and ground bar.

I do not need 100 amps but since I am going through the work I figure why not set it up for possible future needs. There is a large 5000 watt 240v heater and placing a small bathroom which will have 1000 watt 240v heater that I will leave at 40 degrees in the winter. Plan on placing a chest freezer and fridge later. Likely totals would only ever reach 50-60 amp while running a saw with current loads. But again why not setup it for possible future uses.

There is already 2" conduit buried in the ground to pull with rope, so I will not be using direct burial wire. I am not familiar with all abbreviations prior I mentioned UF wire under the house is the 2-2-2-4 AL SER cable and is in place with splices ready.

Ray you mentioned that #2 AL is rated for 90 amps but that I can wire a 100 amp panel with #2 AL rated for 90 amps? I would be using a 100 amp main breaker at sub panel and coming off my main panel is the house. Is this correct?
These are individual wire from the supply store.
 
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Old 08-01-17, 06:02 PM
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I can wire a 100 amp panel with #2 AL rated for 90 amps? I would be using a 100 amp main breaker at sub panel and coming off my main panel is the house. Is this correct?
If a 90 amp breaker is not readily available for your main panel national code alows you to round up to the next available size, in this case 100 amps. The main breaker in the sub panel serves only as a disconnect switch so it doesn't matter as long as it is not smaller than the breaker at the house.
 

Last edited by ray2047; 08-02-17 at 07:35 AM.
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Old 08-02-17, 07:07 AM
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If you feel you can get by with 60A then just use a 60A feeding breaker in the main panel. Breakers over 60A can become rather expensive. As Ray said there is no problem with the 100A main breaker in the subpanel when being fed by a smaller breaker back at the main panel.
 
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Old 08-02-17, 06:56 PM
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If a 90 amp breaker is not readily available for your main panel national code alows you to round up to the next available size, in this case 100 amps.
240.6(A) lists standard amp ratings of circuit breakers. The code reads "available" meaning if one is made. Example: #8 copper is rated 55 amps at 90 degrees. There is no 55 amp breaker so you are allowed to go to 60 amps. All manufactures make 90 amp breakers so you are not allowed to round up to 100 amps.
 
 

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