Sub panel in my detached garage

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  #1  
Old 10-02-18, 07:22 AM
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Sub panel in my detached garage

I'm sure this has been asked before but i've not been able to find the answers to my questions. Anyway...

I want to upgrade the electrical service in my detached garage. Currently it has 2 wires going to it that are tapped off a circuit in the house. The garage has one outlet, several ceiling lights, a garage door opener and switched outside lights.

I want to bury a line from my main panel to a sub panel in the garage. My goals are to "future proof" the garage and have the capacity to install a 240v charging station for an electric car. I also want to power several 120v wood working power tools - table saw, miter saw, air compressor, etc. Lights and door opener as well.

So my questions...

What size and type cable to i need to accomplish this?
What should i look for in a sub panel box? My main is SquareD (200a) so i'd like to stay with that brand.
Is the new panel grounded thru the main or do i need to add a ground rod?

Thanks as always.
 
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Old 10-02-18, 07:48 AM
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You have a couple options -- you could go right to 100A now or you could install a conduit large enough to accommodate 100A and just install smaller wires now. What kind of distance from your main panel to your garage?

A blind swing at it I would do a 90A double pole breaker in the main, and run 2-2-4-6 or 2-2-2-6 aluminum mobile home feeder cable from the main to the garage sub panel. That type of cable needs to be in 1-1/4" (or larger) PVC conduit above ground and inside the buildings, but can be direct buried at 24" deep, or buried in conduit at 18" deep. Set a 100A subpanel in the garage with ground and neutral bars separated. Drive a 5/8" ground rod at the garage bonded back to the garage panel ground bus with solid #6 copper wire. Use noalox paste on the aluminum wire terminations to prevent corrosion.
 
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Old 10-02-18, 07:49 AM
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Start by estimating your planned and future loads - a load calculation. That will help you determine what you need for capacity.

How long is the run? Is the soil sandy or full of rocks? Personally, I prefer running THWN in conduit.

If you already have Square D then it makes perfect sense to stay with them.

You will need a ground rod at the garage. You still need 4 wires to your main panel though.

You'll also have to abandon the current service when you do this. You can't have multiple feeds to an outbuilding.
 
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Old 10-02-18, 08:35 AM
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Got it, good advice.

The run is about 60' and the soil is easy to dig - i don't anticipate any problems digging a 24" deep trench to bury the wire.

I think i'll go with 100a service to the garage.

So is 2246 mobile home cable the same as THWN? Sorry for my ignorance...

The sub panel box... i think i need at least a 6 circuit box... two for the anticipated 240v car charger, leaving 4 for lights and outlets. Is that enough? I don't mind over-designing now to take care of future needs.

I understand i will need to abandon the existing feed, but will keep the switch leg wire to control the exterior switched lights.
 
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Old 10-02-18, 08:58 AM
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THWN is individual wire that always must be installed in conduit. It's a PVC plus nylon insulation on the wire good for use in dry or wet environment, and is usually used on copper wire. It's easy to pull and perfectly good for this type of project as long as you're doing conduit.

Mobile home feeder is an inexpensive prefab cable that has four aluminum wires with XLP/XHHW polyethylene insulation loosely twisted into a cable with no outer jacket. The nice thing about MHF is that it can be direct buried at the bottom of the trench without conduit, but it also carries an indoor fire rating so it can be used inside the buildings in conduit eliminating the need for junction boxes on the exterior of the buildings. Other types of direct burial service cable generally cannot be used indoors due to fire rating so you have to switch cable types on the exterior wall.

One important note about the switch leg is that the circuit needs to be configured so that it's powered from the garage side and only the switch leg runs back to the house. That is so when you kill the feeder breaker to the garage, all power is off in that building.
 
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Old 10-02-18, 11:39 AM
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To be clear, #2 Al has a max rating of 90A when used as a branch feeder. The sizes of MHF in #2 are 2-2-2-4 and 2-2-4-6. My suggestion if this run is in conduit is to use pieces of aluminum XHHW-2 in the same sizes. MHF is a direct bury cable and usually cost a little more per foot than XHHW-2. You will buy XHHW-2 as single conductors and cut to lengths needed. Just for knowledge, MHF is made up of RHH/RHW-2/USE-2 conductors.
 
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Old 10-02-18, 05:43 PM
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Thanks pattenp, i think i got most of that! So what you're saying is that i CAN'T use MHF for a 100a circuit. I can use it with a 90a breaker/circuit...right?

I'm not sure i understand the difference between 2-2-2-4 and 2-2-4-6. Is there a preferred size for my application?

Thanks again guys, big help.
 
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Old 10-02-18, 08:17 PM
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#2 aluminum is limited to 90A, no matter the type. Using cable with a reduced neutral is an issue with heavily unbalanced loads but in your case should not be a problem. The 2-2-2-4 is more the standard to use by having a full sized neutral. The feeding breaker can be up to 90A and the subpanel can have a 100A main breaker because it serves just as a disconnect.
 
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Old 10-03-18, 08:45 AM
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2-2-2-6 or 2-2-4-6 is mostly dependent on what your local supplier carries. Doesn't really matter which one in your case.
 
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