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Old 09-27-2008, 05:12 AM   #1
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100 amp sub panel help


I have just built a attached 24x26 2 car garage and need to put a sub panel in it. The garage also has a loft above which one day I will finish. I was wonder if a 100 am sup panel would be sufficent for both or overkill. Also, it would be fed from a 150 amp service which is in the basement about 55' away.. Is this ok. so far the info I received is that I need to run a #4 4 wire from the main to the sub panel which I beleive is 2 hots 1 neutral and 1 ground all encased together. My main concerns are is the 100 amp off the 150 to much and do I use a 100 amp breaker in the main box to tie in the 100 amp sub panel.
Any help would be greatly appreciated

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Old 09-27-2008, 09:56 AM   #2
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100 amp sub panel help


Hi Chatterbass and welcome to the forum.

The need of having a 100A sub in the garage is dependant on the load that will be placed on it. What are you going to run out there? Any large tools/equipment like compressors, welder, saws, etc.? What's going to be in the loft later? Have you calculated the needed loads and the future loads? (FYI...100A is like running a small house. But again, it all depends on the load that will be placed on it.)

Running 100A sub off a 150A main usually isn't a problem, depending on what all is running at the same time, in both the house and the garage. Again, a total load calculation should be done, this time for the 150A main panel in the house. A 55 foot run to the garage should not pose a problem with voltage drop...you should be ok there.

#4 copper is what you would need for a 100A sub. Code says your Equipment Grounding Conductor (EGC...the ground wire) doesn't need to be larger than a #8 copper. Personally I would use a #6.

Using a 100A breaker in the main to feed a 100A sub is fine. If you used a smaller breaker in the main (for example a 90A) than the sub, the breaker in the sub would be a disconnecting means only, and should never trip...as the breaker in the main would trip before the current ever reached the rating of the sub.

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Old 09-27-2008, 10:50 AM   #3
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100 amp sub panel help


60 amp feeders to attached garages that will have power tools and one man shops is the norm in my area and as SD515 said a load calc should be done. The 'attached' garage sub-panel does not require a main disconnect located in the panel so it can be just a main lug panel if you wish. I prefer some kind of single throw disconnect but its your choice. Small sub-panels like 70 amp or smaller generally are limited as to circuit breakers spaces but will usually accomodate 4 to 8 branch circuits in 2 to 4 spaces at 120 volts using the slim line tandem single pole circuit breakers. Adding double pole 240 volt loads will quickly use up the available spaces. If your just going to have lights and receptacles and cord and plug tools...not welders or several stationary woodworking tools like a 2 or 3 hp table saw and drill presses etc...then these smaller subs should provide the necessary 120 volt capabilty as far as circuit breaker spaces. My personal opinion after considering the load at the garage is to install a 60 amp feeder if the load calc is 20 amps or less than that... as this will give future demand needs if any are ever added... and will accommodate most garages that are also used as one man shops..ie...generally one tool running at one time. You can install only what you need if you like but future needs should be considered.

Some jurisdictions will allow #4 copper for 100 amp feeds to an attached garage from the main service panel...but it is not allowed by national code.

If you can tell us what you feel is the correct amperage for your garage or give us a detailed list of your tools and equipment, lights and receptacle needs and future possibilities as to any load additions we can do this for you.

60 amp feeds to 100 amp panels are pretty common and are usually done when 240 volt and 120 volt equipment will be in use at the garage due to the needed circuit breaker spaces. Or you need several branch circuits due to equipment needs.

As SD515 said... considering the load at the garage is the first step to determining the correct feeder size.

When we get to that point in this thread a diagram will be linked to your thread to use as a guide for your feeder installation.
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Old 09-28-2008, 04:12 AM   #4
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100 amp sub panel help


Good morning. Thank you for the responce. MY plan for the garage are to have a small workshop Consisting of a med size compressor a small welder and various power tools, table saw,chop saw,ect. of course not to be run all at the same time . I also have plans to eventially finish the loft as a game room with a bathroom. And the last thing to be connected to the panel would be the pool equipment. I appreciate all you time and advice in helping me proceed with this project of mine.
One more question I have is it better to have the smaller breaker in the garage so that main trips first or visaversa. Thanks again
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Old 09-28-2008, 11:08 AM   #5
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100 amp sub panel help


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...One more question I have is it better to have the smaller breaker in the garage so that main trips first or visaversa. Thanks again
I would probably run the same size at both ends. You have to run feeder wires from the house to the garage sized for the brkr in the house, so might as well take advantage of all the available current in the garage. Since the garage will be powered entirely off the sub-panel, I would consider adding a few emergency lights to your plans, and have them run on their own circuit. At least one in the loft by the stairs, one at the base of the stairs, and one by the exit from the garage. Maybe a couple more...depends on your layout. Plus I would add a smoke alarm circuit to the plans.
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Old 09-28-2008, 02:50 PM   #6
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100 amp sub panel help


The first thing that comes to mind is if this is a feeder to a panel that will eventually have pool equipment branch circuits... the feeder must be ran in a compliant conduit not a cable like nm-b. Best would be thhn/thwn individual wires in PVC with an insulated ground sized to table 250.122. If the pool is not going to be heated a 60 amp feeder should do you just fine from the description of what you will have for branch circuits to equipment at the garage. The equipment ground for 60 amps would need to be 'minimum' #10 copper. The two hots and neutral need to be #6 awg copper. The neutral by code needs to be white insulation along its entire length, the two hots can be black. If pvc you need 1" to have ease of pulling for a 60 amp feeder. No more than 360 degrees of bends before a pull box or termination point. Stay away from sharp 90 degree fittings use sweeps when at all possible. Your neutral and ground at the sub will be kept serparate ( study the attached diagram) this is accomplished by adding a ground bar kit and not installing the bonding means to the neutral bar of the panel. This is usually a green screw that threads into the panel metal after inserting it through the provided hole in the neutral bar.
If you wish to have a disconnect I would just get a 100 amp main lug sub panel that will accept a back fed double pole breaker of 60 amps. You will need to also buy the hold down kit for the breaker since its is going to be a main. You would connect to the breaker with your feeder hots (backfeeding it) to energize the panel. You would not use the main lugs. Neutral of the feeder to the neutral bar and the equipment ground of the feeder to the ground bar where you will also connect all your branch circuit equipment grounds. Connect all your whites for branch circuits to the neutral bar in the sub panel.

Or you can just buy a 100 amp panel with 100 amp main breaker and feed it 60 amps of capability. It will then just be a disconnect and the 60 amp breaker in the house panel will be the overcurrent protection for the feeder and also used to remove power to the garage for emergency or other reasons.

Or you can just use a main lug panel (no single throw disconnect at the sub) and use the 60 amp breaker in the main house panel for feeder overcurrent and short circuit protection and to remove power from the sub panel in the garage.

You can also run a 100 amp feeder to the garage with a #8 minimum equipment ground and minimum 3# copper thhn/thwn unless they allow #4 copper locally. If pool is heated you probably will need this minimum and the service to your house will need to be capable of handling the load...ie..if 100 amp service to the house you can forget heating the pool....... till you get a bigger service.

As an added note you can if you want protect #6 thhn/thwn in conduit with a 70 amp breaker...60's are common at the big box stores 70's not so common. You are allowed to do this because #6 thhn copper is 65 amps and there is no 65 amp breaker so you can move to the next standard size which is 70 amps.

Please use the below diagram for reference and as a guide...remember your going to be having pool branch circuits eventually from this sub so there is a long list of code requirements when doing that.. so bone up on the things you need to know for your families safety.

I've also attached a diagram of a Homeline 6 space 12 circuit panel that accepts a backfed main to give you a heads up on familiarity with doing that if you so choose.
Attached Thumbnails
100 amp sub panel help-4-wire-feeder-same-building.jpg   100 amp sub panel help-hom612l100scp.jpg  

Last edited by Stubbie; 09-28-2008 at 03:01 PM.
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Old 10-06-2008, 07:40 PM   #7
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100 amp sub panel help


I just want to first thank everyone for all of your help. I found this forum to be very informative and all of the people here very nice.
I have one last question. I have a wire in my home that says (6/3 wg 600v.) can anyone tell me if this is a size 3 wire. It has a black,red,white and ground. Also this wire is copper.
Thanks again for all of the help
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Old 10-06-2008, 07:43 PM   #8
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100 amp sub panel help


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Originally Posted by chatterbass View Post
I just want to first thank everyone for all of your help. I found this forum to be very informative and all of the people here very nice.
I have one last question. I have a wire in my home that says (6/3 wg 600v.) can anyone tell me if this is a size 3 wire. It has a black,red,white and ground. Also this wire is copper.
Thanks again for all of the help
6/3 wg is # 6 AWG times 3 conductors. plus the ground.
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Old 10-14-2008, 11:51 AM   #9
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100 amp sub panel help


TO ALL POST OF THIS THREAD.
Thank you for all of your input. I passed my rough inspection today
you were all very helpfull!!!!
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Old 12-03-2008, 08:14 AM   #10
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100 amp sub panel help


To all subscribed,
Thanks for all of your help. The garage passed my final without a hitch!!!
Everyone here is extremely helpful and I highly recommend this forum for anyone looking for information. Thanks again

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