adding a sub panel to a detached gararage

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Old 07-26-15, 03:20 PM
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adding a sub panel to a detached gararage

Goal is to add some minimal electrical to a detached metal rv/storage garage which is 72 feet from corner of attached garage. There is an existing 50 amp rv plug mounted on the corner of the attached garage which is a dedicated line from the recently redone home main panel with a 50 amp breaker.

Plan is to change the current rv plug to a junction box and run wire underground thru metal conduit (using metal due to shorter depth than pvc as going over a septic drain field) to side of metal garage. I have a straight shot about 72 feet and will use L pieces of conduit at both ends. Goal is to have a 30 amp rv plug on outside of barn and inside to have one security light on outside of building and one light over subpanel inside of building (required by code) and also one plug near panel and two plugs inside building on back wall which is about 38 to 50 feet away from subpanel. Not concerned about additional electrical needs.

Questions:
Told good idea even though panel indoors to use an outdoor rated panel like a square D QO 50amp panel with 10-12 spaces. Not sure that exists...at home depot they had a 100 amp subpanel with 10-12 spaces. If I were to use the 100amp not sure if it comes with a 100amp main breaker for the subpanel or if i can put in a 50amp main breaker for panel or if it matters.

Told i could then run a 2 pole 30 amp breaker with #10 wire to a 30 amp rv plug mounted outside of building. Also a 2 pole 20 amp breaker with #12 wire to the 3 plugs. And last another 20 amp breaker or even a 15 amp breaker for the two lights. Maybe gfi the plugs?

The line in the 72 feet of conduit would be #6/3 with ground.

Also told to run grounding wire from panel to a grounding rod.

With 3 lines totaling 70 amps, 30 + 20 +20 do i need the 100amp subpanel? Not that expensive so thats fine but just want to be sure about the required breaker on the subpanel as i would be pulling from a 50 amp breaker in main house panel.

Any comments, suggestions, ideas would be appreciated. Thanks much.
 
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Old 07-26-15, 04:10 PM
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run wire underground thru metal conduit
EMT can't be used but IMC, RMC, and GRC can be used. How deep can you go?
a square D QO 50amp panel with 10-12 spaces. Not sure that exists.
60 amp is as small as they go. In this situation a good choice would be a 100 amp main breaker panel kit. They come wiithh the main breaker installed and an assortment of branch circuit breakers. The 100 amp main breaker serves as you disconnect only so the 100 amp rating is fine. You will have to buy and ass a ground bar to the panel.
Told i could then run a 2 pole 30 amp breaker with #10 wire to a 30 amp rv plug mounted outside of building.
30a RV receptacles are usually single pole (120 volts).
Also a 2 pole 20 amp breaker with #12 wire to the 3 plugs.
No a single pole breaker for 120 volts would be good. (A multiwire circuit shouldn't be needed for three receptacles.)
The line in the 72 feet of conduit would be #6/3 with ground.
Best to use individual conductors in conduit. Cable would be very hard to pull. If you did use cable it would be UF-b. NM-b can't be used.
With 3 lines totaling 70 amps, 30 + 20 +20 do i need the 100amp
The panel can be any size larger than 50 amps. You do not add the breaker sizes to determine load. You go by actual load.

You will also need one (or two) "x8' ground rod at the subpanel. It will be connected by a #6 to the ground bar of the subpanel.
Sorry but hoever is advising you doesn't have a clue.
 

Last edited by ray2047; 07-26-15 at 05:06 PM.
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Old 07-26-15, 06:16 PM
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Code here allows six inches down for metal conduit so i was going to go down 8-10 inches to be safe on inspection and to allow for some rock and dirt on top of conduit. PVC requires 18 inches and i don't want to chance getting near the drainfield pipes so figure to go with metal.

Sounds good on the 100 amp panel...so you are good with my coming off the 50 amp breaker on house panel and using the existing 50 amp rv plug and turning it into a junction box to service the sub panel?

on the cable for the 72' run i was just trying to go for the easiest way....was thinking of pulling it thru each 10' section of conduit as i put them together...not a good idea? what size wires would you think if using individual?

thanks much
 
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Old 07-26-15, 06:51 PM
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so you are good with my coming off the 50 amp breaker on house panel and using the existing 50 amp rv plug and turning it into a junction box to service the sub panel
Yes.
what size wires would you think if using individual?
#6 THWN.

What type of conduit were you going to use?
 
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Old 07-26-15, 07:09 PM
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not sure other than metal to be able to only go down 6" minimum
 
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Old 07-26-15, 07:12 PM
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electrician i talked to said same #6 thwn but also mentioned if not available thhn

not sure difference in imc, rmc or grc assuming they are metal...i guess i need to check with inspector to see what is ok
 
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Old 07-26-15, 07:38 PM
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Told good idea even though panel indoors to use an outdoor rated panel
Totally not necessary and you would probably regret it before you even get your project finished. Just use a typical NEMA 1 panel like you would use indoors.

Sorry but whoever is advising you doesn't have a clue.
I agree with Ray on this.

If you want to only have 6 inches of cover, use the GRC which is heavywall galvanized threaded steel conduit.

electrician i talked to said same #6 thwn but also mentioned if not available thhn
THHN is usually dual rated as THHN/THWN.
 
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Old 07-26-15, 07:58 PM
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whats the downside of using an outdoor rated panel other than a little more cost? why would i regret it? The reasoning was that the garage is metal uninsulated storage so just in case of a leak or condensation why not use outdoor rated panel. thanks
 
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Old 07-26-15, 08:21 PM
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whats the downside of using an outdoor rated panel other than a little more cost?
I think you will find the cost increase to more than "a little", probably more like double the cost of the NEMA 1 panel.
 
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Old 07-26-15, 08:33 PM
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ok but is that the only concern?
 
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Old 07-26-15, 08:48 PM
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You might find that it has no knock-outs. Some do, on the bottom only, and some don't. If you add any holes in places other than the bottom you need to use Myers hubs and sealing washers or you lose the NEMA 3R rating and therefore it is no longer considered to be weatherproof.

So, unless you are preparing for some big roof leaks it just doesn't need to be "outdoor" rated.
 
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Old 07-26-15, 09:12 PM
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got ya, thanks. In looking at some videos on subpanels can you confirm the following for installation.
In wiring the subpanel the neutral bar should not be bonded to the box and also should not be bonded to the ground bar. The ground bar should be bonded to the box. If there is a green screw near neutral bar it should be removed.

Also, as my source is a 50amp breaker from the main house box i assume that the breaker at the subpanel where the lines come in should also be a 50amp breaker of the same make/model in the house. That is my shutoff for the subpanel. Then below that i can wire in my 30 amp breaker for an rv plug, a 20 amp breaker for outlets inside the building and a 20 amp breaker for the couple of building lights.
thanks everyone
 
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Old 07-26-15, 09:20 PM
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In wiring the subpanel the neutral bar should not be bonded to the box and also should not be bonded to the ground bar. The ground bar should be bonded to the box. If there is a green screw near neutral bar it should be removed.
All true.

Also, as my source is a 50amp breaker from the main house box i assume that the breaker at the subpanel where the lines come in should also be a 50amp breaker of the same make/model in the house.
No. The main circuit breaker in the sub-panel should be of the same make as the panel itself OR an "approved" substitute. It only needs to be of an amperage rating equal to or higher than the supplying circuit breaker (the 50 ampere CB in the house) because it does not provide any protection, it is just used as a master cut out switch.

Then below that i can wire in my 30 amp breaker for an rv plug, a 20 amp breaker for outlets inside the building and a 20 amp breaker for the couple of building lights.
Correct.
 
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Old 07-26-15, 09:21 PM
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In wiring the subpanel the neutral bar should not be bonded to the box and also should not be bonded to the ground bar.
Correct.
The ground bar should be bonded to the box. If there is a green screw near neutral bar it should be removed.
Correct.
as my source is a 50amp breaker from the main house box i assume that the breaker at the subpanel where the lines come in should also be a 50amp breaker of the same make/model in the house.
No. As I wrote in my first reply the main breaker serves only as a disconnect so a 100 amp breaker is fine since it is protected by a 50 amp breaker.
Then below that i can wire in my 30 amp breaker for an rv plug, a 20 amp breaker for outlets inside the building and a 20 amp breaker for the couple of building lights.
Yes. Receptacles must be GFCI protected. Lighting can be on a 15 amp breaker and does not require GFCI protection.
 
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Old 07-26-15, 09:28 PM
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thanks much i missed the 100amp breaker initial response. You all have been really helpful.
 
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Old 07-26-15, 09:32 PM
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Come back and ask us not that electrician if you need anymore help.
 
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Old 07-26-15, 09:53 PM
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will do for sure....thanks again
 
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Old 07-27-15, 10:11 AM
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ok i am going to take you up on a few more questions.

For the 30 amp single pole rv plug...wire size? #10/3 with ground?

For the 20 amp plug circuits....12/2 with ground? The length of run could be about 35-40ft

For the 15 amp light circuit 12/2 with ground as well? Length of run much closer with one light by panel and one about 15-20 feet away.

As this is a metal building with wood trusses and posts and no insulation, I am thinking for protection for the plug and light runs to use pvc conduit to attach to the wood along the walls...Ok?

Are plastic boxes ok or do i need to use metal boxes for some reason?

Once I bring the grc up the side of the metal building I assume i can switch to pvc and take that into the building and into the sub panel?

Thanks guys!
 
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Old 07-27-15, 10:36 AM
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For the 30 amp single pole rv plug...wire size? #10/3 with ground?
No. 10-2.
For the 20 amp plug circuits....12/2 with ground?
Correct.
For the 15 amp light circuit 12/2 with ground as well?
No, you only need 14-2 if a 15 amp breaker. You can use 12-2 but not needed.
I am thinking for protection for the plug and light runs to use pvc conduit to attach to the wood along the walls...Ok?
Yes.
Are plastic boxes ok
They are okay.
Once I bring the grc up the side of the metal building I assume i can switch to pvc and take that into the building and into the sub panel?
Yes. Use a weather resistant junction box as a pull box.
 
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