Wiring to Subpanel Questions

[davesnyder90]

Hello,

I am building a new garage. It will be 230' from my main panel (200A) at my house. I am going to run 2/0 Aluminum Direct Bury Cable to the new panel (125A panel).

Subpanel at Garage Question #1: Attached is a pic of the new box (w/o wiring) at the garage. The breaker installed is going to be used as the main breaker in the panel - electrician at local supply store said this is acceptable. Can someone verify this is acceptable as far as the incoming 'main' breaker?

Subpanel Question #2: I studied voltage drop & my calcs matched calculator on Southwire website. i.e. 230', 2/0 alum direct bury, 120V is good for 60 amps at 3.75% voltage drop. My loads will be a small mig welder (220), small compressor, lights. Should I be using a 100A breaker or a smaller breaker? (There will obviously be a main breaker at the house). The 2/0 just fits in the 100A breaker. (I will be providing shading for the direct bury cables)

Attachment of Neutral Question #3: Attached is a pic of my neutral lugs on my main. Any ideas as to how to expand to accept the 2/0 aluminum to my neutral?
View media item 32163

Attachment of Ground to main panel Question #4: For my ground, in my main, is a bare copper wire that is attached to the neutral at the main. Attached is a pic of the recommended direct bury alum. ground cable. As can be seen in the main panel pic, the top bare copper is the ground. Can I simply insert the new alum ground with the copper ground and tighten the terminal? Or should I simply swage it to the bare copper ground? Or should I attach to a new terminal as mentioned in #3 above? It seems odd that I am attaching a new ground for subpanel to the neutral at the main, but ground is ground.

Thanks so much...this forum is great. I'm new to home wiring but work on wind turbines...much different animals...

 

[alan_3301]

* I am not an electrician, just sharing some thoughts, use at your own risk *

1. I think that is ok, given you use a retaining clip to secure it. (backfed main breakers must be secured) Is what I have heard.
2.No idea
3. Maybe you could pigtail together some of the smaller ground wires, to free up a space. Then move your main ground to that newly freed up space. Then will the 2/0 fit where the ground wire was previously?
4. ?? Do you not have a neutral bus and a ground bus in your main panel?
It is right for neutral and ground to be tied together in the main, but it looks like you only have a neutral bus?

oh and people more knowledgeable than me will probably ask where you are located for code reasons, so you can post that up to save some questions.

 

[davesnyder90]

Thanks for the reply Alan. No, I do not have a ground bus in my main panel. The uninsulated ground wire attaches to my neutral bars and then out the bottom of the panel.

I am located in unincorporated part of San Bernadino County, CA....so governed by San Bernadino County Code.

 

[theoldwizard1]

#1 Assuming lugs at the top are not used that breaker is acceptabel. It looks like those lugs are removable and I probably would.

For #3 and #4, they do make stacked double lugs.

LugsDirect.com

 

[wyliesdiesels]

#1: the breaker in your sub is just acting as a disconnect. The breaker in your main service panel is protecting the feeder. So as long as its the same size or latger than the feeder breaker(main panel), youre set.

#2: u dont change breaker size to accomadate for voltage drop. You actaull upsize the wire. I came up with 7.33v for VD or 3.1%. The NEC recommends 3% but doesnt require this. Keep in mind this is at full load(100a) which u would be hard pressed to hit eapecially if youre a man shop w/ the equipment u listed.

#3 if u have 2 screws available on your neutral bus bar u can add a lug that has 2 ears that sit in the smaller holes of the neutral bus bar.(i cant post pics cause im on a phone but the neutral lug is the same one as pictures in comment 25 in this thread: http://www.garagejournal.com/forum/showthread.php?p=1323549) make sure to get a lug thats listed for your brand of panel.

#4 Neutral and ground are ONLY bonded in the main service panel. The bare wire u see should be going to a grounding electrode such as rods or ufer. This is called the GEC. The ground from your subpanel will also be going to the neutral bar in your main and u will need a lug for this as well!

In addition to the above, your subpanel needs to have separate neutral and ground bars and the neutral bar needs to be insulated/isolated from the panel enclosure and the ground bar. U will also need 2 ground rods at the garage ties together with one continuous wire, usually bare #6 cu, that connects to the ground bar in the sub.

Thanks for the reply Alan. No, I do not have a ground bus in my main panel. The uninsulated ground wire attaches to my neutral bars and then out the bottom of the panel.

I am located in unincorporated part of San Bernadino County, CA....so governed by San Bernadino County Code.

Main service panels dont have ground bars because ground and neutral are at the same potential in main service panels.

 

[pattenp]

For 100A service to the garage use 2/0-2/0-2/0-1 AL Mobile Home Feeder. It comes as 4 pre-assembled conductors.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet15

 

[davesnyder90]

Re OldWizard - Thanks for the info on the doublestacked lugs - that will work perfectly for the 2 new hookups to my neutral at my main - 2/0 ground and neutral by removing 2 of the current lugs in the top of my neutral bonding 'area'. Not sure of proper terminology.

Re Wyliesdiesel - thanks for the info - that is my plan - 2 ground bars at the sub and I already have purchased ground bar for my sub & installed in the pic. I hadn't figured out (found literature) on size wire I needed to connect the ground bars back to the sub panel - as you suggested #6 bare Cu.

I was curious/concerned that I was recommended a 100A breaker - yet I know my load will never be this much - but when I do voltage drop calcs I was outside the parameters (voltage drop) for 2/0 Al direct burial @ 230' for 100 amp CONTINUOUS LOAD...yet I am using 100 amp breaker. Does that make sense as for the proper sizing of the breakers? I.e. I'm putting a larger breaker in than my continuous load service would ever be. I hear what you're saying though - you don't change breaker size for voltage drop - so I guess my main question is - should I install a smaller breaker to meet the recommended voltage drop for the cable I'm installing to adequately protect my service? (hope this makes sense)

 

[davesnyder90]

Re PattenP - hmm - I wonder if it is more expensive using the Mobile Home Feeder Cable than running separate cables? I guess it's all hindsight as I already have the 2/0.

 

[pattenp]

Without going into a lot of detail. The 2/0 AL will be fine at 230' with it protected by a 100A breaker in the main panel. At a full load of 100A the VD is a little over 3% which is no big deal.

 

[pattenp]

What is the Southwire in your picture for? It's #4.

Re PattenP - hmm - I wonder if it is more expensive using the Mobile Home Feeder Cable than running separate cables? I guess it's all hindsight as I already have the 2/0.

 

[davesnyder90]

Re PattenP - Pic #4 - that is the ground cable to run from the main to the sub panel.

 

[pattenp]

Okay. You may have saved a little money because the ground in MHF is #1.

Re PattenP - Pic #4 - that is the ground cable to run from the main to the sub panel.

 

[wyliesdiesels]

Re OldWizard - Thanks for the info on the doublestacked lugs - that will work perfectly for the 2 new hookups to my neutral at my main - 2/0 ground and neutral by removing 2 of the current lugs in the top of my neutral bonding 'area'. Not sure of proper terminology.

Re Wyliesdiesel - thanks for the info - that is my plan - 2 ground bars at the sub and I already have purchased ground bar for my sub & installed in the pic. I hadn't figured out (found literature) on size wire I needed to connect the ground bars back to the sub panel - as you suggested #6 bare Cu.

I was curious/concerned that I was recommended a 100A breaker - yet I know my load will never be this much - but when I do voltage drop calcs I was outside the parameters (voltage drop) for 2/0 Al direct burial @ 230' for 100 amp CONTINUOUS LOAD...yet I am using 100 amp breaker. Does that make sense as for the proper sizing of the breakers? I.e. I'm putting a larger breaker in than my continuous load service would ever be. I hear what you're saying though - you don't change breaker size for voltage drop - so I guess my main question is - should I install a smaller breaker to meet the recommended voltage drop for the cable I'm installing to adequately protect my service? (hope this makes sense)

The ground and neutral bars are installed correctly. Just make sure to remove the green bonding screw on the neutral bar. The #6 is not for connecting the bars to the panel but rather for connecting ground rods to the ground bar.

And no, as I said the breaker size has nothing to do with voltage drop. Breakers protect wire. Voltage drop is lowered by increasing wire size. Dont bother with the breaker in your subpanel, its only a disconnect. Nor the feeder breaker in your main, 100a will be fine!

 

[mdd1986]

Re OldWizard - Thanks for the info on the doublestacked lugs - that will work perfectly for the 2 new hookups to my neutral at my main - 2/0 ground and neutral by removing 2 of the current lugs in the top of my neutral bonding 'area'. Not sure of proper terminology.

Re Wyliesdiesel - thanks for the info - that is my plan - 2 ground bars at the sub and I already have purchased ground bar for my sub & installed in the pic. I hadn't figured out (found literature) on size wire I needed to connect the ground bars back to the sub panel - as you suggested #6 bare Cu.

I was curious/concerned that I was recommended a 100A breaker - yet I know my load will never be this much - but when I do voltage drop calcs I was outside the parameters (voltage drop) for 2/0 Al direct burial @ 230' for 100 amp CONTINUOUS LOAD...yet I am using 100 amp breaker. Does that make sense as for the proper sizing of the breakers? I.e. I'm putting a larger breaker in than my continuous load service would ever be. I hear what you're saying though - you don't change breaker size for voltage drop - so I guess my main question is - should I install a smaller breaker to meet the recommended voltage drop for the cable I'm installing to adequately protect my service? (hope this makes sense)

Technically speaking for a continuous load of a 100 amps you should be using a 125amp breaker based on NEC. However in your situation you will probably very seldom see the full 100 amps do what you have should be ok.

I think what you have should be fine for voltage drop. Like others have said, you size the wire not the breaker based on voltage drop. So if you have 2/0 now and you want a smaller voltage drop you would just bump it up to a 3/0 but could still use the 100amp breaker.

 

[theoldwizard1]

... as I said the breaker size has nothing to do with voltage drop. Breakers protect wire. Voltage drop is lowered by increasing wire size. Dont bother with the breaker in your subpanel, its only a disconnect. Nor the feeder breaker in your main, 100a will be fine!

If you haven't bought the wire (or at least can return it), I would consider going down a couple of steps and using a smaller breaker in you main panel.

Someone double check me, but if you go down to a 60A main panel breaker, you could go down to #1 AWG (not 1/0 !) AL wire and still be at a 3% voltage drop for your distance. #2 AWG (like 2-2-2-4 MHF) would give you a 4% voltage drop @ 60A or a 3% drop @ 50A. Potentially a huge cost savings, if you won't ever need 100A.

With 60A you should be able to run a small plasma arc and a small (< 3hp) compressor, simultaneously, and still have enough for lighting.


As wylie said, the "main" breaker in the subpanel is basically there as a disconnect. It is not really protecting anything. Personally, I would downsize it to no more than the breaker in the main panel (maybe even a bit less) just because I would want it to trip before the main. (That is a long way to walk to reset a breaker if it does ever trip.)

 

[wyliesdiesels]

If you haven't bought the wire (or at least can return it), I would consider going down a couple of steps and using a smaller breaker in you main panel.

Someone double check me, but if you go down to a 60A main panel breaker, you could go down to #1 AWG (not 1/0 !) AL wire and still be at a 3% voltage drop for your distance. #2 AWG (like 2-2-2-4 MHF) would give you a 4% voltage drop @ 60A or a 3% drop @ 50A. Potentially a huge cost savings, if you won't ever need 100A.

With 60A you should be able to run a small plasma arc and a small (< 3hp) compressor, simultaneously, and still have enough for lighting.


As wylie said, the "main" breaker in the subpanel is basically there as a disconnect. It is not really protecting anything. Personally, I would downsize it to no more than the breaker in the main panel (maybe even a bit less) just because I would want it to trip before the main. (That is a long way to walk to reset a breaker if it does ever trip.)

I got the same results as u, though I came up with 3.7% VD for #2al @ 60a, which i assume u rounded up to 4%.