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Old 12-05-2009, 10:33 AM   #1
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Grounding a Subpanel


I have a lot of experience with typical home wiring and main panel installation. However, I am in the process of installing a subpanel; I have never done this before and want to make sure my information is correct. The main panel in my house is 200 amp. I am adding a 100 amp subpanel to my woodworking shop that is in my basement (not a detached building). The subpanel is a standard panel, which I believe can be used as a main load center or subpanel (it does not have the typical main load breaker - just a 100 amp removable circuit breaker that attaches directly to the hot bus bars). I typically work alone, so other than having the lights for the shop on (lights will be from the subpanel), I am only running one piece of equipment at a time and a dust collector - the load should be pretty light most of the time). I was told that I should run #2 3 wire (100 amp entrance wire) from my main panel to my subpanel connecting the two hots to the 100 amp breaker and the neutral to the neutral bus bar. I was told that I would need to ground the subpanel just like I would a regular main panel - continuous loop ground wire to 2 ground rods set 8' apart. I was told to keep the ground and neutral bus bars bonded. Is all of this the correct? I have a couple of additional questions. 1) If I go with this configuration, other research indicates that I should also run a ground to the main panel. Is this correct? 2)Other research indicates that since this panel is in my house and not in a separate building that I should instead run a #2 4 wire (entrance wire?). The two hot wires to the breaker, neutral to neutral bus bar, ground to ground bus bar, break the bond between neutral and ground bus bars, with no separate ground rods. Is #2 wire the appropriate size for this use? By going this route, will my sub panel be adequately grounded through the main panel. I would appreciate any thoughts on this. Thanks. Craig

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Old 12-05-2009, 10:45 AM   #2
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Grounding a Subpanel


Sorry to say that you have received some bad info. Subpanels are required by the 08 NEC to be fed by 4 wire feeds. There used to be an exemption for a 3 wire feed to a detached structure. It has been removed.

Instead of back-feeding a breaker and installing the required hold down clip just install a main breaker if you desire a way to shut the panel down. You can install a main lug panel, but will not be able to shut the panel down except at the main panel.

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Old 12-05-2009, 10:51 AM   #3
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Grounding a Subpanel


Ground rods are only required in a sub in a detached building
I ran #3 for my 100a sub in my attached great room
2 hots, 1 neutral & a #6 green ground
Neutral not bonded to the case at the sub
Grounds & neutrals on seperate buss - not bonded together
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Old 12-05-2009, 10:56 AM   #4
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Grounding a Subpanel


I would suggest you read the sticky above with Stubbies diagrams and it will help you understand your install.
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Old 12-05-2009, 11:58 AM   #5
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Grounding a Subpanel


just in to support the proper installation:

4 wire is required

if you use the back fed breaker as a main breaker, you MUST use a hold down kit to prevent accidental removal.

a main breaker is not required

no GEC (grounding electrode conductor) as you have an EGC (equipment grounding conductor) back to the main panel.


before you buy the wire, talk to us. Not sure what wire you are considering using but to assist you in determining the proper wire, ask.
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Old 12-05-2009, 12:15 PM   #6
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Grounding a Subpanel


Also your subpanel may have come with only a neutral bar.

And there is a "neutral bonding screw" which would come with a panel.

There would need to be a separate ground bar and the 4th "ground wire" would connect to this. And this ground bar would be bonded to the metal case of the subpanel.

The neutral bar would NOT be bonded to the metal case of the subpanel (the neutral bonding screw not installed).

You can buy separate ground bars to add on to panels. Some panels come with the holes predrilled. Otherwise drill your own holes.
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Old 12-05-2009, 01:15 PM   #7
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Grounding a Subpanel


remember 110.14(C)(1)(a) Which will require that you run #1 copper.

And so it goes, R
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Old 12-05-2009, 01:24 PM   #8
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Quote:
Originally Posted by Saturday Cowboy View Post
remember 110.14(C)(1)(a) Which will require that you run #1 copper.

And so it goes, R
#1 for a 100a sub?
Never heard of anyone who has said that ?
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Old 12-05-2009, 01:26 PM   #9
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#1 for a 100a sub?
Never heard of anyone who has said that ?

Read the code section
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Old 12-05-2009, 01:29 PM   #10
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Grounding a Subpanel


I did, read it all...........110.14(c) 1a
there are 4 seperate sections under that
Please provide your justification for having to use #1
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Old 12-05-2009, 01:31 PM   #11
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Grounding a Subpanel


Quote:
Originally Posted by Saturday Cowboy View Post
Read the code section
HOW do you figure this?
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Answers based on the 2008 & 2011 NEC.
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Old 12-05-2009, 01:39 PM   #12
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Grounding a Subpanel


I knew i'd get some flak for posting that Ok this obscure code section was pointed out to me by an Instructor, and he spent an hour proving it to an unbeliving me.

What this says is that unless we can prove that our ALL of our equipment is listed for 75deg or higher(which is hard to prove), we will be forced to use the 60deg collum in table 310.16.

Which by the time we get down to a wire that says >100 is #1copper @110A or 1/0Al @ an ampicity of 100A.

I know that this is not as complete leason as given to me, butmy figers hurt But it wil get you started with a rebuttle.

And so it goes, R
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Old 12-05-2009, 01:43 PM   #13
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Quote:
Originally Posted by Saturday Cowboy View Post
Ok this obscure code section was pointed out to me by an Instructor,
I had a feeling. Like inspectors, instructors are NOT always right.

Quote:
Originally Posted by Saturday Cowboy View Post
What this says is that unless we can prove that our ALL of our equipment is listed for 75deg or higher(which is hard to prove), we will be forced to use the 60deg collum in table 310.16.
This would hard to NOT do in a new installation. When was the last time you installed something NEW that was rated 60 deg C?



Don't take this the wrong way as I am truly just curious. How long have you been in the trade Cowboy?
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Old 12-05-2009, 01:50 PM   #14
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Quote:
Originally Posted by Speedy Petey View Post
I had a feeling. Like inspectors, instructors are NOT always right.
Not always wrong.

Quote:
Originally Posted by Speedy Petey View Post
This would hard to NOT do in a new installation. When was the last time you installed something NEW that was rated 60 deg C?
Lots of things. quite a few of the larger breakers say 60deg on them.

Quote:
Originally Posted by Speedy Petey View Post
Don't take this the wrong way as I am truly just curious. How long have you been in the trade Cowboy?

Like your first comment that is a personal attack instead of a factual rebuttle with a code refrence to back it up. Not that I'm offended far form, But it makes a poor arguement. But to answer your question, I am a fourth year. And yes I realise I don't know anything Thx.

Now to throw another wrench in the works, Give me some time to reasearch it and I will come up with an arugement for the other side.


And so it goes.
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Old 12-05-2009, 01:56 PM   #15
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Grounding a Subpanel


Take a look at 240.4 and see if you can't make a reasonalbe arguement for an instalation as follows:

A 100A sub panel
feed from a 100A breaker
with #2 copper

And so it goes, R

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