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Old 01-23-2007, 09:10 AM   #1
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This doesnt seem right.


I have a question for you guys.

In my garage there is a 100 amp sub panel fed from the top of the main breaker in the house.

The garage is attached. They also ran a seperate ground rod for the box in the garage.

The wire from the main is a two wire with a braded ground that is connected to the neutral in the main panel and in the sub panel. The neutral in the sub panel is not bonded btw.

Is this right?
I think the feeder from the main should be on a breaker the same size or larger than the main breaker in the garage panel right?
Also shouldnt the ground come from the main panel only? Why would they seperate the grounds like that but not the neutral?

If this was on a machine I would know what to do, but at the house neutrals are a little more complicated than what Im used to.
On equipment we create our own neutral.

Thanks fellas.


Last edited by JoulesWinfield; 01-23-2007 at 11:38 AM.
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Old 01-23-2007, 10:32 AM   #2
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This doesnt seem right.


The way we do them here is to have a separate breaker for the sub, rated to the feeder wire. Sub, in attached structure has ground/neutral separate, like you have. Also, no additonal ground is required if attached structure.

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Old 01-23-2007, 10:34 AM   #3
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This doesnt seem right.


Seems that the weirdest thing is not having a separate breaker for the sub in the main panel. The additional ground rod is probably not a big deal. No breaker for the sub is definitely wrong.
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Old 01-23-2007, 11:38 AM   #4
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This doesnt seem right.


So the wire having two hots and the braided wire for the neutral is fine?

It seems like if I take the feeder from the top of the main and run it to a 100amp breaker I should be up to code then.

The wire size is 2awg aluminum.

Not sure why it was done this way, but I want to make sure its right.
It seemed strange to me.
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Old 01-23-2007, 04:55 PM   #5
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This doesnt seem right.


Hello

You have several code viloations for the installation of a sub-panel in the same structure as the dwelling.

These violations are:

1.) Feeding from the line side of the service equipment main breaker (using its lugs) to a sub-panel.
2.) No ocpd protecting the feeder.
3.) A 3 wire feeder (needs to be a 4 wire feeder...ie..(H-H-N-Grd)
4.) No ground rod at the sub is to be used in this application other than the ground rod/rods and possible metal water pipe at the service panel (main panel).
5.) No seperation of equipment ground from the grounded conductor
at the sub-panel. You cant accomplish this without a seperate equipment ground ran to the sub-panel.

#2 al is only good for 90 amps 75C terminations but that should be fine for your purposes.

In the set-up as it exists presently the metal can of the sub-panel can become energized on fault from the feeder or a branch circuit ungrounded conductor and there would be no way to trip a breaker since there is no breaker protecting the feeder and no bond to the can of the sub-panel.

This is no good... you need a 4 wire feeder from a breaker installed in the main panel. You would then install a seperate ground bar for the grounds in the sub-panel. You would not bond the ground and neutral in the sub with a 4 wire feeder and you will not use a ground rod at the sub. If you use the #2 al you would use a 90 amp breaker or less in the main panel.

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Old 01-24-2007, 10:15 AM   #6
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This doesnt seem right.


"same structure as the dwelling"
Is the garage considered the same structure because its attached? It would remain standing if the house was demolished.

So it sounds like what you are saying is I need to run a 3 wire cable with a braided ground to a 90 amp breaker in the main panel.
Then remove the ground rod at the sub panel and ensure that the neutral bar is not bonded.

Shouldnt the breaker in the sub panel be a 90 amp or less then?

I guess not being familiar with residential wiring Im stumped as to why the ground in the sub panel needs to be ran from the service panel.

Last edited by JoulesWinfield; 01-24-2007 at 10:19 AM.
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Old 01-24-2007, 10:44 AM   #7
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This doesnt seem right.


There is no main disconnect in the sub panel. The panel disconnect is in the main panel. 90a in your case.
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Old 01-24-2007, 11:32 AM   #8
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This doesnt seem right.


Quote:
Originally Posted by Brik View Post
There is no main disconnect in the sub panel. The panel disconnect is in the main panel. 90a in your case.
This sub panel does have a main breaker in it (100a).
Is that wrong as well?
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Old 01-24-2007, 04:37 PM   #9
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This doesnt seem right.


Quote:
Is the garage considered the same structure because its attached? It would remain standing if the house was demolished.
Yes, in order for it to be considered a detached garage it must be a stand alone structure and not structurally attached to the house.

Quote:
So it sounds like what you are saying is I need to run a 3 wire cable with a braided ground to a 90 amp breaker in the main panel.
Then remove the ground rod at the sub panel and ensure that the neutral bar is not bonded.
Yes, 3 conductor with ground . If you use copper wire and want 100 amps using table 310.16 you need #3 awg. The ground will be smaller probably a #8 or #6 if you use a 100 amp rated cable. If you use aluminum and want 100 amps you will need #1 awg. If by chance your juristiction allows the dwelling main feeder table 310.15(B)(6) then #4 copper or #2 aluminum will give you 100 amps. Check with your local codes department this will save you a good chunk.

As an added note you do not have to feed the panel 100 amps you could feed it 60 amps with a 60 amp breaker in the main panel. It just depends on what your load requirements will be in that garage. If you just want to run some power tools and lights, and a few outlets , maybe an air compressor then 60 amps will be plenty. That would save you a bit on the cost of the breaker. If you use the #2 al for your 3 conductors and a #8 ground wire then you could always up-size the breaker later if needed.

Btw you never said what kind of wire you have.....is it a cable or individual wire?

Quote:
Shouldnt the breaker in the sub panel be a 90 amp or less then?
Nope, in fact you could have used a MLO panel (main lug only) with no main breaker. As it is your 100 amp breaker will simply serve as a convenient disconnect for the sub. The 90 amp or less that you use in the main panel will determine the allowable load that the sub-can draw and protect the feeder. So essentially any one of the two legs draws more than 90 amps long enough the 90 amp will trip out and the 100 amp will not notice. If both were 100 amps then it would just be a race as to which one would trip first. Personnally I prefer mlo panels for your situation with a stand alone 100 amp disconnect next to the panel. but what you have is fine.

Quote:
I guess not being familiar with residential wiring I'm stumped as to why the ground in the sub panel needs to be ran from the service panel
.

I believe you may be confused as to the purpose of ground rods (lightning and power surges) and the equipment ground (human safety).

The main panel is the only place where you bond the neutral and ground. Remember you have equipment ground and the grounding electrode system (metal water pipe & ground rods) bonded in the main panel to the grounded conductor (service neutral). These are entirely different systems each serving a different purpose. Essentially the equipment ground is for human safety and bonds all metal parts in the electrical system that are likely to be energized in case of a fault such as a short to ground. This allows fault current to flow on the equipment ground back to the main panel where it then travels the service neutral to the transformer center tap. Remember the two are bonded in the main panel. This low impedance path allows so much current to flow that the branch circuit breaker will trip on over current. The fault current will not flow to the ground rods or water pipes as they offer a much higher impedance. If you relied only on a ground rod for your fault current then a breaker will never trip as they arent there to serve that purpose & will not allow current to flow in sufficient amps to trip a breaker. The GES (ground rods. metal water pipes, ground rings etc .. are there primarily for huge voltage events like lightning or power surges. The metal water pipe and ground rod is where that big bare copper wire runs to from the main panel neutral bar. BTW the neutral bar and ground bar are one and the same in the main panel.
Finally if all you had going to the sub-panel was the neutral along with the 2 ungrounded conductors and no equipment ground then the neutral plays double duty both as the return current carrier and the equipment ground fault path to the grounded conductor (neutral) of the service. If you lose the feeder neutral to an open or fault then you lose both. You have no way for a breaker to trip in the sub. A ground rod at the sub is no help.
When you run a 4 wire feeder you have a seperate equipment ground and provides better safety than a 3 wire feeder. The key is to not bond the two in the sub-panel. If you did then return current on the branch circuit neutrals would travel (split evenly) on the feeder equipment ground and the feeder neutral. You do not want any current on the equipment ground unless it is fault current serving to trip a breaker.
This may help....


In this graphic from Mike Holt the metal conduit is serving as the equipment ground but just look at it as an individual wire like the top graphic. The enclosure marked service is your main service equipment whether a disconnect or panelboard with main breaker. The graphic labeled panelboard is your sub-panel. Notice the grounding symbol at the service panel... this is your grounding electrode...ie metal water pipe, ground rod/rods ....notice no current is flowing to the grounding electrode. Also note how all return currents and fault currents end up on the main service panel neutral bar and then travel on the service neutral (due to bonding) to the center tap of the transformer (not shown). Also note there is no ground rod at the sub-panel (both panels are in the same structure). The sub-panel btw is a mlo panel.

In this graphic the main panel is a disconnect main breaker only but could be a main panel with main breaker and branch circuit breakers. The first disconnect the service entrance wires hit is the service equipment everything on the load side after that is a sub-panel.

Hope this clears things up some


Stubbie aka Roger

Last edited by Stubbie; 01-24-2007 at 10:40 PM. Reason: added clarifying information
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Old 01-24-2007, 04:38 PM   #10
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This doesnt seem right.


Quote:
This sub panel does have a main breaker in it (100a).
Is that wrong as well?
No its fine dont worry about it. It will serve as a nice convenient disconnect no matter what the breaker is in the main panel.

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Old 01-24-2007, 05:11 PM   #11
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This doesnt seem right.


Thanks a lot Roger that was a very informative post.
Thats the answer I was looking for.
Dealing with machine wiring I dont get into this type of situation so this helps clear it up.

Quote:
If by chance your juristiction allows the dwelling main feeder table 310.15(B)(6)
Why wouldnt they? The code seems to apply to feeding a panel from the main panel.

Quote:
It just depends on what your load requirements will be in that garage.
For now I just have a compressor, 1/2 hp bench grinder and your general power tools/lighting loads. But I was thinking in the future it would be nice to run power to my shed from this panel. Just a small panel to run a plug and light. Would that even be legal? Never considered it until now.

Quote:
Btw you never said what kind of wire you have.....is it a cable or individual wire?
It is a 2 wire aluminum cable.

Thanks again for your help.
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Old 01-24-2007, 05:17 PM   #12
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This doesnt seem right.


It should be noted. In my area, and MANY others, we can use table 310.15(B)(6) for all feeders. This includes sub-panel feeders.
So we can use #2al fed from a 100 amp breaker.
I bet if a poll were taken of electricians, 80% would be in this group.

This is NOT to start a big debate on the details of these code sections. I typically wholly agree with and am on the same level as Stubbie on most issues. I am merely stating an alternative view.
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Old 01-24-2007, 05:25 PM   #13
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This doesnt seem right.


As Speedy states it is highly likely they allow the table. They do in my area of Kansas. Speedy and I have seen this debated till you know what freezes over. I have never been convinced why one could not use it but none the less it does require approval or at least you need to know if they allow that table... imo.

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Old 01-24-2007, 05:29 PM   #14
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This doesnt seem right.


Quote:
It is a 2 wire aluminum cable
I was hoping for a three wire with grd. Sorry but you will not be able to use it to feed that sub-panel. It must be a 4 wire cable.

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Old 01-24-2007, 05:35 PM   #15
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This doesnt seem right.


Quote:
Originally Posted by Stubbie View Post
I was hoping for a three wire with grd. Sorry but you will not be able to use it to feed that sub-panel. It must be a 4 wire cable.
Thanks, that actually was the original thought that started this whole thread.

Im glad I posted it here cause I learned something today .

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