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Old 05-23-2013, 07:51 AM   #1
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sub panel in a detached garage


i'm feeding a detached garage from house running four wire to garage keeping ground and neutral un bonded in sub panel. Do i need to install ground rods at the new sub panel and if so should I tie them to the main panel ground rods? I have heard if you dont it can cause a potential betwwen grounds

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Old 05-23-2013, 07:57 AM   #2
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sub panel in a detached garage


Drive 2 rods 6' apart, connect to the ground buss that the ground wire from the main panel connects to.

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Old 05-23-2013, 09:47 AM   #3
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sub panel in a detached garage


You are correct in running a separate ground conductor to a ground bar in sub panel . Off this, you only need 1 additional ground rod. And if you have water lines, these also need to be bonded to ground bar.

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Old 05-23-2013, 09:52 AM   #4
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sub panel in a detached garage


If the sub panel does not have a main incoming breaker, then you can't have more than 6 circuit breakers. If you want more breakers than 6, then use a sub with a main disconnect breaker.
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Old 05-23-2013, 09:56 AM   #5
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sub panel in a detached garage


Curiosity..... Can anyone explain the electrical necessity/logic of grounding the subpanel when it is detached.... just extra grounding for a lightening hit ???

TIA
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Old 05-23-2013, 09:57 AM   #6
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sub panel in a detached garage


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If the sub panel does not have a main incoming breaker, then you can't have more than 6 circuit breakers. If you want more breakers than 6, then use a sub with a main disconnect breaker.
or can you use a back-feed with a non-lug panel??
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Old 05-23-2013, 10:03 AM   #7
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sub panel in a detached garage


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Curiosity..... Can anyone explain the electrical necessity/logic of grounding the subpanel when it is detached.... just extra grounding for a lightening hit ???

TIA
The word you want to remember is "potential".

It is important to keep the potential to ground equal at the house service panel and the sub panel structure.
Keep everything at the same potential.

This is accomplished by the grounding conductor (EGC) running between the two structures.
Ground rods at both panels ensure the potential is equal at both structures.

This is why peripheral equipment such as cable, TV and satellite must be connected to the GES. (grounding electrode system) To keep everything at the same potential.

And finally yes. The GES is designed to protect property from lightning strikes.
Nothing more.
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Old 05-23-2013, 10:15 AM   #8
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sub panel in a detached garage


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Originally Posted by J. V. View Post
The word you want to remember is "potential".

It is important to keep the potential to ground equal at the house service panel and the sub panel structure.
Keep everything at the same potential.

This is accomplished by the grounding conductor (EGC) running between the two structures.
Ground rods at both panels ensure the potential is equal at both structures.

This is why peripheral equipment such as cable, TV and satellite must be connected to the GES. (grounding electrode system) To keep everything at the same potential.


And finally yes. The GES is designed to protect property from lightning strikes.
Nothing more.
So why do we not have to pound a ground rod everywhere outside the perimeter of our home where we could come in contact with the home electrical (water faucets and outside recepticals).... does not that present a potential difference to ground. But we only do it for a detached structure.


What's the difference??

TIA

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Old 05-23-2013, 10:32 AM   #9
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sub panel in a detached garage


A lot has changed over the past few years as we learn more about grounding.

Back before WWII, Ufer determined that a concrete pad was a better ground than a bunch of ground rods....the significance for this was not electrical safety, but rather for HF propagation characteristics. In order for an HF or UHF antenna to have the best signal, it needs a really good ground reference. The better the ground plane, the better the transmit and receive.

We only recently learned that the same Ufer works really well as a safety ground.

The primary intent for a ground as noted above is safety in case of a fault and/or lightning strike.

Current code requires at least one rod....assuming you can prove that your ground resistance is 25 ohms or less. In most cases it's not...so it's just cheaper, quicker and easier to run a second rod.

Now...here is the catch....

You have to have additional ground rods if you have a detached panel....but a sub panel in your house in an attached room does not need ground rods....Whats the difference one would say?

The difference is that you can assume an attached structure is going to be pretty close to the main panel. But, a detached garage or any other structure could be 20' away...200' away. So rather than make distance a factor as to whether you need ground rods or not....if it's detached...you put in more ground rods.

As for hooking into your plumbing...two fold....it makes sure the plumbing is grounded to earth potential...and if the plumbing is a better ground than your rod (which is usually the case), then your getting a better ground.

Remember, the key on a fault is to route any fault current to the ground. In a lighting strike, you want the lighting to take the path of least resistance....if it's a direct hit....not a whole lot you can do to save everything...but it will at least reduce the chances of damage and injury.

Remember, everyone is still learning...5 years from now it could be something totally different. For all you know, all AC power will be Bluetooth G1250
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Old 05-23-2013, 10:44 AM   #10
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sub panel in a detached garage


Dawg... Nice write-up... Thank You..

But in theory, shouldn't we have grounds around our perimeter (UFER grounds not considered) where we could encounter our electrical system (Faucets/receps)

Truthfully, I doubt in most circumstances that TWO ground rods get us less than 25 ohm.... and I guess there is not anything magical about 25 ohm anyway... better than 26, not as good as 24.

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Old 05-23-2013, 11:02 AM   #11
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sub panel in a detached garage


In 'theory', it sounds right....but in reality....I doubt it matters....

Remember, a lightning strike takes the path of least resistance.

Lets say you have your antenna grounded to your load center...and then a ground from there goes to your rods. If you take a hit on that antenna...your surge will be split between what ever is connected to the load center ground and your ground rod. Who every has the lower resistance, gets the brunt of the juice.

Now, if your antenna is connected to the ground rod first, then you start adding in the resistance of the wire going up to your load center. Your now improving your chances that you wont get any damage.

Conversely, if you take a lightning hit in the ground near the house, you could get some induced voltage into your system via the ground rods. But, the more parallel paths you have, the more the energy is spread out and dissipated.

Also remember that a surge protector is in most cases a simple MOV across the hot and neutral. It's intent is to clamp any spike across those lines...which is what usually does the damage.

In the case of a lighting strike on your ground system...if the electrical equipment in your house has been built right, any induced voltage on the ground should not hurt it....'shouldn't'..

You know that UL sticker on stuff? It means the device has passed a HiPot test...that is where they apply around 1200 to 2000 Volts between the chassis ground and AC input.

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