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Old 01-27-2015, 08:32 PM   #1
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can some explain why a attached building has a sub panel with the grounding bar and the grounded bar isolated but in a detached building both bars are bonded with ground rods.
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Old 01-27-2015, 08:39 PM   #2
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can some explain why a attached building has a sub panel with the grounding bar and the grounded bar isolated but in a detached building both bars are bonded with ground rods.
A sub panel has the grounds and the neutral separated. Being in a detached building has nothing to do with it being a sub panel. The grounds and neutrals are still required to be seperated.
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Old 01-27-2015, 08:42 PM   #3
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And someone smarter than me may correct this, but I always viewed the ground rods as a safety if the ground wire had too much resistance to operate properly.
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Old 01-27-2015, 08:46 PM   #4
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can some explain why a attached building has a sub panel with the grounding bar and the grounded bar isolated but in a detached building both bars are bonded with ground rods.
Ground rods in a detached sub panel should be connected to the grounding conductor, not the grounded conductor
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Old 01-27-2015, 08:49 PM   #5
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And someone smarter than me may correct this, but I always viewed the ground rods as a safety if the ground wire had too much resistance to operate properly.
Not at all. The ground rod is not a substitute for an equipment grounding conductor. The rods are there to help with voltage surges and lightning. A ground rod, in most cases will never trip a breaker.
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Old 01-27-2015, 08:53 PM   #6
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Not at all. The ground rod is not a substitute for an equipment grounding conductor. The rods are there to help with voltage surges and lightning. A ground rod, in most cases will never trip a breaker.
I didn't mean that they were a substitute I was meaning I thought that they would be there in an emergency if the resistance was too high. I also see that in a surge it would be beneficial too.
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Old 01-27-2015, 09:03 PM   #7
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As I said before they will not help will clearing a fault. Take an 8' rod and drive it in a field. Then take a piece of #12 from a nearby panel and stick it under a 20 amp breaker and connect the other end to the rod. It will never trip that breaker. You will energize the area where the rod is placed.
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Old 01-27-2015, 09:06 PM   #8
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As I said before they will not help will clearing a fault. Take an 8' rod and drive it in a field. Then take a piece of #12 from a nearby panel and stick it under a 20 amp breaker and connect the other end to the rod. It will never trip that breaker. You will energize the area where the rod is placed.
Just a question. Shouldn't it be two ground rods interconnected between each other?

Edit. I am going to conduct an experiment tomorrow with this. Two ground rods and no grounding conductor. I do want to see if it would clear a fault. But as I said I do not consider it a replacement. I consider it supplemental. I would prefer to hit a eufer at the detached garage.

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Old 01-27-2015, 09:07 PM   #9
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Just a question. Shouldn't it be two ground rods interconnected between each other?
Yes, 2 rods and that won't help much either.
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Old 01-27-2015, 09:13 PM   #10
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Does anyone have any suggestions or comments on how they would like me to perform this experiment? I am going to record it so I can upload it here.
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Old 01-27-2015, 09:25 PM   #11
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Does anyone have any suggestions or comments on how they would like me to perform this experiment? I am going to record it so I can upload it here.

From a remote location.
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Old 01-27-2015, 09:26 PM   #12
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all these replies make sense. I know some building inspectors will allow a sub panel in a detached building to not have a grounded conductor just the 2 ground rods 2hot and one neutral with the grounded and grounding bar bonded. but in sub in attached building the neutral has to be floating. does this sound sight.
thanks again for all the feed back so far looking to read more good info
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Old 01-27-2015, 09:36 PM   #13
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Mike Holt's video - 50' ground rod with 120 volts only got 8 amps. Not sure where the other end was. The required 25 ohms at 120 volts means less than 5 amps .
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Old 01-27-2015, 10:40 PM   #14
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The only breaker that will trip on a ground rod most of the time is a GFCI breaker. This is not allowed under the NEC and where used under the IEC most electricians recommend a main GFCI in case the branch fails.
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Old 01-27-2015, 10:43 PM   #15
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Does anyone have any suggestions or comments on how they would like me to perform this experiment? I am going to record it so I can upload it here.
If you are talking about what I said be very careful. One of the members at mike holt did this a year or so ago and he was surprised that the breaker did not trip.
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Old 01-27-2015, 11:10 PM   #16
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A sub panel has the grounds and the neutral separated. Being in a detached building has nothing to do with it being a sub panel. The grounds and neutrals are still required to be seperated.
That answered one half of his question.
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Old 01-27-2015, 11:17 PM   #17
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Does anyone have any suggestions or comments on how they would like me to perform this experiment? I am going to record it so I can upload it here.
Feel free, but An experiment really isn't necassary. Use simple ohms law and do the math. Start out with the legal 25 ohms per nec and tell me how many amps will flow on that wire. Now do it down to 15 ohms. Now 10. Keep in mind you will never acheive 10 ohms to ground with simple rods
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Old 01-28-2015, 02:33 AM   #18
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Feel free, but An experiment really isn't necassary. Use simple ohms law and do the math. Start out with the legal 25 ohms per nec and tell me how many amps will flow on that wire. Now do it down to 15 ohms. Now 10. Keep in mind you will never acheive 10 ohms to ground with simple rods

I have tried for years showing people with cardboard and crayon that a short through a ground seldom if ever trips a breaker. 10 ohms may not quite do it but with a bit of salty urine it can be achieved.
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Old 01-28-2015, 03:17 AM   #19
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For all standard breakers to clear in a safe time frame no more than 0.15 ohms would be needed at a ground rod. Even 1 ohm will be to high.

Consider that most breakers trip instantaneously at 10 times the handle rating. A 50 amp breaker feeding a range will trip instantaneously at 500amps under a fault. In such a case the impedance must be low enough to pass 500amps. 1 ohm would pull only 120 amps, so the breaker would take well over a few cycles to clear. At 1 ohm it would take 20-30 seconds for the breaker to open. Further the voltage will be closer to 120 as compared to a low impedance ground since less current flow equals less voltage drop across the hot feeding the fault. In that time frame the range will be energized close to 120 volts for 30 seconds rather than 2 cycles (2 60ths of a second)

That time makes a difference if someone had contact with the frame.

Under the IEC codes there is a requirement that while a fault is happening it must be cleared within an X amount of time, usually a few cycles. This is called earth fault loop impedance. If the fault can not be cleared within a few cycles it will necessitate either increasing the EGC size to lower impedance or getting the breaker to open faster such as lowering the magnetic trip threshold. In some cases a third option exists like creating an equal potential plane around the appliance where during a fault the voltage of surrounding objects an occupant might have contact with like a floor are raised to a similar potential in reference to the chassis (step touch potential)

Basically grounding/bonding boils down entirely to this:

1. Reducing the voltage between any point(s) of the body a person may be in contact with at the same time.

2. Removing that elevated voltage as quickly as possible.

The combination of these two concepts protect people from harm by limiting the amount of energy that passes through them when something goes wrong.

Everything else is irrelevant, and this is nearly the entire goal of article 250 as well as other codes like CEC and IEC.

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Old 01-28-2015, 03:53 AM   #20
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Feel free, but An experiment really isn't necassary. Use simple ohms law and do the math. Start out with the legal 25 ohms per nec and tell me how many amps will flow on that wire. Now do it down to 15 ohms. Now 10. Keep in mind you will never acheive 10 ohms to ground with simple rods
Very true, but driving a ground rod actually proves how little or how much resistance it has. Even if someone received half an ohm, as much as people would celebrate, that is still would not low enough to be truly safe with any standard breaker.


Nothing beats a copper wire back to the source, nothing.
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