Distance Threshold for Local Ground - Residential

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albator

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I am putting a sub panel in a detached structure. There's already 3 #8 wires going there (2 hot, 1 neutral, no ground). Customer is on a budget, so they would prefer I don't replace the existing wires right now. Probably in 2-3 years from now they will want to upgrade the amperage.

Under which distance am I required to have a unique ground-neutral connection at the main panel/meter?
i.e. have a ground wire from the main panel in the house to the sub-panel.

Above which distance am I required to have a local ground, and tie ground-neutral at the sub-panel?
i.e. stick a rod at the shed.

Does this distance depends on the wire size and average load? Voltage drop?
 
You need a 4-wire feeder to a separate structure and you also need a GES at the separate structure. If you have a metal raceway that may comply as an EGC.
 
Under certain conditions an existing 3 wire feed may be used to power a detached structure. You still need a GES, grounding electrode system at separate building.

New installs require a 4 wire feeder. There is no minimum distance between structures that exempts this rule nor does wire size or voltage drop come into play. An EGC is required and a GES.
 
This is property tax season, they are tight on cash. The plan/wish is to upgrade the wiring to a larger (#6 or #4) 4 wire feeder once the construction is done and they have saved some money. I will then tie the 2 GE together: 2 electrodes, one at the main house, one at the detached building, tied by the ground conductor in the 4 wire feeder.

For now, I only have the existing 3 wire (#8) feeder. It runs in a PVC pipe from the main house. I also have a GES at that detached building. That GES is near the 3 wire entry, and I connected that "secondary" GES to the subpanel and to the neutral, in the detached structure.

My concern is: the neutral is bonded to the "main" GES at the main house. Then the neutral is fed to the detached structure, where it is bonded to another "secondary" GES. Depending on the load, there will be a voltage drop across that neutral. What will happen then?
 
In the future, customer might ask me to upgrade to a larger feeder. I will then route a 4 wire feeder.
I will keep the Ground Electrode at the detached structure, and connect the main ground to the ground wire feeder, to the secondary GE, to the subpanel ground. And I will not connect the subpanel neutral to the ground.

The neutral will then fluctuate around the ground because of the voltage drop, which will be exacerbated by the distance, the load and the wire gauge. Is this expected? Within which limits?

Is there a point at which we should bond neutral&ground in a subpanel?
 
albator said:
In the future, customer might ask me to upgrade to a larger feeder. I will then route a 4 wire feeder.
I will keep the Ground Electrode at the detached structure, and connect the main ground to the ground wire feeder, to the secondary GE, to the subpanel ground. And I will not connect the subpanel neutral to the ground.

The neutral will then fluctuate around the ground because of the voltage drop, which will be exacerbated by the distance, the load and the wire gauge. Is this expected? Within which limits?

Is there a point at which we should bond neutral&ground in a subpanel?
Click to expand...

In answer to the question in the last line.

For a sub panel fed with a 4 wire feeder, no.

Your fluctuating exaberated nuetral with voltage drop or something completely lost me. I have no idea what you are asking there. Sorry.
 
I found the answer in an old post

I found the answer in an old post

That post from Charlie Beck explains it well.
http://www.mikeholt.com/forum/Forum1/HTML/003455.html

"
If you run an EGC to the garage (reference NEC 250.32(b)(1)), you don�t bond neutral and ground at the garage panel. In this case, the current from a ground fault goes through the EGC within the garage back to the sub-panel in the garage, and via the continuation of the EGC back to the main panel in the house. It will trip the circuit breaker feeding the equipment. It might also trip the garage feeder in the house. On the other hand, if you don�t run an EGC to the garage (reference NEC 250.32(b)(2)), you do bond neutral and ground at the garage panel. (The ground rod has nothing to do with this discussion.) In this case, the current from a ground fault goes through the EGC within the garage back to the garage panel, and then via the garage feeder�s neutral leg back to the main panel in the house. Here again, it will trip the circuit breaker feeding the equipment. It might also trip the garage feeder in the house. Whichever method you pick, the human is still protected in the two ways described above.
"

Today, I have a 3 wire feed, I connect the garage GEC to the garage subpanel, and I bond N-G at the subpanel.

In the future, if they want me to upgrade the feeder, I will put a 4 wire feed, keep the garage GE, connect both GE together, and remove the N-G bonding at the subpanel.
 
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