I’ve got a 200 amp service to the house off of which I’ve run a 100 amp subpanel to an outbuilding. I ran 4 wires underground between the panels including the extra ground, and understand that I don’t need to, and shouldn’t, put in separate grounding rods for the subpanel. The caveat is that from this 100 amp subpanel there is an overhead triplex (have I got the terminology correct?) running to yet another outbuilding with a 50 amp panel. Since I don’t know the proper terminology I’ll call this 3rd in line panel the sub-sub. So which grounding strategy is correct/safest??
A) Mixed strategy: Four wires from main to 1st sub with no grounding rods at sub, then 3 wires to sub-sub with grounding rods there in addition to the grounding rods at the main panel.
B) Run 4th wire to sub-sub (can I just twist a insulated ground around the existing triplex?) and only have ground rods at the main panel.
C) Cut out the extra ground between the main and 1st subpanel and put grounding rods at every box.
At this point the mixed strategy is the least work. Is it right/safe??
thanks,
Mark
Replies
"I ran 4 wires underground between the panels including the extra ground, and understand that I don't need to, and shouldn't, put in separate grounding rods for the subpanel. "
No.
EACH and EVERY outbuilding needs it own ground electrode system.
And in most cases the best ground electrode is a pair ground rods at least 6 ft apart. You can use just one ground rod at each location if you test the resistance and it is less than 25 ohms. But that is a specialized test and usually not done.
If there is a metalic connection between building then you need a 4 wire connection. Most common would be a metal water pipe. But cable or phone lines also qualify. And I have heard that some local codes only allow 4 wire connections.
And if either building has livestock there are special grounding rules that I am not familar with.
Otherwise I don't know of any problem with mixing a 4 wire and 3 wire system. But that section of the code is very complex.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
In addition to what Bill said..
If one of the outbuildings is a pumphouse and you use the well casing as a grounding electrode (very good electrode BTW), pay attention to any leakage currents to that electrode so you do not cause electrolysis to eat holes in the casing over the years.
I added another neutral conductor to balance just capacitive rectance current to the well casing - you need to measure with a milliamp meter.
You're assuming steel well casing. Extremely unusual here.
My POCO required me to plant only one ground rod when I added 200A service to an outbuilding. Had a second ready, they said no.PAHS Designer/Builder- Bury it!
Thanks for the comments guys. No pumps, no animals. The local inspector said that with a 4th wire I wouldn't need ground rods at the other structures. I went ahead and hooked up to what was there as an "extra". Since that time I guess I heard bad information -- I was told that if you used a 4 wire system AND had separate ground rods at the subpanels you actually created a dangerous situation--ground feed loop or something???--.
Mark
"I was told that if you used a 4 wire system AND had separate ground rods at the subpanels you actually created a dangerous situation--ground feed loop or something???--. "Or sumpping is about right.First of all the purpose of the Ground Electrode System (ie, ground rods and other electrodes) is to reference the electrical system in the building to a know voltage level (ie, zero) with respect to the ground. Without that you could have enough capacitive coupling through the transformer to the primary side so that while you only have 120 from hot to neutral you might several thousand from neutral to ground. That happens whereever the neutral is bonded to the ground electrode system. And that is at no more than one place in a building, where ever you have a 3 wire supply, either from the power company at the main disconnect or at an outbuilding with a 3 wire supply.The other pupose of a ground electrode system is that it gives a place to discharge surges from nearby lightining strikes. Lightining is a very fast pulse and it does not travel well down a long line. But a ground electrode system is used at each structure.Now inside the structure you have the equipment grounding system, ie. "the ground".Ground loops -First with modern design and signals ground loops are really not a potential problem except with very specialized applications such as medical monitoring.And most people that use the term don't have the slightest idea what it means. They will have a problem make 3 or 4 changes at one time and they claim that they fixed the "ground loop".A ground loop happens when you have 2 different "grounds" and those 2 grounds have a different voltage. Thus when you connect 2 piece of equipment where one is referenced to one ground and the other ground you either have current flowing between the 2 pieces of equipement of the signals between the two is ofset by the difference in voltage.Now to get a voltage difference you need to have current flowing in one or the other grounds.For the equipment ground conductors it is designed that normally there is never any current flowing in the grounds. That is why the ground is allowed to be bonded to the neutral at only one place in the building. If they are bonded at a sub-panel part of the neutral current will also flow through the EGC back to the main panel. There are also other potential problems. The only time that you should have current in ground is when there is somekind of fault. Then the ground is doing it job of safetly directly the current back to the panel and if the fault is large enough then the breaker will trip.Now the connection from the disconnect to the source (POCO or main panel in another structure) does have the neutral in parallel with the ground electrodes/eart conductor. However the ground is a relatively poor conductor and the neutral resistance will be many order of magnatude smaller than the earth resistance so so 99.9999% of the curent will flow in the neutral and not through the ground electrode.Again you only have a problem with there is some defect in the system.A month or 2 ago some one here has a problem. IIRC he was measuring voltage between his electrical ground and water pipes.Turns out that the power company had a defect on their high voltage lines where the neutral connection to the transform was broken and the neutral current was being returned via the ground electrodes..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.