The answer to your last question do you have to drive a ground rod to any accessory building detached from the main building would be a YES, BUT ! The rule you are speaking of is found in 250.32 yet the projects you mentioned are governed by other rules that over ride 250.32 read on to understand better. Remember as you read Chaptes 1 through 3 of the NEC ARE FOR NORMAL WIRING. Chapter 4 pertains only to motors, transformer, and generators, being a specialty wiring condition with different rules that conflict with and over ride rules found in Chapters 1 through 3. Chapter 5 is hazardous wiring classifications also being a specialty wiring condition with different rules that conflict with and over ride rules found in Chapters 1 through 3. Chapter 6 also is a spectial condition chapter for pools, hot tubs, mobile homes and other specialty wiring again being a specialty wiring condition with different rules that conflict with and over ride rules found in Chapters 1 through 3. Chapter
Read the following and I will try and sort out you delemma for you concerning the NEC on each subject you spoke of.
YOU SAID;
if you are going to put in a sub-panel in a free standing building, seperate from the one with the main service...you must drive a ground rod for that paticular sub pane?
REPLY;
Your wording "free standing building" is too gneric to answer yes to the answer would be yes IF Chapters 4 through 8 does not apply.
YOU SAID;
I was taught by one journeyman that you only have to bond it to the main service (ie...bare ground run with hot and neutral conductors) because by driving ground rod you would create a easier path to ground in the event the neutral was lost. Therefore losing the canceling effect and overheating of the sub panels feeders. And something else about single grounding.
REPLY;
250.32 for detached ACCESSORY buildings with a common service as long as Chapters 4 through 8 does not apply for specialty wiring conditions then 250.32 gives you three wiring options. If you only have one branch circuit no matter if multiwire 240 volt or 120 volt then you have an exception allowing that grounding electrode system in teh main house to serve that one branch circuit without a ground rod being installed at the second building. This branch circuit is required a nonfused disconnect or switch that will act as a main form of disconnect to shut down all hot wires entering that detached accessory building however NO FUSES ARE ALLOWED in that form of disconnect. If you install fuses in that disconnect in that accessory building then you just negated the branch circuit definition and created a feeder definition thus negating that exception to the rule for a branch circuit feeding a detached accessory building
If you install a fused panel or disconnect in that detached accessory building then you negate that exception to the rule mentioned above. A branch circuit is that wire that is found after or on the load side of the last overcurrent device [fuse or breaker]. If the building is served by a single 120 volt or multiwire 240 volt branch circuit rated not over 20 amps then you may omit the ground rod installation at that detached accessory building. If you install a fuse or breaker in that detached structure then that wire between the two buildings is now a feeder and you must install a ground rod at that detached accessory building whether that feeder is with or without an equipment grounidng conductor installed with that feeder.
Now I would have agreed with whomever taught you as they did when the 1999 NEC version was active. However tell your instructor to check 250.32.J of the 2002 NEC which now requires that equipment grounding conductor ran with the feeder to be a grounding electrode conductor 250.66 which is a larger conductor than the equpiment grounidng conductor 250.122. 250.32.J created a change in the rules that confirmed that you can not use the grounding electrode system in the main buiding to serve the accessory building because the green or bare wire in that cable is not sized to serve as a grounding electrode conductor. Then the word at each building sets the teeth in the fact that the grounidng electrode must be installed at that second building not served by the main building.
His reasoning is off a bit concerning the overheating etc. The idea of the ground rod at that second building is to attempt to direct lightening directly into the earth ASAP rather than have that bare or green wire carry it to the main building and introduce that lightening striking the accessory building from entering the main building. The hopes are that the rod at the accessory building will obsorb the blunt of a lighteining strike into the earth but if it can not contain all that blunt of power then the neutral and grounding conductor taking that power of the lightening to the main building hopefully will be obsorbed by the second grounidng system found at the main building and the neutral going back to the center tap of the transformer rather than that surge of power entering the main building also.
Remember that you are required to marry the neutral and grounding bar as one entity if your feeder to your panel with overcurrent devices in that accessory building is without a grounding conductor installed with that feeder. Yet if you have an equipment grounding conductor ran with the feeder then your neutral bar in that panel with fuses or breakers in that accessory building must be wired like a sub panel isolating the neutral bar from the grounding bar.
Just to get picky a panel in an accessory buildng is not considered a sub panel. It is a main form of disconnect wired as a sub panel isolating the neutral bar from teh grounding bar, but only if a grounding wire is ran with the feeder. If that accessory panel is without an equipment grouding conductor ran with the feeder then that panel is wired as a main service rated panel marrying all bars together as one entity.
YOU SAID;
Anyway I always thought this was the case for example one job we did was to install a disconnect for a well pump(trailer home). We came out of the service with a 3 wire w/ground (two hots,neutral and bare ground) and trenched it to the pump house set the disconnect and wired it without driving a ground rod.
REPLY;
If you had wired this pump house with a nonfused disconnect then you would have met the required form of disconnect for the pump motor as required in Chapter 4 for motors and at the same time
met the form of disconnect required at that accessory building. The fact that no fuses being in that disconnect form made that wire coming from the main house a BRANCH CIRCUIT meeting the deifintion of a branch circuit being that last wire after the last over current device [aka fuse or breaker]. Then the exception found in 250.32 would apply and no ground rod would have been required at that pump house.
If you had wired this pump house with a fused disconnect then your have created a feeder and you would have had to install the ground rod at that pump house [aka accessory building] whether the feeder was with or without the grounding conductor.
Again with a grounding conductor you would have had to separate the neutral bar from teh grounding bar whether it had been a nonfused branch circuit disconnect or a feeder fused disconnect.
If you did not run a grounding conductor with that feeder between the two buildings then the neutral bar and grounding bar would be married together as one entity.
YOU SAID;
Also same job the panel in trailer was treated as sub panel since the main was considered the disconnect mounted outside on a pole. And again we did not run a ground rod here either. Just seperated the neutrals (isolated) and the grounds were bonded with ground wire run with the hots and neutral. Since it passed inspection I assumed it was the proper way of doing this.
REPLY;
Now you are in a mobile home with a mobile home service disconnect remote from the mobile home. This is an entirely new set of rules special to mobile homes or manufactured homes found in Chapter 6 of the NEC. The rules in Chapter 6 of the NEC overrides 250.32. The rules in Chapter 6 now becomes the rules that you must apply.
In 680 of the NEC you have two types of buildings. If this mobile home is mounted on private lot with a cement block skirting creating a permenent installation then this mobile home becomes a manufactured home and the remote mobile home service disconnect is not requred but instead this mobile home permenently installed on a private lot became a manufactured home and may be wired same as any other dwelling controlled in 230 of the NEC controlling normal services requiring a grounding electrode system at that manufactured home same as any other service is done.
If this home is in a mobile home park or on a stacked set of cement blocks with vinyl skirting then you have a temporary installation remaining to meet the definition of a mobile home. Then as a mobile home the service must be mounted as a detached product remote from, not further than 35 feet yet must be in sight of that mobile home. Then the feeder between that mobile home service disconnect and the mobile home must be four installated wire with the equipment grounding condcutor requried to be insulated ran with that feeder and you were correct the ground rod at the remote mobile home service disconnect serves as the grounding electrode system for that mobile home. The mobile home main service breaker is found in the remote pedistal. No ground rod is installed at the mobile home and an insulated equipment grounding conductor must be a part of the feeder from that remote mobile home service disconnect and that mobile home. These rules all can be found in 680 of the NEC mostly found in 680.32. Article 680.32 overrules Article 250.32
YOU SAID;
Please set me straight, did I learn wrong? Must you always drive a ground rod for sub panels not located within the same structure?
And isn't a disconnect considered a sub panel?
REPLY;
A disconnect can be considered as a main service disconnect or as a nonfused disconnect form but not a sub panel or a main service rated panel or as a subpanel all depending on the use, location in the wiring system concerning the use, and whether with or without overcurrent devices [aka fuses or breakers].
So the answer to this last questions as I said in the beginning of this reply is a definite YES ? If !
Hope you enjoyed that pile of confusion, maybe it helped a bit.
Wg
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