Grounding/Neutral in building structure supplied by feeder

[zcanyonboltz]

I'm running a 1/0 copper feeder to a detached garage. 250.32 says I will need a grounding electrode at the other structure(garage). Where does the code require two grounding electrodes? I know we always run a ufer ground or water pipe ground in residential in addition to the ground rod but I don't see this requirement in 250.

 

[augie47]

250.50 requires all electrodes present be used, one -ten . etc,.
250.53(D)(2) requires a supplemental (second) grounding electrode
250.53(A) requires a supplemental electrode to a rod or pipe unless you cna show 25 ohms or less

 

[zcanyonboltz]

Thank you I'm going to read through those, I may have one question going to try to find the answer myself first. Just now really getting into NEC I'm testing in a couple months and doing the Mike Holts exam prep class and book review.

 

[electrofelon]

I'm running a 1/0 copper feeder to a detached garage. 250.32 says I will need a grounding electrode at the other structure(garage). Where does the code require two grounding electrodes? I know we always run a ufer ground or water pipe ground in residential in addition to the ground rod but I don't see this requirement in 250.

...Also unless you are on a pre- 2008 code, run a separate neutral and EGC to the structure, GEC to the EGC bus right? (I just mention because your title possibly implies something different)

 

[electricalist]

As well as the panel in the garage is a sub panel with grounding conductor (egc) and grounded conductor(neutral) to be isolated .

 

[zcanyonboltz]

...Also unless you are on a pre- 2008 code, run a separate neutral and EGC to the structure, GEC to the EGC bus right? (I just mention because your title possibly implies something different)

Yea that's the other part of the question title I didn't ask in the original post. I will use a separate bus for the grounding and grounded conductors in the garage panel. I just didn't read this in the code book is this in 250?
I gotta get used to the code talk, I'm sure I read the requirement but just didn't register the vocab. Using 2014 .

I' learned from my class and code questions so far that, from 250.66 I'll use a 6 AWG for the grounding electrode conductor, same for the main bonding jumper from 250.102 (C) (1), and then run a 6 awg in the PVC with the 1/0 from 250.122. Then I'll use the 80% multiplier from 310.15 (B) (3) since they'll be 4 conductors in the pipe and use the 75c amperage column since the PVC in ground is considered a wet location.

 

[electrofelon]

Yea that's the other part of the question title I didn't ask in the original post. I will use a separate bus for the grounding and grounded conductors in the garage panel. I just didn't read this in the code book is this in 250?
I gotta get used to the code talk, I'm sure I read the requirement but just didn't register the vocab. Using 2014 .

I' learned from my class and code questions so far that, from 250.66 I'll use a 6 AWG for the grounding electrode conductor, same for the main bonding jumper from 250.102 (C) (1), and then run a 6 awg in the PVC with the 1/0 from 250.122. Then I'll use the 80% multiplier from 310.15 (B) (3) since they'll be 4 conductors in the pipe and use the 75c amperage column since the PVC in ground is considered a wet location.

The code reference on the separate neutrals and grounds is 250.24(A)(5).

The size of the EGC in the feeder is determined by the size of the breaker supplying the feeder by table 250.122.

Also you only need to count "current carrying conductors" for derating. If this is a single phase 120/240 service than you only have 2 CCC.

Finally, A correction on the amperage column: Generally, UF would need to be sized at the 60 degree column and most everything else on the 75 degree. Nearly all conductors now have the -2 rating, i.e. THWN-2, XHHW-2 and this means they are rated 90 degrees wet or dry, so it is the termination provisions of 110.14 that limit you to 75 in most cases.

 

[zcanyonboltz]

Thanks a lot, I didn't think about the neutral not being a CCC but I do see that in 310.15 (B) (5) (a) since its a three wire feeder and not a two wire. Now I can put a bigger breaker on than I was planning.:happyyes: I still will count the grounding and grounded conductor for raceway fill. Running 2" PVC.

 

[petersonra]

The last sentence of 250.32 (B)(1) is important.

 

[zcanyonboltz]

The last sentence of 250.32 (B)(1) is important.

I read that article its very clear. Any idea as to why the "installed grounded conductor shall not be connected to the equipment grounding conductor or the grounding electrode." I am curious why this connection is no longer permitted?

 

[electrofelon]

I read that article its very clear. Any idea as to why the "installed grounded conductor shall not be connected to the equipment grounding conductor or the grounding electrode." I am curious why this connection is no longer permitted?

the concern is if there is another metallic path between the two buildings, then it will form a parallel path for neutral current. I think in general it was good to get rid of the "rebonding" provision, however I would still like an exception for certain situations where there is essentially zero chance of a parallel path.

 

[kwired]

I read that article its very clear. Any idea as to why the "installed grounded conductor shall not be connected to the equipment grounding conductor or the grounding electrode." I am curious why this connection is no longer permitted?

For same reasons we separate grounded and equipment grounding conductors for circuits in same building - NEC just allowed us to treat the supply to separate buildings kind of like service equipment for a long time - with no real good reason, that finally changed with 2008 NEC.

The reason is when you have current in a conductor you have voltage drop. Take a conductor that is connected to a grounded supply and apply enough current to it to cause 3 volts drop over the length of the conductor. At the source end the voltage from that conductor to ground is zero volts, but at the other end it is 3 volts. This doesn't bother users of equipment if it is only connected to current carrying grounded conductors, the white wire of the circuit that never connects to metal frames of equipment. But if you should happen to connect that white wire with 3 volts to ground to the EGC, metal frames, etc. you now have all this exposed metal that carries some shock potential for users if they touch any part of that EGC system and true earth at same time.

Three volts may not sound like enough to be a problem - and often it isn't, but put yourself in a situation where things are wet or damp and that 3 volts can be lethal, and a GFCI won't help as it monitors leakage from the associated circuit conductors, not stray voltages on the EGC.

 

[zcanyonboltz]

Thanks for that explanation! I do remember before the 2008 changes we would separate the grounded and grounding conductors in sub panels.

I did see in 250.32 (A) Exception: That a grounding electrode conductor is not required if the branch circuit including a multiwire branch circuit that supplies the building or structure includes an equipment grounding conductor.

Now the question is, does 1/0 copper on a 150 amp breaker count as a branch circuit or a feeder?

 

[zcanyonboltz]

I think I answered my own question. A feeder from article 100 is between the power supply and the final branch circuit. Since there will be other branch circuits fed by the 1/0 it is a feeder. I will be installing a ground rod.

I am not sure if I need the supplemental grounding electrode at the building or other structure? From 250.52 (D) I think the supplemental grounding electrode only applies to the main service? If this also applies to the building or structure I'll do two ground rods 6' apart.

 

[kwired]

I think I answered my own question. A feeder from article 100 is between the power supply and the final branch circuit. Since there will be other branch circuits fed by the 1/0 it is a feeder. I will be installing a ground rod.

I am not sure if I need the supplemental grounding electrode at the building or other structure? From 250.52 (D) I think the supplemental grounding electrode only applies to the main service? If this also applies to the building or structure I'll do two ground rods 6' apart.

When you have a separate building you need a complete GES for that building, doesn't matter what may be at the building or structure it is supplied from.

Supplemental electrode - is needed any time a water pipe is the sole GE, or if a single rod, pipe or plate is the sole GE. Exception to the rod, pipe or plate is if the first one has 25 ohms or less resistance, but the the way it is worded you must verify that it is low enough resistance.

 

[zcanyonboltz]

When you have a separate building you need a complete GES for that building, doesn't matter what may be at the building or structure it is supplied from.

Supplemental electrode - is needed any time a water pipe is the sole GE, or if a single rod, pipe or plate is the sole GE. Exception to the rod, pipe or plate is if the first one has 25 ohms or less resistance, but the the way it is worded you must verify that it is low enough resistance.

Thank you. Learning a lot, when you're used to being an apprentice doing what you're told to figuring stuff out on your own and reading code there is a bit of curve as well as learning code vocab from trade vocab. I'm studying code about 2 hours a night now.

 

[zcanyonboltz]

Floating neutral still bonded to ground?

Floating neutral still bonded to ground?

So I finished the job put in a ground rod and ran a #6 ground with the 1/0. When I hot checked everything I did a continuity test on the grounding and grounded conductor at the sub panel and I had continuity between the two even though they are floating. I got to thinking about it, there is continuity because the grounding conductor feeding the sub panel is tied in at the main panel with the grounded and grounding conductors on the same bar. So even though they're separate at the sub panel they are still bonded together, correct? So why even float them at the sub?

 

[electricalist]

So there is not a parallel path on the neutral. Neutrals take every path available back to the source dividing its self between all paths which could energize a grounding conductor if you lost the neutral feeding the sub panel.
When I check a dead circuit I usually check neutral to ground for continuity to see if they make it back to the panel.

 

[zcanyonboltz]

So there is not a parallel path on the neutral. Neutrals take every path available back to the source dividing its self between all paths which could energize a grounding conductor if you lost the neutral feeding the sub panel.
When I check a dead circuit I usually check neutral to ground for continuity to see if they make it back to the panel.

So there is not a parallel path on the neutral. Neutrals take every path available back to the source dividing its self between all paths which could energize a grounding conductor if you lost the neutral feeding the sub panel.
When I check a dead circuit I usually check neutral to ground for continuity to see if they make it back to the panel.

I see so if you lost the neutral feeding the sub any current going back on the branch circuits supplied by feeder would go straight to ground, rather than making its way back.
And if when you check neutral to ground for continuity and there is none that tells you the problem is an open neutral anything else you now know when there is not continuity?

 

[electricalist]

It tells me to check the ground to a neutral on a good circuit then if it rings then I assume I'm looking for disconnected circuit,,not hot or neutral. When I thinks it's an open neutral I check hot to ground and if its good voltage I look for an open neutral. An extension cord does wonders for find problems. I have a circuit tracker with 20' extensions leads I can put one in my tester and reference almost anything