ECN Forum Forums Code Related Discussion NEC & other Code issues Size of grounding conductor

Thanks for replying Dave.
I am parelling 3 sets of 350's 3-phase no nuetral so 3-350's per phase and 1/0 ground(maybe repully to another size) I have a Utility supplied Transformer, from there 3 condiuts with set of 350's in each one to a main breaker enclosure with 800 amp breaker, from there 3 more condiuts with a set of 350's each to a motor starter panel. I have a ground rod at all three locations. From the breaker to my motor starter panel I sized my equipment grounding conductor from table 250.122, 1/0 for 800 amps. My ground rods I believe can be #6 copper although I am also useing 1/0. If I am right so far my confusion is comeing from first what to call my grounds from the transformer to the breaker enclosure I believe they are the grounded conductor brought to service equipment. Second how to size this? If I read 250.66 it has a FPN under the first paragraph that leads me to 250.24B for grounded conductor brought to service equipment. If I read 250.24B2 since I am useing 3 different raceways I sized my grounded conductor off of 1 350mcm then to 250.66 which would only require #2 but cant go smaller than 1/0# becuase of 250.24B2.
Am I doing this wrong?

Thanks Ant

BiggladAnt,

Some of the times we tend to over-think things, as I did in the first post.
Duh!


OK, you will need to run a 3/0 in each conduit from the xfr. You will land these on a "ground lug block" fastened to the first enclosure if one is not already supplied. Of course, this is also where the GEC's and EGC's are landed.

The reason and logic of the 3/0 is that the minimum size of one of the conductors needs to be able to handle the full load to be served. In case there was a short circuit the 3/0 will effectively handle the fault current and the over current device will open.


This is the part you were over-thinking!
Once you hit your enclosure as stated above, you have satisfied the gounding requirements of services. From there on out to the equipment, you use 250.122.

IMO;

Now, I know that it does seem illogical at times but there are many factors involved when it comes to being sure a fault current will be handled safely.

The code has seemingly different standards for simular conductors. The difference is where these conductors are used in the system. Typically the service entrance conductors are way more dangerous than the equipment conductors because the over current devices on the service side are much more fault tollerant than the ones on the equipment side. How often have you heard of the POCO being called out because they blew a fuse on the pole due to too much demand?
But, you and I are called out quit often to replace a blown fuse on a three phase machine because they over loaded it.

Best of my recolection, 3/0 comes to mind from practicle experience too.

Hope this helps!

Dave

Dave,

I don't agree. The grounded conductor in each raceway from the transformer to the service disconnect is only required to be a 1/0.
Don

Ok just to be safe I am going to re-pull the 1/0 from the transformer to the main breaker enclosure (this will be easy as its only 30'). I believe I am right by NEC but also I know this is the minimun, I think Ill also ask this same question at Mike Holts forum to see what other replies I get.

Thank you very much
Ant

I stand corrected!

I was mistaken about the 3/0.

It is 1/0.

Sorry, I was a tad tired last night and couldn't get past that parralell part.

Sorry!

Dave

I am parelling 3 sets of 350's 3-phase no nuetral so 3-350's per phase and 1/0 ground(maybe repully to another size) I have a Utility supplied Transformer, from there 3 condiuts with set of 350's in each one to a main breaker enclosure with 800 amp breaker.

These 1/0 are equipment grounds from the utility transformer to The Main Disconect.

I'll take it for granted that there is a current transformer in this main assembly for metering.

Your service ground has to be installed from the Main disconnect to the grounding electrode system (Usually the water supply).

I still think it should be 2/0 copper

Russ

[This message has been edited by russ m (edited 03-29-2003).]

"These 1/0 are equipment grounds from the utility transformer to The Main Disconect." --Russ

I disagree. These may seem like equipment grounds, or an equipment bond jumper, but they're not. They are the required "grounded conductor" of 250.24(5)(B). As a grounded conductor, they have to be sized as Don (rescapt19) said...1/0

ok you guys sorry for my delayed response but what biggladant saying about sawmill i have the excat the same set up and what i did use 1/0 awg copper wire for grounding system and pass the inspection with flying colours and my inspecter told me that the size is the min.that i can use and i told him that i have a chart with grounding wire size matched with service entrence system and he look at my chart it look very good and the metal building main i beam is only 4 feet away from main box and what more the water line is only 5 feet so i can use 1/0 very nice and i went one step futher i add ground rods outside to make sure it is good and it look very good


merci marc
sorry for delay resonde with this one

Elzappr:

Your right.
I forgot we had a Wye transformer.
These are grounded conductors, thanks for setting me straight.

Russ