I am Installing an 800 amp service 480v 3-phase from a 480/277 wye Utility transformer The service is for 3 phase motors only and I am paralleling 3 350kcmill conductors. I believe I am correct on pulling a 1/0 grounding conductor although after looking at an identical job 3/0 was used. Am I OK? Also am I right that from the transformer to the 1st disconnect this would be considered a neutral and be taped white and at the 1st disconnect where I am bonding with the grounding electrode conductor afterwards it would be a green ground?

250.66 2/0 copper or 4/0 alum.

Yes from the Main Disconnect on, the neutral is not required if the circuits are not useing it. A equipment ground is required 250.118 and 250.122.

Russ

By looking at T250.66 the previously given interpretation is incorrect. The left side of the table states the size of largest ungrounded service-entrance conductor or equivalent area for parallel conductors. Since you are using 3-350kcmil the equivalent area will be over 1100 so you would want to use a 3/0 copper.

I am just wondering myslef, If you go to table 310.16 using col. two, 350 kcmil is good for 310 amps
310 time 3 = 930 amps.

Also 350 kcmil times 3 = 1050

if we then go to Table 250.66 and rea Over 600 through 1100 it says to us 2/0 copper.

I would say that's how I would figure it out.

Table 250.66 states equivalent area for parallel conductors. Just because it says 350kcmil does not mean that the equivalent area of that conductor is 350. If we reference Ch. 9, Table 5 it goes in depth on what the specific areas are for various insulated conductors. So depending on what insulation is being used, that will determine what the equivalent area is for the parallel conductors. I went through the calculations of a few of the insulations and made an educated decision that under most circumstances, if not all, the equivalent area of 3 - 350 would be greater than 1 - 1100 of the insulation being used. I hope that answers where I am coming from.

Thanks,
Brian

The insulation thickness has nothing to do with the kcmil of the copper.Use chapter 9 table 8.

Table 250.66 note 1 applies.
350 x 3 =1050 would require 2/0 copper

[This message has been edited by txsparky (edited 03-28-2003).]

I am still confused. Also I could of gotten away with 300mcm but I had 350 to use up. Im not sure if I mentioned that I am parrelling 3-350's in 3 different conduits. For the grounded conductor (from the transformer to the main disconnect). I used 1/0 becuase of 250.24B2. For the equipment grounding conductor (from the main breaker to my Motor Stater Panel) I used 1/0 becuase I have an 800 amp breaker going off of table 250.122 Please tell me if I am wrong and thank you for the prevois replys.

The EGC from the disconnect to the motor starter panel is based on 250.122.

The GEC for the service is based on 250.66



[This message has been edited by txsparky (edited 03-28-2003).]

My grounding electrode conductor from my ground rod to my service equipment is sized from 250.66. 250.66A not required to be larger than 6 copper although I made it 1/0 also. My equipment grounding conductor from my main breaker to my starter enclosure is 1/0 according to TBL250.122 So from My Main Breaker to the Utility Trasformer Are my grounds considered a grounded conductor brought to service equipment? (under 250.66 FPN says see 250.24B) If so wouldnt it be sized based on the size of the ungrounded service-entrance conductor in each raceway according to 250.24B2 for parallel conductors? Or am I confused and my GEC has to be ran clear back to the transformer also?

BiggladAnt,

If I understand your system correctly;

You pull 3-350 phase conductors and a 350 grounded conductor in each conduit(based on a wye xfr., the grounded conductor (neutral) is derived from the xo post on the xfr).

From your first means of disconnect, you would bond the grounded conductor (neutral) to the enclosure. You then need to install a grounding electrode and grounding electrode conductor. The GEC that runs to the grounding electrode needs to be sized at 3/0.
However, that does not mean the ground rod. The ground rod only needs a 6ga. conductor. The other grounding electrode needs the 3/0 run to it. The other electrode would be the water pipe within 5 feet of the entrance of the building or the structural steel. Much confusion is due to many of us thinking of the ground rod as THE grounding electrode.

Hope this helps!

Dave

[This message has been edited by WebSparky (edited 03-28-2003).]

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