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?
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.
Re: Size of grounding conductor#84405 03/28/0303:31 PM03/28/0303:31 PM
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.
Re: Size of grounding conductor#84407 03/28/0304:49 PM03/28/0304:49 PM
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.
Re: Size of grounding conductor#84409 03/28/0308:28 PM03/28/0308:28 PM
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?
Re: Size of grounding conductor#84411 03/28/0311:05 PM03/28/0311:05 PM
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!
[This message has been edited by WebSparky (edited 03-28-2003).]