Sizing grounded electrode conductor

[Mejourneyman39]

First post for me so be gentle. I have been a licensed electrician for 20 yrs and have just decided to get my Masters, don't ask why:happysad: Anyway I am taking a correspondence course at home for the 45 hour requirement and am having brain cramps about figuring out grounded conductor, main bonding jumper, grounding electrode conductor sizing. Below is an example of a question in the book.

Determine the minimum size Cu grounded conductor required for a 120/208 V 3 phase 4-wire service which consists of two parallel 1/0 AWG THW Cu conductors per phase.

Now I have looked over 250.66 and 250.24 c (1) and chapter 9 table 8

A couple of things confuse me one being the question stating (2) parallel 1/0 conductors, and the other being in chapter 9 table 8 where the wire size is in mils.

I know if I had if I had 3 phase 4-wire 480/277 service with three 500 Kcmil Cu THW conductors per phase

per 250.66 500x3= 1500 Kcmil
per 250.24 1500Kcmilx12.5% = 187500 cm
Ch 9 table 8 187500cm=4/0 AWG THW Cu

 

[infinity]

In your first example just take the kcmil area of the 2-#1/0 and add them together, then look in 250.66 for the GEC size.

In your second example you came up with #4/0, a GEC is not required to be larger than #3/0 {250.66} in some cases not larger than #6 (ground rods) or #4 (CEE's).

Welcome to the forum.

 

[Mejourneyman39]

My bad.

The title should not say Grounded electrode conductor it should say grounded conductor (neutral)

And why in the first example would I be using the Kcmil area of 2 1/0 conductors and not 4? There are two parallel conductors 1/0 which would be a total of (4) 1/0 conductors per phase in a 3 phase system? Or am I reading that wrong? This is what drives me NUTS!

 

[Dennis Alwon]

I think you are reading it wrong. There is a parallel for each phase. So if each phase is parallel then you do not look at it as 4 wires. You have 2 wires per phase so it is based on that.

Here is how I see it. For conductor 1100kcm and smaller you can size the neutral as low as the GEC if the load calculation for the neutral allows that. 12.5% is for conductors over 1100KCM

 

[Little Bill]

My bad.

The title should not say Grounded electrode conductor it should say grounded conductor (neutral)

And why in the first example would I be using the Kcmil area of 2 1/0 conductors and not 4? There are two parallel conductors 1/0 which would be a total of (4) 1/0 conductors pelr phase in a 3 phase system? Or am I reading that wreong? This is what drives me NUlTS![/QUOTlE]

Look in 310.4 for conductors in parallel. I would say since you don't know the unbalanced load here you would size the ungrounded conductors the same as the grounded conductors. So you would then need one 1/0 grounded conductor in each raceway.
Welcome to the forum!

 

[Mejourneyman39]

Ok so I guess I am reading the into it more than it is. So let's play devils advocate and say there are (4) 1/0 conductors per phase? I can't seem to come up with anything that makes sense.

 

[Dennis Alwon]

If you have 4 1/0 in parallel -- that means 4 per phase-- then go to chapter 9 Table 8 and look for 1/0. You will see that 1/0 is equivalent to 105600 cir. mil.

105600 X 4 = 422400. The closest wire size would be 500KCM or 500,000.

Go to T. 250.66 and you get 1/0 for the GEC or the smallest neutral possible, assuming copper.

Of course if this is parallel runs in different conduit then you need a 1/0 in each conduit.