CLICK HERE AND JOIN OUR COMMUNITY TODAY, IT'S FREE!
Go Back   Electrician Talk - Professional Electrical Contractors Forum > Electrical Trade Topics > NEC Code Forum

Reply
 
Thread Tools Search this Thread Display Modes
Old 04-15-2009, 10:32 PM   #1
Senior Member
 
william1978's Avatar
 
Join Date: Sep 2008
Location: Charlotte, N.C.
Posts: 11,079
Rewards Points: 2,000
Default What size equipment ground for this feeder?

There is a 1000a breaker feeding an emergency panel the EE has drew 7- 750 mcm per phase and 4/0 ground. The run is about 1100' long 480v. What I'm coming up with is Equipment ground needs to be atleast a 700 mcm. Are ya'll coming up with the same thing?

william1978 is offline   Reply With Quote
Join Contractor Talk

Join the #1 Electrician Forum Today - It's Totally Free!

ElectricianTalk.com - Are you a Professional Electrical Contractor? If so we invite you to join our community and see what it has to offer. Our site is specifically designed for you and it's the leading place for electricians to meet online. No homeowners asking DIY questions. Just fellow tradesmen who enjoy talking about their business, their trade, and anything else that comes up. No matter what your specialty is you'll find that ElectricianTalk.com is a great community to join. Best of all it's totally free!

Join ElectricianTalk.com - Click Here JOIN FOR FREE


Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ElectricianTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

Old 04-15-2009, 10:42 PM   #2
child please.....
 
NolaTigaBait's Avatar
 
Join Date: Oct 2008
Location: Hack City, USA aka New Orleans
Posts: 5,287
Default

i looked at 250.122 and it says 2/0 copper or 4/0 aluminum...

NolaTigaBait is offline   Reply With Quote
Old 04-15-2009, 10:44 PM   #3
child please.....
 
NolaTigaBait's Avatar
 
Join Date: Oct 2008
Location: Hack City, USA aka New Orleans
Posts: 5,287
Default

250.122(f) im not sure i understand, does it mean you need 3x 2/0?
NolaTigaBait is offline   Reply With Quote
Old 04-15-2009, 10:45 PM   #4
Senior Member
 
william1978's Avatar
 
Join Date: Sep 2008
Location: Charlotte, N.C.
Posts: 11,079
Rewards Points: 2,000
Default

Take a look at 250.122 (B)
william1978 is offline   Reply With Quote
Old 04-15-2009, 10:49 PM   #5
child please.....
 
NolaTigaBait's Avatar
 
Join Date: Oct 2008
Location: Hack City, USA aka New Orleans
Posts: 5,287
Default

Quote:
Originally Posted by william1978 View Post
Take a look at 250.122 (B)
you're not increasing the size, just adding more conductors...honestly , i'm not sure of the correct answer
NolaTigaBait is offline   Reply With Quote
Old 04-16-2009, 12:28 AM   #6
Senior Member
 
redbeard43's Avatar
 
Join Date: Mar 2009
Location: Piney Woods of East Texas
Posts: 247
Rewards Points: 150
Default

whats wrong with the 4/0? Why are there 7 -750s?

3 phase?
Pipe 1 ......................Pipe 2
3 hots..................... 3 hots
1 neutral..................1 neutral
1 ground................. 1 ground

1 phase?
2 hots
1 neutral
1 ground

Somethings not adding up
__________________
You have the ring, and I see that your schwarz is as big as mine.May the schwartz be with you!
redbeard43 is offline   Reply With Quote
Old 04-16-2009, 12:56 AM   #7
Registered Member
 
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 4,868
Rewards Points: 2,000
Default

We can assume that the engineer has up sized the conductors for voltage drop, but damn! Seven 750s? Whew! Somebody has got some cheese!

But take it and run with it. If we keep everything proportional, figure what size wire you would need to run seven conductors to achieve 1000 A with. It should turn out to be less than the code minimum of 1/0 for parallel conductors. But size your ground to that, then scale up by the ratio of 750 to that size.
InPhase277 is offline   Reply With Quote
Old 04-16-2009, 05:02 AM   #8
Registered Member
 
Join Date: Feb 2008
Location: orlando florida
Posts: 948
Rewards Points: 500
Default

Well i see alu ? 7 runs of 750 mcm is over kill !!! at 1100 feet there is not that much of a voltage drop at 480 volts 3 phase ? the load you have is not going to be 1000amps whats the load really ?

The ground conductor is per [250.120] or[ 250.4a 5][ or 250.4b 4] take voltage drop into it you increase for that but in your case there is not that much VD on a three phase 480 v at that distance your within the 3% factor on branch circuit .
You would run just one ground in each pipe per breaker size in ampacity.

[310 .16] =75DEG =385 amps x7 = 2695 amps In my book 3 runs of 750 mcm would be enough with a 4/0 ground . But there maybe something i dont know or missing ? 3 x 385 amps = 1155 thats 750 mcm alu ?http://i611.photobucket.com/albums/t...1010049_01.jpg
Look at gear its 7 runs 400 mcm copper at 2000 amps .
In my case we dont need a ground its a service feed 3 hots one neutral each pipe .




Take care ask or RFI the engineer ask if he made a error on his oneline .

Last edited by nick; 04-16-2009 at 06:19 AM.
nick is offline   Reply With Quote
Old 04-16-2009, 09:11 AM   #9
Senior Member
 
Join Date: Jun 2008
Location: Alberta, Canada
Posts: 737
Rewards Points: 504
Default

That seems WAY overkill.
We have a 1600 amp (but 600v) parallel run that is just 2 750's per phase, 8 total and then a ground. Its ~300 meters.
Jeff000 is offline   Reply With Quote
Old 04-16-2009, 03:05 PM   #10
Senior Member
 
electricista's Avatar
 
Join Date: Jan 2009
Location: NC
Posts: 603
Rewards Points: 500
Default

Although I agree with Nick and assuming their is no mistake in requiring 7 - 750 mcm cables then I would say you need 7- 750MCM conduction for EGC in each conduit. Here is why.

As Nick stated, Using aluminum conductors then 7- 750 MCM cables i s equal to 2695 amps- outrageous but not important. What is important is what we would need to accomplish this with 7 parallel feeds. From T. 310.16 we see that 2/0 AL is equal to 155 amps so 7 x 155= 1085. This would satisfy the 1000 amp breaker.

Now T. 250.122 says that for a 1000 amp OCPD we need a 4/0 AL as the egc. Furthermore art. 250. 122 (F) requires this EGC in each conduit.

Art. 250.122 (B) says we need to increase the size of the EGC proportionately.

750 MCM= 750000 circulat mils
2/0 = 133100 circular mils

The difference in these conductors is approx. 5.6 times larger than what would have bben required without VD involved. 750000/133100

Now we take the required 4/0 AL EGC and increase it by 5.6
4/0 = 211600
211600 X 5.6 = 1184960 cir. mils
This is equivalent to 1250 mcm

Now we go back to 250.122(A) and it tells us in no case does the egc need to be larger than the circuit conductors supplying the equipment.

So we are back to 750 MCM AL as the EGC

William I am curious how you got 700 MCM. I double checked my math but I am home sick and may have missed something.
__________________
Alice

"I complained because I had no shoes until I saw a man who had no feet."
electricista is offline   Reply With Quote
Old 04-16-2009, 06:19 PM   #11
Registered Member
 
Join Date: Feb 2008
Location: orlando florida
Posts: 948
Rewards Points: 500
Default

Well the ground does not total up to the paralleled group it is the ground in each conduit alone that must be able to control the fault .
We did some calculating and came up with 4 runs of 750 mcm alu. and you can use the 4/0 ground at 1000amps and at 1100 feet .
cm=1.73 x21.2 x1100x1000/ vd 14.4 == 40343600/14.4 = [2801638.8 ]now divide this by how many paralleled runs you need . 4 runs =750 mcm is fine . this is all you need for that load . How do you come up with 750 mcm explain from what to what is this based on ? take care best to yas

Last edited by nick; 04-16-2009 at 06:49 PM.
nick is offline   Reply With Quote
Old 04-16-2009, 06:30 PM   #12
Senior Member
 
electricista's Avatar
 
Join Date: Jan 2009
Location: NC
Posts: 603
Rewards Points: 500
Default

Quote:
Originally Posted by nick View Post
Well the ground does not total up to the paralleled group it is the ground in each conduit alone that must be able to control the fault .
We did some calculating and came up with 4 runs of 750 mcm alu. and you can use the 4/0 ground at 1000amps and at 1100 feet .
cm=1.73 x21.2 x1100x1000/ vd 14.4 == 40343600/14.4 = [2801638.8 ]now divide this by how many paralleled runs you need . 4 runs =750 mcm is fine . this is all you need for that load . How do you come up with 750 mcm ? We are takling grounding conductor not grounded conductor correct ? take care best to yas
Nick you are changing the question. Given the upsize to 7 pairs of 750MCM from what could be 7 pairs of 2/0 AL you must upsize the EGC proportionately.

Quote:
How do you come up with 750 mcm?
If that question is for me I believe I showed where that came from. The EGC must be upsized in each conduit based on the upsize of the ungrounded conductors.
__________________
Alice

"I complained because I had no shoes until I saw a man who had no feet."
electricista is offline   Reply With Quote
Old 04-16-2009, 06:57 PM   #13
Registered Member
 
Join Date: Feb 2008
Location: orlando florida
Posts: 948
Rewards Points: 500
Default

Well were not changing the question were stating the fact that the 7 runs is not needed at that length so in this case its not correct to start with .

Now you are using 250.122 b increased in size ? from what size ? And how do you get 750 mcm were not giving you a hard time we just might learn something can you show me .take care
nick is offline   Reply With Quote
Old 04-16-2009, 07:17 PM   #14
Senior Member
 
electricista's Avatar
 
Join Date: Jan 2009
Location: NC
Posts: 603
Rewards Points: 500
Default

Quote:
Originally Posted by nick View Post
Well were not changing the question were stating the fact that the 7 runs is not needed at that length so in this case its not correct to start with .

Now you are using 250.122 b increased in size ? from what size ? And how do you get 750 mcm were not giving you a hard time we just might learn something can you show me .take care
We all seem to think the 7 runs are not needed but the specs by the EE call for the 750 MCM's. Whether we agree or not is not pertinent. We were asked to answer the question as stated. I cannot guess what the EE is thinking I can only try and respond to the given info.

That being said you determined to change the number of runs to 4 parallel runs but even that does not enter into the equation.

Let's suppose that the problem states "what size egc would be needed for a run of 750mcm AL conductors for a 125 amp panel.

If we use T. 310.16 we see that a 2/0 AL is all that is needed for this panel. T. 250.122 states that we need a #4 Alum for the 125 amp panel.

The 2/0 has a cir mil of 133100 and the 750 has a cir mil of 750000. So we have upsized our conductors by a factor of 5.6. Now we look at #4 with a cir mil of 41740. Since we have upsized our conductors by 5.6 we must also upsize our EGC by the same proportion. So 41740 X 5.6= 233744 which is equivalent to a 250MCM conductor. We will need to use a 250 MCM conductor for our EGC. art. 250.122(B)

Does this make sense? Do you agree?

Unfortunately the code is not very explicit about where we start from in this upsizing.
__________________
Alice

"I complained because I had no shoes until I saw a man who had no feet."
electricista is offline   Reply With Quote
Old 04-16-2009, 07:36 PM   #15
Registered Member
 
Join Date: Feb 2008
Location: orlando florida
Posts: 948
Rewards Points: 500
Default

Well yes it does make sense and i know the article i just dont have the same thoughts on how the code can say which feeder is picked .

Meaning the one that needs correcting as any feeder could be used for any load with out voltage drop so to me its a pick a choose process which is the one we pick at that moment . Meaning if i picked a 1000 cm wire on most runs we would not have a vd on most runs or loads .

What were looking for is the reason formula that the length in distance to the voltage dropped is the average between the two conductors of different sizes so at a fault in the line at that lower resistance we better change the ground resistance to match the conductors ampacity ? Is this it

yes we understand and how to use it. I see more of a one conduit single feeder type VD ?
But i dont agree with it . take care best to ya good points your ok nice job !

Last edited by nick; 04-16-2009 at 07:50 PM.
nick is offline   Reply With Quote
Old 04-16-2009, 08:27 PM   #16
Senior Member
 
electricista's Avatar
 
Join Date: Jan 2009
Location: NC
Posts: 603
Rewards Points: 500
Default

well this has been a good question. I just looked at the problem from a different stand point and got another answer.

T. 310.16 will allow 3 parallel runs of 750 MCM to feed a 1000 amp load. Thus if we increase the parallel runs to 7- 750 MCM's we increase the size by 2.3.

Now if 4/0 AL is given in T. 250.122 to be used for 1000 amp OCPD then we must increase the 4/0 by 2.3.

4/0 has 211600 cir mils. Multiply this by 2.3 we get 486680 or a 500 MCM conductor as the EGC.

Well I am convinced this section needs clarification and new wording. Where do we start from?? UGH
__________________
Alice

"I complained because I had no shoes until I saw a man who had no feet."
electricista is offline   Reply With Quote
Old 04-16-2009, 08:52 PM   #17
Registered Member
 
Join Date: Feb 2008
Location: orlando florida
Posts: 948
Rewards Points: 500
Default

Well 2/0 alu is what ampacity ? Its really 135 amps not 155 amps . Its funny but most conduits cut or damaged its a fault of all conductors not just one to ground . The load now if shorted or faulted is not just one ground conductor in a parallel group its the phase to phase fault .
Ground faults to me are minimal damage in a electrical short .

But i do see the formula its protecting in the most damaging way possible which is good geeeez what did they do years ago before when no one ran a ground in conduits ? guess they up size the rigid pipe ?

just joken think about it what makes that breaker trip fast . We all think about new code issues or how to look at each others input and we all learn from talking and others input each day .

take care Electricista

Last edited by nick; 04-16-2009 at 09:00 PM.
nick is offline   Reply With Quote
Old 04-16-2009, 09:27 PM   #18
Senior Member
 
william1978's Avatar
 
Join Date: Sep 2008
Location: Charlotte, N.C.
Posts: 11,079
Rewards Points: 2,000
Default

This is how I came up with 700 mcm. They could have used 600 paralleled 3 times which equals 1020 amps, but they used 7- 750 mcm which equals 2695amps which was increased by 2.91666%. A 4/0 ground which is good for a 1000a breaker has 211,600 circular mils X 2.91666% equales 617,165 circular mils which would be rounded up to 700mcm.

Last edited by william1978; 04-16-2009 at 10:09 PM.
william1978 is offline   Reply With Quote
Old 04-16-2009, 09:28 PM   #19
Senior Member
 
william1978's Avatar
 
Join Date: Sep 2008
Location: Charlotte, N.C.
Posts: 11,079
Rewards Points: 2,000
Default

I agree that 7--750's is over kill.
william1978 is offline   Reply With Quote
Old 04-16-2009, 10:11 PM   #20
Senior Member
 
william1978's Avatar
 
Join Date: Sep 2008
Location: Charlotte, N.C.
Posts: 11,079
Rewards Points: 2,000
Default

I belive that I heard that the EE was fired over this screw up. There is about 50 conduits that have the wrong size grounds in them.

william1978 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Ground Fault Location Equipment for Ungrounded AC Larry Tools, Equipment and New Products 8 12-30-2011 09:51 PM
What size underground feeder? wiremeup General Electrical Discussion 6 06-15-2009 09:39 AM
What size ground william1978 NEC Code Forum 14 03-17-2009 03:58 PM
In ground pool gfic equipment EXCEL12 Site Feedback and Suggestions 1 03-15-2009 05:32 PM
Equipment Ground Absolutetruthz General Electrical Discussion 6 06-06-2008 07:23 AM


All times are GMT -4. The time now is 02:34 PM.


Copyright 2006-2014 Escalate Media LP. All Rights Reserved
Our Pro Sites Network
ContractorTalk.com | DrywallTalk.com | HVACSite.com | PaintTalk.com | PlumbingZone.com | RoofingTalk.com