gec for parallel feeders

[toddb1963]

according to table 250.66 note 1 " where multiple sets of service-entrance conductors are used as permitted in section 230.40 exception 2, the equivelant size of the largest service entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set. my question is, if i have say 3 sets of 500 kcmil feeding a service, if i read this correctly, i take the largest conductor from each set and add them together, thus getting 1500 kcmil. now seeing that it is over the 1100 kcmil, i take the 1500 kcmil and multiply by .125 (12 1/2%) i get 187,500. the closest conductor to that size is 4/0. am i calculating this correctly or am i to add the kcmil of all the conductors in ONE raceway (2,000,000 cmil X .125= 250,000cmil)?

 

[roger]

Re: gec for parallel feeders

Todd, you are right in the first part of your question. You only use the combined total of one leg or phase conductor. In other words take the largest ungrounded conductor in all conduits and use this to size the GEC.

This is correct

i take the largest conductor from each set and add them together, thus getting 1500 kcmil. now seeing that it is over the 1100 kcmil, i take the 1500 kcmil and multiply by .125 (12 1/2%) i get 187,500.

Throw out this idea

or am i to add the kcmil of all the conductors in ONE raceway (2,000,000 cmil X .125= 250,000cmil)?

Roger

[ March 14, 2003, 10:24 PM: Message edited by: roger ]

 

[david]

Re: gec for parallel feeders

'now seeing that it is over the 1100 kcmil, i take the 1500 kcmil and multiply by .125 (12 1/2%) i get 187,500. the closest conductor to that size is 4/0. am i calculating this correctly"

I do not want to discourage you from using a 4/0, But if you are sizing the grounding electrode conductor the table says over 1100 you can use a 3/0 copper in other words the largest grounding electrode conductor you have to use is 3/0 copper to be code compliant.

[ March 15, 2003, 10:12 AM: Message edited by: david ]

 

[iwire]

Re: gec for parallel feeders

I would use Table 250.66

It ever requires more than 3/0 copper or 250 aluminum no matter how large the service and do not forget

250.66(A)does not have to be larger then #6 for ground rods, pipe or plate electrodes

250.66(B)does not have to be larger then #4 for concrete encased electrodes.

250.66(C)does not have to be larger then the ground ring conductor.

[ March 15, 2003, 11:06 AM: Message edited by: iwire ]

 

[roger]

Re: gec for parallel feeders

Todd, I was looking to far into the question. By your wording I thought you were sizing the Main Bonding Jumper, 250.28(D) and if this is the case, 4/0 would be correct.

Roger

 

[dereckbc]

Re: gec for parallel feeders

GEC is not required to be larger than a 3/0 AWG. The 12.5% rule is for the MBJ as Roger mentioned.

 

[toddb1963]

Re: gec for parallel feeders

i realized the way i worded the question sounded like i was asking about the mbj and i apologize for that. it was described to me this way by an instructor and i was sure that the way he was describing it was the wrong way to calculate the gec. i knew that 3/0 is the largest ground that i can pull, but when the instructor threw that 12 1/2 % in there and then said to add all the conductors in one conduit i was like "huh?" made me second guess myself and wonder if for 23 yrs i have been doing it wrong. talk about a sphincter factor of ten. thanks.

 

[iwire]

Re: gec for parallel feeders

Originally posted by toddb1963:
talk about a sphincter factor of ten. thanks.

I was getting the same feeling when I could not place the 12.5% with Grounding Electrode Conductors and I did not remember it from MBJ calculations