bobby ocampo
Senior Member
How do we size Grounding Electrode Conductor if the Service Conductor is 2 sets of 1100 MCM? Is 3/0 the maximum Grounding Electrode Conductor in accordance with Table 250.66?:?
Grounding Electrode Conductor
- Thread starter bobby ocampo
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kwired
Electron manager
- Location
- NE Nebraska
The last row in the table just says "over 1100" or "over 1750" for aluminum then says to use 3/0 copper or 250 aluminum GEC.
I read that as meaning 3/0 copper or 250 aluminum is largest GEC you would ever need to run.
I read that as meaning 3/0 copper or 250 aluminum is largest GEC you would ever need to run.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I agree 3/0 is the max grounding electrode conductor one is required to use.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
unless the GE is a CEE, rod, or plate.
kwired
Electron manager
- Location
- NE Nebraska
You could still run 3/0 to those, you just do not have to. Finding termination fittings could be a little challenging.
bobby ocampo
Senior Member
How about the requirement of 250.30 (A)(8)(a) and 250.30(A)(8)(b)
(a) Routing and Sizing. This conductor shall be routed with the derived phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250.66 but shall not be required to be larger than the largest ungrounded derived phase conductor. In addition for phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 121⁄2 percent of the area of the largest derived phase conductor. The grounded conductor of a 3-phase, 3-wire delta system shall have an ampacity not less than that of the ungrounded conductors.
What is the 12.5% for?
(a) Routing and Sizing. This conductor shall be routed with the derived phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250.66 but shall not be required to be larger than the largest ungrounded derived phase conductor. In addition for phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 121⁄2 percent of the area of the largest derived phase conductor. The grounded conductor of a 3-phase, 3-wire delta system shall have an ampacity not less than that of the ungrounded conductors.
What is the 12.5% for?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
they had to decide what the smallest neutral wire could be so someone picked 12.5% at random?
- Location
- Bremerton, Washington
No its based on wire and insulation, how much temp rise and the fault clearing ability. Its a trade off between small sizes, cost and fault ampacity
- Location
- New Jersey
- Occupation
- Journeyman Electrician
The 12.5% is also used for sizing the system bonding jumper in an SDS when the derived condcutors are over 1100 kcmil. Same would apply to a main bonding jumper in a service.
bobby ocampo
Senior Member
- Location
- New Jersey
- Occupation
- Journeyman Electrician
No, #3/0 is the maximum GEC required but bonding jumpers for conductors over 1100 kcmil need to be sized using the 12.5% formula.
So you could have a transformer with 5 sets of 500 kcmil derived (secondary) conductors which equals 2500 kcmil (5*500=2500) and your GEC would be a #3/0, the largest size in T250.66. But the system bonding jumper would need to be sized at 12.5% of the 2500 kcmil. (2500*12.5%=312.5 kcmil) That makes your SBJ a minimum of a 350 kcmil.
bobby ocampo
Senior Member
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Bobby, a system bonding jumper is not a GEC.
As other have stated, a GEC needs never to be larger than 3/0 CU or 250 AL.
bobby ocampo
Senior Member
Sir can you give examples on how to determine the Grounding Electrode Conductor for parallel application?
bobby ocampo
Senior Member
Is there a difference between a Grounding Electrode Conductor and System Bonding Jumper?
Table 250.66 did not say that 3/0 is the maximum.
Does table 250.66 applies only for single conductor per phase?
kwired
Electron manager
- Location
- NE Nebraska
You can't parallel the GEC. But I don't necessarily think that is what you are asking.
If you are asking how to determine GEC size when ungrounded conductors are parallel, then you add total circular mils of the conductor sets to come up with an equivalent single conductor size to use in T250.66.
Two parallel 250's would be the equivalent of a single 500, three parallel 250's would be equal to 750.
- Location
- Illinois
- Occupation
- retired electrician
Yes, see Article 100. Note, you will find System Bonding Jumper under Bonding Jumper, System.
Actually it does...the last line says over 1100 kcmil copper uses a 3/0 copper GEC.
No. Read the text above the two left hand columns.
bobby ocampo
Senior Member
While the last line says above 1100 Kcmil is 3/0 it did not say if it is applicable to single conductor only or multiple conductor only. Otherwise there will be no provision for 12.5%?
Please clarify what the 12.5% is for?
Read some of the previous posts carefully ! Some of the guys have been trying to tell you.
You can also read 250.28, 250.28(D)(1),250.30(A)(2), 250.102(C), and as mentioned Art 100.
You asked about Grounding Electrode Conductors and it was confirmed 3/0 is maximum required.
Others have simply been cautioning you that there are other bonding connections such as System Bonding Jumpers that can require larger than 3/0.
You can also read 250.28, 250.28(D)(1),250.30(A)(2), 250.102(C), and as mentioned Art 100.
You asked about Grounding Electrode Conductors and it was confirmed 3/0 is maximum required.
Others have simply been cautioning you that there are other bonding connections such as System Bonding Jumpers that can require larger than 3/0.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You need to slowly peruse the information in the Table 250.66, including the notes. I've copied some of that information here:
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