Grounding Electrode Conductor

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bobby ocampo

bobby ocampo

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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?:?
 
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.
 
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?
 
bobby ocampo said:
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?
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they had to decide what the smallest neutral wire could be so someone picked 12.5% at random?
 
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
 
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.
 
infinity said:
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.
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Does it mean that 3/0 is not the maximum Grounding Electrode Conductor?
 
bobby ocampo said:
Does it mean that 3/0 is not the maximum Grounding Electrode Conductor?
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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.
 
infinity said:
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.
Click to expand...


Does it mean that 3/0 is not the maximum Grounding Electrode Conductor?
 
infinity said:
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.
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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?
 
bobby ocampo said:
Sir can you give examples on how to determine the Grounding Electrode Conductor for parallel application?
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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.
 
bobby ocampo said:
Is there a difference between a Grounding Electrode Conductor and System Bonding Jumper?
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Yes, see Article 100. Note, you will find System Bonding Jumper under Bonding Jumper, System.

Table 250.66 did not say that 3/0 is the maximum.
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Actually it does...the last line says over 1100 kcmil copper uses a 3/0 copper GEC.

Does table 250.66 applies only for single conductor per phase?
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No. Read the text above the two left hand columns.
 
don_resqcapt19 said:
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.
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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.
 
bobby ocampo said:
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?
Click to expand...

You need to slowly peruse the information in the Table 250.66, including the notes. I've copied some of that information here:

Table 250.66 Grounding Electrode Conductor for
Alternating-Current Systems

Size of Largest Ungrounded
Service-Entrance
Conductor or Equivalent
Area for Parallel
Conductorsa (AWG/kcmil)

Notes:
1. Where multiple sets of service-entrance conductors are used as
permitted in 230.40, Exception No. 2, the equivalent size of the larg-
est service-entrance conductor shall be determined by the largest sum
of the areas of the corresponding conductors of each set.
2. Where there are no service-entrance conductors, the grounding
electrode conductor size shall be determined by the equivalent size of
the largest service-entrance conductor required for the load to be
served.
a- This table also applies to the derived conductors of separately de-
rived ac systems.
b- See installation restrictions in 250.64(A).
Click to expand...
 
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