# Grounding el. Conductor size

#### Bdgelectric

##### Member
I’m trying to verify the grounding conductor size I need to jump the water meter. The primaries are URD #2 AL 15 kv. Secondary’s are 3 sets 500kcmil run in parallel. 3 phase service. If I’m reading 250.66 correctly I am to use 3/0 to go from ground ring in vault to basement water meter. My question is, am I reading this correctly?

#### infinity

##### Moderator
Staff member
What size condcutors are after the service point?

#### Bdgelectric

##### Member
Coming out of CT into trough. Couldn’t get an MDP so I have 6 480v disconnects. 3 @ 200a and 3 @ 150a.

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#### augie47

##### Moderator
Staff member
That doesn't exactly answer Bob's question on the location of the service point and, since you have a transformer vault the service point might be on the 15kv side but assuming it's the secondary side of the transformer.
IF you have (3) sets of 500 that would be equivalent to 1500kcmil and IF the water line qualifies as a grounding electrode the conductor would need to be sized at 12-1/2% which would require a 4/0.
If you are simply bonding a metallic water line that is not a grounding electrode then it would be 3/0.

All this is ASSUMING the service point is the secondary side of the transformer and the conductors you identified are Cu.

#### augie47

##### Moderator
Staff member
It was pointed out I made a mistake in Post #4. The grounding electrode conductor does not need to be larger than 3/0 regardless of service size.
In the 2017 Code bonding jumpers would require the 12-1/2% when the service conductors exceeded 1100 kcmil
(I understand that changed in later Codes)

#### don_resqcapt19

##### Moderator
Staff member
It was pointed out I made a mistake in Post #4. The grounding electrode conductor does not need to be larger than 3/0 regardless of service size.
In the 2017 Code bonding jumpers would require the 12-1/2% when the service conductors exceeded 1100 kcmil
(I understand that changed in later Codes)
That was a one time thing that happened in the 2017 code. In that edition, they made Table 250.66 only apply to grounding electrode conductors, and created a new Table 250.102(C)(1) for all of the other things that we previously used 250.66 for. However, in the 2017 they did not have the 3/0 maximum size for the water pipe and structural steel bonding conductors, and the 12.5% rule applied. This was corrected in the 2020 code.