grounding/bonding of CT cans and meter bases

[stevenal]

I am learning from our metermen that electricians are not always providing bonding when using PVC between CT cans and meter bases. Seems the inspectors are content to leave this in the utility (our) domain. Our metermen are then using the CT secondary neutrals to provide this bonding path. I am making my displeasure with this practice known since we run the risk of creating current loops if an unobserved bond is present, or if one is added later. I wish to make it a requirement that proper bonding be provided prior to our guys wiring the secondaries. The question is then, what size bonding wire to require? NEC speaks of sizing based on overcurrent protection when installed after the service, and based on service conductor size when installed before. But no service conductors run through the meter base, and the instrument transformers have no overcurrent protection. We use #12 for potentials and #10 for currents, so I'm proposing #8 for the bond on the theory that any wire supplying a fault will burn apart prior to the bond wire. Reasonable? Code compliant? Thanks.

 

[waross]

Hello stevenal
Both the Canadian code and the NEC Have exceptions that do not require the equipment grounding conductor to be larger than the circuit conductors supplying the equipment. I would interpret this to mean that #10 would be adequate but I like the extra safety margin that you propose with #8.
respectfully

 

[mc5w]

The CT cabinet should be bonded to the service switch using a conductor that is the larger of either the grounding electrode conductor or 1/8 the cross sectional area of the largest phase conductor. U.S. NEC allows the neutral conductor of a solidly grounded system to be used as the equipment ground on the supply side of a service disconnect but Cleveland Electric Illuminating Company requires the use of a separate equipment ground from the CT cabinet to the service switch. This is so that neutral voltage drop will not create a tingle voltage as the CT cabinet is usually outside of the building. The equipment grounding conductor from the CT cabinet should be a minimum of #6 copper of better yet #4 copper inside of the conduit.

There are some utilities that also require a dedicated ground rod and grounding electrode conductor for the meter socket so that the lightning arrestors in solid state meters will work better. In a lot of commercial applications the service conductors and the grounding electrode conductor ( for the service switch ) have too many bends and so forth to make for a good lightning ground. Even worse, Parma, Ohio requires galvanized rigid conduit from the meter to the service switch for new service and galvanized rigid conduit is useless for conducting lighting.

 

[stevenal]

Waross,
Thanks. I see the the reference to conductor size in NEC 250.102D. Typically these installations are on the supply side of the main disconnect, though. But sizing to the service conductor size (at the meter base where the service conductors do not go) per 250.102C could result in huge bonding wires requiring larger conduits.

mc5W,

I have no question about the size used to bond the CT can, just the remote meter base. Lightning is not a big concern here.

 

[dpc]

The PacifiCorp standards for Oregon show a grounding conductor running from the meter base to the CT cabinet, then another grounding conductor running from the same connection point to the service entrance equipment - but the drawing just says to "size per NEC Art 250". They used to require rigid or IMC conduit between the meter base and CT cabinet, but now they allow PVC if grounding lugs are provided at both ends.

I don't see much served by making the grounding conductor too much larger than the potential leads coming in, other then mechanical strength. But if you work your way through the NEC requirements, it could be interpreted to require bonding using an bonding jumper sized per Table 25.66, which seems like serious overkill.

Article 250.92 seems to requires bonding of the meter enclosure.

 

[waross]

Hello stevenal
I see the problem. I don't think that this rule was meant to be applied to remote meter sockets.
I have a suggestion that may be acceptable.
I understand that in the past the meter bases were grounded by the conduit. I would imagine that it woulkd be 1" rigid or 1 1/4" rigid.
The Canadian code table for bonding raceways and equipment, and the table for grounding service raceways and service equipment show #8 copper in the same column as 1' rigid and #6 copper in thye same column as 1 1/4" rigid.
Does the NEC have similar tables?
Historically, conduit has been adequate to ground the meter bases. The challenge is to maintain the equivalent protection with a copper conductor, and to not increase the potential liability or hazard over the historical methods of grounding.

I would suggest that a #6 copper conductor can be justified as being eqivalent to the historical method of grounding this equipment.
I would further suggest that you contact the appropriate code committee for a clarification and/or amendment.
I don't believe that the code is in accordance with the intent of the code committee.
On the one hane, an ungrounded meter base represents a potential hazard.
On the other hand, sizing the ground conductor to the main service conductors to protect #12 and #10 conductors, that were in the past protected by 1 1/4" rigid conduit is not reasonable.
respectfully