C.T. Service meter pan bonding

[russ]

Re: C.T. Service meter pan bonding

Somebody get those trees out of the way!! I can't see the forest!!

Ok Ryan
Just one more thing to ask.
If this circuit was in conduit, would the conduit be a bonding jumper?

[ January 19, 2004, 07:22 PM: Message edited by: russ ]

 

[rick5280]

Re: C.T. Service meter pan bonding

Russ, you said "I'm just using 250.122s own wording."shall not be required to be larger than the circuit conductors supplying the equipment".

If you tap off of a feed does the ground run with the tap have to be sized to the circuit you tap off of? or the size of the tap wires?"

The whole point here is that these conductors are connected to a ct, or current transformer, which reduces the utilities voltage to a lower voltage (usually 120v) for metering equipment. To base the sizing of the bonding conductor on the conductors ahead of the ct is wrong.

Now in the ct cabinet, if there is one, you would use the larger size bond conductor.

The reason utilites see a #4 most often is that is what they use, most often. This does not mean you are required to use it, though it does cut down on stock!!


To summerize, from the ct cabinet to the ct meter pan you run a #12 (usually) bond wire, with no additional bonding at the ct meter pan, like you would do for a normal meter pan(feed through type).

The CT cabinet would have the neutral of the incoming service connected to the cabinet, grounding electrode conductor, and the #12 bond to the ct meter pan.

You may even have pts in the cabinet, which could also be connected to the metering equipment.

Rick Miell

 

[rwinslow]

Re: C.T. Service meter pan bonding

I went with the #4 and it made everyone here happy. Also just a side note, the meter pan is manufactured for the POCO and has their logo on it. The bonding lug is a Burndy/FCI KA2U which is only rated for a maximum of a #2.

However, the reason I posted this question was that I could not get a concrete answer as to what is required by the NEC. There were a lot of good points brought up but still no hard evidence. The POCO transformer is actually 1500 KVA (future growth). So in theory since the CT's originate at the 1500 KVA transformer, the bond should be sized to the transformer's rating? Not very practical.

In turn I believe that it is true that #4 is the most common bond for meter pans, so since it is seen at other installations, they want to see it in this situation also.

 

[iwire]

Re: C.T. Service meter pan bonding

Originally posted by rwinslow:
I went with the #4 and it made everyone here happy.

Sometimes that is just the best way to go, keep everyone happy.

I am curious to know how this CT meter pan could be energized in a way that would require a bonding conductor larger than the conductors suppling it?

[ January 21, 2004, 06:28 AM: Message edited by: iwire ]

 

[charlie]

Re: C.T. Service meter pan bonding

A CT meter pan does not contain service conductors.

Assuming this is transformer rated metering (I think someone said it was) and the voltage is 480 or higher, PTs will be used somewhere. IPL will install PTs in the metering enclosure along with the meter. The wires that feed the PTs are not fused and we want to be sure that the enclosure will remain at the same potential as all the other equipment in the event of a fault. The #10 Cu. that feeds the PTs is expected to burn clear without disturbing the #4 Cu. grounding conductor.

Bottom line, if you do not install the #4 Cu. that we require, you do not get power. IMO, if we did not have those rules, 250.66 would apply.

 

[iwire]

Re: C.T. Service meter pan bonding

Charlie it must be one of those different location kind of issues.

The majority of buildings I work in have CT metering and not one has the bond wire we are talking about, the raceway is the bond.

Here is the Green book info for the POCO I work with the most (National Grid)

933. Instrument Transformer Secondaries A continuous rigid metal raceway shall provide an uninterrupted metallic bond between the transformer cabinet and the test switch cabinet for instrument transformer secondaries.
Elbows with removable covers are not acceptable.

A. Provide one and one-quarter (1?) inch conduit for secondary conductor runs that are less than 20 feet in length.

B. Provide one and one-half (1?) inch conduit for secondary conductor runs that are twenty (20) to fifty (50) feet (one hundred (100) conductor feet) in length.

The maximum distance between meter and current transformers shall be fifty (50) feet. Conduit shall be furnished and installed by the Customer.

Secondary wiring will be furnished and installed by the Company. See Figure 941-2.

Is it possible out CT meter pans are different?

Ours are nothing more than a socket to hold a meter nothing else other than some switches.

Bottom line everyone does it differently if I did put a 4 AWG bond wire in the raceway the POCO would have me take it out.

 

[iwire]

Re: C.T. Service meter pan bonding

I agree of course that 250.102(C) requires supply side bonding jumpers to be sized by 250.66.

Where I disagree is that the NEC requires a CT meter pan to be bonded.

250.92 Services.

(A) Bonding of Services. The non?current-carrying metal parts of equipment indicated in 250.92(A)(1), (2), and (3) shall be effectively bonded together.

(1)The service raceways, cable trays, cablebus framework, auxiliary gutters, or service cable armor or sheath except as permitted in 250.84.

(2)All service enclosures containing service conductors, including meter fittings, boxes, or the like, interposed in the service raceway or armor.

(3)Any metallic raceway or armor enclosing a grounding electrode conductor as specified in 250.64(B). Bonding shall apply at each end and to all intervening raceways, boxes, and enclosures between the service equipment and the grounding electrode.

I do not see that any of 250.92 covers a CT meter pan.

A CT meter pan, at least in this area contains no service conductors and is not interposed in the service raceway.

It may be a great idea to add a bond wire if your POCO allows it