Medium Voltage Service Disconnect

[Mr. Bill]

I need some help with NEC requirements for medium voltage (12.47kV) installations.

Situation: Existing building is getting an addition which requires additional capacity. The utility pole has a utility disconnect switch. There's an overhead service to a customer owned pole with a disconnect on top. The service goes down the pole in rigid conduit and underground to an existing vault. There is an existing, antiquated knife blade disconnect located in the vault on the primary side. Further down the wall is some fusing next to each transformer. The existing customer owned transformers are located in this transformer vault. (3 can type transformers) These are getting removed and a new pad mounted transformer is being located next to the vault.

The inspector is requiring a new disconnect, with adjacent OCP, on the primary side of the transformer that is "Listed as a Service Disconnecting Means compliant with NEC 230.205." and that "the Utility Service Point and customer owned Service Conductors could require the Primary Service Disconnect to be located at the Utility Pole or Property line."

I was planning on saving the customer some money and since there's a lot of extra space now in the tranformer vault I was going to install a non-enclosed, wall mounted, knife blade switch with all three ungrounded legs opening at the same time. The inspector told me they will not accept any type of switch located in the transformer vault. The vault is about 12'W x 18'L x 8'H.

1) If I'm replacing the transformer why do I need to upgrade the service disconnect? Isn't existing equipment grandfathered in?

2) Does UL even list medium voltage switches? Does NEC require this switch to be listed if nothing exists? I cannot find any information on the UL website about this type of equipment and none of the manufacturers I know about state that their equipment is UL listed. No one asked for evidence that the tranformer was UL listed during plan review. Why is the switch special.

3) I understand that a service entering a building needs a service disconnect at the point it enters, but I don't see anything about needing a disconnect at the property line when there's a customer owned transformer. Is this right?

 

[bob]

Situation: Existing building is getting an addition which requires additional capacity. The utility pole has a utility disconnect switch. There's an overhead service to a customer owned pole with a disconnect on top. The service goes down the pole in rigid conduit and underground to an existing vault. There is an existing, antiquated knife blade disconnect located in the vault on the primary side. Further down the wall is some fusing next to each transformer. The existing customer owned transformers are located in this transformer vault. (3 can type transformers) These are getting removed and a new pad mounted transformer is being located next to the vault. ?

Isn't an antiquated switch a reason to install a new one? Since the transformers are removed is this still a transformer vault?

The inspector is requiring a new disconnect, with adjacent OCP, on the primary side of the transformer that is "Listed as a Service Disconnecting Means compliant with NEC 230.205." and that "the Utility Service Point and customer owned Service Conductors could require the Primary Service Disconnect to be located at the Utility Pole or Property line."

The utility may need a visible open switch for clearance purposes. The OCP
will not provide this feature. I do not see the need for the OCP. See if the inspector will except a 3 phase gang switch between the utility and the riser pole.

The existing customer owned transformers are located in this transformer vault. (3 can type transformers) These are getting removed and a new pad mounted transformer is being located next to the vault.
3) I understand that a service entering a building needs a service disconnect at the point it enters, but I don't see anything about needing a disconnect at the property line when there's a customer owned transformer. Is this right?

The way you have stated the problem it sounds like you are installing a pad mounted transformer outside the vault, on the outside of the building.

 

[Mr. Bill]

Isn't an antiquated switch a reason to install a new one? Since the transformers are removed is this still a transformer vault?

The switch is never used so I saw no point to update it. Just because something in antiquated doesn't mean it has to be replaced. Most stuff only gets updated because it needs to be. Do you replace every Federal Pacific panel you see in a building just because it's old? Even if your scope of work doesn't touch the panel?

There are still 3 transformers located in this vault for another building.

The utility may need a visible open switch for clearance purposes. The OCP will not provide this feature. I do not see the need for the OCP. See if the inspector will except a 3 phase gang switch between the utility and the riser pole.

I don't fully understand this statement. I got the impression that they wanted a pad mounted switch with fusing. I was told that the switch I planned to use looked too similar to an isolating switch rather than a disconnect switch. Even though the cutsheet for the swich calls it a disconnect switch. 230.208 calls for OCP on the load side of the switch.

The way you have stated the problem it sounds like you are installing a pad mounted transformer outside the vault, on the outside of the building.

The pad mounted transformer is located outside. A good distance from the building. But the Code section he quoted refered me to building service entrances. 230.70 I don't understand how that requires me to locate this switch at the property line.

 

[wirenut1980]

Sorry, but I am having trouble visualizing the setup as you described it...maybe a quick and dirty one line diagram would help. So the utility service point is on the 12.47 kv overhead system, then the 12.47 kv runs underground and will feed one pad mount transformer, and 3 transformer banks in the vault? So it sounds like a service disconnect is required to be installed between the service point and upstream of all transformers it serves in accordance with Section VIII of Article 230. Does the utility have rules on where the disconnect should be? It sounds like they already have a disconnect on their side of the meter...

I am not sure what the inspector is saying is wrong either. Is it that you want to have 2 service disconnects but they are not grouped together?

 

[kingpb]

You will not find UL listed medium voltage equipment. On that note, make darn sure that the inspector understands this and is willing to accept equipment built to IEEE and NEMA standards. I've was invovled in a dispute like this once in Idaho. We ended up having to have a thrid party testing agency come and verify the equipment was as it was supposed to be.

At MV, I don't think you'll find anything listed as Service Entrance Rated.

 

[gedwards]

See the following links:
http://ecatalog.squared.com/pubs/El...witchgear, Metal Enclosed/9840/6040CT9201.pdf
or http://www.sandc.com/webzine/2005/120505_1.asp

I agree with wirenut, as the customer is purchasing power @ 12.47 and the drop is feeding a PMT and a bank of single phase xfmrs. Per Article 230, part VIII it appears that you need to provide either a PMS or a multi section MV switchgear line-up to provide a means of disconnect of the 12.47 feeder into the buiding. I have a feeling that things might be different if the vault was owned by the local utility and not the end customer.

As for a UL label, a UL Label can be placed on Metal Enclosed Switchgear, although it is typicaly at an additional charge. There are some AHJ's in my area that will not accept any piece of equipment on a job if it does not carry a UL label.