Direct connection to transformer secondary with two or more feeds

[dollinger]

This is a 13.8KV:480 2000KVA pad mounted transformer with a primary fuse protected by pole mounted 65 amp fuses.

I usually feed to a panel board or a LV switch gear (depending on ratings) from the secondary of the transformer. I would like to feed two 1200 amp MCC. Therefore I would normally feed the MCC from a breaker in the switch gear. However the customer engineer does not want the switch gear. If either MCC goes down, the process goes down so his point is that if he ever wants to service the MCC mains, he will open the primary switch. The MCC's would have main breakers and each MCC would have a separate set of cables from the transformer secondary to feed the MCC. Both MCC would be located in the same room. He says that the primary fuse will protect the cables on short circuit and the MCC main breaker will protect the cables from overload. The MCC rating would be 65KA the short current on the secondary is 40KA or less.

I think this shouldn’t be done unless the there is a single MCC to avoid two services into the building and the cables and terminations on the MCC breaker should be rated for the full secondary current that likely would flow until the primary fuse blows which would be about 3700 amps. But I’d prefer to limit the MCC bus to 1200 amps to limit the available energy. I don’t want to make the customer spend money on switchgear he doesn’t need and I need to give him a strong case if I’m going to insist. Am I being too conservative? What say you? Are there parts of the code I can point too?

 

[alehman]

I don't think you mention what code you are using. If you are in the NEC world, there are several rules that permit unprotected secondary tap conductors (see article 240). The NEC would not recognize the primary fuses as providing any protection for the secondary conductors, and I would not try to size them that way.

It sounds as if you may need to hire a professional to assist with this installation.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rbulsara]

alehman's advice is right on money.

Also you did not mention if this a customer owned transformer or utility co.'s. Utility companies do often do this. You of course need to meet requirements/approval of two services then.

If this is not a true service, there are still tap rules that may apply as alehman said.

I do favor your design, but what customer wants is also doable, within Code limitations. Review the code and pick your battles.

Rafiq Bulsara

http://www.srengineersct.com

 

[dollinger]

Sorry I did not give enough details. Let me add some more. The Utility will have pole mounted fuses with an underground 300 foot feed to the pad mounted delta-wye transformer primary. The transformer secondary would be fed underground to the building where conduit will go up the outside wall and enter the building to the MCC room. The distance of the runs would be 20-50 feet. This is an industrial plant where the majority of the loads will be motors.

This would be installed according to the NFPA70 NEC and the utility company owns the transformer. The service point from the utility is therefore the secondary of the pad mount transformer. The utility specifies number of conduits, number conductors, and size of conductors that can be run from the transformer. The utility will allow as many feeds as desired as long as their constraints are met but they do not apply the NEC code past their delivery point. The problem for me is that the utility guy has basically told the customer it is ok to not have the LV switch gear on the secondary so I have to justify the extra expense. I think it is good practice to have some type of switch gear, panel board, etc but I can’t really find that the code doesn’t allow it either. I’ve been told the local inspector is not likely to challenge this either way.

According to article 100 of NEC, service conductors are defined as “the conductors from the service point to the service disconnecting means." So each MCC would be fed by a set of conductors from the utility delivery point to the Main breaker in the MCC. The main breaker without any switch gear would become the disconnecting means. Therefore it would appear from this definition the conductors feeding the MCC could be the service conductors. Article 240.1 defines a tap conductor as “a conductor other than a service conductor…” which does support that these are service conductors as long as no taps are taken off the service conductors.

Article 230.40 requires only set of service conductors but has an exception 2 " where two to six service disconnecting means in separate enclosures are grouped at one location and supply separate loads from one service drop or lateral, one set of service conductors shall be permitted to supply each or several such service equipment enclosure.” This exception allows more than one service into the building if each service has a service disconnecting means. It appears the article intends the service disconnecting means to be in separate means enclosures. Article 240.21 (c) (3) (3) states “all over current devices are grouped” (but this applies to secondary conductors less than 25 feet). The question is if a main breaker section of each MCC is intended to be considered a separate service enclosure?

Article 230.42 requires the ampacity of the service conductors to be rated for the sum of the non-continuous loads plus 125% of the continuous loads. Or the sum of the fore mention loads if the OCD and its assemblies are listed for 100% of their rating. Article 230.90 requires the over current protection to be in series with each ungrounded conductor but makes exception for motors and for two to six breakers or fuses.

Since this is a delta-Wye transformer In 240, 1 (c) (1) does not allow the primary fuse to serve as the protection for the transformer (as would be permissible if this were a delta-delta transformer).

Article 240.21 specifies the location of the OCD at the supply point except in exception 240.21 C states “… each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without over current protection at the secondary, as specified in 240,1 (c) (1) – (c) (6)”.

In 240,1 (c) (3) (2) where the conductors are less than 25 feet long, the ampacity of the conductors is required to be not less than the secondary current rating of the transformer AND the sum of the OCD does not exceed the ampacity of the secondary. Since the MCC breakers rating would be far less than the secondary current rating, the secondary cable would have to be sized for the secondary current rating if the conductors are less than 25 feet.


In 240.1 (c) (4) (2) if the conductors are outside (with no cable length limitation stated) and terminate at OCD, then the OCD must limit the current to the ampacity of the conductors. In 240.1 (c) (4) (4) (b) it states” the disconnecting means is installed inside, nearest the point of entrance of the conductors”. The 2011 code is proposed to state: “structure, or terminate within service equipment at a location inside nearest the point of entrance of the service conductors. This suggest the service conductors can be of unlimited length outside but the code requires the distance to be minimized to the greatest extent possible (IE: just inside the wall) inside if the length of the service conductor is over 25 feet. The OCD do not have to be “grouped” as required in the less than 25 feet section.

My conclusion is that it appears the code would permit the each MCC to be fed with its own service conductor provided the service conductor was sized to the secondary current rating of the transformer regardless of length as long as the OCD is installed inside “nearest the point of entrance” I find no definition of nearest point of entrance in the code. Article 230 only requires the service conductor to be sized based upon the loads while article 240 requires the service conductors to be sized based upon the secondary current rating of the transformer. Have I missed anything in the code or are there any reasons either pro or con for installing the Disconnect/OCD before the MCC given the customer doesn’t care about isolating individual MCC?

 

[rbulsara]

Just to be on record, I do not have the patience to read such a long post and it falls outside my self-imposed limits of time and effort for free advice!

Rafiq Bulsara

http://www.srengineersct.com

 

[dollinger]

Then just read the conclusion in the last paragraph which is concisely summarized!

 

[rbulsara]

Review the rule that allows up to six service disconnects. It sounds like that your installation could meet that. Local inspector will have the final say.

You are responsible for the design after the service meter. Upstream of that you need to meet the utility co.s requirements, for facilitating what they need on the customer premises.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Where are the metering transformers installed?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dollinger]

The metering will be on the primary of the transformer and the delivery point will be at the secondary.

 

[alehman]

Metering on the primary usually means the customer owns the transformer. Are you sure?

If the transformer is owned by the utility, these are services and article 240 does not apply. By definition, service conductors do not have overcurrent protection at the source and are sized for the load. The rating of the transformer is not considered.

As Rafiq said, you can have up to 6 disconnects per service provided they are grouped. You can have multiple services if required for capacity (over 2000 amps total).

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[dollinger]

Yes I'm sure the metering is on the primary of the transformer. The power company rep verified this and their spec clearly states so. Also, the spec clearly states the secondary is the delivery point. Section 240 and section 250 appear to require the service conductor to be rated for the transformer secondary see below:

In 240,1 (c) (3) (2) where the conductors are less than 25 feet long, the ampacity of the conductors is required to be not less than the secondary current rating of the transformer AND the sum of the OCD does not exceed the ampacity of the secondary. Since the MCC breakers rating would be far less than the secondary current rating, the secondary cable would have to be sized for the secondary current rating if the conductors are less than 25 feet.

 

[alehman]

If the transformer is owned by the utility, these are "service conductors". Article 240 does not apply to service conductors.

Sorry, I'm done here. As I said earlier, please consider hiring a professional to provide proper engineering. Just as it would not be prudent (or legal) for me to design a building structure from a code book although I understand the basic principals, neither is it proper (or legal) for you to design an electrical system if you don't have the proper education and experience.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.