13.2 kv and new transformer

[bob]

This is a question that was posted on another forum. I would appreciate any comments to my response:

"We are installing a 13KV to 480/277 volt 1000kva 3phase transformer for an MCC in an idustrial facility. We are using 3 Paralleled 500mcm per phase (1200amp secondary). Where do you size the ground for this installation? For this size conductor Table 250-66 calls for a 3/0 grounding electrode conductor. It will be protected on the primary side for the secondary conductors. Table 250-122 list 3/0 for a 1200amp overcurrent protection device, if this applies since it doesn't actually have a 1200amp protective device.
First question: Which table would you use to size the ground for the secondary?
Second question: If it is 3/0 which seems to be indicated, can you parallel 2 2/0 instead of the 3/0?

Response
The post seems to indicate that the primary 13kv and the transformer are customer owned. If so, then the 13kv is the service and I assume there is a primary disconnect switch at the site some where. The conductors secondary, 3 500 kcm per phase, are then feeder conductors. Since 3 # 500 75C conductors are rated at 380 x 3 = 1140 amps, there is a violation of 240.3.C. The secondary needs to be 3 # 600 kcm or equal to match the 1200 amp main. 240.21C(1) thru C(6) also comes into play. The length of the conductors was not listed but the requirements of C2, C3, C4 where applicable. All of these require that the conductors be rated at 1200 amps.

The quote "It will be protected on the primary side for the secondary conductors. Table 250-122 list 3/0 for a 1200amp overcurrent protection device, if this applies since it doesn't actually have a 1200amp protective device." The main OC protection is the transformer primary fuse as stated. If you assume that the transformer fuse is 200% of the FLA as allowed by table 450.3A then (1000 kva)/(1.73x 13.2) = 88 amps or a 90 amps fuse or 2055 kva. Relating this to the secondary you have 2055 kva/(.48 x 1.73) = 2475 amps. According to table 250.122 the EGC is 350 kcm. In addition, this conductor must be able to withstand the fault current imposed on it. Assuming the transformer has 5% impedance, the available fault current is about 24000 amps using infinite buss. The time current curves of the primary fuse will determine the the conductor withstand rating.
Edited to show secondary voltage 277/480 volts

[ September 27, 2004, 01:49 PM: Message edited by: bob ]

 

[nvcape]

Re: 13.2 kv and new transformer

My guess is that this is a wye secondary. Table 250.66, for Grounding Electrode Conductors, requires 3/0 copper or 250 aluminum. There is no provision listed for reducing size and parallel conductors. If it is a wye secondary, the neutral is required to the MCC and also secondary protection (240-4(F)). If, as you stated, there is no secondary protection (240-4(F)), is it delta-delta, then why the question?

 

[charlie b]

Re: 13.2 kv and new transformer

Sorry, Bob, but I disagree with several aspects of your response. First of all, Table 250.122 does not come into play here. The fact that it has an entry of 3/0 in the row that corresponds to the secondary rated current is not relevant in any way.

Next, the fact that the secondary has a rated current of 1,203 amps does not mean that you cannot use a set of 3 conductors of 500 MCM 75C copper. The conductor is based on the load, not on the capacity of the transformer. The question did not give us the calculated load. If the calculated load is below 1,140 amps, then the proposed secondary conductors are acceptable.

Next, the point of connection to the utility is not relevant. We don?t know the size and overall load of the site. For all the question tells us, the utility could supply the site at 25,000 volts, and the owner might have several unit substations that convert 25kV to 13.2kV.

Here?s my take on the installation.

The transformer creates a new Separately Derived System. The center point of the WYE secondary must be connected to planet Earth via a Grounding Electrode Conductor. That conductor is to be sized, per Table 250.66, no smaller than a 3/0 copper.

Now I must ask how this installation can be permitted to have primary protection, but no secondary protection. Table 450.3(A) only gives one circumstance under which you can protect the secondary with the primary. That one case is a ?supervised location,? with the primary fuse set no higher than 250%. However, Table 450.3(A) does not say this, but that one case is required to be a delta-delta (3 wire secondary). To verify that requirement, see 240.21(C)(1). So tell me, what is the case here?

 

[bob]

Re: 13.2 kv and new transformer

Charlie
Thanks for the reply. No need to apologize for disagreeing.
The only information I have is what is posted:

"We are installing a 13KV to 480/277 volt 1000kva 3phase transformer for an MCC in an idustrial facility. We are using 3 Paralleled 500mcm per phase (1200amp secondary). Where do you size the ground for this installation? For this size conductor Table 250-66 calls for a 3/0 grounding electrode conductor. It will be protected on the primary side for the secondary conductors. Table 250-122 list 3/0 for a 1200amp overcurrent protection device, if this applies since it doesn't actually have a 1200amp protective device.
First question: Which table would you use to size the ground for the secondary?
Second question: If it is 3/0 which seems to be indicated, can you parallel 2 2?/0 instead of the 3/0?"
The point I was trying to clear up is this:
If the customer takes service at 13.3 kv, should these conductors be considered the service conductors. I am assuming that there would be a
main disconnect switch. If this is true then the
should not the transformer secondary be considered
a feeder and all of the feeder rules and tables
be applied? If not, then I would agree with your
statement "The transformer creates a new Separately Derived System. The center point of the WYE secondary must be connected to planet Earth via a Grounding Electrode Conductor. That conductor is to be sized, per Table 250.66, no smaller than a 3/0 copper."
I do not know how the primary fuse can protect the secondary conductors.

 

[wirenut1980]

Re: 13.2 kv and new transformer

Charlie b, Could the secondary protection of the transformer come from the main circuit breaker on a downstream main distribution panel or MCC (a main disconnect), or would the protection have to be located at the transformer secondary?

Dave

 

[charlie b]

Re: 13.2 kv and new transformer

You can protect the secondary of the transformer with a breaker on a downstream panel. Can you also protect the secondary conductors with that same downstream breaker? That is a matter of meeting the Tap Rules of 240.21(C).