I have a 3PH 4W 225KVA step down outdoor transformer fed from a 480V panel board with a 400A breaker. The secondary of the tmfr. 102/208V (2 parallel feeders of 4-600KCMIL, 1-3/0 gnd) is installed underground and terminates on a single 800amp breaker just above slab inside a building. Length of secondary is 90 feet. Is this an appropriate application of NEC article 240 (c)(4)"outside secondary conductors"?
Secondary disconnecting means
- Thread starter agwynn
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Secondary disconnecting means
I might throw a flag on the play.
It sounds like you might be using aluminum. I infer that from the selection of the EGC (being 3/0, whereas a 1/0 would be enough for copper). A 600 MCM aluminum conductor has an ampacity of 340 amps, so a pair would be good for 680 amps. The next higher standard overcurrent device rating is 700 amps, not 800. Article 240.21(C)(4)(2) states that the overcurrent device must ?limit the load to the ampacity of the conductors.?
The rest of the installation sounds fine. If you are using copper, then I will ?pick up the flag.? But if it is aluminum, then you need a 700 amp breaker.
I might throw a flag on the play.
It sounds like you might be using aluminum. I infer that from the selection of the EGC (being 3/0, whereas a 1/0 would be enough for copper). A 600 MCM aluminum conductor has an ampacity of 340 amps, so a pair would be good for 680 amps. The next higher standard overcurrent device rating is 700 amps, not 800. Article 240.21(C)(4)(2) states that the overcurrent device must ?limit the load to the ampacity of the conductors.?
The rest of the installation sounds fine. If you are using copper, then I will ?pick up the flag.? But if it is aluminum, then you need a 700 amp breaker.
Re: Secondary disconnecting means
Some background info...Reason for questioning this install comes from punch list received by electrical consultant on a public works job. The consultant's drawings show a WP 800Amp disconnect installed on transformer. Submittals were submitted with 800AMP breaker installed in panelboard instead of at transformer. So basically, protocal was skipped (without receiving written permission to change the installation a red flag has been raised by the consultant). The project is finished with what I thought was a code worthy installation and I am trying to obtain that permission now in order to close out the project. The consultant is now citing NEC 240 (c) 1, 2, and 3 as a citation of illegal install. If code is his only concern, my desire is a correct interpretaion of these items 1-4 under section (c). And yes the secondary conductors are copper.
[ September 14, 2004, 08:44 PM: Message edited by: agwynn ]
Some background info...Reason for questioning this install comes from punch list received by electrical consultant on a public works job. The consultant's drawings show a WP 800Amp disconnect installed on transformer. Submittals were submitted with 800AMP breaker installed in panelboard instead of at transformer. So basically, protocal was skipped (without receiving written permission to change the installation a red flag has been raised by the consultant). The project is finished with what I thought was a code worthy installation and I am trying to obtain that permission now in order to close out the project. The consultant is now citing NEC 240 (c) 1, 2, and 3 as a citation of illegal install. If code is his only concern, my desire is a correct interpretaion of these items 1-4 under section (c). And yes the secondary conductors are copper.
[ September 14, 2004, 08:44 PM: Message edited by: agwynn ]
Re: Secondary disconnecting means
I feel if the install was designed like you stated originally it would be fine.The problem is the installer did not install the design.The disconnecting means would have been required near where the feeder entered the building regardless of the disconnect at the xfmr secondary.I don't think any reply will settle this dispute between the designer and the installer.
I feel if the install was designed like you stated originally it would be fine.The problem is the installer did not install the design.The disconnecting means would have been required near where the feeder entered the building regardless of the disconnect at the xfmr secondary.I don't think any reply will settle this dispute between the designer and the installer.
Re: Secondary disconnecting means
It sounds like the design issue may not be an issue. The contention lies with the interpretation of 240 (c)1,2,3, and4. Bottom line: given the installation that was performed, is the disconnect at the transformer required for OCP at the secondary? Put another way, with the OCP at the panel, does the secondary need OCP at the transformer that complies with 1, 2, or 3 of section (c)?
It sounds like the design issue may not be an issue. The contention lies with the interpretation of 240 (c)1,2,3, and4. Bottom line: given the installation that was performed, is the disconnect at the transformer required for OCP at the secondary? Put another way, with the OCP at the panel, does the secondary need OCP at the transformer that complies with 1, 2, or 3 of section (c)?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Secondary disconnecting means
There was no foul on that play.
The installation that you have described appears to me to be in compliance with 240.21(C)(4). I believe that you do not need an additional OCPD at the transformer, in order to protect the secondary conductors.
I pick up the flag.
There was no foul on that play.The installation that you have described appears to me to be in compliance with 240.21(C)(4). I believe that you do not need an additional OCPD at the transformer, in order to protect the secondary conductors.
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