Two LV feeders from a secondary side transformer - Does it meet 240.21(C) (4) & 450.3?

J

jesusrperezd

Member
Location
California
Occupation
Engineer
Hi all, I have read several similar posts, but still have some questions. I'm exploring possible distribution schemes for a client in California, and I want to be sure the concept I have in mind is code compliant regarding transformer and conductor protection.

There are two existing switchboards, 800A and 1600A, with main breakers each, located in different buildings. The initial thought is to add a customer owned Pad Mounted transformer outside 12kV/208-120V, and provide two LV feeders from the secondary side of the transformer to each switchboard.

My interpretation is that the scheme meets the following:

NEC 240.21 (C): "A set of conductors feeding a single load, or each set of conductors feeding separate loads..." . That means that two feeders are acceptable.

240.21 (C) (4) indicates the conditions for the outside transformer; those are met.

450.3 (A) is also met: Supervised location, primary setting less than 300% (by CB and protection relay), secondary protection is not required.

An engineer said that we need to provide a switchboard downstream of the transformer with a main breaker and two branch breakers for the two switchboard feeders to comply with the code. Additionally, a post I read here mentioned that the secondary conductor is a tap, and you can't tap a tap conductor. I have been reading the code to see where the above statements are based, but found nothing. Any comment on this will be valuable. Another thing I need to verify is the correct feeder size for both switchboards.
 
jesusrperezd said:
An engineer said that we need to provide a switchboard downstream of the transformer with a main breaker and two branch breakers for the two switchboard feeders to comply with the code. Additionally, a post I read here mentioned that the secondary conductor is a tap, and you can't tap a tap conductor.
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If your conductors connect directly to the transformer terminals you would not need the additional switchboard.
 
Any thoughts on the feeder size? I don't see any special requirement different from 215.2. At first look, the 800A switchboard feeder would be 3x350kcmil, and the 1600A switchboard feeder 5x500kcmil. For those sizes and quantities (8 conductors per phase), I'm unsure if the connections can be made at the transformer cabinet (LV side).
 
jesusrperezd said:
Any thoughts on the feeder size? I don't see any special requirement different from 215.2. At first look, the 800A switchboard feeder would be 3x350kcmil, and the 1600A switchboard feeder 5x500kcmil. For those sizes and quantities (8 conductors per phase), I'm unsure if the connections can be made at the transformer cabinet (LV side).
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Good question. You might need to switch to less sets and larger conductors. 600 kcmil gets you down to 6 conductors per phase which might be workable with 6 barrel lugs.
 
They also make transformer buss extenders which will accommodate more conductors.
(I've seen them but never purchased..infinity likely has a link)
 
augie47 said:
They also make transformer buss extenders which will accommodate more conductors.
(I've seen them but never purchased..infinity likely has a link)
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Good point. We would have our switchboard company customize them based on the number of conductors and type of terminals. If using crimp connectors you can get stackable ones. This is 10 sets on the neutral (200%) with an addition piece of bus.1000000470.jpg
 
jesusrperezd said:
Is that something provided as part of the transformer?
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When we order the transformer the supplier sets it up based on the size and number of conductors, etc. So yes in this case they provided the stacked terminals and the necessary bus extension to make the 10 neutrals fit on the transformer stab.
 
jesusrperezd said:
Any thoughts on the feeder size? I don't see any special requirement different from 215.2. At first look, the 800A switchboard feeder would be 3x350kcmil, and the 1600A switchboard feeder 5x500kcmil. For those sizes and quantities (8 conductors per phase), I'm unsure if the connections can be made at the transformer cabinet (LV side).
Click to expand...
My first suggestion regarding # of conductors per phase would be to reach out to the manufacturer of the transformer and ask them. You may need to add a tap box if there are too many.
 
jesusrperezd said:
An engineer said that we need to provide a switchboard downstream of the transformer with a main breaker and two branch breakers for the two switchboard feeders to comply with the code. Additionally, a post I read here mentioned that the secondary conductor is a tap, and you can't tap a tap conductor. I have been reading the code to see where the above statements are based, but found nothing. Any comment on this will be valuable. Another thing I need to verify is the correct feeder size for both switchboards.
Click to expand...
The engineer is wrong. Also, the secondary conductors from a transformer are taps, and you can't tap a tap conductor, but form a code perspective, if a tap is a 3/0 and you then split it in to two 3/0's to two different locations, they are not "tapping a tap." Those two conductors would be extensions of the same tap, not a secondary tap of a tap conductor. (hope that makes sense). 240 is where all of this is.
 
Strathead said:
The engineer is wrong. Also, the secondary conductors from a transformer are taps, and you can't tap a tap conductor, but form a code perspective, if a tap is a 3/0 and you then split it in to two 3/0's to two different locations, they are not "tapping a tap." Those two conductors would be extensions of the same tap, not a secondary tap of a tap conductor. (hope that makes sense). 240 is where all of this is.
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Do you mean both feeders should have the same size and conductors?
 
jesusrperezd said:
Do you mean both feeders should have the same size and conductors?
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No, and I see where my head was deeper than my writing. And as I started to clarify, I now have a question. For you, what I meant was, I couldn't run, for example, a set of 600KCMIL out of the transformer and then split that in to two sets of 3/0 that terminate in 200 A breakers. But< now wonder....
If I run parallel sets of conductors to a tap box and then split it off to multiple services that seems to be tapping a tap, but I know it is done. Can we do this for a separately derived system as well. Say, 2500 amps of conductors to a tap box, 2500 amps of conductors to a service and 100A conductors to a fire pump? I am guessing that is ok, but I could not go 2500 amps to a tap box and 1000A to one gear and 1500 to another gear. At least one of the runs must be fully rated the entire length.
 
Strathead said:
No, and I see where my head was deeper than my writing. And as I started to clarify, I now have a question. For you, what I meant was, I couldn't run, for example, a set of 600KCMIL out of the transformer and then split that in to two sets of 3/0 that terminate in 200 A breakers. But< now wonder....
If I run parallel sets of conductors to a tap box and then split it off to multiple services that seems to be tapping a tap, but I know it is done. Can we do this for a separately derived system as well. Say, 2500 amps of conductors to a tap box, 2500 amps of conductors to a service and 100A conductors to a fire pump? I am guessing that is ok, but I could not go 2500 amps to a tap box and 1000A to one gear and 1500 to another gear. At least one of the runs must be fully rated the entire length.
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I think the answer lies it the fact that transformer secondary conductors are not by definition a "tap" so they need only comply with the 240.21(C) rules.
 
augie47 said:
I think the answer lies it the fact that transformer secondary conductors are not by definition a "tap" so they need only comply with the 240.21(C) rules.
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There's no restriction regarding the different feeder sizes, let's say a feeder is 2AWG and the other is 500kcmil, each of them ends on an OCPD that has a setting lower than or equal to the ampacity of each feeder.


What if the connection is intended downstream in a tap box? lets say the 500 kcmil is extended to the box, and then split to 2AWG and 500kcmil. The first section of the 500 kcmil from the transformer terminals will be the "Secondary conductors"; the other 500kcmil section, is it a feeder? and the 2AWG would be a tap?
 
jesusrperezd said:
The first section of the 500 kcmil from the transformer terminals will be the "Secondary conductors"; the other 500kcmil section, is it a feeder? and the 2AWG would be a tap?
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All of the 240.21(C) options require the secondary conductors to end in an OCPD. How could you make a connection ahead of an OCPD and still be in compliance?
 
jesusrperezd said:
There's no restriction regarding the different feeder sizes, let's say a feeder is 2AWG and the other is 500kcmil, each of them ends on an OCPD that has a setting lower than or equal to the ampacity of each feeder.


What if the connection is intended downstream in a tap box? lets say the 500 kcmil is extended to the box, and then split to 2AWG and 500kcmil. The first section of the 500 kcmil from the transformer terminals will be the "Secondary conductors"; the other 500kcmil section, is it a feeder? and the 2AWG would be a tap?
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Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.
 
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