- Occupation
- Licensed Electrician
No. Transformer secondary conductors have to land in a single overcurrent.
I am tapping out on understanding tapping taps...
- Thread starter cottora
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No. Transformer secondary conductors have to land in a single overcurrent.
- Location
- Connecticut
- Occupation
- Engineer
I would think the requirement to land in a single overcurrent device would depend on which subpart of 240.21(C) is being applied. 240.21(C)(4), (5) & (6) require a single overcurrent device, but 240.21(2) & (3) do not.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Is this transformer (per NEC) a separately derived system or is it a utility transformer and the secondary conductors are actually service conductors?
Seems most transformers that could accept a set of 600 amp conductors would also have room for three sets of 200 amp conductors. Many people would install parallel conductors in most cases for a 600 amp circuit.
PaulMmn
Senior Member
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- Union, KY, USA
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- EIT - Engineer in Training, Lafayette College
If, as jap says, 'B' is unnecessary, and we're using 200A conductors from the 600A disconnect to the 200A OCPDs in the panels, does that mean that the 200A OCPDs are 'protecting' the upstream conductors?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
That is the point of having tap rules and the provision those rules allow, there are limitations in those rules because the conductor is not protected at/below it's ampacity on the supply side.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Yes, but according to the OP they have _10_ sets of this setup, meaning they want 10 sets of 600A conductors which then each split to 3 200A panels. So if they want to run everything back to the transformer separately they need _30_ conductors per phase.
cottora
Senior Member
- Location
- Atlanta, GA
This is the issue. It is more conductors than we have room. We thought about three smaller transformers to split the pulls, but I think we end up in the same place. We had been pricing breakers, but 600 amp inline fuses aren’t too pricey (it still seems unnecessary and will increase install cost).
cottora
Senior Member
- Location
- Atlanta, GA
This may be a dumb question but how are multi-port Polaris Connectors/Blocks not in violation of the tap rule? I have seen multiple installations where a conductor hits a Polaris connector and branches out to multiple legs.
- Location
- Connecticut
- Occupation
- Engineer
What do you think the violation is?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
The connector itself is not a violation they can be used for making taps, you are not in compliance with the tap rules because you didn't land the 600 amp tap conductor at an individual overcurrent device.
What would be compliant is making three 200 amp taps from a 600 amp feeder, but you are tapping a conductor that is already a feeder tap.
cottora
Senior Member
- Location
- Atlanta, GA
It seems that branching from such a device is essentially the same as what the original post contemplated (at least from my memory, there was no OPCD device, just a splice point that headed out to different equipment)
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I missed that, so we have 6000 amps worth of conductor already leaving the transformer?
I looked back at OP and it says 120/240 volts and mentions only three wires so I'm guessing single phase.
You really have that large of a transformer? ~1440kVA in a single phase unit?
Just because you have 200 amp conductors to each individual panelboard doesn't mean you must have 6000 amps of feeder to supply them, size the common portions according to load calculations.
Put in whatever overcurrent device that will be appropriate for protection of the transformer secondary and then make feeder taps off that, don't have to bring all the taps to one location either can run the feeder along the length of the building and make a tap near where each 200 amp panel is located. Depending on circumstances might save on copper anyway.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
problem is your applying it to a conductor that is already a "tap conductor" and are "tapping a tap" which is not really allowed in any of the tap rule situations.
If these were service conductors there is no overcurrent protection involved until the service disconnect, so the tap rules don't apply to service conductors, you maybe have seen similar to what you are wanting to do on service conductors - and reason I asked in my first reply in this thread if this is service or feeder application, being a medium voltage supply to the transformer it does raise the possibility some that it may be a service application. If so then what you are doing is fine - as long as all those 200 amp panels are not serving the same building - that would then be violation of 230.2
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
The required OCPD short-circuit interrupting ratings will likely be less if you have multiple smaller transformers rather than one larger one. I assume that this was considered in the comparison between them that you mentioned?
cottora
Senior Member
- Location
- Atlanta, GA
Thanks! You are probably correct that what I saw was from a Utility transformer and therefore service feeders.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I'd like to try to summarize and ask an additional question.
You have a single 120/240 single phase transformer, which will be feeding _30_ panels, each with a 200A main breaker.
The conditions of the installation (outside taps of unlimited length??) are such that you could in principal go from the transformer with 30 conductor sets (each properly protected at 200A) to the 30 panels and meet code, however landing 30 conductor sets on the transformer would not be practical.
You _want_ to run 10 sets of 600A conductors to an intermediate point where you will have power distribution blocks to divide the 10 600A sets to 30 200A sets. The problem is that this 'smells' like a tap of a tap, and doesn't meet the explicit requirements of the NEC tap rules which require a single OCPD at the load end for the particular taps you wish to make.
Is the above correct?
IMHO a 600A conductor protected at the load end with 3 separate 200A OCPD is actually _safer_ then a 'legal' 600A tap protected at the load end by a single 600A OCPD. Because of the way diversity factors accumulate, a panel with a 200A calculated load _likely_ has a real loading that is a smaller % of full capability than one designed at 600A. On top of this, if there is a short circuit downstream of one of the 200A breakers, I would expect lower fault current and lower I^2T on the 600A conductor.
Given the above, if you can show that the design would be 'to code' using 30 separate 200A taps, I think you could make a strong argument to the AHJ to allow a variation and permit the design you actually wish to do. My question: is this a reasonable assessment?
-Jon
You have a single 120/240 single phase transformer, which will be feeding _30_ panels, each with a 200A main breaker.
The conditions of the installation (outside taps of unlimited length??) are such that you could in principal go from the transformer with 30 conductor sets (each properly protected at 200A) to the 30 panels and meet code, however landing 30 conductor sets on the transformer would not be practical.
You _want_ to run 10 sets of 600A conductors to an intermediate point where you will have power distribution blocks to divide the 10 600A sets to 30 200A sets. The problem is that this 'smells' like a tap of a tap, and doesn't meet the explicit requirements of the NEC tap rules which require a single OCPD at the load end for the particular taps you wish to make.
Is the above correct?
IMHO a 600A conductor protected at the load end with 3 separate 200A OCPD is actually _safer_ then a 'legal' 600A tap protected at the load end by a single 600A OCPD. Because of the way diversity factors accumulate, a panel with a 200A calculated load _likely_ has a real loading that is a smaller % of full capability than one designed at 600A. On top of this, if there is a short circuit downstream of one of the 200A breakers, I would expect lower fault current and lower I^2T on the 600A conductor.
Given the above, if you can show that the design would be 'to code' using 30 separate 200A taps, I think you could make a strong argument to the AHJ to allow a variation and permit the design you actually wish to do. My question: is this a reasonable assessment?
-Jon
cottora
Senior Member
- Location
- Atlanta, GA
This is good information. Thank you! We can probably leave room in the enclosure that host the Power Distribution Blocks and add fuses if they require (within 10 feet of the transformer).
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
Forgive me, but can someone help me here. Is this entire thread based on the "Xfmr" being customer owned ? or is the argument the same if the "Xfmr" in the diagram in post #9 is "Utility" owned or service transformer ?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Big difference if it is utility owned and the conductors in question are service conductors.
First big thing - the tap rules of 240.21 don't apply to service conductors.
Uh Ohhh,,,, there's that sentence I mentioned in post #19 that should be giving you that "guilty NEC code violating feeling". :lol:
JAP>
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