Transformer secondary w/ tap conductors
- Thread starter cppoly
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petersonra
Senior Member
- Location
- Northern illinois
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- engineer
Note the part in red. this does not really have anything to do with transformers. it is just part of the tap rules.
DARUSA
Senior Member
- Location
- New York City
But if you use a 4/0 wire on the line side of the disconnects you will have no problems!!!!
Assuming that you can feet the 4/0 in the DS lugs!!!
Also why not run the 6 directly to the transformer!!!
Assuming that you can feet the 4/0 in the DS lugs!!!
Also why not run the 6 directly to the transformer!!!
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Primary overcurrent protection is 125 amps.
Primary to secondary ratio is 2:1.
If we have 125 amps on the primary we have 250 amps on the secondary.
4/0 75C conductor is only good for 230 amps. Next higher standard overcurrent device would be 250 - this would probably be ok, but would only apply where the primary is allowed to protect the secondary - which is pretty much limited to two wire single phase secondaries or 3 wire delta secondaries, so the 4/0 is considered a tap conductor for all other applications.
As said, you can not tap a tap. Running the 6AWG feeder taps all the way to the transformer terminals would be allowed, if the allowable length of the tap conductors in the application is not exceeded.
Want to add: the one line does not say the transformer secondary is 120/240, it just says it is 240. The author likely made a mistake there trying to get his point across, when what he has drawn is technically not a violation.
Primary to secondary ratio is 2:1.
If we have 125 amps on the primary we have 250 amps on the secondary.
4/0 75C conductor is only good for 230 amps. Next higher standard overcurrent device would be 250 - this would probably be ok, but would only apply where the primary is allowed to protect the secondary - which is pretty much limited to two wire single phase secondaries or 3 wire delta secondaries, so the 4/0 is considered a tap conductor for all other applications.
As said, you can not tap a tap. Running the 6AWG feeder taps all the way to the transformer terminals would be allowed, if the allowable length of the tap conductors in the application is not exceeded.
Want to add: the one line does not say the transformer secondary is 120/240, it just says it is 240. The author likely made a mistake there trying to get his point across, when what he has drawn is technically not a violation.
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- Location
- Illinois
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- retired electrician
You can't use 240.4(B) for transformer secondary conductors, so the author is correct. See 240.21(C).
kwired
Electron manager
- Location
- NE Nebraska
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- EC
I knew someone would find something that would disallow that, but you are right.
I still think I am on track with my thought process though. Lets say the secondary is 250 copper instead of 4/0 and everything else unchanged. Then I believe there would be no violation in that drawing as is. Do you have anything to shoot that down with?
If the secondary were labeled 120/240 then I think the secondary conductor is a feeder tap no matter how large it is, but with the single voltage as mentioned in the drawing, I don't think we have a feeder tap - (if the 4/0 were to be at least one size larger), until we hit the 6AWG conductors.
- Location
- Illinois
- Occupation
- retired electrician
If the transformer in question is a two wire to two wire or a delta/delta then the installation would be code compliant if you increased the 4/0 to 250 kcmil.
2 or or 3 wire, I don't see how that changes...
240.21 Location in Circuit. Overcurrent protection shall
be provided in each ungrounded circuit conductor and shall
be located at the point where the conductors receive their
supply except as specified in 240.21(A) through (H). Conductors
supplied under the provisions of 240.21(A) through
(H) shall not supply another conductor except through an
overcurrent protective device meeting the requirements of
240.4.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Now go farther down in the section you quoted and tell us what you think of what it says in part (C)(1).
All noteworthy but still not compliant. 240.21(C)(1) says primary protection must be in accordance with 450.3. Table 450.3(B) limits primary only protection of the transformer to 125% of rated current. Drawing shows 90A primary with 125A fuse... that's 139%.
iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
Doh!!! I stand corrected (by prompt).
I was thinking of the latter part of 240.21(C) general statement which says 240.4(B) is not permitted... but Table 450.3(B) Note 1 doesn't reference 240.4(B).
- Location
- Illinois
- Occupation
- retired electrician
Yes, you make a good point. I just see the secondary conductors of a two wire to two wire transformer as protected at their ampacity and not really tap conductors. I know the code does not say that.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
I still say the last sentence of 240.21(C)(1) tells us that a two wire or three wire delta secondary is not a tap conductor.
- Location
- Connecticut
- Occupation
- Engineer
Four wire (delta-wye) secondaries are not tap conductors either. But it is irrelevant because the code does not say you cannot "tap a tap." It says Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an over current device.
This would include feeder taps, branch circuit taps, busway taps, motor circuit taps, as well as conductors from generator terminals, battery conductors and transformer secondary conductors.
That's the way I read it also.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
OK I'll buy into that with that explanation. The two wire secondary must either feed a load or an overcurrent device as the first thing beyond the secondary conductor.
Thanks for the replies, but I'm still unclear of the outcome. Is the figure from post #1 an acceptable installation or no?
- I see that in that figure the transformer OCPD is fine. The primary has a maximum of 125% OCPD (next size up permitted) in table 450.3(B)
- The #2 feeders on the primary have an ampacity of 115 amps and are protected with 125% OCPD (next size up permitted)
- OCPD is not required on the secondary of the transformer since 125% OCPD is provided on the primary
- OCPD is not required on the secondary conductors because the distance is less than 10' and the #6's terminate in OCPD's with a lower rating than the conductor ampacity.
- The taps are not tapping a tap since transformer secondary conductors are not considered a tap
Any comments?
- I see that in that figure the transformer OCPD is fine. The primary has a maximum of 125% OCPD (next size up permitted) in table 450.3(B)
- The #2 feeders on the primary have an ampacity of 115 amps and are protected with 125% OCPD (next size up permitted)
- OCPD is not required on the secondary of the transformer since 125% OCPD is provided on the primary
- OCPD is not required on the secondary conductors because the distance is less than 10' and the #6's terminate in OCPD's with a lower rating than the conductor ampacity.
- The taps are not tapping a tap since transformer secondary conductors are not considered a tap
Any comments?
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