Transformer secondary w/ tap conductors

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bob

Senior Member
Location
Alabama
Because the connection to the transformer is considered a tap. As you know, you are not allowed to tap a tap, although I see nothing really
unsafe with this connection.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
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.

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!!!
 

kwired

Electron manager
Location
NE Nebraska
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.
 
Last edited:

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...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. ...
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
You can't use 240.4(B) for transformer secondary conductors, so the author is correct. See 240.21(C).

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.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have to agree with Don and the drawing. 240.21 main paragraph states "conductors . under 240.21(A) through (H) .....shall not supply another conductor....."
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
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 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.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
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
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.

Now go farther down in the section you quoted and tell us what you think of what it says in part (C)(1).
 

Smart $

Esteemed Member
Location
Ohio
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.

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
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
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%.

Looking at T450.3.B (note 1), what size fuse would you recommend?

ice
 

Smart $

Esteemed Member
Location
Ohio
Looking at T450.3.B (note 1), what size fuse would you recommend?

ice
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).
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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.
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
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.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
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.

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.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
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
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.

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.
 

cppoly

Senior Member
Location
New York
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?
 
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