tap on secondary conductor of transformer

B

binwork91

Senior Member
Location
new york
Occupation
electrical engineer
I saw an existing building has a tap on conductor of secondary side of transformer. see picture.
I know it is code violation per NEC (2008)
240.21
(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.


My question is: since it is old building, I wonder if there any older code allow to do as picture show?
1757538515323.png
 
binwork91 said:
I saw an existing building has a tap on conductor of secondary side of transformer. see picture.
I know it is code violation per NEC (2008)
240.21
(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.


My question is: since it is old building, I wonder if there any older code allow to do as picture show?
Click to expand...
If you see all the context rules of that rule, you'll see that it's for the special case of qualifying transformer topologies (my term), where overloads align across corresponding windings between primary & secondary. Anything with a wye or centertap, is not a qualifying topology, and requires each circuit group of transformer secondary conductors to terminate in a dedicated OCPD/disconnecting means. For secondary conductors in general, if 240.21(C) says they don't need OCPD, what it really means is they don't need OCPD immediately at the source, but rather are allowed to extend a limited distance until it's practical to install your OCPD equipment.

This has been the rule since NEC1999. Even in NEC1996, there has been a requirement for the other end of transformer secondary conductors to terminate in an OCPD/disconnecting means. Qualifying topologies, and the more permissive rule for them, were added in NEC1999.

The situation where tapping what appear to be transformer secondary conductors would be allowed, is the secondary of a utility-owned transformer. In that context, all conductors classify as as service conductors, instead of 240.21(C) conductors or 240.21(B) taps, until they get to the service disconnect(s).
 
binwork91 said:
I saw an existing building has a tap on conductor of secondary side of transformer. see picture.
I know it is code violation per NEC (2008)
240.21
(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.


My question is: since it is old building, I wonder if there any older code allow to do as picture show?
View attachment 2579538
Click to expand...
How do you "know" it is a violation? As Augie implied, it is only a violation if other conditions of that code section are not met. Specifically 10' combined conductor length for the 3/0-#4 conductor.
 
augie47 said:
Your secondary conductors would need to originate at the transformer and not"tap" then meet the appropriate tap rule dependent on length,
Click to expand...
Though you won't see these exact words in NEC, there is a saying that you can not tap a tap. Is pretty much true for all the 240.21 tap situations.

Transformer secondary conductors are "taps". As you mentioned all those disconnects can be supplied by tap conductors from the transformer but can not be supplied via a tap conductor that is supplied via another tap conductor.
 
kwired said:
Though you won't see these exact words in NEC, there is a saying that you can not tap a tap. Is pretty much true for all the 240.21 tap situations.

Transformer secondary conductors are "taps". As you mentioned all those disconnects can be supplied by tap conductors from the transformer but can not be supplied via a tap conductor that is supplied via another tap conductor.
Click to expand...
I was going to ask you to prove this. Once again at over 60 years old, I learn I was wrong again. The exact words are in 240.21:

B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.
 
Strathead said:
B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.
Click to expand...
240.4(B) is the round up rule.

The rule that prohibits taps of a tap is in the parent text of 240.21.
240.21 ... 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.
Click to expand...
 
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