Tapped Conductors per 240.21(B)(2)

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wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Here is the situation:

  1. A 1200A breaker set at 960A
  2. 3 sets of 500mcm (1140A) run from the breaker to an MCC for a length of 500ft
  3. 25ft from from the breaker, the 500mcm is tapped with 2 sets of 500mcm (760A) and runs to another MCC 25ft in conduit

It appears to meet the code requirements but does it since not all 3 conductors per phase are tapped but only 2 per phase? I could not find anything specific in the code, unless I missed it, where an equal number of conductors must be tapped.

This is on a 480Y/277V circuit.

Thanks in advance for reading and/or replying.
 

dkidd

Senior Member
Location
here
Occupation
PE
Here is the situation:

  1. A 1200A breaker set at 960A
  2. 3 sets of 500mcm (1140A) run from the breaker to an MCC for a length of 500ft
  3. 25ft from from the breaker, the 500mcm is tapped with 2 sets of 500mcm (760A) and runs to another MCC 25ft in conduit

It appears to meet the code requirements but does it since not all 3 conductors per phase are tapped but only 2 per phase? I could not find anything specific in the code, unless I missed it, where an equal number of conductors must be tapped.

This is on a 480Y/277V circuit.

Thanks in advance for reading and/or replying.

As far as an actual code section, see 310.10(H). It may not be apparent at first, but if you draw it out and look at the various paths in the original conductor set that supply the tap, you will find they are not the same length.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
One of the requirements for using parallel conductors is that they be

(5) Be terminated in the same manner

pretty hard to claim all three conductors are terminated in the same manner when they aren't.

no reason you could not tap off the 3 conductors with 2 conductors as long as all five conductors were tied together at the tapping point.
 

Carultch

Senior Member
Location
Massachusetts
Here's an example showing why you cannot break the symmetry, and thus must tap equally onto all conductors in parallel .

Consider a 500A service, made up of two parallel sets of 250 kcmil. Suppose we desire to connect 200A of PV, which is 3/0. Ignore the 1.25 safety factor, for simplicity, i.e. assume that this is already included in all applicable ampere figures. The 250 kcmil service conductors run a distance of 500 ft from the transformer to the service disconnect, and the PV is tapped onto set #1, within 1 ft of wire length from the service disconnect. That's a voltage drop of about 10 volts, under full load.

Calculate the current in each resistor (representing one of the parallel sets of service conductors):
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