Phased Installations

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xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
Hello, thank you for the opportunity to learn and contribute. My question today is why do companies not install services and feeders in phased installations? For example: All "A" phase in one conduit, All "B" phase in one conduit, All "C" phase in one conduit?
 
Because special steps have to be taken to make sure induction doesn't cause heating in the surrounding metal. The magnetic fields generated by the current flowing in the conductors is cancelled by being in close proximity to the other conductors of opposite phase. But when separated into individual conduits, you end up with several small transformers in effect, inducing eddy currents in the metal enclosure.
 
xformer said:
For example: All "A" phase in one conduit, All "B" phase in one conduit, All "C" phase in one conduit?
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One reason being that (except for a very limited type of installation) it is prohibited by the NEC for the reasons mentioned. For utility network compartments we have done it that way with fiberglass conduits.
 
It is somewhat common I believe with larger services and feeder circuits installed underground in non metallic raceway with multiple parallel conductors and this is one place NEC does allow this kind of thing. Makes it easier to keep each individual conductor of a parallel set the same length as well.
 
Makes for neater installations at terminations and helps keep parallel conductors the same length,
I believe Rob has some great pictures of such installations.
 
xformer said:
Hello, thank you for the opportunity to learn and contribute. My question today is why do companies not install services and feeders in phased installations? For example: All "A" phase in one conduit, All "B" phase in one conduit, All "C" phase in one conduit?
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2023 nec Read the 300.3(b)(1) exception. And 300.5(I) exception 2
ISO phase is only permitted underground non metallic raceways
 
This thread has a picture of an isolated phase installation done correctly. You have to meet the requirements of non metallic conduit, no metal circling individual phases, etc.

Three Phase piped separately

Can piping each of the wires in a three phase feeder in separate pipes cause overheating? Yes and in most cases will be a code violation.
forums.mikeholt.com
 
don_resqcapt19 said:
I know that is how everyone reads the code, but the language in 300.3(B)(3) permits you to do an isolated phase installation anywhere you want to.
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You’re correct after reading 300.3(b)(3) it does say that ! So why does the 300.3(b)(1) exception only mention underground installations if that’s the case?? Especially since 300.5(i) exception 2 is specific to underground installations anyways ?
 
Jaybone812 said:
You’re correct after reading 300.3(b)(3) it does say that ! So why does the 300.3(b)(1) exception only mention underground installations if that’s the case?? Especially since 300.5(i) exception 2 is specific to underground installations anyways ?
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I tried to delete the exception with a proposal for the 2008 code, but the panel insists that the exception is the only permitted installation of an isolated phase system.
3-11 Log #2229 NEC-P03 Final Action: Reject
(300.3(B)(1) Exception)
____________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Delete the following:
Exception: Conductors installed in nonmetallic raceways run underground
shall be permitted to be arranged as isolated phase installations. The raceways
shall be installed in close proximity, and the conductors shall comply with the
provisions of 300.20(B).
Substantiation: The exception is not required. Such installations are permitted
by 300.3(B)(3) in all locations, not just underground locations.
Panel Meeting Action: Reject
Panel Statement: This exception is necessary to permit installations where
nonmetallic raceways are installed in close proximity with all of Phase A in one
raceway, all of Phase B in another raceway, all of Phase C in another, and all
the grounded conductors in another raceway. Where this is useful is between
underground manholes so the phase conductors can exit the raceways and be
easily racked within the manhole, taking up less space, and making it easier to
do testing and maintenance on the installation. This would not be permissible
without this exception.
Number Eligible to Vote: 13
Ballot Results: Affirmative: 13
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It appears that panel 3 did not really read the language in 300.3(B)(3).
 
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don_resqcapt19 said:
I tried to delete the exception with a proposal for the 2008 code, but the panel insists that the exception is the only permitted installation of an isolated phase system.
quote]
3-11 Log #2229 NEC-P03 Final Action: Reject
(300.3(B)(1) Exception)
____________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Delete the following:
Exception: Conductors installed in nonmetallic raceways run underground
shall be permitted to be arranged as isolated phase installations. The raceways
shall be installed in close proximity, and the conductors shall comply with the
provisions of 300.20(B).
Substantiation: The exception is not required. Such installations are permitted
by 300.3(B)(3) in all locations, not just underground locations.
Panel Meeting Action: Reject
Panel Statement: This exception is necessary to permit installations where
nonmetallic raceways are installed in close proximity with all of Phase A in one
raceway, all of Phase B in another raceway, all of Phase C in another, and all
the grounded conductors in another raceway. Where this is useful is between
underground manholes so the phase conductors can exit the raceways and be
easily racked within the manhole, taking up less space, and making it easier to
do testing and maintenance on the installation. This would not be permissible
without this exception.
Number Eligible to Vote: 13
Ballot Results: Affirmative: 13
Click to expand...
It appears that panell3 did not really read the language in 300.3(B)(3).
[/QUOTE]
@don_resqcapt19 it appears your correct because 300.3(b)(3) mentions both raceways and trenches so the the statement when they resolved your PI is incorrect 300.3(b)(3) would make the example they use compliant as well
 
don_resqcapt19 said:
How so? You probably have the same number of current carrying conductors in the raceways either way.
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Well with traditional routing you'll typically have four conductors per conduit, but only three CCC. With ISO phase, with 4, you'll have to derate. Of course you could install just three per pipe and install as many as you need. Just with larger size services, like this would most likely be used for, it's going to be more conduits, but smaller. Maybe that's fine.
 
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