My question is in a three phase parallel fed system does each parallel feed A, B & C have to be ran together in the same conduit per NEC or is it not specifyed how they are ran? It has always been our pratice to run A, B & C together in the same conduit for each parrallel feed. But the question was asked about running A&A, B&B and C,C together I have always been told that would cause heating problems between the phase's.
Parallel conductors
- Thread starter CEU
- Start date
- Status
- Location
- Massachusetts
CEU said:
Yes, look at 300.3(B)
It can cause heating of metal raceways and or enclosures.
There is an exception to 300.3(B)(1) that will allow isolated phase installations only underground in non metallic conduit.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Be sure to read 300.20 as well.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
When you have a conductive material (such as a metal raceway) in the presence of a varying magnetic field (such as one generated by an AC current), a current will be induced in that conductive material. That is the basic principle behind a transformer. So if you put only the A phase in a metal conduit, it will cause a current to flow in the conduit. If you put two or more phase A conductors (and no other phases) in a metal conduit, it will induce a higher current in the conduit. That conduit will experience I^2R heating in the same way that a wire does. Current flow causes heating.
But if you put an A, a B, and a C in the same conduit, their magnetic fields tend to cancel each other out. Therefore, there is no (or very little) magnetic field seen by the conduit, and no (or very little) current induced in the conduit.
That is the reason for 300.3(B).
But if you put an A, a B, and a C in the same conduit, their magnetic fields tend to cancel each other out. Therefore, there is no (or very little) magnetic field seen by the conduit, and no (or very little) current induced in the conduit.
That is the reason for 300.3(B).
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I would run all phases in each conduit even if they were non-metallic. Remember that, if they terminate at a metallic enclosure, the holes must be joined by slots.
- Location
- Massachusetts
LarryFine said:
Not necessarily, the metal enclosure will likely have an open bottom.:smile:
The only time the exception allows isolated phase arraignments is for underground installations.
I have personally used that exception twice for two different 3000 amp services, I left a pad mount transformer ran underground in PVC and popped up under the lugs in the switch gear.
The advantage in doing this is that it saves trying to chris cross 32 heavy conductors.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
That's why I said "at", rather than "in", an enclosure.iwire said:
Lightweight! :wink:
- Location
- Massachusetts
LarryFine said:
I don't understand. :smile:
Yeah.
Those are all 600 coppers hanging out still left to be terminated after finishing the service conductors coming up on the left.
The lugs where only about 12" above the floor, it would have been a mess doing it the normal way.
Awg-Dawg
Senior Member
- Location
- Dayton Ohio
Nice installation.
What does the xfmr feed?
What does the xfmr feed?
- Location
- Massachusetts
Awg-Dawg said:
The original 208Y/120 3000 amp service/switchboard. We re-fed it with 2000 amps as we had reduced the load on that switchboard.
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