Anwar Pasha
Member
Is there any regulation in NEC that forbids to put THREE single core cable of of a 3 phase line in a single PVC Pipe?
Where to look in NEC??
- Thread starter Anwar Pasha
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Buck Parrish
Senior Member
- Location
- NC & IN
Probably, what type of install are you doing?
Anwar Pasha
Member
Under ground duct bank
- Location
- Illinois
- Occupation
- retired electrician
That is a very very common installation and there are, in general, no code issues with that type of install. In fact. 300.3(B), requires all of the conductors of a circuit to be in the same raceway, with some limited exceptions.
Hameedulla-Ekhlas
Senior Member
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- AFG
You may can or may can not according to NEC or any other but it has some disadvantages please read the following.
A single core cable creates concentric magnetic fields radiating outward from the conductor, when this magnetic field encroaches on adjacent ferrous metal local heating is produced as a result of Eddy Currents. I was once called to a customers premises where the 300A meter tails passed through individual drilled holes in the metal trunking at the mains switchgear. With the switch room in darkness it was possible to see that the trunking was glowing a cherry red colour with the heat. This is why single core cable must not be passed through separate holes in metalwork, all cables must pass through the same slot or hole so that the resulting magnetic fields around the phases/neutral conductor cancel each other out. Multi core cables are not a problem because all the magnetic fields from the adjacent conductors are cancelled out within the cable.
The main reason for the trefoil formation in heavy current applications is to reduce local heating caused by magnetic Eddy Currents which set up around single core cables.
If three single cables are laid side by side the magnetic fields from the middle cable will to a certain extent be cancelled out by the magnetic field from the two outer cables.
The magnetic field from the outer cables however, have no no magnetic field on their outer side to cancel out and therefore these magnetic fields can react with steelwork to produce local heating.
When cables are laid in trefoil formation this is usually done by spacer blocks to prevent cables actually touching, the concentric magnetic fields around each conductor all intersect equally and especially if all cable are equally loaded then there is no imbalance and therefore no magnetic field being produced.
Obviously having three cables placed together in contact is not a good idea and this would require a design current reduction in the cable. That is why trefoil formation cable should always be used with spacers. In bygone days these were a wooden block with three holes drilled in them in a triangle formation, then the block was sliced through the holes to allow them to be located on the cables
When single conductor cables are installed, two type losses take place in the cable support system. One is magnetic hysteresis loss and another is eddy current loss. Hysteresis loss can be avoided by installing non-ferrous materials for cable support system. The eddy current loss can be minimized by putting 3-single conductor cable in trefoil formation and by supporting it by using trefoil cable clits. Generally trefoil cable clit is made of stain less steel (non ferrous). This kind of support system also helps to prevent cable damage from mechanical forces developed during the short current flowing through the cable.
A single core cable creates concentric magnetic fields radiating outward from the conductor, when this magnetic field encroaches on adjacent ferrous metal local heating is produced as a result of Eddy Currents. I was once called to a customers premises where the 300A meter tails passed through individual drilled holes in the metal trunking at the mains switchgear. With the switch room in darkness it was possible to see that the trunking was glowing a cherry red colour with the heat. This is why single core cable must not be passed through separate holes in metalwork, all cables must pass through the same slot or hole so that the resulting magnetic fields around the phases/neutral conductor cancel each other out. Multi core cables are not a problem because all the magnetic fields from the adjacent conductors are cancelled out within the cable.
The main reason for the trefoil formation in heavy current applications is to reduce local heating caused by magnetic Eddy Currents which set up around single core cables.
If three single cables are laid side by side the magnetic fields from the middle cable will to a certain extent be cancelled out by the magnetic field from the two outer cables.
The magnetic field from the outer cables however, have no no magnetic field on their outer side to cancel out and therefore these magnetic fields can react with steelwork to produce local heating.
When cables are laid in trefoil formation this is usually done by spacer blocks to prevent cables actually touching, the concentric magnetic fields around each conductor all intersect equally and especially if all cable are equally loaded then there is no imbalance and therefore no magnetic field being produced.
Obviously having three cables placed together in contact is not a good idea and this would require a design current reduction in the cable. That is why trefoil formation cable should always be used with spacers. In bygone days these were a wooden block with three holes drilled in them in a triangle formation, then the block was sliced through the holes to allow them to be located on the cables
When single conductor cables are installed, two type losses take place in the cable support system. One is magnetic hysteresis loss and another is eddy current loss. Hysteresis loss can be avoided by installing non-ferrous materials for cable support system. The eddy current loss can be minimized by putting 3-single conductor cable in trefoil formation and by supporting it by using trefoil cable clits. Generally trefoil cable clit is made of stain less steel (non ferrous). This kind of support system also helps to prevent cable damage from mechanical forces developed during the short current flowing through the cable.
- Location
- Illinois
- Occupation
- retired electrician
I may have misread the question. I assumed that there were 3 single core cables, but one of each phase in the raceway. There are issues, as Hameedullah, said if you put only one phase in each raceway.
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