I just started a job on an industrial paper mill and we are using cable tray and power cables. We were doing a parallel run of three cables with three phase conductors in each cable(non-shielded). I was wondering if instead of using a conductor from each cable per phase, the each cable could each act as a parallel run? I understand it may not be the best practice to do this but I am not seeing the harm, since they are in a cable tray side by side and would essentially be three large conductors. I desire further enlightenment.
Paralleling wire
- Thread starter JerL
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See 300.5(I) plus the exceptions.
cadpoint
Senior Member
- Location
- Durham, NC
well I was going to go with 215.4 (B) and 300.20
Oh great! my code book is at work and all I'm getting is code references. I would love to know the reasoning, and theory behind it yea, or nay. I understand that there may be some issue of inductive heating, but this is not putting each conductor in a separate conduit. They are laying side by side in an open cable tray. I don't see the difference between this and three non-cabled conductors laying side by side in a cable tray.
if I posted a 'can you do this?' in a NEC forum, I would expect to get a bunch of code references back.....just sayin...
perhaps you can use the online nec if you dont have your book with you.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
What you are intending to do is called Isolated phase installations. See 300.3(B)((1)exception. You will see the restrictions put upon this type of installation. Some POCOs will not permit these types of installations for services anymore.
The isolation of phases will cause the electrons to bounce around generating heat in the conductors. The heat generated will lead to failure of the installation.
That is why we have300.3(B). One wants to keep the conductors of a circuit as close together as possible.
If individual conductors are installed, see 300.20.
The isolation of phases will cause the electrons to bounce around generating heat in the conductors. The heat generated will lead to failure of the installation.
That is why we have300.3(B). One wants to keep the conductors of a circuit as close together as possible.
If individual conductors are installed, see 300.20.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Here is a link to the online 2008 NEC:
http://www.nfpa.org/freecodes/free_access_document.asp?id=7008SB
Just register and follow the links.
Conductor spacing
Conductor spacing
Conductor spacing
Tight grouping of balanced circuit conductors reduces the net magnetic field surrounding the circuit conductors. The sum of the balanced circuit conductor's magnetic field cancel each other. This canceling effect is aided when the balanced circuit conductors are tightly bundled, which is required by the NEC.
Increasing conductor spacing, the individual conductors have a net unbalanced magnetic field which travels through the air and through surrounding iron to it's balance point, where the sum is again zero. This anomaly is increased by grouping the same phase together rather than the same balanced circuit and this effect increases with the length of the run.
Inductance is the storage of energy in a magnetic field. Increasing the magnetic field storage area increases inductive effects. The feeder can behave less like a conductor and more like a load. This is the cause of inductive heating in iron which has a magnetic field density thousands of times the permeability of air.
Increasing conductor spacing also increases the capacitance of the line, the storage of energy in an electric field. These are nonlinear effects which increase also with the frequency. Briefly this increases all the bad things you are trying to avoid.
Conductor spacing
Conductor spacing
Tight grouping of balanced circuit conductors reduces the net magnetic field surrounding the circuit conductors. The sum of the balanced circuit conductor's magnetic field cancel each other. This canceling effect is aided when the balanced circuit conductors are tightly bundled, which is required by the NEC.
Increasing conductor spacing, the individual conductors have a net unbalanced magnetic field which travels through the air and through surrounding iron to it's balance point, where the sum is again zero. This anomaly is increased by grouping the same phase together rather than the same balanced circuit and this effect increases with the length of the run.
Inductance is the storage of energy in a magnetic field. Increasing the magnetic field storage area increases inductive effects. The feeder can behave less like a conductor and more like a load. This is the cause of inductive heating in iron which has a magnetic field density thousands of times the permeability of air.
Increasing conductor spacing also increases the capacitance of the line, the storage of energy in an electric field. These are nonlinear effects which increase also with the frequency. Briefly this increases all the bad things you are trying to avoid.
Should add
Should add
Should add that if the loads on the feeder have a lot of nonlinear (non sinusoidal) power supplies, vfd's, switching or rectifying power supplies, inductive heaters, these loads will draw a lot of higher frequency components, waveform distortion, on the line which will interact badly with added inductive and capacitive elements of the feeder.
The feeder is arranged to reduce the total impedance of the conductors.
Should add
Should add that if the loads on the feeder have a lot of nonlinear (non sinusoidal) power supplies, vfd's, switching or rectifying power supplies, inductive heaters, these loads will draw a lot of higher frequency components, waveform distortion, on the line which will interact badly with added inductive and capacitive elements of the feeder.
The feeder is arranged to reduce the total impedance of the conductors.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
Here is a picture of what one would look like.
Notice the underground supply.
Notice there is no metallic parts, including lock nuts
Notice the open bottom switchgear
Notice the underground supply.
Notice there is no metallic parts, including lock nuts
Notice the open bottom switchgear
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
Here is a picture of a burned-out isolated phase installation.
Notice the metallic bottom switchgear
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