12 kV conductors from pole to underground transition

[smarteng]

Hello Sir/Madam

When transitioning from pole to underground, can we have all 3 phase wire in one conduits or do we need to have one conduit per phase. ( overhead conductor :ACSR, underground : 15KV shielded cable ) Also, whats better practice for underground medium voltage 12 kV line, separate conduit per phase or all cable in one conduit.

Thank you.

 

[charlie b]

Putting all phases (and the neutral and ground, where applicable) in the same conduit is the rule. Exceptions (other than parallel runs) are rare. See 300.3(B).

 

[smarteng]

Putting all phases (and the neutral and ground, where applicable) in the same conduit is the rule. Exceptions (other than parallel runs) are rare. See 300.3(B).

Thank you sir. But Rule applied only below 1000V per 300.2(A)

 

[mkgrady]

There is no advantage to putting them in seperate conduits

 

[JFletcher]

Your installation may be covered more by the NESC than the NEC.

 

[electrofelon]

Your installation may be covered more by the NESC than the NEC.

That's what I was thinking. I think that because some utilities let electricians run primary. If that's the case be sure to check with poco and do it to their specs.

 

[JoeStillman]

My past experience included an encounter with a utility company that required what they called "iso-phase" installations - one phase per raceway. They also had strict voltage drop rules. If you've ever marveled at the impedance difference between "iso-phase" and normal installations, you'll see why this meant that services longer than 50' were impossible in this utility's territory.


I think the iso-phase rule arose from a desire to maintain service to the 1Ø customers on the other two phases when a backhoe operator dug up one of the phase legs. This legacy rule was still around because nobody understood it well enough to stop enforcing it.


For 3Ø customers, iso-phase is pointless.

 

[beanland]

Pulling Calculations

Pulling Calculations

Hello Sir/Madam

When transitioning from pole to underground, can we have all 3 phase wire in one conduits or do we need to have one conduit per phase. ( overhead conductor :ACSR, underground : 15KV shielded cable ) Also, whats better practice for underground medium voltage 12 kV line, separate conduit per phase or all cable in one conduit.

Thank you.

First rule, iso-phase, one phase per conduit, never do it with metal conduits. With metal conduit, always 3-phases each conduit. But, where PVC is used, the choice to use one larger conduit versus 3 smaller ones can have to do with pulling. It is a lot easier to pull one cable in one conduit where there is a long pull. Also, with large wire, over 1000kcmil 15kV, where you have to use an 8-inch conduit, three 4-inch are a lot cheaper.

 

[electrofelon]

My past experience included an encounter with a utility company that required what they called "iso-phase" installations - one phase per raceway. They also had strict voltage drop rules. If you've ever marveled at the impedance difference between "iso-phase" and normal installations, you'll see why this meant that services longer than 50' were impossible in this utility's territory.

I wouldnt think there would be much difference in impedance if the NEC rules for isolated phase installations are followed.

 

[GoldDigger]

I wouldnt think there would be much difference in impedance if the NEC rules for isolated phase installations are followed.

The inductance of an isolated conductor carrying AC can be significantly higher than for the same conductor in a three phase bundle, even when there is no ferromagnetic material involved.

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[electrofelon]

The inductance of an isolated conductor carrying AC can be significantly higher than for the same conductor in a three phase bundle, even when there is no ferromagnetic material involved.

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I guess it comes down to what exactly is meant by "bundle" or "grouped". On one end of the spectrum we could have direct buried conductors that prob ly would be installed the same way in either case. What about iso phase pvc conduits that are right next to each other?

 

[Sahib]

What about iso phase pvc conduits that are right next to each other?

Inductance is more when separation distance between iso phase PVC conduits is more and so is voltage drop.

 

[Fnewman]

This voltage level is often encountered in large industrial facilities. A typical design would be a three phase pothead on the pole connected to 3/c armored cable for direct burial to a unit substation primary switch.

 

[kingpb]

Hello Sir/Madam

When transitioning from pole to underground, can we have all 3 phase wire in one conduits or do we need to have one conduit per phase. ( overhead conductor :ACSR, underground : 15KV shielded cable ) Also, whats better practice for underground medium voltage 12 kV line, separate conduit per phase or all cable in one conduit.

Thank you.

Depends on who owns the line. If it's the utility you will need to follow there requirements. If it's privately owned then follow NESC.