Usage of drainage tubes for electric cables

[AusLee]

Hello,

Is it possible to use normal drainga tubes (PVC or uPVC i do not know for sure) to run electric cables carrying a couple of hundred amps? The size of the ducts allow the code requirement of 60% space, but what about dielectric strength and if it is accessible then why are there corrugated ducts that seem to be exclusive for electrical cables?

Thanks.

 

[davidbeach]

Conduit is designed and manufactured so that the inside of the conduit will not damage the insulation of the conductors. No such precaution is taken for water pipe or tube.

 

[alehman]

Dielectric strength is not typically an improtant factor because the conductors are supposed to be insulated (the same types of wire are used in steel or PVC conduit).

PVC conduit is rated for the use of conductors at elevated temperature. Fittings are different than pipe fittings. Elbow radii for conduits are designed for pulling conductors. Materials and testing standards are altogether different. Most electrical codes, inspectors and insurance companies require electrical raceways be listed and approved for electrical use (in the U.S. that would commoly be by Underwriters Laboratories).

Can it be made to work? Yes. I have seen it. Is it recommended? No.

 

[AusLee]

Thank you for your replies.

I have 4 ducts and i must run 3 phases and neutral to the load.

The neutral and each phase are 4 conductors.

The qustion is, which is the best configuration:

A] Duct 1: 4 conductors of phase A, Duct 2: 4 conductors of phase B, duct 3: 4 conductors of pahse C, duct 4: 4 conductors of neutral + earthing conductor.

or

B] Duct 1: {A1, B1, C1, N1}, Duct 2: {A2, B2, C2, N2}, Duct 3: {A3, B3, C3 ,N3}, Duct 4: {A4, B4, C3, N4, PE}

?

 

[jghrist]

B

Configuration A will increase impedance (and voltage drop), result in unbalanced phase impedances (and unbalanced currents), and if the cable is shielded, increase shield circulating current.

See comments on NEC 310.4, Conductors in Parallel, in the NEC Handbook.

 

[AusLee]

Thank you, B was my choice as well.

I did not understand this in NEC 310.4
"Conductors of one phase, polarity, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, or grounded circuit conductor to achieve balance."

This note has been added in the 2005 edition, could you please explain in 2 words?

 

[davidbeach]

You can use XHHW for A and THHN for B, but all A must be XHHW and all B must be THHN. It also means that all A must be of the same length, but not necessarily the length that all B must be.

Your option A could work if there is no ferrous metal that would surround one or more ducts but not all. If there is ferrous metal around part of the total number of conductors you will have severe heating problems.