Phased Conduit Installation

[xformer]

What is the definition of close proximity according to 300.3B1 Ex.?
Also, what are the effects on any ferrous material in a duct bank and how does one calculate the effects?

 

[beanland]

Impedance

Impedance

My take on "close proximity" is the standard design drawings shown in the NEC for duct banks, typically 7.5" on centers.

Even if you are using PVC conduit, there is no reason not to install A-B-C-N-G in each conduit so that the net magnetic flux from those cables is minimized.

If you insist on using non-magnetic conduit and running A-A-A all in the same conduit, etc. then you will need to perform finite element modeling to determine the net magnetic fields and the amount of current induced in rebar.

There are plenty of horror stories about people using RGS conduit and installing one phase in each conduit, the conduit heats up and melts the wire.

When a transformer manufacturer installs a bushing in a transformer tank, they typically install a stainless steel panel for the bushings to penetrate to avoid the magnetic effects on the steel tank.

 

[Julius Right]

Because the reactance between phases depends on the distance between conductor center-lines
in order to reduce the reactance and reduce the voltage drop, the phase cores have to be how close one to another it is possible [even in touch one to another].
If a single phase cable passes through a closed ferromagnetic loop then the magnetic field density in this magnetic material is elevated and the magnetic losses heat the loop.
In steady state it hard to say this will burn the ferromagnetic ring but will heat the concrete around.
In short-circuit case the heat is elevated and the ferromagnetic material could burn.
The main losses are the eddy current losses and the hysteresis losses.
In steady state the generated heat will be evacuated totally through concrete and soil.
In short-circuit case the heat will rise the steel temperature only [adiabatically].