installation of cables
installation of cables
(OP)
My cable sizing calculations (to NEC standards) have told me that I need 4 x triplexed circuits in one trench for a particular situation. For ease of installation it would be desirable to have 3 'groups such that the arrangement has the 4 'reds' grouped together, likewise with the other two phases. I think there could be a problem here though in terms of increased reactance, and overheating of cables. If someone could confirm my suspicions that would be appreciated.






RE: installation of cables
RE: installation of cables
This will be an issue if the phase bundles enter the switch gear through seperate holes. It will be a more serious issue if the phase bundles pass through steel conduit for mechanical protection.
I have seen serious heating damage to a 400 amp switch with a 200 amp load.
The switch was fed by parallel conductors in rigid steel conduit. "A""A""B" in one conduit and "B""C""C" in the second conduit.
respectfully
RE: installation of cables
RE: installation of cables
Not necessarily the same cable or voltage-level you are using, but some of the info might be helpful.
Regards
Ralph
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RE: installation of cables
Rmullin, Schneider puts out a good document about heating in cables due to skin and proximity effects. It outlines the eaxample of what you are talking about. We have also seen with IR cameras examples where people run all the same phases together in a flat formation, and wonder why their cables are runnning very hot.
The brochure is at the following site.
http://
Ausphil
RE: installation of cables
If you have 4 current carrying conductors in an underground duct you will only get 1.6 times as much current as 2 conductors. If running 4 conductors in parallel using isolated phase and neutral configuration you are better using 2 conductors per duct such that you have AA AA BB BB CC CC NN NN GG groupings. Isolated phase is frequently used for underground installations as it makes equal length conductors a lot easier to achieve.
However if you are placing 12 current carrying conductors in the same trench in a direct burial configuration, you can only have 1/2 as much current per conductor than with 3 conductors directly buried. This is because if you go over 4 conductors the soil acts as the heat disspation bottleneck hance the derating is in square root proportion to the number of current carrying conductors.
How you can have problems equalizing conductor lengths is that these 2 conduits
* +
* +
* +
* ++++
*
*********
have different lengths but these 2 conduits
* +
* +
* +
**********
+
+
++++
have the same lengths. The second corner requires that the conduits be staggered like this when looking in cross section:
AA AA CC CC GG
BB BB NN NN
or something like that.
If in a direct burial installation you have phase groupings
AAAA BBBB CCCC NNNN GG
you could also transpose your phase groupings so that for 1/3 of the run you have
BBBB CCCC AAAA NNNN GG
and for another 1/3 of the run you have
CCCC AAAA BBBB NNNN GG
which will equalize reactance and reduce induced voltage in the neutral and equipment grouding conductors.
Mike Cole