Importance of Symmetrical Arrangement of Parallel Cables

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FaradayFF

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Greetings,

EPRI and other standards recommend installing parallel runs of cables in a symmetrical arrangement. For example, if we've got 3 sets of feeders routed from Switchgear to MCC, it is recommended to run phase conductors in their own conduit(A1A2A3 - B1B2B3-B1B2B3).

What are the intended, practical benefits of following these guidelines?
Thanks,
EE
 
It would surprise me to learn that this was a recommended practice. Could you verify your source?

NEC 300.3(B)(1) tells us to run parallel sets as A1B1C1N1G, A2B2C2N2G, A3B3C3N3G. It has an exception that allows what you describe, but only if the conduits are non-magnetic and are run underground. The issue is that with all phases in the same conduit, their magnetic fields tend to cancel each other out. This means they will not induce a current in the conduit that surrounds them, and therefore a magnetic conduit can be used. If you put all of a single phase in a magnetic conduit, there will be current induced on the conduit itself, resulting in high temperatures and likely a fire.
 
charlie b said:
It would surprise me to learn that this was a recommended practice. Could you verify your source?

NEC 300.3(B)(1) tells us to run parallel sets as A1B1C1N1G, A2B2C2N2G, A3B3C3N3G. It has an exception that allows what you describe, but only if the conduits are non-magnetic and are run underground. The issue is that with all phases in the same conduit, their magnetic fields tend to cancel each other out. This means they will not induce a current in the conduit that surrounds them, and therefore a magnetic conduit can be used. If you put all of a single phase in a magnetic conduit, there will be current induced on the conduit itself, resulting in high temperatures and likely a fire.
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Correct. And putting phase conductors in their own steel conduits will increase the inductive reactance a substantial amount.
 
Where installing single conductors for parallel circuits in cable trays, the code requires them to be grouped with one conductor from each phase and the grounded conductor, if there is one, in each group.
392.20(C) Connected in Parallel. Where single conductor cables comprising each phase, neutral, or grounded conductor of an
alternating-current circuit are connected in parallel as permitted in 310.10(H), the conductors shall be installed in groups consisting of not more than one conductor per phase, neutral, or grounded conductor to prevent current imbalance in the paralleled conductors due to inductive reactance. (/quote]
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synchro said:
Correct. And putting phase conductors in their own steel conduits will increase the inductive reactance a substantial amount.
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Thank you for the explanation, that makes a lot of sense. I looked again at my EPRI reference, and I stay corrected. They recommend running triplex(three-conductor cable) cable in its own conduit in instances where one triplex per phase is required and the conduits are laid out parallel to each other in a horizontal plane.
 
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