parallel runs of cables

[electricguy-1]

In parallel runs I know that I usually have two different conduits with each set of conductor runs and I calculate the ampacity accordingly.
Someone asked me the question if I run parallel runs is the ampacity to feed a load configured differently than if I had a single run of conductors to the load? In other words is the ampacity higher or lower as opposed to one conduit with conductors in it?

 

[charlie b]

Re: parallel runs of cables

The ampacity of a service, feeder, or branch circuit depends on a number of variables. These include cable type, cable size, whether installed in free air, conduit, or other raceway, whether buried in an underground ductbank, the ambient temperature, and the number of current-carrying conductors in the same raceway. If you feed a given load in one conduit, as opposed to parallel conductors in two conduits, the ampacity in each case will depend on this same set of variables. The design process generally starts with determining the current drawn by the load, then selecting the conductor size. If the conductor size is too large for your tastes, then you can go with a paralleled set of smaller conductors.

Generally, in above-ground installations, if you use two sets of conductors in parallel in two conduits, the total ampacity is twice that of a single set of conductors in one conduit. On the other hand, if you try to run two sets of conductors in parallel within a single conduit, then you get less than double the ampacity, because you need to derate for more than 3 current-carrying conductors.

That being said as an introduction, can you clarify the nature of your question? Are you dealing with two different sizes of conductors in the one-conduit case, as opposed to the two-conduit case? If so, you would need to provide more details about the proposed installation.