Conduits Overheat in Three-Phase System

You bet is can. You should have 2 A 2 B 2 C + Neutral in the same conduit. The other phase need to be A B or C and neutral in another conduit.

What about if you don't run the neutral but only an earth conductor in each conduit? thanks

Same applies. You need to look at the maximum current through each conductor in the conduit, & add them (taking the cable gauge into account) to get the total heating effect.

If your loads are properly balanced, there won't be much current in the neutral anyway, so it won't make much difference if it's not in the conduit.

inquirer,

In most cases, the neutrals and the earth conductor are not optional.

It would appear that you are looking for a way to make use of an existing conduit that is too small for the task.

If you provide more specific details about the source, voltage, length of conduit run, conduit size, type of wire insulation, and indicate why you want to or must use the existing conduit and any other specifics you think might apply, then you will get much better suggestions for workable solutions.

In general, conduit heating occurs three ways; ambient (surrounding) temperatures, I²r resistive heating (load current dependent), and inductive heating.

The inductive heating is controlled by including all the circuit conductors in the same conduit so they interact with each other rather than with the conduit. Omitting any of the conductors to a particular load and running them in a separate conduit leads to the conduit getting the inductive heating that would have been canceled by the missing complementary conductor.

An exception would be the earthing bond conductor. That would not be a problem to run separately if you absolutely must reduce conductor mass in the conduit, since it does not regularly carry a current load.

My 2¢, CJM

The IEE regulations here in the UK .... gives maximum currents for each type of cable that they can carry and also the number of cores you can run in conduit taking into account how the conduit its installed.. clipped to walls, in freespace, burried in wall and under lagging. All 3 phases shoul be balanced for your load.. if they are single phase supplies then the cores should be rated for the maximum current capacity. You can alos take diviristy into account for the installation depending upon the type of loads eg, lighting, sockets, heating applications eg. fires , washinging machines, ovens etc. If you can supply more details about instalation, loads, volatages, cables, type and sizes used and also how the conduit is installed. I will look up the intallation for yo can check what you have meets the recommend speifications for here in the UK.

Certainly, With all three phases equal the magnetic fields produced by the three phase currents will all sum to zero and there will be no circulating current in the metallic conduit. If the circuit is unbalanced, the magnetic fields will not sum to zero and you can get rather monstrous circulating currents and heat the conduit right up (at least to the point where the insulation on the cables is damaged, then you get a new phenomenon called fire)...