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Conductors in Parallel and Division of Current

Conductors in Parallel and Division of Current

Conductors in Parallel and Division of Current

(OP)
I am planning on running two sets of four single core #4/0 conductors in parallel PVC conduits.  The raceways and conductors will be identical in size and length.  Since the individual conductors will be pulled through each raceway, the conductor orientation and hence reactance may be slightly different in each set of conductors.

The Canadian Electrical Code states the following:
"The orientation of single-conductor cables in parallel, with respect to each other and to those in other phases, shall be such as to minimize the difference in inductive reactance and the unequal division of current."

Is it possible to achieve this requirement using conduit as a raceway?

RE: Conductors in Parallel and Division of Current

I understand that that rule applies to cables run individually in free air, on surfaces, in and on cable trays etc. where the cables are continuously available for spacing and the cables may be run in a suggested orientation, usually in a flat configuration of one or more layers.
In conduit, if the cables are installed in a straightforward manner by a competent crew, they will naturally take a configuration that minimizes reactance. The presence of full size neutral conductors may have a slight effect, when neutrals are smaller than the phase conductors they often are squeezed out of the trefoil bundle and the bundle cancels well.
BUT, construction crews don't usually worry about reactance in conduits. The close spacing gives good cancellation even when the physical arrangement is not perfect.

Quote:

Is it possible to achieve this requirement using conduit as a raceway?
An experienced crew using cables that are straight will often feed the three phase conductors into the conduit in a trefoil configuration with the smaller neutral fed in beside the bundle. There will be some flattening of the trefoil in the bends but mostly the configuration will stay somewhat trefoil. The smaller neutral conductor often does not enter the bundle.
With a same size neutral, there will be some mixing but it never seems to be an issue in the field.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Conductors in Parallel and Division of Current

(OP)
Thanks for sharing your experience Bill.  I am planning on going ahead with this configuration.  I'll make sure the contractor is aware of the installation requirements.

Do you think there is value in testing the current division between the two sets during commissioning of the plant?

RE: Conductors in Parallel and Division of Current

There is always value in learning about the actual versus the theoretical.  But I would not plan on making any changes based on the measurements.
 
Also weigh the possible danger in making hot measurements against the value of the information.

RE: Conductors in Parallel and Division of Current

I suspect that the errors inherent in measuring the current may be greater than any difference due to small differences in impedance. Running conductors in parallel is common. Checking the impedance of parallel conduit runs is never done. You may find that when you use a clamp-on ammeter to measure heavy currents in the presence of the the strong magnetic fields of the surrounding conductors that the position of the conductor in the jaws and the orientation of the meter relative to the other conductors will cause the reading to vary. this will be a greater error than reactance differences.
As for testing, the quality of the connections may be more important. I may check the connection temperature with an IR spot meter. A connection that is running unexplainably hotter than the others may be a sign of future trouble.
How much hotter is a matter of judgment and experience.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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