Ampacity of ducted vs direct-buried touching cables

Generally, the current rating of direct buried cables will be higher than those enclosed in ducts.

Also, cables touching or closely spaced will have a lower current than those not touching or separated.

A special case exists that is an exception to the above assumptions due to the sheath bonding arrangement.

If the cables are touching, the current rating of cables in ducts can be higher than direct buried cables when the sheath is solidly bonded.

ELEK Cable HV Software has been used for the modelling.

Example Calculations:

A trefoil installation with individually ducted active conductors would have a higher rating than a directly buried trefoil installation.

Below is an example of a 132 kV, 1600 sq. mm single-core copper cable with a single-point bonding arrangement.

The ducted circuit rating is 1240 A compared to the direct-buried trefoil rating of 1210 A. The difference is minimal but is the opposite of what is expected. Why does the ducted circuit have a higher rating than the direct-buried circuit?

Answer: The spacing introduced for individually ducted circuits reduces the mutual heating effects between the phases.

This happens for individually ducted circuits since phase separation is required to install each phase conductor into a dedicated conduit.

Increased separation reduces mutual heating. The separation reduces the proximity effect and electromagnetic coupling between the cable sheaths/screens, reducing the eddy current losses.

This is an observation of the calculated sheath loss in the shown example. The sheath/screen total loss factor is from the eddy current loss. This loss factor is significantly greater for the direct-buried trefoil.

Losses for the direct-buried trefoil (Sheath loss factor = 2.74058)

Losses for the individually ducted trefoil (Sheath loss factor = 1.17313) 

The fact that the rating increased for the individually ducted trefoil means that the reduction of mutual heating is much more significant compared to the effect of an additional external thermal resistance from the addition of conduits in the installation.

It is important to note that this happens for single-point and cross-bonded systems because there are no circulating currents in these bonding systems. While a solidly bonded circuit would also experience the same reduction in eddy current losses when individually ducted, there is still a circulating current loss factor component of the sheath/screen total loss factor. The circulating loss factor significantly increases when phase separation is increased.

References:

[1] ELEK Cable High Voltage V7.5 software