Distance between underground circuits?

[majesus]

Questions on Underground raceways.

I have two separate motor feeds (call them circuit1 and circuit2) where they shall run underground. The CEC gives installation configuration diagrams on underground raceways for one circuit based on IEEE 835 standard. However, for two circuits, can I just parallel the runs, keeping the same cable distance apart (190mm) between circuit 1 and circuit 2 as shown in red pen for the following drawing?

Also if anyone know this: I noticed for direct buried cable configurations. If the cable runs are even (say 2, 4 or 6 cables per phase) the distance apart is equivalent for all (190mm). But if the cable runs are odd (say 3 or 5 cables per phase) then between the parallel runs, the distance is 610mm. I am just curious to know why this is done?
I have circled the distance in question in the following diagram.

 

[alehman]

It's all about mutual heating. Underground conductors running at load heat the surround soil or whatever. That heat is transferred to the other conductors. That's why the code illustrates specific arrangements. I'm not familiar with the CEC - does it give a minimum separation distance for duct banks or de-rating factors?

 

[majesus]

Yup, it is related to heating...
I just wanted to know if I can assume equivalent distances with a secondary circuit. There is no mention of it in the CEC.

Also I was curious about why odd parrallel runs have a distance of 610mm between runs and even runs have a distance of 190mm.

This section of the CEC is in Appendix B, it is a convenient illustration (including ampacity tables for each configuration) of installing underground cables without going into much calculations. It is conservative thou. I have never really gone through the iterations as described in IEEE 835. It would be very good practice.

 

[swz202]

Try to answer your second question:

First,CEC diagram B4-1 doesn't show 3 or 5, 7...paralell runs at all, it just shows different senarios of 1/2/4/6 paralell runs.

Second, diagram B4-1 is intended to be used together with table D8A/B--D15A/B, to give you the maximum allowable conductor ampacities under each senarios.

 

[jghrist]

However, for two circuits, can I just parallel the runs, keeping the same cable distance apart (190mm) between circuit 1 and circuit 2 as shown in red pen for the following drawing?

I'm not familiar with the CEC, but in the NEC, the general answer to your is no. There is more mutual heating in your penned in sketch than there is in the original. In the NEC, if your installation doesn't match the figures, you have to change the installation or calculate the ampacity under engineering supervision using Neher-McGrath.

 

[majesus]

Thanks guys for feedback, a few comments:

CEC diagram B4-1 doesn't show 3 or 5, 7...paralell runs at all, it just shows different senarios of 1/2/4/6 paralell runs.

FROM CEC, Section 4, Rule 4-004 in Appendix B of Page 361. Fifth paragraph down from Rule 4-004:
"It is recommended that ampacities for three single conductors per phase and for five single conductors per phase with spacings, directly buried in the earth, be selected from Table D8A or D8B or Table D9A or D9B for the installation configurations of Diagrams B4-1, Detail 5 and Detail 7 respectively."

I'm not familiar with the CEC, but in the NEC, the general answer to your is no. There is more mutual heating in your penned in sketch than there is in the original. In the NEC, if your installation doesn't match the figures, you have to change the installation or calculate the ampacity under engineering supervision using Neher-McGrath.

Roger that... Thank you

 

[7anoter4]

hi majesus
For two parallel circuits[5 cables/phase] you have to derate 10% more [for the same cable type and size] with respect to the one circuit rated current[ampacity].
BTW: the distance of 190 mm from ducts [or conduits] centerline is the minimum possible: 3" minimum concrete between ducts and 4.5"duct overall dia [for a 4" nominal duct] that means 7.5"=190.5 mm. For 6" ducts you need 3"+6.5"=9.5"=241.5 mm. =approx.250 mm.
For direct buried cables if the horizontal distance is the same between all cable [190 mm- detail 4] the maximum permitted current[ampacity] will be 7% less then if between column 3 and 4 the distance will be 610 mm[detail 5].
Regards