×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cable in Duct Bank De-rating

Cable in Duct Bank De-rating

Cable in Duct Bank De-rating

(OP)
Dear all,

I am aware that this topic has been discussed in many previous threads however none of the threads I saw has indication for de-rating of cables with SI cross-sections i.e. mm.sq.

Any reference? or is it valid to directly transform from AWG\kcmil to mm.sq.

Thank you

RE: Cable in Duct Bank De-rating

If I understood correctly you're asking about reduction factors for current carrying capacities of cables, with cross section expressed in sq.mm. If so, you can find it in IEC 60364-5-523 standard.
Regards,
Miki

RE: Cable in Duct Bank De-rating

Well, the physics doesn't change, but the standards and codes do. You probably need to find the applicable code and use that as a basis. In the US, the original reference is the Neher-McGrath paper on duct bank heating. The concepts and theory behind this are applicable regardless of which system of units you are using.

More info here: http://www.calcware.com/cwnmcalc2.htm or see Wikipedia. Note - I have no affiliation with Calcware and have never used it.

dpc

RE: Cable in Duct Bank De-rating

(OP)
Thanks for the reply.

I notice that underground cables in concrete duct-bank are only mentioned in NEC. Is there any reference that considers this method as well. IEC mainly considers ducts in ground or directly buried while from NEC it is concluded that having different configurations of duct banks such as with 6 or 9 ducts leads to different permissible loading level which is not addressed in the IEC or did I miss it somewhere.

P.S: my question is related to MV cables 13.8 kV and 34.5 kV single-core.

Thank you.

RE: Cable in Duct Bank De-rating

I don't believe the NEC is limited to concrete-encased duct banks. The sections shown in Annex B indicate "earth or concrete backfill".

RE: Cable in Duct Bank De-rating

(OP)
Hello dpc,

I didnt mean that the NEC is limited to concrete encased ductbanks however the de-rating factors for such laying method are only shown in the NEC. Concrete encased ductbank is not shown in IEC unless i missed seeing it.

Thanks.

RE: Cable in Duct Bank De-rating

You are right, Adam. First, IEC 60364-5-52 it is only for low voltage installation [cables].
Second, Reference D it is destined for buried conduits. NEC [Neher &McGrath] consider concrete RHO=85 [dgr.C*cm/W].In this case if Earth RHO>concrete RHO then the ampacity acc.to IEC will be less. If the shield is grounded only at one end, the difference will be negligible even up to 45 kV cable.

RE: Cable in Duct Bank De-rating

(OP)
Thanks all for the input.

So how to calculate the ampacity of cables if we have the input from the manufacturer for directly buried if we are going to have a concrete encased duct bank up to 3 x 3 configuration?

I presume now that NEC is the only available reference apart from using specific programs such as ETAP or CYME.

Thanks

RE: Cable in Duct Bank De-rating

Talk with one of the major cable manufacturers. They generally have the software to compute ampacities for duct bank configurations and sometimes will do this gratis for customers. Or you can get the Neher-McGrath paper and calculate manually.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources