NEC 347-11 - Fill ratio

[Avadh]

We are looking for cable Fill ratio variation effect on ampacity of the cable,for cable rating from 1.1Kv to 230 Kv,diameter 16mm - 140 mm,PVC/PE or lead sheath single cable in single HDPE Conduit or three single conductor cables in Trefoil arrangement in Single HDPE Conduit as per chapter 347 -11.

Please comment.
Regards
Avadh - India

Removed the e-mail addresses. Please use the PM system to exchange personal information and e-mail addresses. Thanks, Charlie

[ June 22, 2005, 06:50 AM: Message edited by: charlie ]

 

[rcwilson]

Re: NEC 347-11 - Fill ratio

Cable Corporation of India has a Technical circular that appears to answer this.

In the US, we do not consider the fill ratio in determining the permissible loading for cables in conduit. Programs for calculating underground cable ampacities (based on Neher-McGrath calculation method) do not account for any difference in heat transfer between cable and the conduit due to the % fill of the conduit.

Another way to look at this is that your ampacity calculations are based on a series of assumptions about the installation, about the thermal rho of the soil, the other heat sources in the area, the ambient temperature, the load profile, burial depth, etc., etc. These assumptions make the answer accurate to +/-10% at best. Adding a 0.05% correction because there is some air space in the conduit does not add any useful data.

If your designing your HV cables this close to maximum rating, it?s too close.