Derating Factors for Cable

1. Draft Indian Standard - BIS
   csc-xxxxix/dh/uh/p&d/2009-2010
   Cable Sizing-As per IEC standard - eazyword

Good ol' British Standard 7671 gives full protocol.

Sir,

The cable size has to be large enough so the heat losses into the surrounding soil do not heat the cable beyond the temperature the insulation is rated for. Knowing the configuration of adjacent ducts in the cable bank and the temperature rating of the insulation are both essential. Keep searching, whether in the resources already suggested by the other posts, or in the NEC (USA), which has a number of tables for different duct bank configurations, or other sources. Realize, however, that these tables or charts are only a part of the answer.

--JMM

Dear Mr. SARASWAT,

A simple search and look at CR4 SEARCH will give plenty of replies/responses.

Pl. look at the link given below.

http://cr4.globalspec.com/search/sitesearch?do=show&srchobjs=t%2Cc&order=asc&fs=12&sort=textmatchrank&srch=CABLE%20DERATING%20FACTORYou can

DHAYANANDHAN.S

...which makes this thread a duplicate.

Dear Mr. PWSlack,

Indeed, It is repeat.

This topic discussed 5 times earlier, and the link given by me is one such repetition.

DHAYANANDHAN.S

The derating factors of power cables are dependent on many factors depending on ambient temperature,depth cable trench,soil conditions distance of the cable length and more .Normally cable manufacturers provide the derating factors of their cable.

http://www.cablesizer.com/help/principles/

cable sizing.

" 3.1 Base Ampacities

International standards and manufacturers of cables will quote base ampacities for specific types of cable constructions (e.g. copper conductor, PVC insulated, 0.6/1kV voltage grade, etc) and a base set of installation conditions (e.g. ambient temperature of 40°C, installation in conduit / raceways, etc). It is important to note that these ampacities are only valid for the quoted types of cables and base installation conditions.

3.2 Installed / Derated Ampacities

When the proposed installation conditions differ from the base conditions, derating (or correction) factors can be applied to the base ampacities to obtain the actual installed current ratings.

International standards and cable manufacturers will provide derating factors for a range of installation conditions, for example ambient / soil temperature, grouping or bunching of cables, soil thermal resistivity, etc. The installed current rating is calculated by multiplying the base current rating with each of the derating factors, i.e.

I_c = I_b . k_d

where I_c is the installed / derated ampacity of the cable (A)

I_b is the base cable ampacity (A)

k_d are the product of all the derating factors

For example, suppose a cable had an ambient temperature derating factor of k_amb = 0.94 and a grouping derating factor of k_g = 0.85, then the overall derating factor k_d = 0.94x0.85 = 0.799. For a cable with a base ampacity of 42A, the installed / derated ampacity would be I_c = 0.799x42 = 33.6A."

Plesase read IS 1255 1983 .This gives the IS standard for selection and laying of under ground cables.

Dear All

Please note I have not asked about how to do the cable sizing and even also not asked about of the basic of derating factor. I have seen the topics discussed earlier. I have specifically asked about the grouping factor for the under ground cable as per indian standard. this is given in IS 3961 fig 1 to 4 . which is not sufficient to derive the data.