Space for Power Cables Laying Through PVC Duct

Have you referred the technical catalogue of cable manufacturer? it gives guidance on various derating factors depending on the ambient conditions for cable laying.

yes, tht factor affect the current carrying capacity only, and its enough also. it can carry up to 480ampere in 50degree temp.

The usual language is based on how many conductors in a conduit/raceway; you have three, filling ~75% of the 110mm conduit. I think you need larger.

First of all I don't think the cable diameter is 95 mm. Usual 3*240 sqr.mm 3.3/6.6 kV armored cable diameter is only 76 mm [or less!]. If the duct thickness is only 5.3 mm [DIN 8061-10 bar -] then inner dia.is 99.4 mm and 60% is 77 mm [border line].

According to NEC maximum permissible cable cross section it is 53% from rated [4"=101.6 mm]=74 mm.

I guess that you don't realize that there are wiring standards for your location that address this very issue. You should find out what standards apply to your location. Since you did not know that these standards existed, you are not qualified to do this work.

Best source is to check the tables in the "National Electrical Code" book. If not listed in those tables, they give the formulas for determining maximum fill for conduits. If possible also get the "Guide to the NEC Code" book". It helps a lot and explains it in more useable words.

If a code book is not available, it is also within the National Fire Protection Association codes as section #70.

Good Luck, Old Salt

Use a code book.

If you can't find one in your local library, then check with your local electrical union.

There is ALWAYS a need for heat dissipation. What idiot told you otherwise?

You need to refer to the cable manufacturer's specs for the derating factor applicable to the installation conditions. Remember that different types of cable have different current carrying capacities (PILC, oil cooled, XLPE, etc)

If the cable run in concrete embedded duct bank I don't think the insulation or jacket thickness or other layers-as shield, screen concentric neutral, armor or serving [even the duct size] are so important in temperature rising.The main temperature drop will be in the concrete and the earth [it depends on earth RHO, depth and temperature].

BZZZ. Wrong

May be I exaggerate a little. If we take from Nexan Co. catalogue-according to BS-a 3.8/6.6 (7.2) kV Three Core armored copper conductors cable and run in a 4" duct [concrete thermal resistance of 1 k.m/w] 0.8 m depth in 120 k.m./w 30 dgr.C earth -we'll get 350 A ampacity [for 90 dgr.C xlpe insulation].The temperature drop [including 30 dgr. in earth temperature drop] will be as following:

Insulation: 11.55%; jacket, armor and serving: 10.88%, from cable to inner surface of the duct: 14.80%; the thickness of duct: 3.64%; the concrete and earth: 59.13%.The percent will be the same [approx.] if we use 6" duct instead of 4".

Your remarks about thermal conductivity of different layers are very interesting.

No doubt the heat will flow along the metallic layers-as shield or armor. However, nor IEC 60287 neither Neher & Mc Grath takes this in calculation [except the eddy current or circulating losses induced here].

I think also-as you said- the duct diameter is dictated from mechanical consideration.

By-the-way, my affirmation that the concrete and earth thermal resistances are more important it is only in this specific case.

The cable installation conditions and the subsequent de-rating factors depend upon the temperature tolerance of the cable insulation. XLPE can withstand higher temperatures than, for example PILC cable, and can therefore be run at higher currents.