De-Rating Factor for Power Cable in Tunnel

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prashanth Member Join Date: Jan 2008 Location: BANGALORE Posts: 6	De-Rating Factor for Power Cable in Tunnel 02/19/2008 4:30 AM
	what is the De rating factor for power cable is running in tunnel In one of my project, i calculated cable size between 33kV switchgear and PDT transformer in normal over head cable routing.distance between 33kV switch gear and PDT transformer is around 900 meters. The problem is we have to cross the main road approx 25 meters ,so we have to run thru the tunnel of size 25 Meter (L) X 2250mm (H) X 1400mm(W).(both side closed with vent pipe for air circulation) if i take the de rating for the tunnel ,then i have to go for 2R X 1C x 630sq mm.that means it will double my total length approx 14000 Meters instead of 7000 meters Can we ignore the 25 meters Tunnel length i checked up the STD (IEEE ,IEC ,IS etc) also. __________________ prashanth N K
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Abbas Associate Join Date: Sep 2007 Posts: 49 Good Answers: 1	#1 Re: De rating Factor for Power Cable in tunnel 02/19/2008 5:11 AM
	Dear Friend, Here is the sub-clause 523.8 of IEC 60364-5-52: Electrical installations of building, Wiring system. I think it is also applicable for your case. " 523.8 Variation of installation conditions along a route Where the heat dissipation differs in one part of a route to another, the current-carrying capacity shall be determined so as to be appropriate for the part of the route having the most adverse conditions " You can improve the current carrying capacity in the tunnel by increasing the tunnel H and W, or considering more distance between cables, or .... __________________ A thousand-mile journey begins with the first step.
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Bill Guru Join Date: Dec 2005 Location: Clemson, South Carolina Posts: 1722 Good Answers: 18	#2 Re: De-Rating Factor for Power Cable in Tunnel 02/19/2008 11:02 AM
	"The problem is we have to cross the main road approx 25 meters ,so we have to run thru the tunnel of size 25 Meter (L) X 2250mm (H) X 1400mm(W).(both side closed with vent pipe for air circulation)" If the tunnel is well-ventilated, it can be ignored. Can air be forced through the tunnel to keep it the same temperature as the ambient air? __________________ We have met the enemy and he is us . . . Walt Kelly
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prashanth Member Join Date: Jan 2008 Location: BANGALORE Posts: 6	#3 In reply to #2 Re: De-Rating Factor for Power Cable in Tunnel 02/19/2008 10:24 PM
	Hello bill thanks for ur reply can u please provided more info on this subject __________________ prashanth N K
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Bill Guru Join Date: Dec 2005 Location: Clemson, South Carolina Posts: 1722 Good Answers: 18	#6 In reply to #3 Re: De-Rating Factor for Power Cable in Tunnel 02/20/2008 4:16 PM
	According to most electrical codes, the current carrying capacity of conductors must be derated as a function of temperature. Look up the derating factors for the code you're subjected to in your part of the world. If the temperature of the tunnel can be ventilated so that the temperature is close to ambient, outside air temperature, then there is no need to derate the conductor. The size of the tunnel is so large that it is not considered to be a conduit. __________________ We have met the enemy and he is us . . . Walt Kelly
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Anonymous Poster	#4 Re: De-Rating Factor for Power Cable in Tunnel 02/20/2008 7:45 AM
	Depends, what else is in the tunnel? Steam pipes? You need information on the maximum ambient temperature seen in the tunnel, What is the insulation on the cable rated temperature? 75 deg C? 90 deg C? What is the load on the cable, Typically motors need a larger sized cable (We usually multiply the FLC by 1.25 and size the cable for the adjusted FLC...
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Anonymous Poster	#5 Re: De-Rating Factor for Power Cable in Tunnel 02/20/2008 2:00 PM
	According to my calculations you can ignore the tunnel, since it is a small portion of the run anyway. Don't worry nobody will check your work. Unless someone dies.... That's a bummer, Hate when that happens....
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sumit salunke Participant Join Date: Oct 2009 Posts: 1	#7 Re: De-Rating Factor for Power Cable in Tunnel 10/17/2009 7:13 AM
	you should take 1000mm2 power cable because it laying on ground
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Abbas (1); Anonymous Poster (2); Bill (2); prashanth (1); sumit salunke (1)

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