I am working on a project for a client and we are putting in an underground ductbank, (4) - 4" conduits, 2 rows and 2 columns. 2 out of the 4 conduits will have 5kv 3/c ciruicts, 1 of the remaining 4 will have a fiber optic cable and the remaining will be a spare, the duct will be about 9' deep to site obstructions, how and what is the best step to derate due to depth?
Derating due to depth of Duct Bank
- Thread starter ajbeatty3
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Duct-Bank Derating
Duct-Bank Derating
it is not so much the depth of the duct bank but what surround it (concrete encased / direct burried), etc. it is the temperature at that depth that you will need to know but you will also need to know the RHO factor, etc, the problem is the heat generated by the cables and how to dissipate this some materials handle this better than others but with heat you need to de-rate the cables the IEEE brow book is a good place to start. Hopes this helps.
Duct-Bank Derating
it is not so much the depth of the duct bank but what surround it (concrete encased / direct burried), etc. it is the temperature at that depth that you will need to know but you will also need to know the RHO factor, etc, the problem is the heat generated by the cables and how to dissipate this some materials handle this better than others but with heat you need to de-rate the cables the IEEE brow book is a good place to start. Hopes this helps.
Or with table 310.79, and 310.60(C)(2)(b), and figure 310.60 along with the note at the bottom you can take the ampacity and derate it 6% per foot for every foot over 30".
That's at least 36% derated, which may be pretty tough to make up for.
Steve
That's at least 36% derated, which may be pretty tough to make up for.
Steve
kingpb
Senior Member
- Location
- SE USA as far as you can go
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- Engineer, Registered
Good info Steve, glad to see someone understands the need to derate for depth.
9' depth? That will be a tough one to deal with, not to mention a deep manhole to get the water sloping to one end or the other.
9' depth? That will be a tough one to deal with, not to mention a deep manhole to get the water sloping to one end or the other.
kingpb said:
Kingpb:
I won't actually claim to understand why we need to derate for depth. On one hand, I can kind of see it, but on the other hand, doesn't the ground conduct the heat away just as well as the air would?
And it just seems like by the time the heat goes through 30" of soil, whatever is past the 30" wouldn't make much difference.
Steve
steve66 said:
I'm starting to think I have been doing this wrong. Since you are supposed to derate 6% per foot, I have always multiplied 6 * the extra burial depth, and then derated by that number.
For example, assume a 470 amp conductor, with a 8.5' burial. That's an extra 6' of depth (8.5' - 30" = 6').
So 6*6 = 36%. I would derate by 36% to get an ampacity of 300 amps.
If you burry a cable an extra 17' deep, that would be 17*6 = 102%, which leaves 0 amps capacity. So the cable is useless.
Maybe I am supposed to do it this way:
470 amps * 0.94 = 441 amps (derate 6% for the 1st extra foot)
441 amps * 0.94 = 390 amps (derate the new ampacity by another 6% for the second extra foot)
390 amps * 0.94 = 366 amps (3rd foot)
366*.94 = 344
344*.94 = 324 amps.
So doing the math the second way gives 324 amps instead of 300 amps.
And doing it this way, for an extra 17', the conductor ampacity would still be 36% of its original ampacity. I think that makes a lot more sense than 0 amps.
Can anyone say for sure which method is right?
Steve
kingpb
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- SE USA as far as you can go
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- Engineer, Registered
6% per 1ft, after the maximum 30" to top of duct for those configurations given. So, taking 9' as the top of the duct bank, then:
(108" - 30") x 6%/12" = 39% reduction in current carrying capacity
If you were to use the 17' depth mentioned, then it would be:
(204" - 30") x 6%/12" = 87% reduction in current carrying capacity
Working back wards, @ 230" or 19.167' depth, the cable would be derated 100%.
My understanding behind the depth, is that due to the mass of the earth and the deeper you go, the earth temperature is more constant and therefore becomes difficult to dissipate the heat of the ductbank. Sort of like putting insulation around the ductbank. The closer proximity to the surface allows the heat as it goes upward to reach the surface and dissipate.
(108" - 30") x 6%/12" = 39% reduction in current carrying capacity
If you were to use the 17' depth mentioned, then it would be:
(204" - 30") x 6%/12" = 87% reduction in current carrying capacity
Working back wards, @ 230" or 19.167' depth, the cable would be derated 100%.
My understanding behind the depth, is that due to the mass of the earth and the deeper you go, the earth temperature is more constant and therefore becomes difficult to dissipate the heat of the ductbank. Sort of like putting insulation around the ductbank. The closer proximity to the surface allows the heat as it goes upward to reach the surface and dissipate.
beanland
Senior Member
- Location
- Vancouver, WA
Depth
Depth
The thing to remember is that over time the earth absorbs the heat from the conductors until an equilibrium is reached. Once this happens, ALL heat ends up being lost through the earth-air interface on top. The deeper the cables, the more soil the heat has to go through to get out the top.
Neher-McGrath (1952) wrote the essential paper on the subject. However, there are plenty of computer programs that can compute ampacity at varying depths.
Knowing the thermal resistance of the system (duct, encasement, backfill, moisture, native soil, etc.) is required to perform a decent estimate of ampacity.
Depth
The thing to remember is that over time the earth absorbs the heat from the conductors until an equilibrium is reached. Once this happens, ALL heat ends up being lost through the earth-air interface on top. The deeper the cables, the more soil the heat has to go through to get out the top.
Neher-McGrath (1952) wrote the essential paper on the subject. However, there are plenty of computer programs that can compute ampacity at varying depths.
Knowing the thermal resistance of the system (duct, encasement, backfill, moisture, native soil, etc.) is required to perform a decent estimate of ampacity.
broadgage
Senior Member
- Location
- London, England
Only slightly O/T but I believe that the great power failure in Auckland was caused by the failure of buried cables (direct buried not in ducts)
It was said in the enquiry that insufficient allowance had been made for the poor thermal conductivity of the local soil and the high ambient temperatures.
I believe that subsequent research showed that if the surface temperature of the cable exceeded a critical figure (55 centigrade I think) then the soil would eventually dry out, and become a thermal insulator, which would result in the cables becoming still hotter.
JOKE; Q, what did Aucklanders use for lighting before candles?
A, electricity!
It was said in the enquiry that insufficient allowance had been made for the poor thermal conductivity of the local soil and the high ambient temperatures.
I believe that subsequent research showed that if the surface temperature of the cable exceeded a critical figure (55 centigrade I think) then the soil would eventually dry out, and become a thermal insulator, which would result in the cables becoming still hotter.
JOKE; Q, what did Aucklanders use for lighting before candles?
A, electricity!
beanland
Senior Member
- Location
- Vancouver, WA
Aukland CDB Power Outage
Aukland CDB Power Outage
The CDB outage was caused by underground pipe-type cables getting into thermal runaway from excessively dry conditions. It is a good conservative practice to determine the thremal resistivity of the dry soil and use that in the design. Or, to avoid dry-out, limit temperatures to 55C or lower. The resulting ampacity may be similar. Excellent CDB report available on-line.
Aukland CDB Power Outage
broadgage said:
The CDB outage was caused by underground pipe-type cables getting into thermal runaway from excessively dry conditions. It is a good conservative practice to determine the thremal resistivity of the dry soil and use that in the design. Or, to avoid dry-out, limit temperatures to 55C or lower. The resulting ampacity may be similar. Excellent CDB report available on-line.
kingpb said:
That's the way I've always done it too, but like I said, I not sure that's right.
When you add 1' foot to the burial depth (42" total), you derate the original ampacity by 6%. There's no question about that.
But then add another 1' to the burial depth (54" total). Do you derate by another 6% of the original ampacity? Or do you derate by 6% of the ampacity at 42"?
I don't think the code is 100% clear.
Steve
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