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cable ampacity in ductbanks

cable ampacity in ductbanks

cable ampacity in ductbanks

(OP)
Hi,

I have following questions for cable sizing laid in ductbanks:

Whether to consider soil temperature or ambient air temperature while calculation of cable ampacities in ductbanks?
Further how to consider small air gap in ducts carrying cables ?

Thanks.

RE: cable ampacity in ductbanks

Use soil temperature. It does not vary much with ambient air temperature.

There are several programs that calculate the cable temperature or allowable amp loading by using the Nehr-McGrath calculations. AmpCalc and ETAP are two that we have used. You enter the the duct bank configuration, size and loading on each cable in the ducts, depth of burial,and other parameters to come up with the proper calbe size for the load or the proper ampacity for the given cable. The models account for the heat transfer from the cables to the duct air space and from the air to the duct wall.

If you have access to the National Electrical Code, look at Appendix B for examples of underground duct bank calculations.

RE: cable ampacity in ductbanks

I agree with rcwilson, the ambient temperature for directly buried or in underground duct is earth ambient temperature.
Usually the cable in ductbank ampacity is calculated following IEC 60287 or Neher and McGrath methods. Never-the-less it is not specifically indicated both consider earth temperature as ambient. NEC Annex B recommends IEEE 835/1994 for detailed calculation.
IEEE 835/1994 on ch.3.1.5 Temperatures is stated: “Ambient temperatures were selected at 25 °C earth ambient for buried cables and 40 °C for air.”
Still I don’t understand what you mean by “small air gap in ducts carrying cables”. May you be more specific?






RE: cable ampacity in ductbanks

I think you meant the prisoner air between the cable[s] and the duct (pipe)- θm. This air temperature one needs to know in order to calculate the cable-to-duct [conduit or pipe] thermal resistance.
According to IEC 60287-2-1 ch. 2.2.7.1 Thermal resistance between cable and duct (or pipe) T4′.
T4’=U/(1+0.1*(V+Y*Θm)*De)
According to Neher and McGrath [eq.41):
Rsd=n’*A/(1+ (B+C*Tm)*Ds’)
Tm=mean temperature of the medium(air)in degrees centigrade.
Neher takes it as 60oC.
According to IEC you may recalculate it considering the temperature drop from inner part of the duct-through duct thickness,concrete and surrounding earth-T4*Losses .
Losses=Total Losses in all cables included in a duct [inclusive shield, dielectric, armor losses]
or to extract from Tc [conductor rated temperature] the temperature drop through insulation, bedding, jacket and from cables to inner side of the duct [T4’*Losses].
If conductor temperature:
DΘ= (I2R + ½ Wd) T1 + [I2R (1 + Λ1) + Wd] n T2 + [I2R (1 + Λ1 + Λ2) + Wd] n (T3 + T4) conductor temperature
then:
DΘ1=DΘ-(I2R + ½ Wd) T1 + [I2R (1 + Λ1) + Wd] n T2 + [I2R (1 + Λ1 + Λ2) + Wd] n T3 maximum trapped air temperature
DΘ2=[I2R (1 + Λ1 + Λ2) + Wd] n (T4) minimum trapped air temperature
θm=(DΘ1+DΘ2)/2

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