Ampacity of URD

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Julius Right said:
It seems a cable could overcome 31.8% overload in cable duct for 4 hrs.-for instance- but transformer only 5%.

See IEEE 5796/1993 Table 6 compare with IEEE 242/2001 9.5.2 Overload capacity[for cable].

I thought if one intends to replace the actual transformer with another larger and keep the secondary cable.
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Can you copy and paste the table?
 
See attachment[if it succeeded!]
 

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mbrooke said:
You mean a trafo can take an overload because it has thermal mass, but not a cable?
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A transformer has inherent thermal mass. A cable in free air does not have enough thermal mass to be a factor in an overload situation as opposed to a fault situation. But the same cable in a duct has an ambient temperature which is dependent on the temperature of the surrounding earth, which in turn does have a significant thermal mass in addition to a thermal resistance.
 
I agree with you, GoldDigger. I took a cable of more than 250 mcm XLPE insulated in duct bank. If one will take 250 mcm PVC insulated, in air [no conduit] the maximum overload permissible will be only 11%.[still more then for a standard dry-type distribution transformer.]
 
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