Cable sizing to bs7671

[STONE85]

Hello,
In regards to cable calculations to bs7671. If we have a cable that needs to run through a duct bank for a small section of its run say 5m and the remainder >80m is in a subway. When we derate can we ignore the duct bank portion?

Thank you

 

[che12345]

Mr. STONE85 (Industrial)(OP)20 Jul 23 05:49
"...In regards to cable calculations to bs7671. If we have a cable that needs to run through a duct bank for a small section of its run say 5m and the remainder >80m is in a subway. When we derate can we ignore the duct bank portion?"
1. NO. Reason e.g. for thermal insulation cable length is > 0.5m , shall be derated by a factor 0.5. It is noted that duct bank is NOT the same as in insulating material, therefore the derating factor may deviate from 0.5; but the length of 0.5m shall apply.
2. this short length (5m) operating temperature must NOT > the temperature rating of the insulation.
Che Kuan Yau (Singapore)

 

[7anoter4]

N.E.C.[USA] states other way:
NEC [USA]
310.15 Ampacities for Conductors Rated 0–2000 Volts.
(A) General.
(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where different ampacities apply to portions of a circuit, the
higher ampacity shall be permitted to be used if the total portion(s) of
the circuit with lower ampacity does not exceed the lesser of 3.0 m
(10 ft) or 10 percent of the total circuit.

 

[7anoter4]

BS 7671/2008 permits no exception. See Appendix 4. However, it is only informative.