Cable ampacity derating

[TimDta]

Cable derating for Australian installation ; however general comments/answers are welcome.
1)Is it advisable to design & install a 415VAC cable where cable’s derated ampacity is equal to the circuit’s full load current ? Examples, practical experiences regarding cable failures because of incorrect derated ampacity, incorrect cable sizing will be helpful.

2)Often cables are run in ladders but the entire cable route, because of practical reasons, cannot be run in ladders; in such instances, cables are routed through sections of (say PVC) conduits. Is there any thumb-rule/ballpark figure(i.e. the conduit length as a % of total cable route length), beyond which an ampacity derating factor will be applicable?

3)A specific example: A 5 meter cable is routed in free space(let’s say the derating factor is zero in this case) with 4.5 meter of this 5m cable being routed through a PVC conduit. Should there be any ampacity derating applied in this instance? If yes, what is the recommended derating figure or are there other explanations (e.g. if this 5 meter cable is terminated to a switchboard busbar, then the switchboard busbar will act as a heat sink) which could be used to avoid the ampacity derating factor?

 

[waross]

Those questions are generally answered in your code book.
The first issue with improper derating is often a rejection by the Authority Having Jurisdiction. Removing the small cable and replacing it with a larger cable is an expensive way to size cables.
Once you properly size the cable for the 4.5 meters in PVC, the remaining 0.5 meters in free air will be acceptable.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikiBg]

IEC standard takes into account derating factors in accordance to critical part of the cable route, regardless of length of that most critical part.
There isn't derating factor for different way of cable laying, but current ampacity given in table is different if other way of laying is relevant.

 

[7anoter4]

NEC Art. 310.15 Ampacities for Conductors Rated 0–2000 Volts.
(2) Selection of Ampacity.
" Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance
equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less."