Cable Size

[Newcomer2005]

Hi everyone,

I am new graduate in electrical, if my question is very simple please forgive me in advance.

I am looking at ontario Electrical safety code, Table 1.
It says for single conductor 300MCM cable the ampacity is 480A (85-90 celcius)

On table 2 it says for 300MCM not more than 3 conductors in cable or raceway the ampacity is 295 (85-90 celcius).

On table 5B correction factors for table 1 it says for 3 conductors in contact the derating factor is 0.85

The question is this :
I am expecting that according tables 1, and 5B on table 2 the ampacity for one multiconductor cable (1c-3#350 MCM) be 480*0.85= 408A not 295A. and why table 2 is needed at all if the procedure for calculation is right.

I think I am missing something between multiconductor cables and single conductors.

Any help and references would be highly appreciated.

Regards,

Newcomer2005

 

[LionelHutz]

Table 1 is for an individual conductor in free air. If you read some of the rules, this table also covers individual conductors with an aluminum jacket.

Table 5B is a correction factor when individual conductors are put together in free air.

Table 2 is for conductors inside a raceway or cable, or in other words when there's another jacket or conduit covering the individual conductors.

Basically, the difference between single conductors and conductors in a cable or raceway is that the heat from the conductors in a cable or raceway has to travel through another barrier before reaching free air and the cables are all together inside this insulating barrier.

 

[Newcomer2005]

LionelHutz,

What is the ampacity of these two cases:

1)One cable three conductors (1-3/c-300MCM) in pvc conduit
2)three 300MCM cables (3#300MCM in pvc conduit

Thanks and Regards,

Newcomer2005