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First, please go buy a copy of the NEC.
Since I know you will do that, I am going to help with the tables:
Use 400.5(A)(1) for normal 60 degree C cables.
Use 400.5(A)(3) for ampacity adjustment for more than 3 current carrying conductors.
Use 520.44 for 75 and 90 degree multicables where diversity is 50% minimum.
For explanation of diversity, see my article in the CB Wiki under Diversity, Electrical.
Cheers
ST
Thanks Steve,
I found a 1998 copy of the CEC buried in our shelves of manuals, and i noticed some differences between NEC and CEC... Ive attached the CEC copy. View attachment 9390
Did that table come from the CEC or is it someone's "interpretation"?
While the notes reference "not more than 3 conductors in cables", the table lists 4/0 copper at 235 amps--which would be wrong for a single conductor 90 degree 4/0 feeder. That cable is rated 405 amps in the NEC. Also, the correct code term would be "3 current-carrying conductors".
I would be very suspect of this table--it looks like someone has mixed in portable cables and wire in conduit. Also the wording of note 5 is especially "un-code-like".
This is another reason to have the code document itself in front of you--then there is no question of the wording.
Finally a 1998 CEC is old!
ST
That was from the organization that manages entertainment safety(Not a government organization) in my province.
I found the 2012 Code Amendment PDF, and it has a ratings table in it as well. It seems that they are mixed in. Ive attached the page in question.
The BC Safety Authority is the trades AHJ in BC. View attachment 9399
It could be related to the fact that down here we have a common set of wiring rules with New Zealand but different current carrying capacity tables because the ambient temperature is warmer here than there.
Likewise, Canada is probably cooler and so can eek a bit more capacity out of the cable safely
The temperature ranges in Canada are extreme, in South West Ontario we can see -20 centigrade in the winter to + 35 centigrade in the summer, these are outdoor temperatures of course, indoors we heat and cool our buildings.
Please note that the Provincial Electrical Codes are the governing documents, they basically follow the Canadian Electrical Code but each province has the authority to adopt codes and standards they wish. the same approach applies to building, fire and gas codes etc. It is best practice to use the applicable provincial code for where you are working.
The adoption of the Canadian codes into the provincial codes normally allows for equipment to be used in all provinces - but it is not foolproof. Where I work we continue to have to modify mchine tools that meet the Canadian requirements but do not meet Ontario requirements before they can be used.
The conductors are the same. The difference is how efficiently the heat from the conductor is transferred to the environment around it. The amount and type of insulation affects it, as well as whether it is in conduit or free air. If conductors get severly hot, the resistance increases which then increases the amount of heat produced, a viscious circle.
A wise electrician taught me to never pile or bunch portable cable that is handling significant current. It needs air around it to avoid head build up. This is especially true when excess cable is rolled up.
I do not recall ever seeing this written about, but there is one factor on finer stranded conductors that must have a detrimental effect on it's rating. We make the assumption that all strands are in conduction, but anybody that has put a connector on rubber insulated cable has run across the oxidization of strands. I can't help but believe that even with a good crimp, all strands are not brought into conduction. I don't know whether the problem has got to do with the manufacturing process, or oxygen permeation of the rubber itself, but it cannot benefit conduction. Even new stock may appear this way, although I have seen some that incorporates a thin plastic wrap between the insulation and the conductor that appears to fair much better. Compare this to THHN, where the copper is almost always shiny and clean.
I have to wonder if any studies have been done on this. In fact, I have to think that there must have been studies and I just never happened on one.
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