Conduit Fill

[Kao Chen]

OK, I'm going to ask a real beginners question but it's a topic in which I've never read or heard a good explanation.

How does the proximity of multiple current carrying conductors, Table 310.(B)(3)(a), apply when looking at the conduit fill tables of Chapter 9?

 

[Kao Chen]

Sorry, 2011 NEC.

 

[north star]

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All "current carrying conductors" emit a certain amount of

heat when the circuit is being used.......The larger the

conductor, the more the amount of heat is being

transmitted through them.........That heat has to be dissipated

somewhere inside the conduit \ raceway........To be able

to effectively dissipate the heat, ...there is a limit to the

number of conductors that can be installed inside the conduit

\ raceway.



& = &

 

[cda]

"""How does the proximity of multiple current carrying conductors, """

Not an electricalian , but doesn't ?

 

[ICE]

Conduit fill and more than three current carrying conductors in a raceway are not related.

I don't know, but I've been told:

Current flow in a conductor produces a magnetic field around the conductor. AC current produces an expanding and collapsing magnetic field. If a conductor is moved through a magnetic field, electricity is created. That is how generators operate. The same thing occurs if the wire is stationary and the magnetic field is moving. Therefor, the expanding and collapsing magnetic field induces a current flow in the wires next to it. That current isn't worthwhile and will not do work. So that extra worthless current impedes the current that will do work. Somebody that understands this stuff will have a name for it.

 

[cda]

Electro Magnetic Pulse??

 

[north star]

@ 8 @ 8 @



O.K., ...I went out on the "world wide webby" and found some more

info on this topic.

**Kao Chen**, your listed Table reference is incomplete, so I am

assuming that you meant Table 310.15(B)(3)(a), from the `11 NEC,

...which has to do with "the number of current Carrying

Conductors In A Raceway Or Cable".

This explanation is from the IAEI:



"You will notice in the title of Table 310.15(B)(16) that it is based on

three current-carrying conductors, so if we exceed that number we

have to do an ampacity adjustment................The idea here is that when

conductors carry electricity, they heat up according to the load.

Since our table has only taken into account three conductors, we have

to lower the values allowed when we use additional current-carrying

conductors....................The table to reference is Table 310.15(B)(3)(a),

where we find that if we use 5 current-carrying conductors, we would

have to apply an 80% correction factor to the values given in

310.15(B)(16).......................So, a 2 AWG aluminum XHHW-2 conductor

would be good for 80 amps instead of 100 amps if there were only

3 conductors......................A note that has to be covered here is found in

the second paragraph of 310.15(B), which states the adjustment or

correction of a conductor is allowed to be applied to the rated

temperature value of that conductor.........................So if you have a

conductor which falls in the 90 degree column, then you can start

with that ampacity value, as long as the final calculated ampacity value

does not exceed the temperature limitations of the termination points.

If your conductor starts in the 90-degree column and you derate it

to a value that exceeds the temperature rating of the equipment

( which is generally rated at 60 or 75 degrees C ), you are not

allowed to use that conductor at a value above the termination

ampacity..............So, for the example of 2 AWG XHHW-2, if you

derated it from 100 amps to 80 amps, but we were terminating it

onto equipment that was only rated for 60 degrees C, we wouldn’t

be allowed to run it at an ampacity higher than 75 amps.................This

works throughout the ampacity adjustment portion of the code;

however, because you can start at the actual conductor temperature

rating, it frequently allows us to run smaller conductors overall due

to the improved insulation properties of the higher rated conductors."

Here is the link to this article:

http://iaeimagazine.org/magazine/2013/11/16/article-310-conductors-for-general-wiring/

@ 8 @ 8 @

 

[Paul Sweet]

As far as I know, Table 310.(B)(3)(a) and the conduit fill tables of Chapter 9 are unrelated. As others explained, Table 310.(B)(3)(a) is to limit heat buildup from multiple conductors. The conduit fill tables of Chapter 9, as well as the limitation on 360 degrees of total bends between pullboxes, are to limit the force required to pull conductors and prevent damaging them.

 

[north star]

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Paul,

I agree with you in that the Tables are not directly related,

but IMO, ...there is the intent to limit the number & size of

conductors in a particular sized raceway for heat buildup in

that raceway; as well as, the number of bends allowed on

the conductors themselves.

Just my 2 cents worth.

Also, ..."thanks" for your input !..........I thought we might

have some more input on this topic.

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