2 conductors in a conduit

Status
Not open for further replies.

aramg83

Member
Why is it that 2 wires are only allowed to fill a conduit to 31% whereas more than 2 are allowed til fill it to 40% of its internal area?
 

haskindm

Senior Member
Location
Maryland
You probably have not pulled much wire. 2 wires will easily jamb each other and make it hard to pull, 3 or more wires will nest together an be easier to pull given the same percentage of fill.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
It's simple geometry:

Notice that the smallest percent fill, on the table is for two conductors (no more than 31% of the conduit?s interior volume). The reason for this is that two conductors, of the same size, collectively form an oval shape. One, or any number of cables greater than two, will tend to form a circular shape (see figure ZZ).

http://www.lanshack.com/DesigningConduitRuns.aspx
 

Strife

Senior Member
Because you're thinking square. Think round.
Also because you're thinking the 31% rule is for the same reason as the 40% rule.
The 40% rule is for heat dissipation, the 31% rule is to be able to fit the wires in.

For the sake of simplicity, take two wires 1" thick (nominal diameter). Now draw a circle around them. Your outside circle will have a diameter of 2".
Now take 4 wires put two on top, two on the bottom and draw a circle around them. Then draw a square around them, the corners of the squares will reach the circle. So the diagonal of the square is the diameter of the circle. Low and behold, even though we doubled the area occupied, we only increased the diameter by 40%. The circle that 2 wire occupy is 2" while the circle 4 wire occupy is only 2.8".
Now based on same assumption let's backward engineer a pipe to fill 40% with 2 wires. You'd have 0.11 between the pipe and the wire on each side (less than 1/8 of an inch), or .22 space between the wires and the pipe wall (less than a 1/4" of an inch. I'd say that is mighty hard to pull, good luck if you ever try it.
On the other hand the conduit sized for 40% fill with 4 wires will give you 0.18 from either side of the wire, or 0.36 altogether. That's almost double the space between wires and the wall of the conduit.
Like I said, you're assuming the percentage rule in both cases is for heat dissipation (or for being able to fit them in), when in fact they are for two completely different reasons.

Why is it that 2 wires are only allowed to fill a conduit to 31% whereas more than 2 are allowed til fill it to 40% of its internal area?
 

iwire

Moderator
Staff member
Location
Massachusetts
The 40% rule is for heat dissipation, the 31% rule is to be able to fit the wires in.

Like I said, you're assuming the percentage rule in both cases is for heat dissipation (or for being able to fit them in), when in fact they are for two completely different reasons.


I do not think heat dissipation has to do with either percentage, it is all about fitting the conductors in without damaging them.

Derating address heat and it applies regardless of fill. In other words, five 12 AWGs in a 1/2 conduit would get the same derating as five 12 AWGs in a 4" conduit.
 

Strife

Senior Member
I do not think heat dissipation has to do with either percentage, it is all about fitting the conductors in without damaging them.

Derating address heat and it applies regardless of fill. In other words, five 12 AWGs in a 1/2 conduit would get the same derating as five 12 AWGs in a 4" conduit.

Yeah, kinda thought that after I posted, might be wrong there (have to think some more), but he still thinks square when he should think round:)
 
Status
Not open for further replies.
Top