Jam Ratio

[timm333]

Does jam ratio for conduit sizing apply only to three or more equal sized conductors/cables. What if the conductors/cables are not of the same size?

 

[Gac66610]

ok may i ask what you mean by "jam ratio"?
are you referring to conduit fill?
how many you can "jam" into one conduit?

 

[don_resqcapt19]

FPN No. 2: When pulling three conductors or cables into a raceway, if the ratio of the raceway (inside diameter) to the conductor or cable (outside diameter) is between 2.8 and 3.2, jamming can occur. While jamming can occur when pulling four or more conductors or cables into a raceway, the probability is very low.

It appears to apply to 3 conductors of the same size.

 

[mike7330]

ok may i ask what you mean by "jam ratio"?
are you referring to conduit fill?
how many you can "jam" into one conduit?

I believe he is talking about note 2 in ch 9 tables. I don't have my code book with me but he is correct about asking.
The thought is that when 3 cables are pulled into a conduit they will flaten out and jab in the conduit
(Not to scale.. 1inch id with 3 cables with .35 od when laying flat they are wider then in id of the pipe)

 

[Gac66610]

Did not know that, i'm a resi

Back to skwell :dunce:

:lol:

 

[bob]

The fact that you have 1 cable that is larger can still cause the jamming. The cables tends to flatten out as they are pulled through the 90 degree turn. There are means
to ensure the cable remain in the triangular configuration while making this pull.
The sites below have some good information. The tables assume the 3 cables are the same.

http://www.polywater.com/jamming.html]
http://ecmweb.com/mag/electric_simple_calculations_cable/

Timm have you been through this process before?
Very good question.

 

[timm333]

I have not had this problem in the field, but still want to be sure. There is lot of literature available on the internet about jam-ratio. But there are two questions unanswered.

1. Does jam-ratio apply only to 3 conductors, or more than 3 conductrors as well?
2. Does jam-ratio apply to conductors/cables of the same size (in same conduit), or to different sized conductors/cables (in same conduit) as well?

 

[infinity]

The three percentages for conduit fill in Chapter 9 should take care of all jam ratio scenarios. That why for one conductor you're allowed 53% fill, for two conductors 31% fill and for 3 or more 40% fill.

 

[kwired]

I have not had this problem in the field, but still want to be sure. There is lot of literature available on the internet about jam-ratio. But there are two questions unanswered.

1. Does jam-ratio apply only to 3 conductors, or more than 3 conductrors as well?
2. Does jam-ratio apply to conductors/cables of the same size (in same conduit), or to different sized conductors/cables (in same conduit) as well?

I think jamming issues can exist in those other situations but with 3 conductors same size and the correct to raceway size ratio is probably when it happens the easiest and has most effect on pulling force needed.

 

[don_resqcapt19]

The three percentages for conduit fill in Chapter 9 should take care of all jam ratio scenarios. That why for one conductor you're allowed 53% fill, for two conductors 31% fill and for 3 or more 40% fill.

Rob,
I don't understand what you are telling us here. Can you expand on it?

 

[timm333]

Infinity is saying that if you follow NEC table 1 of chapter 9, then the problemtic range of jamming ratio (2.8-3.2) will be automatically avoided.

But the thing is that if this is true, then why did they put informational note no. 2 at the bottom of table 1 of chapter 9.

 

[K8MHZ]

Infinity is saying that if you follow NEC table 1 of chapter 9, then the problemtic range of jamming ratio (2.8-3.2) will be automatically avoided.

But the thing is that if this is true, then why did they put informational note no. 2 at the bottom of table 1 of chapter 9.

So 40% would be a jam ratio of 2.5, correct?

 

[don_resqcapt19]

The percentage fill is nothing more than a percentage fill. It has nothing to do with the jam ratio.

 

[K8MHZ]

The percentage fill is nothing more than a percentage fill. It has nothing to do with the jam ratio.

And can't be converted mathematically from one to the other?

 

[timm333]

No these are two different things. 40% is the conduit fill. And the jam-ratio is the ratio of inner-diameter of the conduit to the outer-diameter of cable.

EXAMPLE: what is the jamming ratio (or jam ratio) for three single conductor THHN #2AWG cables in a 1" rigid metal conduit?

SOLUTION: The inner diameter of 1" conduit = 1.063". The outer diameter of one (not three) single conductor #2AWG cable = 0.3840. so the jam ratio is 1.063/0.3840=2.76.

As jam ratio of 2.76 lies outside the preblemtic range (2.8-3.2) so the installation is acceptable.

 

[K8MHZ]

No these are two different things. 40% is the conduit fill. And the jam-ratio is the ratio of inner-diameter of the conduit to the outer-diameter of cable.

EXAMPLE: what is the jamming ratio (or jam ratio) for three single conductor THHN #2AWG cables in a 1" rigid metal conduit?

SOLUTION: The inner diameter of 1" conduit = 1.063". The outer diameter of one (not three) single conductor #2AWG cable = 0.3840. so the jam ratio is 1.063/0.3840=2.76.

As jam ratio of 2.76 lies outside the preblemtic range (2.8-3.2) so the installation is acceptable.

I think I got it.

So, the jam ratio would be the same for 2 #2AWG conductors in a 1" conduit as three, correct?

 

[don_resqcapt19]

I think I got it.

So, the jam ratio would be the same for 2 #2AWG conductors in a 1" conduit as three, correct?

There is no issue with a jamming possibility when you only have two conductors in the raceway. You have to have at least three. Take a look here.

 

[K8MHZ]

There is no issue with a jamming possibility when you only have two conductors in the raceway. You have to have at least three. Take a look here.

I realize that as the FPN was specific to three conductors. My question was purely mathematical.

Good info on the link though.

 

[infinity]

Rob,
I don't understand what you are telling us here. Can you expand on it?

I thought that I remembered reading something that said those Chapter 9 percentages were related to safely installing conductors in raceways without damage including the possibility of jamming. Can't recall where but found this:

8-201 Log #436 NEC-P08 Final Action: Reject
(Chapter 9, Tables 1 & 4)
____________________________________________________________
Submitter: Ben Stuckey, Piper Electric Co., Inc.
Recommendation: In Chapter 9, Tables 1 and 4 of the 2005 NEC the following
revision is recommended:
Where 2 conductors are listed, the percentage of conduit fill should be in proportion
to 1 conductor and over 2 conductors, not less. Therefore, the percentage
of conduit fill for 2 conductors would be revised to 46 percent of conduit
fill.
Substantiation: Why would more than 2 conductors be permitted to occupy
more conduit space than only 2 conductors? Shouldn?t this be a lesser percentage
in order to dissipate heat more effectively? The percentage of conduit fill
should be in proportion to the amount of conductors in the conduit. Therefore,
the percentage of conduit fill for 2 conductors would be revised to 46% which
would relate directly to the percentage of conduit fill for 1 conductor and over
2 conductors.
Panel Meeting Action: Reject
Panel Statement: Heat dissipation is only one concern when installing conductors.
In a raceway, ease of installation and removal of conductors, and jamming,
are also required to be taken into account.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 12

 

[don_resqcapt19]

A quick look at rigid and thhn conductors shows that three 1/0s in an 1 1/2", 3-4/0s in a 2", 3-300kcmil in a 2.5" and 5-500kcmil in a 3" all have a high probability of jamming. It is interesting that in each of these cases, the maximum fill for the size of conductor that produces a probability of jamming when installing 3 conductors is 4.