Conduit Sizing

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480sparky said:
Wouldn't that pressure translate into friction? And more force needed to overcome that friction?

Seems it's always harder to pull through 200' of pipe when theres's 2 or 3 90s in it as opposed to the exact same length but a straight shot.
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Yes it is harder to pull around bends but, as Bob said, assuming that the bend meets the minimum radius, the required pulling force does not go down when you increase the radius. Yes the friction per unit of length in the bend does go down, but there is more length in the larger radius bend. The total friction is about the same between the standard radius and long radius bends.
 
Rockyd said:
Try pulling four coax cables in a 3/4" EMT through a standard 90, now "open one up" on a corner, and see if you personally think its different. Just like offsets, less number of degrees used, normally faster, and more betteh bro:)
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Off hand it sounds like your overstuffing the EMT.


FWIW it is not just my opinion


Observations:

The radius of bend does not effect the pulling tension.

The size of raceway does not effect the pulling tension or the sidewall pressure. This assumes that the raceway is not overfilled and is in compliance with the NEC.

The sidewall pressure is effected by the coefficient of friction and the radius of the bend.

The larger the radius the less the sidewall pressure.

For long pulls with larger conductors sweep bends are usually necessary to lower the sidewall pressure to within allowable limits. For these types of pulls radii of standard benders cause the sidewall pressure to exceed the maximum in most cases.

The end that is the farthest away from the bends will have the least pulling tension.

Bends have a multiplying effect on the incoming tension to the bend.

Although there are no ICEA data for pushing in conductors at the feed end there is substantial evidence that this method has a substantial effect on decreasing the total pulling tension at the pull end.
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http://www.electrician2.com/electa1/ductwirepull.html


http://ecmweb.com/mag/electric_simple_calculations_cable/
 
Help me out here Don, why do a lot of utilities require sweep bends on their lateral runs?

Edit to add- Thanks Bob, excellent links.
 
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Rockyd said:
Edit to add- Thanks Bob, excellent links.
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No problem, I am just passing on what I learned awhile ago from Don. Those links came in handy when an engineer on a job I was running wanted me to provide sidewall pressure estimations with my proposed conduit routing and pulling plan. :smile:
 
At three on a Saturday afternoon, and they decide they (client) need one more screen, it's 38 miles to HD (for 1" pipe) would you do 4 coax in a 3/4"? Trust me, I normally wouldn't do it this way, but there are trying times...
 
Rockyd said:
At three on a Saturday afternoon, and they decide they (client) need one more screen, it's 38 miles to HD (for 1" pipe) would you do 4 coax in a 3/4"? Trust me, I normally wouldn't do it this way, but there are trying times...
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Yeah, it's always easier when sitting at a computer to mention shoulds and woulds.
Any way, Art. 820 (CATV) isn't subject to 300.17 (raceway fill). So it seems ok.
 
Rockyd said:
At three on a Saturday afternoon, and they decide they (client) need one more screen, it's 38 miles to HD (for 1" pipe) would you do 4 coax in a 3/4"? Trust me, I normally wouldn't do it this way, but there are trying times...
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Isn't there an increased risk of damaging/deforming the dielectric.
I am under the impression that the round shape of the dielectric is critical to the performance of the cable.
Is this true?
 
ch_0124 said:
i was thinking that you did the pie times r times r to get the circumference and that was your cross sectional area but wasn't sure.
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Actually it's Pi x Radius squared.

Multiply the radius times itself and then multiply that amount times 3.1416.

You've got the right numbers but the wrong sequence.

"Pie are square..........Wrong......Pie are round, cornbread are square:grin:...that's a joke.

steve
 
Isn't there an increased risk of damaging/deforming the dielectric.
I am under the impression that the round shape of the dielectric is critical to the performance of the cable.
Is this true?
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Don't know, my neighbor is a CATV guy for the local company. He just said it was a tight fit, don't do it often, and that yes, he got a clean signal. SOLD! I normally don't do the comm work. he is twice as fast as most of the guys around, so happy that he does clean work on CAT5, CATV, Alarm systems,etc. Whenever he needs pipe ran, I'm first on his list:). So it works out sweet.
 
ch_0124 said:
When sizing conduit with multiconductors, chapter nine says to treat it as a single conductor.
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Chapter 9 says to treat multiconductor cables as though they were single conductors.

Would you just measure the diameter of the cable and convert it to square inches and then use the tables in chapter nine or would you find the circumference of the cable?
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You are correct to convert to square inches. Measure the diameter of the cable, then convert it to square inches using Pi x Radius x Radius (which is exactly the same as Pi x the Radius squared). As mentioned in other posts, for an elliptical shaped cable, you use the longest width (the major diameter) as the diameter for your calculations.

So, 0.5" diameter round-like multiconductor cable has a radius of 0.25", and an area of 0.196 sq. in. (Pi is 3.14, so 3.14 x 0.25 x 0.25 = 0.196).


I am so tired of trying to pull cables into conduits that were sized by wadding up cables and seeing if they will fit into a conduit because they don't know or don't care about the 40% fill the code recommends.
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kent
 
Forgot to finish...

I am so tired of trying to pull cables into conduits that were sized by wadding up cables and seeing if they will fit into a conduit because they don't know or don't care about the 40% fill the code recommends.
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The sizing of conduits should have been done on the design end. For smaller jobs, they should have been done at least before the conduit was installed.

Also, too many times I have seen electrical contractors arrive at my pipeline Company's large jobsites with cable types other than what was specified, usually larger cables. The reasons are usually:
1. they have some leftover from another job
2. they want to use it because they got a good price on it, or
3. most often>> they want to use direct burial cables in the conduit instead of what we specified because the tougher & thicker outer jackets take more abuse and are less likely to fail megger tests during construction (means they pay less attention to reaming out conduit cuts).

However, our company inspectors aren't too sharp when it comes to electrical (mostly concerned with our pipeline concerns) and they allow it. Then I come to the site and explain how they blew all the conduit fills and probably had to really pull those runs to get them installed.

kent
 
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