Measuring Circumference of a bundle of disimilar cables

[Tarjan]

Hi All,
First post. First of all, I want to thank you all for the knowledge I have gleaned from this forum. I've only been in the electrical field for a few years and MANY times I have come here to help beat the learning curve (which at times has been steep). So, thank you!

My question:
Does anyone have a formula or a trick of determining the circumference of a bundle of different diameter cables?
We are pulling in some medium voltage cable; two runs of (3) #750 AL & 1/0 CU ground and one run of (3) 4/0 AL & 1/0 CU ground. We need some new grips. Per Greenlee's Grip Selection Guide (Cooper's too), they say to "...bundle the cables together, measure the circumference and refer to chart..." That would work if we had the cable on hand. We won't get the cable until just before the pull but we were hoping to get the grips ahead of time. I could just get one grip for each cable but I'd rather bundle them up in one grip per pull.

Now, if all of the cables were the same diameter, Greenlee's selection chart works great (I can even do the math to figure that out). But the kicker comes in when adding the smaller diameter ground to the mix. By guesstimating, I could go too big or too small.

If anybody has a trick for calculating this, without cable in hand, I'd love to hear it. Sorry if I rambled or it the answer is so simple.

Thanks,
Tarjan

 

[Smart $]

What are the spec' diameters of the conductors?

No easy way to calculate... but drawing up a bundle in CAD and measuring the circ' is pretty straightforward.

 

[wwhitney]

Does anyone have a formula or a trick of determining the circumference of a bundle of different diameter cables?

When you say circumference, do you mean (1) the length of a rubber band stretched around the bundle, or (2) the circumference of the smallest circle inside which the bundle would fit?

Either way, if you have the manufacturer's specs for the outer diameter of the wires you are going to use, then you could just draw out a cross section of the bundle and measure the appropriate circumference. You could draw it at full scale, or maybe 2x, using a compass. [Or use a CAD program as Smart$ suggests.]

BTW, which packing is going to be more compact: (a) all three larger conductors touching each other in a triangular configuration, with the fourth smaller conductor outside touching two of the larger conductor, or (b) the smaller conductor in the center, touching all three larger conductors?

It's not too hard to calculate circumference (2) of configuration (b): if the two wire diameters are X > Y, then it is Pi * (2X + max(Y, (-1 + 2/sqrt(3))*X)). When Y <= (-1 + 2/sqrt(3))*X, this is definitely the optimal packing.

Cheers, Wayne

 

[cadpoint]

There's an app for that; search for Southwire app.

I have Partial Reel on I-Phone.

 

[Julius Right]

The measuring is the better.
See [for instance]:
Kellems® Wire Management Products
http://www.farnell.com/datasheets/1816388.pdf
See on page T-26:
Multiple Cable Selection Charts for Cables and Wires of Unequal Diameters
"1 . Find the Grip Circumference Range by measuring the circumference of the bundle of different diameter cables to be gripped (see illustration) . 2 . Divide the bundle circumference by 3 .14 to determine the diameter . 3 . Choose a grip offering a range of cable diameters the same as the cable diameter."
In order to calculate the grip rated diameter it is difficult to find an accurate formula.
I used an approximate one based on filling factor-for maximum diameters ratio of 3
d=sqrt[sum(Area(i)*5/pi()]
where Area(i)=pi()/4*d(i)^2

 

[Tarjan]

Thank you all for your input. it just so happens that the Eaton/Arrow Hart Rep just happened to walk in my door yesterday and he helped kick it up the line to their in-house engineer.
Thanks Gentlemen,
Tarjan

 

[don_resqcapt19]

I would not even consider trying to put the three 750s in a pulling grip along with a 1/0. I would use a grip for the three 750s, and serve the 1/0 to the pulling rope along with eye of the pulling grip.

 

[Smart $]

I would not even consider trying to put the three 750s in a pulling grip along with a 1/0. I would use a grip for the three 750s, and serve the 1/0 to the pulling rope along with eye of the pulling grip.

I agree in principle, but I'd bring it through the grip and serve it up with the eye.

 

[don_resqcapt19]

I agree in principle, but I'd bring it through the grip and serve it up with the eye.

That may work, but I find it easier to just put the small wire on the outside of the grip.

 

[Smart $]

That may work, but I find it easier to just put the small wire on the outside of the grip.

Easier, yes. In the end, whatever works is okay by me... :thumbsup: