Minimum Bending Raduis for MV Shielded Cable

[rob569]

Ok, 300.34 reads,

300.34 Conductor Bending Radius.
The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

The top potion is discussing single conductors and the potion discussing the multiconductors or multiplexed shielded is what we are dealing with.
As strange as it my seem the contractor, believes that the calculation refers to one cable, but I contend that it is refering to all 3 cables in the multipex (triplexed is what the cables are configured). The difference is great when you speak of all three cable diameter, instead of one of course. In this case the single conductor 500kcmil EPR Shielded Cable diameter is 1.38 by the manufacturer detail, that would mean that the bend radius would be somewhere near 49" inches. I just wanted to know the correct interpretation of the code. Is it the one single conductor or all three in the multiplex configuration? My calculation would look like this:

MV cable diameter = 1.38" X 3 (number of MV cables in the multiplex) = 4.14"

12X4.14" = 49.68" bending radius

The code is definitely concern with maintaining the protection of the MV cable, by stating of the two calculation take the one that has the greater bend radius. So it is clear they are not trying to give any allowances on the bending of the cable and I am talking about a 4160V cranking motor.

The bend radius may not come into play today or even a year from know, but maybe a few years down the line, my company would be absorbing the cost of replacing the cable and a shut down of the Units, if the MV cable is damaged because of a bad install.

 

[drbond24]

Ok, 300.34 reads,

300.34 Conductor Bending Radius.
The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

The top potion is discussing single conductors and the potion discussing the multiconductors or multiplexed shielded is what we are dealing with.
As strange as it my seem the contractor, believes that the calculation refers to one cable, but I contend that it is refering to all 3 cables in the multipex (triplexed is what the cables are configured). The difference is great when you speak of all three cable diameter, instead of one of course. In this case the single conductor 500kcmil EPR Shielded Cable diameter is 1.38 by the manufacturer detail, that would mean that the bend radius would be somewhere near 49" inches. I just wanted to know the correct interpretation of the code. Is it the one single conductor or all three in the multiplex configuration? My calculation would look like this:

MV cable diameter = 1.38" X 3 (number of MV cables in the multiplex) = 4.14"

12X4.14" = 49.68" bending radius

The code is definitely concern with maintaining the protection of the MV cable, by stating of the two calculation take the one that has the greater bend radius. So it is clear they are not trying to give any allowances on the bending of the cable and I am talking about a 4160V cranking motor.

The bend radius may not come into play today or even a year from know, but maybe a few years down the line, my company would be absorbing the cost of replacing the cable and a shut down of the Units, if the MV cable is damaged because of a bad install.

The number closer to 4 feet is the correct one. If you bend a 500 MCM triplexed cable at a 4 inch radius (if that is even possible) you will destroy it.

It does say overall diameter. What part of that is your contractor having trouble with?

 

[steve66]

Is there a shield around all three conductors??

If not, it looks to me like you would use the larger of:

12 x 1.38", or
7 x the entire cable diameter.

Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.

steve

 

[walton]

If in doubt, contact the cable manufacturer and ask them what the recomended bending radius is for that type cable. They should have bending radius, side wall pressure and pulling tension calcs for every cable they make.

 

[rob569]

Is there a shield around all three conductors??

If not, it looks to me like you would use the larger of:

12 x 1.38", or
7 x the entire cable diameter.

Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.



steve

This is EPR shielded cable and that number was the manufacturer's O.D. for the cable. I didn't make that up. They are ran in a 4" rigid conduit, but it comes out of the pipe and they put a pretty tight bend on it, my Siemens rep. really brought it to my attention, I just followed up on it.

 

[rob569]

The number closer to 4 feet is the correct one. If you bend a 500 MCM triplexed cable at a 4 inch radius (if that is even possible) you will destroy it.

It does say overall diameter. What part of that is your contractor having trouble with?

No they did not have a 4" bend radius, they orginally had like a 10" or 12" and they mess with it and it went to almost 15", but that is stil not enough, even if you take the smaller of the 2 calculations. I think this is one of those cases where no one actually question it and now all I'm hearing is I've seen worst. If they modify it and put it in tray coming out of the conduit, they would have to meet the requirement, becuase of the tray they would have to use.

 

[rob569]

Anyway my question was if you have the cables in a triplex, three cables wrapped in a triangle configuration (this is what they would call a multiplexed single conductor cables) how would you calculate it. Like I did or using only one cable out of the three, I think given how the code reads you would use the three totaled diameters together and then multiply that.

 

[Smart $]

Is there a shield around all three conductors??

If not, it looks to me like you would use the larger of:

12 x 1.38", or
7 x the entire cable diameter.

Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.

steve

The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

The overall diameter would be 2.97.

It's kind of hard to physically measure the overall diameter of triplex

7 ? 2.97 = 20.8
12 ? 1.38 = 16.56

Minimum bend radius is 20.8

 

[drbond24]

Anyway my question was if you have the cables in a triplex, three cables wrapped in a triangle configuration (this is what they would call a multiplexed single conductor cables) how would you calculate it. Like I did or using only one cable out of the three, I think given how the code reads you would use the three totaled diameters together and then multiply that.

Given those two choices, the answer is neither. It is certainly not the diameter of just one of the conductors. However, the overall diameter of the cable is also not the sum of the individual diameters. The number you want is somewhere in between.

The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

The overall diameter would be 2.97.

It's kind of hard to physically measure the overall diameter of triplex

7 ? 2.97 = 20.8
12 ? 1.38 = 16.56

Minimum bend radius is 20.8

It is still too early in the morning for me to test this. I've never actually calculated it. I've always drawn the cable in AutoCAD and then just put a circle around it and measured the diameter of that circle. I don't have AutoCAD handy at the moment to do that now.

Your best bet is to just ask the manufacturer what the overall diameter of the cable you have is.

 

[rob569]

The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

The overall diameter would be 2.97.

It's kind of hard to physically measure the overall diameter of triplex

7 ? 2.97 = 20.8
12 ? 1.38 = 16.56

Minimum bend radius is 20.8

So you think the way the code reads it is suggesting that you take just one cable diameter out of the triplex and then calculate that. Interesting. I was waiting for that intereptation, I guess I will have to see what the manufacturer says about this now.

 

[rob569]

Given those two choices, the answer is neither. It is certainly not the diameter of just one of the conductors. However, the overall diameter of the cable is also not the sum of the individual diameters. The number you want is somewhere in between.



It is still too early in the morning for me to test this. I've never actually calculated it. I've always drawn the cable in AutoCAD and then just put a circle around it and measured the diameter of that circle. I don't have AutoCAD handy at the moment to do that now.

Your best bet is to just ask the manufacturer what the overall diameter of the cable you have is.

Well, I check the cable size and it is what you see there in my first post. Why would you need autocad to do the wire calculations. It is pretty simple we just need to know if the code meant take one cable out of the troplex or all the cables out of the triplex, if they only meant one, then why even right the second part of the code they could have included the multipleexed in the single cable paragraph. Dont forget we are not trying to bend one cable here but three individual cables group together.

 

[drbond24]

Well, I check the cable size and it is what you see there in my first post.

You put the size of a single conductor in your first post. You need the overall diameter of the cable. Ask the manufacturer for that.

Why would you need autocad to do the wire calculations.

You don't. I was being conversational.

It is pretty simple we just need to know if the code meant take one cable out of the troplex or all the cables out of the triplex, if they only meant one, then why even right the second part of the code they could have included the multipleexed in the single cable paragraph. Dont forget we are not trying to bend one cable here but three individual cables group together.

This has been answered 5 times at this point if you count the one further up in this post. You need the overall diamter of the cable. Smart $ was showing you how to calculate it using the information you have because, like he said, it is very difficult to physically measure it. I suggest you just call the manufacturer. Either way, you need the overall diameter of the cable (that was the 6th time).

OVERALL DIAMETER (just to get to lucky 7).

If you don't get it yet, then I suggest you don't understand the question. The overall diameter of the cable, like I said before, is not the diameter of one conductor and it is not the diameter of all three conductors. If you want someone to pick one of those answers and nothing else, you'll be waiting a long time. It would be like saying "What color is the sky, green or yellow?"

Imagine if you slid the triplex into a large paper towel tube. Now imagine that the paper towel tube that you have is exactly the right size for the cable to slide into with no extra room. The diameter of that imaginary paper towel tube is the number you are looking for. It will be larger than the diameter of one conductor but smaller than the diameter of all three conductors added together. It will be the overall diameter of the triplex.

 

[weressl]

Ok, 300.34 reads,

300.34 Conductor Bending Radius.
The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

The top potion is discussing single conductors and the potion discussing the multiconductors or multiplexed shielded is what we are dealing with.
As strange as it my seem the contractor, believes that the calculation refers to one cable, but I contend that it is refering to all 3 cables in the multipex (triplexed is what the cables are configured). The difference is great when you speak of all three cable diameter, instead of one of course. In this case the single conductor 500kcmil EPR Shielded Cable diameter is 1.38 by the manufacturer detail, that would mean that the bend radius would be somewhere near 49" inches. I just wanted to know the correct interpretation of the code. Is it the one single conductor or all three in the multiplex configuration? My calculation would look like this:

MV cable diameter = 1.38" X 3 (number of MV cables in the multiplex) = 4.14"

12X4.14" = 49.68" bending radius

The code is definitely concern with maintaining the protection of the MV cable, by stating of the two calculation take the one that has the greater bend radius. So it is clear they are not trying to give any allowances on the bending of the cable and I am talking about a 4160V cranking motor.

The bend radius may not come into play today or even a year from know, but maybe a few years down the line, my company would be absorbing the cost of replacing the cable and a shut down of the Units, if the MV cable is damaged because of a bad install.

Follow the manufacturers installation instructions.

 

[drbond24]

Follow the manufacturers installation instructions.

Are you suggesting that the manufacturer will instruct them to do something other than follow the NEC? Or are you saying to ask the manufacturer and they will give you the NEC compliant bending radius?

I hope its the latter. That's the one I like.

 

[drbond24]

The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

The overall diameter would be 2.97.

It's kind of hard to physically measure the overall diameter of triplex

7 ? 2.97 = 20.8
12 ? 1.38 = 16.56

Minimum bend radius is 20.8

Ok, I finally had a chance to work through this and I agree 100%. I got 2.155 so you're more accurate than me.

OP, this means that if you know the diameter of one of the single conductors within the triplex, all you have to do is multiply that number by 2.155 and you will have the overall diameter.

Therefore, in your case:

1.38 * 2.155 = 2.97 inches.

I know Smart $ already came up with that number, I'm just confirming that it is correct. Read Smart's post to get your minimum bending radius.

 

[jdsmith]

Are you suggesting that the manufacturer will instruct them to do something other than follow the NEC? Or are you saying to ask the manufacturer and they will give you the NEC compliant bending radius?

I hope its the latter. That's the one I like.

If you're buying reputable cable, not "Joe's MV-105 cable" being sold under the Omniwire name or something like that, the manufacturer's recommendation will be NEC compliant and save you the calculating.

 

[rob569]

If you're buying reputable cable, not "Joe's MV-105 cable" being sold under the Omniwire name or something like that, the manufacturer's recommendation will be NEC compliant and save you the calculating.

No the cable was bought from Aetna:roll:

 

[weressl]

Are you suggesting that the manufacturer will instruct them to do something other than follow the NEC? Or are you saying to ask the manufacturer and they will give you the NEC compliant bending radius?

I hope its the latter. That's the one I like.

The cable should be installed in accordance with the manufacturers instructions and certainly to the NEC. Whichever requirement results in a larger diameter, that should be followed.