# Bending Wire



## travis13 (Oct 12, 2012)

Two of my coworkers are in a heated exchange on the topic of bending wire too sharply and hurting ampacity or creating a hot-spot in that particular section of the wire. I'm a second year apprentice and can't answer them, so I figured I would ask the experts. Does bending wire sharply change the characteristics of the wire?


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## Black Dog (Oct 16, 2011)

travis13 said:


> Two of my coworkers are in a heated exchange on the topic of bending wire too sharply and hurting ampacity or creating a hot-spot in that particular section of the wire. I'm a second year apprentice and can't answer them, so I figured I would ask the experts. Does bending wire sharply change the characteristics of the wire?


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.


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## Black Dog (Oct 16, 2011)

300.34 covers that and I would say yes you will damage the cable if you bend it too much..


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## AK_sparky (Aug 13, 2013)

short answer...yes.


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## travis13 (Oct 12, 2012)

So physically you can damage the wire, but will it change current flow


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## Black Dog (Oct 16, 2011)

travis13 said:


> So physically you can damage the wire, but will it change current flow


The current will be the same but the wire will get hot at the bend....


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## NacBooster29 (Oct 25, 2010)

In addition to the conductor being damaged. The jacket may break. Causing a potential for trouble.


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## ponyboy (Nov 18, 2012)

I've seen countless transfer switch installs where the conductors, usually 500-750, were bent in perfect 90° angles. Ran for years and years


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## Black Dog (Oct 16, 2011)

ponyboy said:


> I've seen countless transfer switch installs where the conductors, usually 500-750, were bent in perfect 90° angles. Ran for years and years


Probably they were never loaded up..


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## Mshow1323 (Jun 9, 2012)

NacBooster29 said:


> In addition to the conductor being damaged. The jacket may break. Causing a potential for trouble.


I think you've nailed it. It's probably based on insulation breakdown


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## lannjenks (Feb 4, 2014)

This is from an article written about lightning protection but I thought it was related. Not sure how much stress a tight radius bend would cause from normal circuit current, but my guess is nothing too significant.

"Lorentz forces increase with sharper bends (lower radius) and are resisted by the size and strength of the wire material. As a rule, bends in lightning grounding wire should have at least a 10" or 30-cm radius. To eliminate bends, lugs should be positioned in line with the wire. Lugs which are in line with the ground wire are more effective than cosmetically satisfying lugs in line with an architectural component which force a directional change in the wire."


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## Wirenuting (Sep 12, 2010)

Black Dog said:


> Probably they were never loaded up..


I've seen 60+ year old "Tightly" bent conductors in LB's and the failure rate is higher in heavily loaded lines. 

Now as to were the failure was,

Inside the radius and crushed tight to the metal = 95% 

Outside radius and not touching anything = 5% or less. 
The interesting thing I saw on these failures was that the fault would burn or cut thru the LB's cover. These were not caused by contact and yet the voltage was 208 or 480. 

I always assumed that it happened due to a transient spike from outside the facility. 

I've also seen heavily loaded conductors vibrate enough to cut thru the insulation. These typically took 10 years or more to happen. They were also rotten installs were people didn't think to file edges or just crammed wires into boxes and troughs.


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## just the cowboy (Sep 4, 2013)

Installed a panel not to long ago and the instruction that came with it said "do not over bend wires to main breaker". don'tremember if it mentioned code reason or not but it made a ugly panel.


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## ponyboy (Nov 18, 2012)

Black Dog said:


> Probably they were never loaded up..


Oh but they were. We regularly run our generators during the summer to help out the local co-op


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## wildleg (Apr 12, 2009)

links: 


http://www.amercable.com/doc/catalogs/tigertips/PrimaryCausesofCableFailure.pdf


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## don_resqcapt19 (Jul 18, 2010)

300.34 is in the "over 1,000 volt" part of Article 300. It does not apply to lower voltages and is not really concerned with damage to the conductor itself. The issue is damage to the insulation. There is no issue with bending the actual conductor. It does not change its ampacity.

As far as lightning protection system conductors, the larger minimum bending radius has to do with the high voltage "jumping" off the conductor at a sharp bend.

Damage at sharp bends in conduit bodies is almost always from the damage caused by the pressure of the conductor insulation against part of the conduit body. Over time the insulation is displaced and you have a ground fault issue.


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## denny3992 (Jul 12, 2010)

ponyboy said:


> I've seen countless transfer switch installs where the conductors, usually 500-750, were bent in perfect 90° angles. Ran for years and years











Installed in 80s... But i wouldnt have bent them so hard...


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## Jhellwig (Jun 18, 2014)

http://www.greenlee.com/products/BENDER%40dCABLE-RATCHET-(796).html?product_id=15475

And it will bend 600kcmil. 

Less than 600 volt you will not have a problem unless it is touching something at the bend but that has to do with the installation being poor.

Look at any bus bar and they will be bent tight yet never burn up.


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## NacBooster29 (Oct 25, 2010)

Jhellwig said:


> http://www.greenlee.com/products/BENDER%2540dCABLE-RATCHET-(796).html?product_id=15475
> 
> And it will bend 600kcmil.
> 
> ...


Bussing isn't stranded, relying on surface contact of individual strands to carry the load. Different animal.
If the cable is bent in a way which distorts the conductor you may be having an issue with heating up. 
While I say this I have personally seen hundreds of installs with cables bent using the hydralic bender. And none burn up. 
Maybe it has to be a perfect storm of heavy load, and poor surface contact to start the heat build up.


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## Jhellwig (Jun 18, 2014)

NacBooster29 said:


> Bussing isn't stranded, relying on surface contact of individual strands to carry the load. Different animal.
> If the cable is bent in a way which distorts the conductor you may be having an issue with heating up.
> While I say this I have personally seen hundreds of installs with cables bent using the hydralic bender. And none burn up.
> Maybe it has to be a perfect storm of heavy load, and poor surface contact to start the heat build up.


The only way the cable could heat up is if it stretched as it was bent. Wire doesn't care how you distort it as long as you don't reduce the cross sectional area.


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## Mich drew (Mar 3, 2013)

If you have ever installed 600's in a 4" LB you've probably exceeded the bending radius. But it is done all the time. If you have been in the trade any length of time, and your honest about it, you have probably at some point used a 2 x 4 to assist you in urging the last couple of inches into your fitting so the cover would go on.


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## Big John (May 23, 2010)

I say BS. I don't know of any mechanism that would cause a conductor to be reduced ampacity just because it's bent. Further, I do a lot of infrared and have never seen even the slightest heat increase do to conductor bends.

Sharp bends can cause physical damage to the insulating jacket by over-stretching. In high voltage applications the sharp point focuses electric fields which creates extra stress on the insulation. As far as I know those are the only reasons to pay attention to bending radii.


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## Stickshaker (Jun 29, 2012)

A lightning strike would be like suddenly loading a pipe with a 1,000,000 psi when its been running at 480 psi for years. That tight ass bend is is going to be a great bursting point. I've seen it a hundred times. Ok, I've never seen it but it sounds logical...


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## Paulusgnome (Mar 28, 2009)

I was taught not to use sharp 90 degree bends, and still recall being made to re-wire a switchboard that I had done with sharp bends so that they were all softened out.
However, earlier this year I was doing the NZ electrician's practical assessment for my electrical registration, and on this same assessment we had electricians from South Africa and Ireland too. These guys were adamant that when they were tested back in their home countries the examiners insisted on sharp bends, and would have failed them for making the bends too gentle! Our assessor didn't have an opinion either way provided it was neat and tidy.
My own opinion is that sharp bends do look neat, but if you make them too sharp the insulation on the outside of the bends can get stretched, may break or perforate and become a danger.


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