# inrush



## backstay (Feb 3, 2011)

electricalcontractor said:


> I have 3 sets of 350 aluminum feeders in a high rise about 400' from the source 208V 3 phase, horizontal 250' and vertical about 150'. Inrush when compressor number 2 on chiller kicks in causes feeders to shake, any one have a solution, I have considered replacing the feeders with copper but customer is concerned with cost.


Can you soft start the compressor, you didn't indicate how big it was.


----------



## electricalcontractor (Apr 7, 2011)

unable to soft start, 800amp service, checked amperage at distribution panel had 330A when 2nd compressor started up


----------



## nitro71 (Sep 17, 2009)

Can't see that changing what they are made of will fix shaking. Do you have stress relief points on the run? You might be able to better secure the conductors at those points.


----------



## electricalcontractor (Apr 7, 2011)

tried that, someone even tried to quiet the wire shaking by drilling holes in each conduit and tie strapping the feeders to the conduit


----------



## BBQ (Nov 16, 2010)

nitro71 said:


> Can't see that changing what they are made of will fix shaking. Do you have stress relief points on the run? You might be able to better secure the conductors at those points.


The reason the conductors are shaking is high current durring start up. A soft start could reduce the high current.

As far as I know the options are:


Reduce starting current (soft-start, VFD, increase voltage, etc)

Increase the capacity of the conductors

Live with it.


----------



## nitro71 (Sep 17, 2009)

BBQ said:


> The reason the conductors are shaking is high current durring start up. A soft start could reduce the high current.
> 
> As far as I know the options are:
> 
> ...


How does increasing the capacity of the conductors decrease the "shaking"?


----------



## BBQ (Nov 16, 2010)

nitro71 said:


> How does increasing the capacity of the conductors decrease the "shaking"?


I don't know the science behind it but it seems conductors only noticeably vibrate or slap when loaded beyond their ratings.


----------



## nitro71 (Sep 17, 2009)

BBQ said:


> I don't know the science behind it but it seems conductors only noticeably vibrate or slap when loaded beyond their ratings.


Interesting, I understand that inrush can cause conductors to shake. Had imagined it had to do with magnetic fields. But really don't know if that is it. Maybe upsizing just makes them heavier and harder to move?


----------



## brian john (Mar 11, 2007)

BBQ said:


> I don't know the science behind it but it seems conductors only noticeably vibrate or slap when loaded beyond their ratings.


I would think, additional weight, and possible less VD in the conductors at high inrush.

Strapping to the conduit seems a bit unusual.


----------



## 76nemo (Aug 13, 2008)

brian john said:


> I would think, additional weight, and possible less VD in the conductors at high inrush.
> 
> Strapping to the conduit seems a bit unusual.


 
Very unusual. Look at BBQ's third option. Live with it. Do your term's suffer? Doubt it. Although vague, very well put, "live with it".......


----------



## Mike_586 (Mar 24, 2009)

BBQ said:


> I don't know the science behind it but it seems conductors only noticeably vibrate or slap when loaded beyond their ratings.


There's not really much tough science behind it. The linear coefficient of copper and aluminum aren't that much different either. Not enough to mean a huge difference, but copper would be a little better.

The biggest factors that come into play here is the amp rating of the cable, how far its run, and how big and fast the change in load is. The sudden change in heat being generated is causing linear expansion. Copper conductors with the same amperage rating would be a little better, without a doubt, but I don't think it would eliminate slap. Like BBQ said, increase the capacity, what material used wouldn't make a huge difference.

BBQ's already given you the answers.


Reduce starting current (soft-start, VFD, increase voltage, etc)
Increase the capacity of the conductors
Live with it.



brian john said:


> Strapping to the conduit seems a bit unusual.


 Directly strapping a 350 that's slapping would be a cause for concern in my opinion. 

I don't think it would be a matter of if, but when the cable wrap eventually breaks from it being subjected to big temperature fluctuations and tugged every time the unit cycles on and off. The other question would be if the tie wrap fails before it begins wearing through the insulation on the conductor. It might take months or even years, but I don't see it holding up over time.


----------



## Mike_586 (Mar 24, 2009)

electricalcontractor said:


> I have 3 sets of 350 aluminum feeders in a high rise


It just occurred to me.... 


Parallel runs? 
Is it only one of the three sets slapping?
Are all the runs the same length? 

You might want to make sure all your runs are the same length before trying anything else.


----------



## Big John (May 23, 2010)

They drilled holes in the conduit? So, now when the conductors jump around they're doing it inside a giant cheese grater? Good solution.

-John


----------



## electricalperson (Jan 11, 2008)

why is a soft start not an option?


----------



## AFOREMA1 (Nov 23, 2009)

electricalperson said:


> why is a soft start not an option?


Already suggested a few posts up


----------



## Wirenuting (Sep 12, 2010)

On a smaller scale, when running a feed for an AC unit across a basement , put and offset in the pipe run to hold the wires tighter. Stopped the jump & buzz on start up. 

At one 15 story building they had repulled a vertical run for one of our chillers. Every 3rd floor was a junction box. The original build had a rubber stopper installed to hold the wire. When they replaced the run they didn't reinstall the stoppers. Those wires jumped every time the chiller started. 
We reinstalled the strain relief stoppers and the buzzing went away.


----------



## sparky970 (Mar 19, 2008)

electricalcontractor said:


> tried that, someone even tried to quiet the wire shaking by drilling holes in each conduit and tie strapping the feeders to the conduit


hahahaha


----------



## guest (Feb 21, 2009)

electricalcontractor said:


> tried that, someone even tried to quiet the wire shaking by drilling holes in each conduit and tie strapping the feeders to the conduit





Big John said:


> They drilled holes in the conduit? So, now when the conductors jump around they're _*doing it inside a giant cheese grater? *_Good solution.
> 
> -John


If I were the AHJ I would red tag it because of the drilled holes. Unless it could be proven that ALL of the holes were properly deburred inside the pipe.

So now they get to replace all the damaged runs of conduit and might as well upsize the conductors while they're at it.


----------



## RePhase277 (Feb 5, 2008)

Pour the conduit full of ... ScotchKote.. That ought to cushion the conductors:laughing:


----------



## piperunner (Aug 22, 2009)

Well now lets see first take the drilled pipe out and replace it i personally would have never .

Second your not going to believe this but if its in rush vibration from your start up and there the correct size feeders who cares . If your really troubled by this or the owner or who ever is paying the bills wants it fixed read on .

To stop wire from vibration if it was magnetic which is rare but just say when they pulled it into that raceway years ago if they pulled it in a long length of emt hanging above check your conduit support it may not be the wire look at it .

Now just once many years ago long length feeders in a TG LEE milk plant here in florida we had running above exposed conduits 4 runs each of 1600amp 480v 3 phase which is 16 x 4 inch conduits on a rack 450 feet long stuff paralleled runs we had a vibration issue kinda like yours you could hear it like a loud bee flying along the rack during loading time .

Solution was made by engineer pull them out roll them up and repull them but twist or tri- plex the conductors this will cancel any left over magnetic issues .

It will also tighten up a long length of lose wire running a long way which can vibrate things due vibrate .

Guess what it worked !But it was costly and that oldman was sweating bullets when we turned power back on after a major shut down .


----------



## tgreen (Sep 14, 2010)

the noise, and slaping in the pipe is because of the inrush. what is happening is because of the surge of current through the conductors on start up the wires will actuall contract slightly. the slapping inside the pipe is caused by this sudden tension of the wires. having the wires straped to the inside of the pipe is dangerous. because the wire is still moving and the straping can cause breakes in the insulation. 
the remove this issue the proper solution is not to secure the wire in the raceway, but it is to reduce the inrush current on the conductors, or increase the capacity of the conductors. paralleling the conductors will reduce by half the current per conducotrs. you can also increase the conductor size which increases the capacity. i would look at adjusting up 2 wire sizes, or like what was stated before a soft start will reduce the inrush, so that you can use the existing conductors. either solution is expensive. 
there is no danger to the existing conductors to leave them the way they are, as long as they were properly sized in the first place. this is mearly a modification the make the owner happy.


----------



## BBQ (Nov 16, 2010)

tgreen said:


> the noise, and slaping in the pipe is because of the inrush. what is happening is because of the surge of current through the conductors on start up the wires will actuall contract slightly. the slapping inside the pipe is caused by this sudden tension of the wires.


I am going to ask for some reference for this 'contraction' of the conductors.


----------



## tgreen (Sep 14, 2010)

my reference is from observatinal experience. while i was working in a large steel mill the welders were using an air arc to remove old welds. as they would strike an arc you could watch the cable tense up and then release.


----------



## Jmohl (Apr 26, 2011)

Conductors expand and contract with current conditions. Large inrush gives you a noticible movement in the lines. Used to use a suicide plug to short receps in the plant because none of the panels were labeled correctly. You could hear the wire bounce down the pipe. Another example would be chromate cutting wire, when it's hot, it will sag. When it's cool, it is taut. if you put too much current across it, it will pop.


----------



## BBQ (Nov 16, 2010)

I have no doubt conductors move when current is applied to them, I doubt they contract due to current. Of course they do expand and contract due to heat.


----------



## tombs (May 26, 2011)

I believe conductors being tied together inside the conduit goes against code as well.


----------

