running parallel wires

Status
Not open for further replies.
zappy

zappy

Senior Member
Location
CA.
Just trying to learn whats involved, and how much you can be off. Like is every wire cut exactly the same before you pull it always? Can another wire be longer then another? by how much? Is there a % like +/- 1"? Thank you for your help.
 
zappy said:
Just trying to learn whats involved, and how much you can be off. Like is every wire cut exactly the same before you pull it always? Can another wire be longer then another? by how much? Is there a % like +/- 1"? Thank you for your help.
Click to expand...


The NEC says 'they shall be the same length'.

They really should add a percentage of tolerance to that section as in the real world no two conductors are 'the same length'
 
We try and get them the same length and route the the same at either end. I have seen more issues with conductors routed improperly, then with length.

Also had current sharing issues with conductors the exact same length and installed properly.
I did not think it was an issue but the inspector did, the conductors at full load were within their
current carrying capacity but about 7-10% current difference, the inspector (Army Engineer) insisted
on perfect load balance between all conductors. After several days of testing and the EC trying different
things the inspector finally relented.
 
brian john said:
We try and get them the same length and route the the same at either end. I have seen more issues with conductors routed improperly, then with length.

Also had current sharing issues with conductors the exact same length and installed properly.
I did not think it was an issue but the inspector did, the conductors at full load were within their
current carrying capacity but about 7-10% current difference, the inspector (Army Engineer) insisted
on perfect load balance between all conductors. After several days of testing and the EC trying different
things the inspector finally relented.
Click to expand...

I belive I have seen somewhere that concutor lengths should be within 10% of each other.
 
Length

Length

As already said length by code "to be the same" and as was said in the real world very unlike they are "exactly" the same. Where I see the biggest problem is with routing. Normally, when pulling in conductors you pull them in have some extra (unless you have a wire stretcher in your tool box), you route them as neatly as possible which usually requires cutting off some on the phase closest to where the conductors enter the box, reason being (probably not stating something most of you dont already know) especially with larger conductors you the way they deflect you are hard pressed to get them in the lug(s ) with out putting to much tension on them. Anyhow, at this point you may need to cut off some conductor, hence they are now not the same length. I have never read anything about a +/- % but I would say as stated before on a longer run it may not be a huge deal but if your going from a xfmmr to a panel very close you may run into issues.

In other words its up to the wire gods...
 
Pierre C Belarge said:
I wonder Bob, how would/could that be enforced?
Click to expand...
Pierre,
How is the existing rule enforced? If it was really enforced we get red tags on every pallellel installation as they are never exactly the same length. The inspectors are permitting a reasonable difference in lenght now.
 
Otdr

Otdr

Is that similar to an optical time domaine reflectometer (OTDR) :) used one many years ago testing fiber...never thought to use something like that to measure length on parallel conductors, which we have never been called on in an installation nor do I recall it ever being checked.
 
Pierre C Belarge said:
Check out this link
http://en.wikipedia.org/wiki/Time-domain_reflectometer
Click to expand...

Very interesting.

Can these be used or are they typically used in industrial settings to locate faults on cables so that it may be possible to replace only part of the cable as opposed to the whole thing?

Dont you have to know the speed at which the pulse will propagate determined by the cables sqrt (L/C) if I recall correctly? How do you determine this for a cable in the field?
 
Would it really matter if the cables are the same length and the impedance of the cable was off by 10%?

Length has no bearing as long as impedance is the same. But real world say's that it would be physically possible to have the exact same impedance unless you resort to significant figures.

So, what would be the acceptable tolerance?
 
Pierre C Belarge said:
Hmmm, that is an interesting thought. I would venture to say that maybe the CMP might have thought about this in the past.

I wonder Bob, how would/could that be enforced?
Click to expand...



IMO as the code section is written today it is imposable to comply with or enforce, it can't be done.

If there was a tolerance provided contractors could comply with the section.

Tolerances are normally provided for many things, look at the specifications for an multimeter and most of them have tolerance. :)
 
kingpb said:
Would it really matter if the cables are the same length and the impedance of the cable was off by 10%?
Click to expand...
If the circuit is fully loaded, that could result in the shorter conductor operating above its T310.16

Length has no bearing as long as impedance is the same. But real world say's that it would be physically possible to have the exact same impedance unless you resort to significant figures.
Click to expand...
Yes, code rule should probably reference the impedance of the conductors and not the length. A problem with a termination can make a big difference in the impedance and cause the same problems as different lengths.
 
Status
Not open for further replies.
Top