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.
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.
That is not what the NEC states. It states equal length. 310.4(B)I belive I have seen somewhere that concutor lengths should be within 10% of each other.
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'
Pierre,I wonder Bob, how would/could that be enforced?
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.
A TDR test could verify that the conductors were the same length.
They do not sell these in the USA.....only kidding. Ask the average electrician what a TDR tester is...
O.K. I'll ask it :grin:
What is a TDR and how does it work?
Check out this link
http://en.wikipedia.org/wiki/Time-domain_reflectometer
I belive I have seen somewhere that concutor lengths should be within 10% of each other.
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?
If the circuit is fully loaded, that could result in the shorter conductor operating above its T310.16Would it really matter if the cables are the same length and the impedance of the cable was off by 10%?
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.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.