The code says the paralleled conductors in each phase, neutral, or grounded circuit conductor shall be the same length. Now that is virtually imposable to make them 'exactly' the same length so all you can do is try to make the runs as equal as possible.
The longer the total length the less critical this issue becomes.
IMO a 6" difference on a 200' run is no big deal, a 6" difference on a 24" run is unacceptable.
Bob (not recommending a 6" difference)
[This message has been edited by iwire (edited 09-04-2005).]
Bob Badger Construction & Maintenance Electrician Massachusetts
See 2002 NEC 310.4 (1) No tolerances given. You could see for yourself if theres any imbalance by calculating the resistances, but depending on the size of the conductors, I don't see that 1" could matter.
[This message has been edited by Redsy (edited 09-04-2005).]
When this issue was addressed during my apprenticeship days, (a long time ago) it was made clear to us that EXACT LENGTH was not the real issue, rather that running the parallel conduits by the same route and being as near as practically possible were the goals of the code. There are situations where it could be tempting to run the conduits by different routes to the same point. If you do that the conductors cannot be judged to be the same length. When the wire is cut for you at the wholesale supplier, how well do you trust the length anyway? Do you unroll them all and measure? Do you use an electronic device to measure?
Let's look at this from a different poit of view....
Any imbalance, caused by the differing resistances of the wires, will be expressed in heat.
I suppose that resistance measurements could be taken- but there is still the mattter of tolerances.
One thing that does not change is the temperature rating of the terminations. I suppose it could be argued that if the circuit can run three hours at full load, without exceeding 75 degrees C at the terminations, you're 'within tolerance.'
The current will divide in an inverse relationship to the lengths. If you have a 49' run and a 51' run, the 49' run will carry 51% of the total load and the 51' run will carry 49% of the total load. In most cases a difference of 5% or even more in length will not cause any problems, but as others have said there is no tolerance in the code and would be a violation. Don
Reno, I've seen the aftermath of a gross difference in distance of parallel feeders (about 5 feet on 4/0) one raceway got so hot the paint all burned off of the wall behind it. Wish I'd had a camera then, melted globs of insulation running out of the end of the conduit. One of the guys I was working with at the time said that since ti was AC power the Eddy currents contribute to the heat as well.