Parallel feeder fault and the result if there is difference in conductor lengths
- Thread starter Wyo721
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Strathead
Senior Member
- Location
- Ocala, Florida, USA
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- Electrician/Estimator/Project Manager/Superintendent
what is there was a 1 millimeter difference in length? Would that violate the code also?
That code is capricious and bugs me every time I think about it.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
No parallel conductors are the same length, the requirement is not practicable. As Roger stated the overall length of the run is the critical factor. IMO the NEC should allow a tolerance like no more then X% difference between the parallel conductors to make it considerate of real world conditions.
Strathead
Senior Member
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- Ocala, Florida, USA
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- Electrician/Estimator/Project Manager/Superintendent
:thumbsup:
I agree - not practicable. An allowable tolerance would seem like a reasonable suggestion. But then how would/could that be checked/policed/inspected after installation?
the method used to ensure they are exactly the same length?
if you have 500' run within 1' that is a violation?
chris kennedy
Senior Member
- Location
- Miami Fla.
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- 60 yr old tool twisting electrician
I recently was at an existing service that had parallel 350's for neutrals. One was 6' long, the other 9' long.
Had 11A on one and 1A on the other.
Had 11A on one and 1A on the other.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I don't think the rule is followed much. Lay all your conductors out on the ground and cut to the same length then install. It won't look pretty. You can tell just by looking at an install if they were actually cut to the sam length.
I don't know what your code would say about that.
How would you measure it to determine if it was a violation was my question.
there had to be something else going on
total i = 12
R = ohm/ft
current divider
one branch i = 12 x 6R/(15R) = 4.8
other i = 12 x 9R/(15R) = 7.2
length delta should have little to do with ph-ph sc i
La and Lb = length of faulted phases
i sc = v/((La + Lb) x R/ft)
obviously for a gnd fault the longer will be a bit lower
R = ph ohm/ft, Rg = egc ohm/ft
gnd Rg ~ 50% higher so = 1.5R
i gf a = (v/sqrt3)/((La + 1.5Lg) x R)
i gf b = (v/sqrt3)/((Lb + 1.5Lg) x R)
ratio i gf a/ingf b = (Lb + 1.5Lg)/(La + 1.5Lg)
as L>0 the limiting factorbis Lg
assume La = 10', Lb = 8' and Lg = 10'
i gf a= 23/25 x i gf b = 0.92 x i gf b
for a 20% ph L delta the shorter cond i gf will be 8% higher
as length >inf
assume 100' with La = 100, Lb = 90 and Lg = 100
i gf a= 240/250 x ib = 0.96 x i gf b
ia will be 4% lower or ib 4% greater for a ph L delta of 10%
Last edited:
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
Even if you used precision techniques to make the cable lengths the same, you would still have production tolerance on the diameter of the conductors themselves.
But to answer the OP, different lengths would mean different impedance, which would mean that the fault current would be unevenly distributed between the conductors.
-Jon
But to answer the OP, different lengths would mean different impedance, which would mean that the fault current would be unevenly distributed between the conductors.
-Jon
Yes. But unevenly enough to be a problem?
- Location
- Illinois
- Occupation
- retired electrician
There was something in addition to the length difference at work there. Based on just the length, the 6' one should have 60% of the current and the 9' one should have 40% of the current.
(Sorry, Ingenieur, I posted without scrolling down and reading your post)
was your comment?
what on here is 'necessary'?
it is one thing to state something
another to illustrate it
take it, or leave it, no one is compelled to review it
- Location
- New Jersey
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- Journeyman Electrician
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