I have a question about paralleling cables.
I know the U.S. NATIONAL ELECTRICAL CODE states parrallel cables must all be
the same length.
however in practice it is not always practical to have them all exactly the
same. what is an acceptable allowance on the difference?
the situation I have is 12 parrallel cables per phase 12.5 kv 2410 amps
3 phase 60 cycle.
36 cables total, 12 per conduit. quanity of six, six inch ridgid galvenized
2 A, 2 B , 2 C cables in each conduit.
With a 330 foot length will a difference in length of 6 feet in cables of the
same phase cause an unacceptable inbalance of current??
I am not an enginer, but an electrican. I have seen people spend an extra 50
or more hours in a situation like this to get the cables all the exact length.
Is this nessasary??
From what I read and calculate, using the impedance of the cables at 2410
amps/12 = 200.8 amps per conductor.
effective impedance of 500 MCM cable in steel conduit shortest 330 ft. long =
effective impedance of 500 MCM cable in steel conduit longest 336 ft. long =
the rest of the cables are all between this in proportional lenghts 300' 8",
301' 16" 302'4", ect due to the difference in length of the bends in conduit.
the formula I have found is
I long cable = I total x Z short cable/(Z long cable + Z short cable)
I short cable = I total x Z long cable/(Z long cable + Z short cable)
The shortest cable would carry a current of 202.6 amps
the longet acurrent of 199 amps
Is this the correct way to figure the current inbalance???
I understand all cables must be the same insulaton type, conductor type, same
type of conduit ect.
If you assume equal temperatures, you will be conservative in the current
split. That is, you will calculate more current in the shorter conductor than
will actually be the case. The shorter conductors will carry more current
because the resistance and inductance will be less. With higher temperature,
the resistance will increase, reducing the difference between the short and
There should be an engineer that designed that job. Find him or her and
have he or she give you the answer. If he says they all have to be the
exact same length then ask for chapter and verse.
It dosen't sound well engineered to me.
The length can vary but duct configuration, fill arounf the duct ( dirt,
sand, concrete etc.) and the loading cycle are things that need to be
He's using 12 cables at 200 amps each when he could have use 6 rated at 400.
Six phase conductors in one conduit have to be derated to 80 % of of that
with 3 conductors.
If he does want the cables to be the exact same length then start what ever
paperwork you have to get more money for the job.
It would be a well designed duct bank if every conduit it it were
exactly the same length. In reality they are always different by 5 or
6' ( could be more).
So would you measure the longest run and cut all cables to that
length? You might have to put loops in the cables that were to long (
tricky with 15 kV cable). Ypu could cut to the shortest length and use
a cable streacher on the shroter ones.
Cables that are different by a few feet in a run of 360 will stabalize
and operate fine for ever if they are designed correctly. The cable
in the group that will cary the highest percentage of the load can be
identifed and evaluated. If it's going to operate at 110% of it's
rated load then the cables may fail one after another.
Brain may be lazy but he may be doing the work of an even lazier
engineer. If the route is not the same for each cable then the
engineer needs to verify on the drawing ( of somewhere ) that he has
evaluated the situation and it is acceptabel. If not he needs to
desing the duct bank, vaults and nmination sections of the gear such
that each cable can be cut the same length with 1/16".
If the engineer doesn't say whats good then the inspector may want
them to be within 1/32", something that may be impossible. So the real
question is what is the "Same".
I will refer you to a situation. We had 4-500 kcmil per phase in cable tray
about 26 feet in total length. The cables were in a flat configuration with
4 A-phase cables, 4-B Phase cables, 4-C-phase cables. The individual A and
B phase and B and C phase cables that were next to each other carried about
350 amperes. The outside cables for A-phase and C phase carried about 30
amperes. This was on the Alaska's north slope in about 1982 on the Houston
pad. Word got around and engineers from all over the slope came by to look
at the readings. They simply could not believe their eyes. They all agreed
that it was an inductance problem where the adjacent cables had lower
inductance and therefore carried most of the current. The Code now requires
that parallel cables in cable trays be bundled to prevent this.
Now on the exact same length for parallel cables - I have often wondered
about this. I know for a fact that most electricians do not cut the cables
to the exact length but cut them for the convenience of installation and for
looks. I have done this myself on occasion. We are supposed to cut the
same amount from each set of parallel conductors for each phase, but we
seldom do. I have not seen or read any literature about how this effects
the current carrying of the conductors. I am sure it is not as bad as the
situation that I previously described.
Seems to me that it would be better to bundle conductors in groups of 3 or 4
(if neutral is carried) and then parallel the three phase "feeders."
Each feeder would with be twisted (random) or have the conductors
This would cancel out most inductive coupling effects and make the current
division in reverse proportion to length.
In the original post, if each feeder were in its own conduit, one feeder
being a few percent longer than other would reduce its current by only a few