Assuming that the phase loads are balanced and there is no neutral
current or ground current and that the resistance is that per conductor:
Then each line has 900 amps flowing through a total resistance of 0.015
x 30 ohm.
Thus each line loss is I^2(R) = 900x 900 x 0.015 x30
The total loss is three times that, ie 900 x900 x0.015 x30 x 3 watts.
--
Sue

Ohm's law is quite useful here. power = current squares x resistance.
Power = 900 x 900 x 0.015 x 30
which my calculator gives as 364.5kW
(& probably 3 times that if there is 900 amps flowing in each line)

0.15 ohms per mile comes to 0.15/5280 x 1000 = 0.0284 ohms per 1000
ft.
This corresponds to 400 Kcmil at 75 degrees C copper using table 8 of
the NEC.
The ampacity of 90 degree C 400 kcmil is 380 amperes. Utility
overhead lines are run at 120 degrees and assume a 40 degree C
ambient.
Even at this higher temperature, it appears without further
calculations that 900 amperes is much too high of a current for this
conductor.

hmmm, now we're thinking!
local nuke here (Dominion) wants nrc special dispensation to run the rods
higher to increase plant output beyond originally spec rating.
engineering says orginal specs were conservatively written to ensure
original permitting was accepeted without severe review.
safety measures were reviewed to assure any unusual incident occurence could
be met within nrc standard.
ok, so now somebody in newsgroups wants to know if some xmiss line can
handle more than it was designed for....
am i paranoid, or is the element of pure profit causing bending of rules and
compromise of safety??
ok, now lets find out if the BWR really can huff and puff its way over the
hil.......... :-))
wrote:

0.15 ohms per mile comes to 0.15/5280 x 1000 = 0.0284 ohms per 1000
ft.
This corresponds to 400 Kcmil at 75 degrees C copper using table 8 of
the NEC.
The ampacity of 90 degree C 400 kcmil is 380 amperes. Utility
overhead lines are run at 120 degrees and assume a 40 degree C
ambient.
Even at this higher temperature, it appears without further
calculations that 900 amperes is much too high of a current for this
conductor.

0.15 ohms per mile comes to 0.15/5280 x 1000 = 0.0284 ohms per 1000
ft.
This corresponds to 400 Kcmil at 75 degrees C copper using table 8 of
the NEC.
The ampacity of 90 degree C 400 kcmil is 380 amperes. Utility
overhead lines are run at 120 degrees and assume a 40 degree C
ambient.
Even at this higher temperature, it appears without further
calculations that 900 amperes is much too high of a current for this
conductor.
------------------
I would suggest that the person who set this problem just extracted some
numbers out of the air. numbers together without considering that they were
not realistic. However they suffice to deal with the concept at a beginner
stage.
Also, I am not too sure that I would want to run ACSR (as opposed to copper
which no utility uses) at 120C. There will be an irreversible deterioration
in conductor life which will happen every time the conductor reaches this
temperature.

--

Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer

And with such temperatures isn't there also a problem with sagging in the
line? ISTR that running very hot lines can lead to the expansion of the
line and sagging to dangerous levels.
But the OP had said 0.015 ohms per mile, so it must be larger than 400
kcmil...
daestrom

IIRC, the big outage back in '03 began with a heavily loaded
transmission line (on a hot day) sagging into some trees.
'Dangerous levels' is somewhat ambiguous. All power lines sag more at
higher temperatures. Their design must take this into account when
allowing for clearances, insulator string swings, etc. Then, its up to
the maintenance folks to keep trees clear of the lines and operations
not to overload them beyond their thermal limits.

--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
------------------------------------------------------------------

In addition, with ACSR, there can be a problem with "birdcaging" due to
differential expansion of the steel core and the aluminum conductor. I don't
think that this is completely reversible- correct me if I am wrong. I am
going on memory of a long ago look into the effects of overloads
(particularly repeated ones) on overhead conductors.
You reminded me of a situation many years ago (over 50) in Quebec when, on a
hot day with well above normal loading due to a combination of things there
were repeated mysterious outages on a 240KV line.
Mysterious until a local farmer called in and complained that the wires were
hitting his barn roof.
This is an exercise in some circuits course where the intention was to find
out whether the student could apply I^2R to a 3 phase line- as opposed to a
single phase line. The realism of the actual numbers given isn't important
at this stage (sure, it would be nice to use data pertaining to a real
conductor just to say there is an real application for this) - the concept
(which Troy apparently hadn't grasped) is the point.
The side track is more interesting than the original question.

--

Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer

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