"mike hide" wrote in message
...

"guest987" wrote in message
news:uWuyd.546239$Pl.492525@pd7tw1no...
"Duane Bozarth" wrote in message
...
[...]
Despite your best attempts, eventually there virtually
surely will be
a leak and water will find a way in. Only
underground-rated cable
should be used in an underground run despite the
conduit. Normally
(unless there is an access tunnel in an industrial site,
for example)
conduit is used to protect the cable from the above
ground junction to
the required depth then the cable is laid in a trench.
A protective
barrier is sometimes used over the cable, but not
normally full run
buried in conduit. It's not an immediate danger, but
eventually it is
virtually certain to get water...

I see. So conduit was never meant to be a substitute for
rubber-sheathed
underground-rated cable in the first place. And here
people were
conveying
to me that rubber-sheathed underground-cable was something
new in that it
didn't require conduit underground! So I chose conduit
thinking that was
the 'tried-and-proven' standard method of laying
underground cable. I now
see I got it wrong. Well, the cable hasn't been tied in
to the power yet
(decided to wait until I can get the cash to hire a
licensed electrician
for
the inside-of-house wiring --mainly for insurance
reasons). Guess I'll be
pulling out all the romex (as well as the
single-strand-wires) from the
conduit and replacing with underground-rated cable then.
I have junction
boxes underground too where power gets split to serve two
separate
destinations. (The rubber-sealed junction box covers, I
had reinforced
with
silicone sealant --but come to think of it, silicone does
in time lose
some
of its effectiveness), Guess I'll have to modify the
cable layout scheme,
in order to serve all of the separate power destinations,
without those
junctions...


So according to the above does that mean that all pvc
underground
installations will eventually fail .I.E. water pipes
sprinklers etc.......?
mjh

All such installations will not fail...but some will fail..
depending on the circumstances. Even copper and galv steel
pipe fails after a time. When I put a job in, I put it in
with the failure mode in mind.

For instance I wouldnt run pvc water lines in a
basement..where physical damage could flood the basement...
but I might use a more durable cross linked plastic in the
basement. Or copper which can fail but its failure mode is
pin hole leaks not breaking in half. ..same considerations
with electrical. We use PVC underground.. it doesnt
corrode. But its not nearly as strong against getting hit by
a shovel as ridgid conduit.. rigid metal conduit however can
rust through... in a salt water infiltrated area it wont last
more than a few years.... in other areas metal would be your
best choice.. none of this is fully coverable in the NEC
code.

The code states though that one must use his head. In some
cases, industrial sites for instance, I run electrical in
grossly oversized schedule 40 galvanized steel water pipe
under areas where later trenching is likely..with a concrete
bulk head at each end..so that a back hoe can hit it without
tearing it up. Cost? Not much... 40' of 3" pipe and few
sacks of dry mix is under a few hundred dollars installed.

If its 240 volts in light gage wire the risks are lower than
say 480 volts direct from a large transformer in 000 wire...
that can easily be fatal if hit underground... so the heavy
480 goes in a lot differently than light amperage 240 vac say
#6 or lighter wire... distance from the utility service is
also an issue. the closer you are to the meter (and the heavy
utility feeders, and the heavier the utility transformers
(they are being paralleled lately to serve larger loads) the
greater the 'bolted' short amperage is... this is explosive
the closer you get to the heavy feeders....

..... at a distance from the heavy feeders on ligher wire,
there is not enough current carrying capacity in the wire to
create an explosion when shorted. (Google search term on
that range of issues would be 'arc flash, explosion'... this
can easily exceed a few sticks of dynamite... hard to believe
aint it?).

These arc flash issues are becoming more relevant now than
previously due to the utility company's need to parallel
service drop transformers to service a load... what happens is
that the combined transformer circuit breaker is now *grossly
oversized...so a dead short doesn't trip them in milliseconds
as before, but allows hundreds of thousands of amps to flow to
the short for half a second or a second or longer before
tripping increasing the danger and magnitude of the explosion
exponentially.

New regulations are in the works to code the floor areas in
industrial and large commercial buildings for instance...
around utility service main panels and transformers .... so
that only licensed electricians are permitted...with warning
lables applied detailing the nature of the transformer circuit
breakers and 'bolted short' time to trip ratings.



Phil Scott