Allowable pressure on an 11KVA cable
Allowable pressure on an 11KVA cable
(OP)
I am planning some temparory works to repair a bridge.
Part of these works require a 180t crane and there is an 11KVA cable running though the middle of the site.
The cable owner has stated that we are not to damage it (no ....), however has left it up to us to do this and will not provide any useful info on allowable stress on the cable.
I plan to protect the cable using compacted earth.
So I need to know what is the maxium allowable settlement and pressure on the cable.
Further more what else do i need to know/consider?
regards
Part of these works require a 180t crane and there is an 11KVA cable running though the middle of the site.
The cable owner has stated that we are not to damage it (no ....), however has left it up to us to do this and will not provide any useful info on allowable stress on the cable.
I plan to protect the cable using compacted earth.
So I need to know what is the maxium allowable settlement and pressure on the cable.
Further more what else do i need to know/consider?
regards






RE: Allowable pressure on an 11KVA cable
The NEC Table 300.50 indicates the minimum required cover-for direct-buried 30 in is required, for instance.
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
For something the size of a 250 Ton crane, I'd still lay down timber mats.
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
For "SOIL PRESSURE CAUSED BY POINT LOAD":
SigmaZ=3*P*z^3/2/pi()/R^5
where
SigmaZ=soil pressure at Z [ft.] depth lbs./ft^2
P=point load [lbs.]
R=sqrt(Z^2+LENGTH^2)
for a depth of 30 inches a force of 350000 lbs. [160 metric tons]
will produce 8376 lbs./ft^2
If the cable diameter would be 2 inches the force exercised on a foot length could be:
8376*2/12=1396 lbs.
Compared with maximum permitted side pressure of a cable around a pulling wheel
it is less than permitted as side-pressure: 1500 lbs./ft. for a tape-shielded cable.
RE: Allowable pressure on an 11KVA cable
The resulted pressure has to be more: 3075 lbs./ft^2
that means only 160000 lbs.[73 metric tons] is the point load in order to produce 8376 lbs./ft^2
If the truck of 180 tons stays on 3 wheels[at least] and we may divide the force by 3 then 30 inches depth could be fair.
RE: Allowable pressure on an 11KVA cable
Unless this is a paved roadway or concreted apron it is unlikely that the soil itself will be able to support the ground loadings you suggest. In softer ground the cable will be subject to tensile stress as the surrounding soil compresses and the cable will stretch as it deflects vertically. Most manufacturers publish maximum pulling tensions for applications where the cable is installed in a duct; this might be a starting point for calculating the maximum loading if the soil conditions are known.
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
RE: Allowable pressure on an 11KVA cable
Regards,
Lyle