Answer for USA, Canada and countries running a 60 Hz supply service.
For permanently installed equipment in tunnels I find no special limitation on what can be used. I take this to mean that high voltage conductors in rigid non metallic conduit may be installed in a tunnel if other provisions of the code are followed, such as using schedule 80 PVC when the conduit may be subject to physical damage.
As for mobile or portable equipment, non metallic raceways may not be used in tunnels.
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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
Conduit is just a medium to get the wires from one place to another. There is no voltage restrictions in low voltage wiring (750 volts or less). When it comes to conduit fill, electricians are governed by the electrical code conduit fill tables. The conduit is sized to accommodate the conductor size for a specific load.
According to the wording ,2 different phase wires "can" run through the same conduit. Not only can they, but if they are part of the same circuit they must. And in cases where conductors are paralleled, meaning more than one conductor per phase as is common in commercial and industrial installations, you put one conductor of each phase and a neutral, if one exists, and a ground in each conduit. Not doing so, meaning to put all of one phase in one conduit, causes a voltage to be induced into the conduit and a tremendous amount of heat builds up. Even when using pvc or other non-metallic conduit, somewhere along the way you have something metallic and have the same problem. To simplify, you must put all conductors associated with the same circuit in the same conduit.
Reduces fire hazard. Jcastillo
The minimum cover requirements for direct buried conduits in vehicular areas is governed by the voltage in the conduit. Under 750 volts 600 mm (24 inches), over 750 volts 1000 mm (36 inches).
In the electrical industry there are many different types of sleeves used. There are insulation sleeves, linesman's high voltage sleeves and conduit sleeves to name a few. The insulation sleeve is used for the protection of electrical conductors. Linesman's sleeves are used for extensions to the protective rubber gauntlets that they wear when working on high tension wiring systems. Conduit sleeves are used when a broken conduit needs to be repaired in situ.
Conduit is just a medium to get the wires from one place to another. There is no voltage restrictions in low voltage wiring (750 volts or less). When it comes to conduit fill, electricians are governed by the electrical code conduit fill tables. The conduit is sized to accommodate the conductor size for a specific load.
noyou can not
According to the wording ,2 different phase wires "can" run through the same conduit. Not only can they, but if they are part of the same circuit they must. And in cases where conductors are paralleled, meaning more than one conductor per phase as is common in commercial and industrial installations, you put one conductor of each phase and a neutral, if one exists, and a ground in each conduit. Not doing so, meaning to put all of one phase in one conduit, causes a voltage to be induced into the conduit and a tremendous amount of heat builds up. Even when using pvc or other non-metallic conduit, somewhere along the way you have something metallic and have the same problem. To simplify, you must put all conductors associated with the same circuit in the same conduit.
There are no voltage regulations. A rigid conduit system used two locknuts and one bushing at every connection to a junction box unless the box has a hub. One outside the box and the other locknut inside the box. These are tightened against each other to make the connection secure and to scratch the paint on the outside of the box to make continuity of the metallic system. Continuity of the system is a must as this is the return path for any short circuit that might occur. Rigid connections to fittings require no locknuts as the conduit's threaded end screws into the fitting.
5% from source to farthest load.
There are two things here to keep in mind. If you are trying to add more conductors to an existing conduit run use the cross sectional area of the conduit to see if more wires can be fitted. The second thing that's relates to length is the voltage drop at the load. To size the conduit work backwards from the load amperage to size the wires. If a long distance is involved a voltage drop calculation will be needed. Then select the conduit size to accommodate the wires recommended for that load amperage. The length of conduit run is not brought into the equation on de rating wire size it is always about keeping the voltage up at the load end of the run by increasing the wire size and selecting the proper conduit size for that wire.
Reduces fire hazard. Jcastillo
derating
The voltage regulator is in the alternator.
3 % (+ or -) on LT side is generally permissible. (in India)
If you are talking about the primary wires to the transformer then yes they have to be encased in concrete.
The question can't be answered without knowing what the voltage is. That is because the allowed voltage drop is 5% of the supply voltage, so you need to know the supply voltage. Then the wire calculation aims to find out what the minimum size of wire is that produces that voltage drop or less.