30 KV Underground Cable Sizing

Ignore.

You should have the copy at hand. Asking strangers to do this work is unethical. Cablesizer | Cable Sizing Calculations to IEC and NEC Standards

Cable Sizing Calculation - Open Electrical

The cable must be sized for the most stringent condition. Though it might be possible to splice in larger cable for the under-the-road places, it might not be practical.

Seems to me the main consideration here all things being proper, is the thermal conductivity of the backfill...now to me the cable placement could be calculated to render the thermal characteristics equal in some circumstances....going from trefoil to widely spaced flat formation...

https://www.nexans.com/Corporate/2013/60-500_kV_High_Voltage_full_BD2.pdf

http://www.ijera.com/papers/Vol4_issue9/Version%206/W4906159166.pdf

NEC 2017 Art.310.60 if the ducts total length it is more than 3 m the ampacity in duct has to be considered.

310.60 Conductors Rated 2001 to 35,000 Volts.

(A) Ampacities of Conductors Rated 2001 to 35,000 Volts

(1) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception: Where different ampacities apply to portions of a circuit, the higher ampacity shall be permitted to be used if the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit.

IEC 60364-5-52[for cables up to 1000 V] does not accept exception:

523.8 (523.7) Variation of installation conditions along a route

Where the heat dissipation differs in one part of a route to another, the current-carrying capacity shall be determined so as to be appropriate for the part of the route having the most adverse conditions.

Thanks alot for your answer can you please share the same phrase of IEC but cables volt more than 1000 V (high voltage cables)

I won't get into cable sizing but to cross two roads, try a moaling company, I've had it done with a water pipe & it saved a stack of money.

If you ignore the ducted portion of cable and the that portion of cable runs hotter than the rest, what will happen??

It is not that if fault occurs in that 15mtrs portion, the rest of the cable will continue to function??

The total feeder will be down, isn't it!!

No. Size it for the ducted section. And don't use concrete. CB sand is used.

You can use as stated previously, directional bore or a pipe jacking system to install a duct(s).

If its a busy road section, be smart and install two poles, gang links, overhead conductor and two terminations, far more cost effective, quicker and safer and offers points of isolation for future faults if the system is MV. HV, 132kV and above stay underground.

Dear all

I really appreciate your answers, it almost covered all the aspect about the issue i request information

British Standard 7671 gives full protocols on cable sizing.

You need to determine the thermal conductivity of the back fill around the entire length of the cable,and use the worst case(hottest) point to determine the cable size.

There are formulas to calculate the soil thermal conductivity in the NEC.

A chain is only as strong as it's weakest link and proper equipment procedures for feeding and placing the cable must be followed. to prevent damage to the cable.

Installation should be supervised by a fully qualified company with experience in the field.

A 30 KV underground splice is very costly,and introduces another potential failure point.

This is a very hazardous project,and should not be performed by non professionals seeking how-to instructions from a forum.

Good luck on this venture,but don't just cross your fingers and hope for the best..get professional help.