I believe, the larger gauge is required to reduce the skin effect. One large wire would have all current flowing on the outside (essentially). Multiple wires will distribute the current more evenly. Just a guess but that is my two cents. Also, solid wire in larger gauges is harder to work with (did they actually think of the workers in writing that part of the NEC? I doubt it, hehe)
NEC requires stranded wire be used above a certain gauge. I dont know what that is off the top of my head. But working in substation I am sure that is why they were ordered pulled out.
The bend radius is probably a concern depending on the application. Stranded copper has a larger bend radius and therefore can be pulled through conduit with sharper turns.
If you're talking about control wiring, the main reason for avoiding solid conductors probably has to do with wire terminations.
Most terminals in substation equipment are designed for use with stranded wire or compression lugs. Solid conductor terminations will be less reliable.
Stranded wire is also easier to work with and pull.
We typically specify solid copper wire for lighting and receptacle circuits and stranded for everything else.
The NEC 2002 Handbook says of Article 310.3 (requiring stranded conductors in #8 and larger):
"Larger-size conductors are required to be stranded to provide greater flexibility. This requirement does not apply to busbars and the conductors of Type MI mineral-insulated metal-sheathed cable. In addition, the bonding conductors of a common bonding grid of a permanently intalled swimming pool are required to be solid, nonstranded conductors of 8 AWG or larger, according to 680.26(C)."
Stranded copper wires have greater flexibility compared to solid ones. Solid wires also break easily compared to stranded ones. Have you tried cutting/breaking a small solid wire without a cutter and you just bend it (using your fingers) a lot of times until the bent area heats up then breaks? You try that maneuver with a same size but stranded wire and you will realize that it's a lot harder.
-Max
As regards use of solid wire for lighting circuits, it is generally allowed because wire costs are less and terminations and splices used are generally designed to work with solid wire.
I really don't think flexibiity is a critical factor when dealing with #12 and #10 control circuits. Stranded wire will be easier to pull, but most substation runs are pretty short.
I have seen a lot of solid control wire in very old substations, but it isn't used anymore, at least in the US.
For substation wiring(not for controls), stranded copper wire is used to reduced the vibrations caused by the currents passing thru at high voltages. Solid wires exhibit more vibrations compared to stranded ones. This is because, each strand in a stranded cable vibrates separately. However since the strands are laid tightly with each other, the vibrations in each strand counteracts with each other, therefore, reducing significantly the overall effect(vibration).
-Max
GOOD MORNING FOLKS FROM THE LAND OF OZ - THE M.I.M.S. TYPE OF COPPER CABLE CALLED PYROTENAX HAS BEEN USED EXTENSIVELY HERE ESP IN THE AREAS OF INTRINSICALLY SAFE / FLAME PROOF WIRING IN GASOLINE/OIL TYPE FUEL PLANTS - ALSO IN THE AREA OF ELEVATOR MAINS. THIS TYPE IS SOLID CORE WITH SINGLE SOLID COPPER CONDUCTOR WITH MAGNESIUM POWDER INSULATION IN BETWEEN - USUALLY IN THE 500A -2000A SIZE . THE SMALLER SIZES ARE AVAILABLE IN MULTI CORE FOR CONTROL WIRING / SMALL LIGHTING / POWER ETC . SINGLE IS USED ALSO FOR UNDERFLOOR HEATING EMBEDDED IN CONCRETE . TALK TO YOUR HEAVY INDUSTRIAL ELECTRICIAN AS OPPOSED TO THE HOUSE WIRING ELECO . HE WILL ALSO HAVE INSTALLED SOLID COPPER BUSS BAR INSTALLATIONS . SOMETIMES ALUMINIUM IS ALSO USED - THE COMMENT ON SKIN EFFECT WAS INTERESTING AS WE USE 208V / 115V-400HZ SYSTEMS ON MOST LARGE + MILITARY AIRPLANES - ONLY TRUE INTERNATIONAL ELECTRICAL STD I HAVE EVER SEEN INC THE TYPE OF PLUGS + SOCKETS - TRUST THAT ASSISTS J.C.
Flexible/stranded wires have the advantage of flexibility over solid conductors.But sometimes too much flexibility is a disadvantage -eg wiring inside a panel .The leads will not remain in place and give a shabby appearance.Then a solid conductor is an advantage.
Many electrical engineers have the notion that a stranded wire of same dia as a solid conductor will have less skin effect and will have a higher current carrying capacity.This is a misnomer.AC current will see the stranded wire same as a solid rod and current will tend to concentrate on outer layers.In fact skin effect will be more for stranded wire due to the stranding factor .
But all this will change once the strands are insulated from one another by an insulation say ,enamel or paper.Then skin effect is completely eliminated or reduced and still better results are obtained when the strands are transposed.This is the application in Roebel bars used in large generators and motors and Continuously Transposed Cable used in the windings of large Power transformers
I would think the skin effect would be worse with stranded wire that was individually insulated since the strands are not touching, all the current divides amongst each individual strand. Whereas the no insulation on individual strands would tend to act like a solid conductor. I do not understand why you say skin effect is less with stranded than solid conductors. Also, just to note, I never thought stranded had a higher current carrying capacity than solid.
Also, when terminating large gauge wire to terminals, we use conductive tape to hold the strands together. But the stranded often needs tightening more often than solid.
Some thoughts.
The NEC does not usually apply to substations. Parts of privately owned substations may, but in general the NESC and/or the whims of the local inspector cover substations. In my experience seldom does a local inspector look at a substaion, usless it's just for a tour.
Solid aluminum cable with XLP insulation is commonly used for URD system in sizes up to 2/0. It would seem that solid cable would be harder to pull but in practice it is not. The stiffness of the insulation is greater that that of the conductor so the change from stranded to solid doesn't really affect the overall stiffness.
On reason for solid conductors was water intrusion in the conductor and the "treeing" and failure of insulation.
Cable pushers work well with solid cable. It may sound strange to push a cable but it works more like installing a speedometer cable or brake cable on a bike.
Response to Buzzp:
My understanding is skin effect is higher for non insulted sranded wire because of stranding effect .When srands are insulated from one another current flows in all strands equally (an assumption wrong in many cases and hence the need for transposing )there by reducing the skin effect substantially.
