MEDIUM AND HIGH VOLTAGE CABLE, 5 kV THROUGH 69 kV

The Layers That Make MV Cable

Medium-voltage cable is built in layers, and every layer has a job. A stranded copper or aluminum conductor sits at the center. A semiconducting conductor shield follows it, then the insulation, EPR or XLPE, then a semiconducting insulation shield, then the metallic shield of concentric neutral wires or copper tape, then the jacket.

The two semiconducting layers are what separate medium-voltage cable from building wire. They smooth the electric field at the conductor surface and at the insulation surface, which is why a nick in the insulation shield matters more than a scuff on the jacket. A cable with damaged insulation shield is a cable with a problem.

• 5 kV, 8 kV and 15 kV for distribution feeders, pad-mounted transformers and industrial mains.
• 25 kV and 35 kV for utility distribution and larger industrial services.
• 46 kV and 69 kV for subtransmission, usually single conductor on heavy reels.
• EPR insulation: flexible, tolerant of moisture, more forgiving to splice.
• XLPE and tree-retardant XLPE: higher dielectric strength, stiffer, common in URD.

Insulation Wall Thickness and What It Tells You

Insulation is specified by wall thickness and by insulation level. A 15 kV cable at the 100 percent level carries 175 mils of insulation; at the 133 percent level it carries 220 mils. The thicker wall is chosen for circuits exposed to higher switching surges or impulse levels, and it costs more to make.

That is why the class and the level are asked for before anything else. A 15 kV 500 MCM cable at 175 mils and one at 220 mils look alike on a reel and are not the same product. The tag normally prints both, and the wall can be measured at a cut end when the tag is gone.

Concentric Neutral, Tape Shield and Jacket

Concentric neutral is the common shield for utility distribution cable: copper wires applied helically over the insulation shield and sized as a fraction of the phase conductor. A one-third neutral is standard on many 15 kV circuits. A full neutral carries more fault current and contains more copper.

Tape shield is thin copper tape applied with overlapping turns, used where fault current is lower and a compact cable matters, such as substation and industrial work. Some constructions use a longitudinally corrugated copper shield with drain wires. The jacket over the shield, usually PVC or polyethylene, is what keeps water out of the shield.

• Concentric neutral in one-sixth, one-third or full neutral.
• Tape shield with overlap, for lower fault current duty.
• Longitudinally corrugated shield with drain wires for substation work.
• Jacketed and unjacketed constructions, which change where the cable may be installed.
• Direct-burial URD MV: aluminum conductor, concentric neutral, jacketed.

Reels, Weight and Handling

Medium-voltage cable is heavy, and the reel is part of the handling problem. A 60 inch wooden reel carrying several thousand feet of 15 kV cable is a machine lift rather than a two-person lift, and a reel must never be dropped on its flange. A dropped reel can crush the inner layers and leave cable that tests badly.

Reels are lagged for transport and should stay lagged until installation. Store them upright on firm ground, with the drum clear of the dirt where possible. The minimum bend radius for shielded cable is generous, on the order of a dozen times the outside diameter, and a reel stored on its side for a season will have taken a set.

• 48, 60, 72 and 96 inch flanges, with 60 inch the common utility size.
• Wooden reels are returnable in some supply chains, so keep any deposit tag.
• Note lagging condition and whether the reel has been dropped or stored on its side.
• Cable ends should be sealed against moisture, since water travels down a stranded conductor.
• Say what can lift the reel: a forklift with the reach, or a crane for the largest sizes.

Photograph and Note

Photograph the tag, the reel face and one cut end. The tag states voltage class, conductor size, insulation level, shield type and length. The cut end shows the layers in order, which confirms what the tag claims and reveals the wall thickness.

Cable is what we buy, and only cable. Transformers, switchgear, breakers, motor control centers, generators, chillers and machinery are outside the offer, including the cans and switches at the end of the cable. Underground distribution is covered with URD and USE-2, conductors under aluminum wire, and the quote form takes the reel details.

• The reel tag, flat and readable, with length and voltage class.
• The full reel, so flange diameter and remaining cable are visible.
• A cut end, showing conductor, insulation wall and shield.
• Any jacket damage, exposed shield or flattened cable on the outer layer.
• Splices or terminations still attached, and whether kits are on the reel.