Medium-Voltage Cables

NOTE This standard governs shielded extruded-dielectric power cable rated 2.4 kV through 35 kV, from the reel to the energized circuit: the construction the cable is ordered to, the listing it carries, the basis its conductor size rests on, the limits it is pulled within, the splices, terminations, and separable connectors it is made up with, how its shield is grounded, how it is protected and identified in manholes, the factory tests, the field withstand test that releases it for energization, and the warranty and spares that follow it. (1.1)

NOTE Medium-voltage cable fails at the places a person made it: the splice, the termination, and the length that was pulled too hard or bent too tight. The factory dielectric is proven on every reel before it ships, and almost every in-service failure traces to field work, which is why this standard spends most of its length on the splicer, the pull, the shield, and the withstand test rather than on the cable itself. (1.2)

NOTE Two decisions fix the cable: the voltage class, which follows from the system voltage, and the insulation level, which follows from how the system is grounded and how long a ground fault is allowed to stand. Both are stated here as derived values with their rule, so that a project that knows its system voltage and its grounding method gets the cable that system needs, and a project that wants a heavier cable selects it deliberately. (1.3)

NOTE The following are governed elsewhere and are outside this standard: (1.4)

NOTE Type MV is the only medium-voltage cable NFPA 70 recognizes for premises wiring, so every circuit on the Owner's side of the service point under that code carries the listing; utility cable built to the ICEA utility specifications is the same construction without the listing evaluation, and it is selected by an Owner whose distribution system is operated as a utility system exempt from the code. (4.2.3)

NOTE A splice is an insulation system built by hand around a conductor, in a manhole, at the rate the schedule allows, and the quality of the stress control it establishes is set by the person who scored the insulation shield and shaped the pencil, not by the kit; the demonstration is how a project finds out before the first production splice is buried that a splicer's hands match the certificate. (4.3.5)

NOTE A duct bank drains toward its manholes and a manhole holds water for part of every year in most of the country, so the splice at its floor spends its life submerged whatever the drawings assumed; a splice kit rated for immersion seals the jacket interface against that water, and one that is not lets it reach the shield within a few seasons. (5.1.3)

NOTE The solid insulation of the cable does not care about altitude, but the air along the surface of a termination does: its dielectric strength falls as the air thins, so a termination that has the creepage IEEE 48 requires at sea level flashes over at a lower voltage on a high site unless its skirts are lengthened. (5.2.3)

NOTE A cold jacket and a cold insulation shield crack rather than stretch at a bend, and the crack is under the jacket where no inspection finds it; the cable then passes its withstand test dry and fails when water reaches the shield. (5.3.3)

NOTE The class names the band of system voltages the insulation geometry was designed for, not a margin over the operating voltage: a 4.16 kV system is a 5 kV class cable, a 12.47 kV or 13.8 kV system is a 15 kV class cable, a 24.9 kV system is a 25 kV class cable, and a 34.5 kV system is a 35 kV class cable. A class above the one the system needs buys a thicker insulation and a larger cable, which some Owners select on a 4.16 kV system so that one 15 kV class cable, one kit family, and one splicer certification serve the whole plant. (6.1.4)

NOTE A ground fault on one phase raises the voltage on the two unfaulted phases toward the phase-to-phase value for as long as the fault stands, and the insulation level is the thickness margin for that duration: 100 percent is the solidly grounded system whose breaker trips at once, 133 percent is the resistance-grounded system that alarms and runs for up to an hour, or any system an Owner wants a thicker wall on, and 173 percent is the ungrounded or resonant-grounded system that runs faulted until someone finds it. A level above the one the grounding needs costs a thicker insulation and a larger cable, and 133 percent on a solidly grounded system is the margin many facility Owners select against the fault that is cleared slower than the study assumed. (6.2.3)

NOTE The insulation thickness follows from the voltage class and the insulation level by the tables of the cable standard, so it is not a project selection; the two values above fix it, and the cross-section drawing is where the reviewer confirms that the cable offered carries the wall those two values require. (6.3.2)

NOTE Copper carries a given current in a smaller conductor, pulls a smaller and lighter cable into a smaller duct, and terminates in any medium-voltage lug or elbow made; aluminum costs less per ampere at the larger sizes, needs a conductor one or two sizes larger and the duct and the lug to match, and depends on a bimetallic or aluminum-rated connector and a connector preparation that the splicer does right. Facility distribution is usually copper and utility distribution is usually aluminum, and both are made in every class and construction under this standard. (7.1.4)

NOTE Concentric round stranding is the full-diameter conductor every lug and connector is dimensioned around; compressed stranding reduces the diameter by about three percent and fits the same connectors; compact stranding reduces it by about ten percent, which yields a smaller cable and a lower pulling tension in a crowded duct, and it requires a connector or a lug sized for the compact diameter, which is a different part number from the one for the same size concentric conductor. Manufacturers offer compressed or compact as their standard construction by class, so no one stranding is the norm across the scope. (7.3.3)

NOTE A water-blocked strand stops water that enters at a damaged jacket or an open end from travelling along the strand interstices to the next splice; it adds cost, and it adds a cleaning step at every termination that the kit instructions describe. It is used on direct-buried and submerged cable and on utility concentric neutral cable, and it is left out of most facility duct bank cable, so the field carries no default. (7.3.5)

NOTE Ethylene-propylene rubber stays flexible at low temperature, is easier to train and terminate in a tight cable compartment or manhole, tolerates water in the insulation without treeing, and carries a higher dielectric loss, which matters on a long run at 25 kV and above and is of no consequence at 5 kV and 15 kV; tree-retardant cross-linked polyethylene has the lower dielectric loss, is stiffer, costs less per foot at the larger sizes, and depends on its tree-retardant additive and a clean, dry extrusion to resist water treeing over its life. Both are listed Type MV in every class under this standard, both are made with a 105 °C rating, and facility and utility practice divide between them by region and by Owner history. (8.1.2)

NOTE The 105 °C rating is the construction nearly every manufacturer now ships in both compounds, and it buys an emergency overload capacity and a thermal margin the 90 °C rating does not; its higher ampacity column is claimed only where the lugs and the equipment are identified for it, which most switchgear and transformer terminations are not, so on most projects the 105 °C cable is sized from the 90 °C column and the extra rating is margin rather than capacity. (8.1.6)

NOTE The two semiconducting layers are what make a shielded cable: the conductor shield smooths the field over the strand surface and the insulation shield holds the outer surface of the insulation at ground so that the stress is radial and contained. Every splice and termination begins by removing the insulation shield to a precise length and ending it in a stress cone, and a shield that strips unevenly or leaves residue leaves a point of concentrated stress where the kit cannot control it. (8.2.4)

NOTE A helical copper tape shield is the facility construction: it is thin, it adds little to the diameter, it carries charging current and the short-circuit current its thickness and overlap allow, and it terminates with a solder or constant-force-spring ground connection that every kit provides for. A longitudinally corrugated tape shield is a continuous copper sheet folded around the core with a sealed overlap, which carries more fault current than a helical tape, blocks radial water ingress, and bends less freely. Concentric round wires are the utility construction: the wires are a neutral as well as a shield, they carry a full-neutral or a fractional-neutral current and a high fault current, and they add diameter and weight. Flat copper straps carry a higher fault current than a tape at less diameter than wires and are a utility and industrial construction. Which construction a project uses follows from the fault current the shield must carry and from whether the shield is also the system neutral. (8.3.2)

NOTE A full neutral carries the phase current of a single-phase or an unbalanced circuit and is the construction of a single-phase utility lateral; a one-third neutral is the three-phase utility feeder construction, where the three neutrals together equal one phase conductor; the one-sixth and one-twelfth neutrals are the three-phase constructions at the largest conductor sizes, where a one-third neutral would be far larger than any fault or unbalance needs. A concentric neutral cable of any fraction shall comply with ICEA S-94-649 and shall be ordered with the neutral size stated. (8.3.6)

NOTE A polyvinyl chloride jacket is the facility jacket: it passes the flame tests cable tray and building installation require, resists oil and most chemicals, strips cleanly for a kit, and is the lowest in cost; it is the stiffest of the group in cold weather and the least abrasion resistant in a long duct pull. The polyethylene jackets are the utility jackets: they are tougher in a pull, absorb almost no water, and tolerate direct burial and cold better than PVC, and they do not pass the flame tests, so they stay out of buildings and tray. Chlorinated and chlorosulfonated polyethylene are the industrial jackets, flame-resistant and oil-, ozone-, and sunlight-resistant together, at a higher cost. A low-smoke zero-halogen jacket is specified where the cable runs through an occupied space, a tunnel, or a transit or data facility whose Owner limits the smoke and the acid gas a fire produces, and it is softer and less abrasion resistant than the others. (9.1.2)

NOTE Three single conductors are pulled into one duct, trained separately in the manhole and the cable compartment, and spliced and terminated one at a time with single-conductor kits, which is the facility duct bank construction; a triplexed assembly is three single conductors cabled together at the factory, which pulls as one cable, lays in tray and in direct burial as one, and terminates with the same single-conductor kits; a three-conductor cable carries three shielded conductors under one jacket, with or without bare grounding conductors in the interstices, which is the tray and armored construction and the one used where a single round cable is wanted through a gland or a sealing fitting, at a larger diameter and a larger bending radius than the three singles it replaces. (9.2.2)

NOTE Unarmored cable in duct, conduit, or tray is the construction of nearly every facility feeder, and the raceway provides the mechanical protection. Interlocked armor over a three-conductor or triplexed core makes the cable a Type MC assembly listed to UL 1569 as well as UL 1072, which is run in tray and on supports without a raceway in an industrial plant and is terminated through an armor fitting; aluminum armor is the lighter and nonmagnetic armor for a three-conductor core and galvanized steel the stiffer and more crush-resistant one. A continuous corrugated welded aluminum sheath is an armor and a moisture barrier in one, used where the cable is exposed to water under pressure, to rodents, or to crushing, at the highest cost and the largest bending radius. (9.3.2)

NOTE A polyethylene jacket does not pass the vertical tray flame test, so a cable in tray or exposed indoors is PVC, CPE, CSPE, or LSZH jacketed, and a cable that runs from a duct bank into a tray carries the flame-rated jacket over its whole length or is spliced to a flame-rated cable where it enters the building. (9.4.2)

NOTE The underground ampacity tables of NFPA 70 and IEEE 835 are computed at a soil thermal resistivity of 90 °C·cm/W and an earth ambient of 20 °C at a 100 percent load factor, which is why those are the values that apply without knowing anything about the site; a dry sandy or volcanic soil reads 150 °C·cm/W or more and takes a third off the ampacity, a wet clay reads 60 and adds to it, and a southern site or a duct bank beside a steam line runs warmer than 20 °C. Where the soil has been measured, the measured value governs, and a site whose soil is unknown and whose duct bank is loaded near its rating is a site whose soil is worth measuring before the cable is bought. (10.4)

NOTE A cold-shrink termination is a pre-stretched silicone or EPDM body released onto the prepared cable from a removable core, which needs no heat source, fits a range of cable diameters per kit, and is the termination most facility work now uses in a cable compartment; a heat-shrink termination is a set of tubes and a stress-control mastic recovered with a torch or a hot-air gun, which costs less per kit, fits a wider diameter range, and depends on the splicer applying heat evenly in a space where a torch may not be welcome; a pre-molded slip-on termination is a molded body pushed over the insulation, sized closely to one cable diameter, and installed in the least time where the cable is the size the body was made for; a housed outdoor termination carries the external insulation on porcelain or polymer skirts and is the termination of a riser pole, an outdoor bus connection, and a transformer with cover-mounted bushings; a hand-taped termination is the stress cone built up from tapes by a splicer trained in the method, which fits any cable and takes the longest and is the most dependent on the hand that built it. (11.1.3)

NOTE A Class 1 termination provides stress control, full external leakage insulation, and a seal against the environment, and it is the one most projects use at every termination so that one kit family serves indoors and out; a Class 2 termination provides stress control and external leakage insulation with no seal, and a Class 3 termination provides stress control only, which suits a termination inside a sealed or conditioned compartment where no moisture reaches it and a smaller body is wanted. (11.1.6)

NOTE A 200 A loadbreak elbow mates to a 200 A bushing well insert or integral bushing and is operated with a hot stick to make and break load current, which is the interface of a pad-mounted transformer and of a loop-fed switch; a 200 A deadbreak elbow mates to the same interface and is operated only de-energized; a 600 A deadbreak T-body bolts to an apparatus bushing, carries the current of a feeder or a loop and the cable sizes a 200 A elbow does not accept, and is operated only de-energized; a T-body with a loadbreak reducing tap adds a 200 A loadbreak interface on the T-body for a test point, a grounding elbow, or a surge arrester. The interface follows the equipment, which is why the field derives from it. (11.2.2)

NOTE An elbow-type surge arrester at the open point of a loop protects the cable from the voltage doubling that a surge produces at an open end, where the switchgear arresters at the source do not reach; the arrester is a cable accessory at that point and is governed here, while the arresters on the transformer and in the switchgear are governed by the standards for that equipment. (11.2.5)

NOTE A cold-shrink splice is a pre-stretched body released from its core over the joint, with no heat source in the manhole and a diameter range per kit; a heat-shrink splice is recovered with a torch or hot-air gun over a mastic stress-control system at a lower kit cost and a greater dependence on even heating in a confined, often wet, structure; a pre-molded splice is a molded body slid over the joint and sized closely to one cable, which is the fastest to install on the cable it fits and the one most often used where the splice must be a rated submersible splice; a hand-taped splice is built up from tapes by a trained splicer and fits any pair of cables, including two different constructions, at the longest installation time. (11.3.3)

NOTE The work of a splice happens in the half-inch at the end of the insulation shield, where the radial field inside the cable turns into the field at a free surface. A knife nick in the insulation at that edge, a trace of semiconducting dust on the cleaned insulation, or a wipe pulled back from the shield onto the clean surface each leaves a conducting path or a void where the stress is highest, and each one fails months after it passes the withstand test. The instruction sheet in the manhole, the tools, and the dry tent are what keep the splicer's work on the geometry the kit was designed around. (11.4.7)

NOTE A shield grounded at every termination and splice carries a circulating current induced by the phase current, which heats the shield and reduces the ampacity by an amount that grows with the conductor size and the spacing between phases, and it holds the shield at ground everywhere, so that no standing voltage appears on it and no sheath voltage limiter is needed; this is the arrangement of nearly every facility circuit at 500 kcmil and below and in a duct bank. A shield grounded at one end carries no circulating current and gives up none of its ampacity, and it leaves the far end at a standing voltage proportional to the current and the run length that a sheath voltage limiter must hold during a fault, which is the arrangement of a large single-conductor circuit on a short run, such as a transformer secondary at 4.16 kV or a generator lead. Cross-bonding transposes the shields at sectionalizing joints so that the induced voltages cancel over three sections, and it is the arrangement of a long transmission-class circuit, available under this standard for the long campus feeder whose ampacity a grounded shield would take too much of. IEEE 575 is the guide for all three. (12.2)

NOTE The standing voltage limit sets how long a single-point grounded run can be before its far end is a shock hazard to a person who touches the shield in a manhole: a low limit keeps the run short or forces a larger spacing between grounding points, and a higher limit is accepted by an Owner whose manholes are entered only by qualified persons who treat every shield as energized. Utilities commonly work to a limit near the low end of the range and industrial Owners to a higher one, so the field carries no default. (12.5)

NOTE Arc-proofing tape wrapped over a cable and its splice in a manhole contains the arc of a fault on that cable long enough for the protection to clear it, so that the fault does not burn through the jacket of the cable racked beside it and take a second circuit, or the whole structure, out with the first. Arc-proofing every structure protects every circuit against its neighbor and against a fault that starts in its own splice; limiting it to shared structures protects the circuits that have a neighbor to lose and leaves a single-circuit manhole bare; omitting it is the practice of an Owner whose circuits never share a structure or who accepts the loss of a second feeder when a splice fails. A campus loop with two feeders in every manhole is the case the first option was written for. (13.2)

NOTE A stainless steel tag survives decades in a flooded manhole, is read after a fire in the structure, and costs the most per tag; an engraved plastic tag is legible, resists water, and is the usual tag in a dry cable compartment and in a handhole; a printed heat-shrink sleeve cannot fall off and is applied before the cable is terminated, which suits a cable compartment, and it cannot be added to a cable already in place without cutting it loose. Many Owners use stainless steel in the manholes and plastic or sleeves in the equipment, so the field carries no default and the project may enter one material per location on the identification schedule. (14.4)

NOTE The jacket legend is the manufacturer's identification of the cable and the tag is the project's identification of the circuit; the first tells a splicer what cable is in hand and the second tells an operator which breaker to open before touching it, and a cable that carries only the first is a cable that is traced by lifting manhole covers. (14.7)

NOTE The cable is tested disconnected because the equipment at each end has its own withstand test at its own voltage, and a transformer winding, an arrester, or a voltage transformer left on the circuit either fails the cable's test or is damaged by it. The insulation resistance before and after bracket the withstand: a reading that drops across the test is the sign of a splice that the voltage damaged without breaking down. (15.2.5)

NOTE A very low frequency sinusoidal test stresses the insulation with an alternating voltage at 0.1 Hz, which a portable set can supply to a long cable because the charging current at that frequency is small, finds a defective splice or termination by breaking it down under test rather than in service, leaves no space charge behind, and is the method ANSI/NETA ATS and nearly every facility project now use; the cosine-rectangular waveform is the same test with a different set and a slightly different voltage table in IEEE 400.2; a damped AC test charges the cable and lets it ring down through an inductor at a power-frequency-like rate, which is gentler and is usually paired with partial discharge measurement; a power-frequency test is the factory test applied in the field, which needs a resonant test set sized to the cable capacitance and is seen on short runs and at the largest facilities; a direct-current test needs the smallest set of all and was the standard method for decades, and IEEE 400 advises against it on extruded insulation that has been in service because the space charge it leaves can shorten the life of aged insulation, while ICEA still publishes direct-current field voltages for new cable; a project that performs no withstand test relies on the factory test, the insulation resistance, and the splicer's qualification, which is the practice of some Owners on a short run that was pulled in one length with no splice. (15.3.2)

NOTE A 30 minute very low frequency test is the duration IEEE 400.2 recommends for an acceptance test, long enough for a defect at the test voltage to grow to breakdown and the one most projects specify; a 60 minute test is the duration some Owners specify for a splice-heavy campus loop or for 25 kV and 35 kV cable, at the cost of a longer outage per phase; IEEE 400.2 permits the test to be shortened to 15 minutes where it is monitored by a tan delta measurement that holds stable, which is the duration a project with a monitored withstand selects. (15.3.7)

NOTE A withstand test gives a pass or a breakdown and nothing between; the diagnostic tests give a number. Tan delta grades the whole insulation of a phase, finds water treeing and a contaminated or wet splice as a loss that rises with voltage, and sets the baseline every later maintenance test is compared against; partial discharge mapping finds the one splice among twenty in which a void or a shield edge is discharging at a voltage above operating but below withstand, which is the splice that fails in its second year, and it locates that splice to the manhole. A higher inception floor fails more splices and catches more marginal ones; a lower floor accepts a discharge that incepts closer to operating voltage. Both tests add a set, a technician, and time per phase, and the project that has them is the one whose splices were made in the field in the hundreds. (15.4.4)

NOTE The published limits depend on the attachment and on the cable: a pulling eye attached to the conductor carries a tension set by the conductor area and material, a basket grip over the jacket carries a much lower tension set by the jacket and the shield, and the sidewall pressure at a bend is set by the tension at that bend divided by the bend radius and limited by what the insulation shield and the metallic shield tolerate without deforming. The manufacturer publishes all three for each construction, and the calculation is the only way to know before the pull whether a long run with two bends near the take-up end is within them. (16.2.3)

NOTE A tension reading on every pull is the only proof after the fact that the cable was not overstressed, it costs a load cell on the winch and a line in the log, and it is what most Owners of a medium-voltage system require; recording only the pulls the calculation shows to be near the limit saves the setup on the short and easy pulls and leaves the easy pull that went wrong unrecorded; no recording is the practice of a project whose pulls are all short and hand-fed, and it leaves the pulling log as the crew's word. (16.2.7)

NOTE A cable bent below its minimum radius is a cable whose insulation shield has been stretched on the outside of the bend and buckled on the inside, and whose metallic tape has opened or wrinkled; neither recovers when the cable is straightened, and the gap in the shield is where the field concentrates for the rest of the cable's life. The radius matters most at the duct mouth, where the cable turns up the wall, and at the termination, where the installer is tempted to make the cable fit the compartment. (16.3.5)

NOTE Water that enters an open cable end travels along the strand and under the shield for tens of feet, where no inspection finds it and where it sits against the insulation shield and the insulation until it trees through them; the end cap is the whole of the cable's protection between the extruder and the splice, and a reel whose cap has been lost in a yard for a season is a reel whose first fifty feet are cut off before the cable is used. (17.7)

NOTE Commencement at shipment is the manufacturer's usual term, and it consumes the warranty while the reels sit in the yard and the duct bank is poured; commencement at energization or substantial completion moves the coverage to the period the Owner is operating the circuit, and the manufacturer prices the added months into the cable. (18.4)

NOTE A failed splice on a medium-voltage feeder is an outage that lasts as long as it takes to get a kit for that cable in that voltage class to the site, and a kit for a 15 kV compact 500 kcmil copper cable is not a stocked item at a supply house; the spare kits under this standard are the ones that let the Owner's splicer or the Contractor remake the failed splice the day the fault is located, and the spare reel length is for the Owner whose feeder cannot wait for a mill run. (19.3)