The art of protecting electrical systems, part 18: protecting wires and cables
Part 18 of our continuing series on protecting electrical systems begins a series of articles on protection of electrical systems components, starting with conductors.
Editor’s Note: From 1965 through 1970, Consulting-Specifying Engineer ’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection. Due to the immense popularity of the 31 installments in the series, the authors, George Farrell and Frank Valvoda, PE, reprised the series in an updated version beginning in the Feb. 1989 issue of CSE. Over the years since the last installment ran in the late ’90s, we have received many requests to re-run this series. Mr. Valvoda passed away in Dec. 2001, and his long-time friend and editorial partner, George Farrell, passed away in early 2006.
Part 18 of our continuing series on protecting electrical systems begins a series of articles on protection of electrical systems components, starting with conductors.
Designing reliable electrical systems requires consideration of many interrelated details. It is not enough to depend only on protective devices, which, by definition, function after the fact. Engineers need to seek means to prevent equipment and component failures, and they need to understand how equipment selection and application affect system reliability.
The next several articles in this series look at protection of electrical system components, such as switching equipment, transformers, generators, and utilization equipment. We begin with an examination of wire and cable.
Within facilities subject to the National Electrical Code (NEC), basic rules for wire and cable application are contained in Article 310—Conductors for General Wiring. Except where bare or “covered” conductors are specifically permitted, NEC 310-2 requires all conductors be insulated.
Covered conductors are encased in material composition or thickness not recognized by NEC as electrical insulation. Their use has decreased greatly since development of 600-volt, plastic-insulated conductors, which are generally cost-competitive with most covered constructions.
NEC Section 310-5 and Table 310-5 establish minimum sizes of conductors for various voltages. These minimums follow Insulated Cable Engineers Assn. (ICEA) guidelines, which most manufacturers follow.
Minimum conductor size increases as voltage rating increases in part because size is determined by the unit voltage stress over their circumference, reducing unit stress on the insulation. For example, if a #32 AWG conductor were used in a 5-kilovolt (kV) cable, the small wire would act almost like a knife-edge, concentrating the stress, damaging standard types of insulation, and eventually causing premature failure.
Section 310-5, Exception 9 of the 1990 NEC was renumbered as Exception 8 in the 1993 code. It gives separate minimum wires sizes for Type V (varnished cambric) insulation. However, Type V cable was dropped from Table 310-13, which lists acceptable constructions. Whether the omission was intentional or accidental will need to be determined. Clarification will be included in a future article.
NEC Paragraph 310-13 requires wire and cable insulation materials and application conform to Tables 310-13, -61, -62, -63 and -64. Basic information about thermoplastic and thermosetting conductor insulations helps define their use and limitations.
Thermoplastics are not true solids. At normal temperatures they are highly viscous liquids, and they flow to some degree at any temperature. At high temperature(as in overcurrent or short-circuit-current conditions), thermoplastics return to their liquid state and drip off the conductor. However, properly selected and applied, thermosplastic insulation is reliable and cost-effective.
The Fine Print Note (FPN) to Paragraph 310-13 cautions thermoplastic insulation may stiffen at temperatures less than -10 C (14 F) and care must be exerted during installation. Thus cables often are warmed by passing low-voltage current through them just prior to installation. When possible, however, cables should be stored in heated warehouses for several days prior to installation during cold weather. At extremely cold temperatures such as -29 C, more flexible rubber or rubber-like insulation is a better choice. Recommended minimum installation temperatures for insulations are given in Table 1.
Minimum Temperature at Which Cables Should Be Installed
PVA Artic Grade
(Data provided by The Okonite Company)
The FPN also cautions thermoplastic insulation may be deformed at normal temperatures when subjected to pressure, requiring that care be used during installation and at points of support. Excess pressure can occur when minimum-size conduit bodies are used to make short-radius bends. When larger, stiffer conductors are installed, it is sometimes difficult to close the conduit body cover because the wire is not completely drawn into it. Electricians have been known to pound on them with a heavy maul.
In other cases, conduit body covers have been installed using extra-long screws to force covers into position so normal-length screws can be used to close them. These practices exert too much pressure on the thermoplastic insulation, and it may fail after a short time. Storing cable reels on soft ground or on their sides also may cause problems.
Polyvinyl chloride (PVC) and polyethylene (PE) are principal thermoplastic insulations. They often incorporate additives to achieve specific characteristics. For example, chlorine or bromine may be added to PVC to decrease its flammability and propensity to propagate flame.
Because PVC is the more economical of the two, it is most widely used for 600-volt construction wire. However, PVC insulation is soft, deforms easily, and is easily damaged by conduit burrs and other sharp edges. Nylon jackets found on wires such as THHN and THWN add abrasion resistance and physical strength to the insulation, but they will not accommodate poor workmanship.
Polyethylene is a superior high-frequency insulation and is used extensively in coaxial cable and similar applications. In power cables, it is more common to see thermosetting cross-linked polyethylene rather than the thermoplastic form.
Thermosetting insulations start life as complex plastics. Setting or vulcanizing agents are added, along with fillers, coloring compounds, and other materials. Two common thermosetting insulations are ethylene-propylene rubber and cross-linked polyethylene. Prior to development of modern peroxide compounds, sulfur was widely used as a setting agent. Because sulfur and its compounds attack copper and aluminum, conductors were tinned. With newer insulations, tinning usually is not required.
Thermosetting materials are extruded on conductors in a manner similar to thermoplastics, but the cable then undergoes an additional curing process. In most applications it is maintained at a temperature that activates the thermosetting agent. Once the process is completed, the insulation will not return to its liquid state. When heated, it softens to a point but maintains its strength until temperatures are high enough to carbonize the insulation.
Overload protection for insulated conductors prevents premature insulation failure and reduces fire risk from excessive surface temperatures. Most manufacturers try to achieve a 20-year “normal” insulation life for products applied within industry standards.
Most insulations are made of organic compounds, one exception being mineral-insulated cable (type MI). As soon as the product is manufactured, the insulation begins aging, primarily due to oxidation. The speed at which a compound ages is determined by temperature. The speed of most chemical reactions doubles for each 10 C increase in temperature. If a compound has a 20-year life at 75 C, anticipated life at 85 C would be about 10 years, and at 95 C five years. Contaminants such as vapors of gasoline, benzene, and similar active solvents may affect insulation life.
At some temperatures, organic compounds oxidize so rapidly that they carbonize and become conductive within a few seconds. This is known as insulation “breakdown.”
Research by the Massachusetts Institute of Technology (MIT) and ICEA determined maximum insulation temperatures under short-circuit conditions for thermoplastic insulations is 150 C, and for thermosetting insulations, 200 C. Each time a conductor is heated to this temperature and immediately begins to cool, MIT and ICEA found the insulation lost about 2% of normal life as determined by loss of elasticity and dielectric strength. Extrapolating these values for higher temperatures is not permissible, as some insulations carbonize and fail immediately at only slightly higher temperatures.
This is not a case against PVC insulation, but an argument for correct application. The ICEA publishes charts indicating the maximum short-circuit current a conductor can withstand. There has been much controversy about the application of this information. A discussion of the ICEA charts will be included in a future article, along with recommendations on how to correctly use this information.
However, in determining the maximum operating temperature permitted for insulated conductors, NEC also takes into consideration the conductor’s “heat-sinking” properties. Electrical circuits obey all the laws of thermodynamics. Heat flows from points of high temperature to points of lower temperature. Control equipment such as safety switches and contactors; protective devices such as circuit breakers and fuses; and other devices such as receptacles, lighting switches, etc. have test standards based upon a specified size and length of wire in the test circuit. Because resistance of the equipment being tested generates a certain amount of heat, this heat must be dispersed, or the equipment will be damaged. One way heat dissipation occurs is by conduction to circuit conductors.
Paragraph 110-14(c) in the 1993 NEC addresses the subject. Underwriters’ Laboratories Inc. (UL)-listed equipment is approved for maximum terminal temperatures. If the equipment is listed for use in 100-amp or smaller circuits or has terminals designated for #1 AWG and smaller wire, the ampacity of attached conductors must be based on 60 C wire as shown in Table 310-16, unless the equipment is listed for use with 75 C wire, and its terminals are so marked. In such cases, ampacity may be based on 75 C ampacity if the wire is rated 75 C or higher.
Equipment listed for use with circuits greater than 100 amps or with terminals rated for wires larger than #1 AWG may base wire ampacity on the 75 C column provided the wire is rated for at least 75 C.
While conductors with insulation rated 90 C and higher may be operated at the ampacity shown in Tables 310-16 through 310-19 and Appendix B, no UL-listed equipment has terminals suitable for use with conductors operating at an ampacity higher than permitted for 75 C wire. What then is the value of these conductors? In many cases, high-temperature insulation is required to meet the operating environment’s needs.
In other cases, especially in long cable runs from manhole to manhole or pull box to pull box, 90 C or higher cables can be used at their higher ampacity for the bulk of the run. Then, using a 90 C-rated power block, the number of conductors can be increased to meet the terminals’ 75 C requirements. The transition must be made at least 10 ft from the equipment where the cable is to be terminated.
In still other cases, some switching equipment UL listed for use at 100% of its rating with terminal ampacities based on 75 C ampacity is required to be connected with 90 C wire (but a 75 C ampacity) because of high ambient temperatures. These requirements are shown on the equipment as part of the UL listing but may not be as clearly indicated in catalogs or specification information. When specifying such equipment investigate the type of conductors required.
The rule in Article 220, “Branch Circuit, Feeder, and Service Calculations,” must be followed when determining minimum conductor size. Conductors must have a permitted ampacity adequate to carry the loads as determined by Article 220. Paragraph 220-10(b) of the 1984 and 1987 code required feeder conductors to have ampacity equal to 125% of the continuous load plus 100% of the non-continuous load after permitted demand factors were applied. The 1990 code revised the requirements, and feeders for non-motor loads need only have an ampacity equal to 100% of both continuous and non-continuous loads.
While technically permissible, it is not always possible to meet code requirements for conductor protection as required by Article 220-3. The conflict arises since Section 220-10(b) requires overcurrent protective devices to be sized at 125% of the continuous load plus 100% of the non-continuous load and Section 220-3 requires conductors be protected in accordance with ampacities as given in Section 310-15, unless otherwise permitted in Section 220-3(a) through (m).
If, for example, a load consists of 100 amps continuous load and 100 amps non-continuous load, the code permits a 3/0 THWN cable, as it has a 200-amp rating. Unless the overcurrent protection device is approved for use at 100% of rating, it is required to have a rating of 125% of the continuous load (125% of 100 amps, or 125 amps) plus 100 amps of the non-continuous load, or a total of 225 amps. However, 240-3(b) permits the next larger standard overcurrent device rating to be used only when the conductor’s ampacity does not correspond with the standard amp rating of a fuse or circuit breaker. Because 200 amps is a standard rating for both fuses and circuit breakers, the next larger rating cannot be used. If the conductor had an ampacity of 201 amps, 225-amp overcurrent protection would be permitted. To meet the requirements of Section 310-15, the wire would have to be 4/0, rated 230 amps. This will be discussed in greater detail when the series looks at Article 240.
Paragraph 220-22, “Feeder Neutral Load,” was revised slightly in the 1993 code to clarify that the neutral of a four-wire, wye-connected, three-phase system or a three-wire circuit consisting of two-phase wires and the grounded neutral of a four-wire, three-phase, wye-connected system must be sized the same as the phase conductors for that portion of a circuit’s load consisting of components that may generate harmonics. These include electric-discharge lighting, electronic computer/data processing, or similar equipment.
The next article in the series will continue examination of NEC requirements for selection, application, and protection of wire and cable.
[SIDEBAR] Current only part of the Story
The primary concern for wire and cable protection has been to prevent damage from overheating due to current. However, rated current is only one of many interrelated factors that must be considered to ensure conductors can carry the amperes stated in Article 310. Many of these factors are clearly and specifically included in other National Electrical Code (NEC) articles.
However, several considerations are included only by inference and must be heeded. Many of these factors are covered in Article 110—Requirements for Electrical Installations. Article 110 requires consideration of such things as:
Product suitability for an environment or application
Heating effects under normal and abnormal conditions likely to arise in service
Whether listed or labeled equipment is being used in accordance with appropriate instructions
Presence of deteriorating agents
Protection from physical damage such as crushing or sharp edges
[SDIMoisture: a hidden danger
Every effort must be made to prevent moisture from entering a cable. As cables are heated and cooled, especially when stored outdoors, they “breathe.” That is, air in the stranded conductor interstices expands and contracts.
Moisture and airborne contaminants may be drawn into the cable, where they often condense. A reel of cable stored outdoors for several months was found to contain over a pint of water. If water has been trapped in the conductor when it was energized, it would have damaged the insulation.
Other factors can damage insulation, especially during installation, and will be covered in a future article in this series.
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