NEC RULES FOR CONDUCTORS

NEC rules for the ends of a wire differ from those for the middle. (Adapted from Practical Electrical Wiring, 20th edition, © Park Publishing, 2008, all rights reserved).

The key to applying these rules, and the new NEC Example D3(a) in Annex D on this topic is to remember that the end of a wire is different from its middle. Special rules apply to calculating wire sizes based on how the terminations are expected to function.

Entirely different rules aim at assuring that wires, over their length, don’t overheat under prevailing loading and conditions of use. These two sets of rules have nothing to do with each other—they are based on entirely different thermodynamic considerations.

Some of the calculations use, purely by coincidence, identical multiplying factors. Sometimes it is the termination requirements that produce the largest wire, and sometimes it is the requirements to prevent conductor overheating.

You can’t tell until you complete all the calculations and then make a comparison. Until you are accustomed to doing these calculations, do them on separate pieces of paper.


Current is always related to heat.
Every conductor has some resistance and as you increase the current, you increase the amount of heat, all other things being equal. In fact, as is covered in Sec. 110 of Div. 1 and elsewhere, you increase the heat by the square of the current.

The ampacity tables in the NEC reflect heating in another way. As the reproduction of NEC Table 310.16 (see Table 18 in Div. 12) shows, the tables tell you how much current you can safely (meaning without overheating the insulation) and continuously draw through a conductor under the prevailing conditions—which is essentially the definition of ampacity in NEC Article 100: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Ampacity tables show how conductors respond to heat.
The ampacity tables (such as Table 18 in Div. 1) do much more than what is described in the previous paragraph. They show, by implication, a current value below which a wire will run at or below a certain temperature limit.

Remember, conductor heating comes from current flowing through metal arranged in a specified geometry (generally, a long flexible cylinder of specified diameter and metallic content). In other words, for the purposes of thinking about how hot a wire is going to be running, you can ignore the different insulation styles.

As a learning tool, let’s make this into a “rule” and then see how the NEC makes use of it: A conductor, regardless of its insulation type, runs at or below the temperature limit indicated in an ampacity column when, after adjustment for the conditions of use, it is carrying equal or less current than the ampacity limit in that column.

For example, a 90 C THHN 10 AWG conductor has an ampacity of 40 amps. Our “rule” tells us that when 10 AWG copper conductors carry 40 amps under normal-use conditions, they will reach a worst-case, steady-state temperature of 90 C just below the insulation.

Meanwhile, the ampacity definition tells us that no matter how long this temperature continues, it won’t damage the wire. That’s not true of the device, however. If a wire on a wiring device gets too hot for too long, it could lead to loss of temper of the metal parts inside, cause instability of nonmetallic parts, and result in unreliable performance of overcurrent devices due to calibration shift.

Termination rules protect devices.
Because of the risk to devices from overheating, manufacturers set temperature limits for the conductors you put on their terminals. Consider that a metal-to-metal connection that is sound in the electrical sense probably conducts heat as efficiently as it conducts current. If you terminate a 90 C conductor on a circuit breaker, and the conductor reaches 90 C (almost the boiling point of water), the inside of the breaker won’t be much below that temperature.

Expecting that breaker to perform reliably with even a 75 C heat source bolted to it is expecting a lot. Testing laboratories take into account the vulnerability of devices to overheating, and there have been listing restrictions for many, many years to prevent use of wires that would cause device overheating. These restrictions now appear in the NEC.

Smaller devices (generally 100 amp and lower, or with termination provisions for 1 AWG or smaller wire) historically weren’t assumed to operate with wires rated over 60 C such as TW. Higherrated equipment assumed 75 C conductors but generally no higher for 600-volt equipment and below. This is still true today for the larger equipment. (Note that medium-voltage equipment, over 600 volts, has larger internal spacings and the usual allowance is for 90 C, but that equipment will not be further considered at this point.)

Today, smaller equipment increasingly has a “60/75 C” rating, which means it will function properly even where the conductors are sized based on the 75 C column of Table 18, Div. 1.

SUPER CONDUCTORS BASIC INFORMATION AND TUTORIALS

What Are Super Conductors?


The ideal superconducting state is characterized by two fundamental properties, which are the disappearance of resistance when the temperature is reduced to a critical value, and the expulsion of any magnetic flux in the material when the critical temperature (Tc) is reached.

Superconductivity was first discovered in the element mercury, in 1911. Other elements have subsequently been found to exhibit superconductivity and theories have been developed to explain the phenomenon. The critical temperatures for these materials were typically about 10 K (−263°C), which meant that they had to be cooled with liquid helium at 4 K.

In general these materials have been of academic interest only because they could only support a low current density in a low magnetic field without losing their superconducting properties. In the 1950s a new class of materials was discovered. These are the metallic alloys, the most important among them being niobium titanium and niobium tin.

The highest critical temperature achieved by these materials is 23.2 K and they can be used to produce magnetic flux densities of over 15 T. The main commercial application for these low-Tc superconductors is for magnets in medical imaging equipment which require the high fields to excite magnetic resonance in nuclei of hydrogen and other elements.

The magnet or solenoid of the magnetic resonance imaging (MRI) unit has an internal diameter of about 1.2 m and the patient to be examined is put into this aperture. The image from the resonance test shows unexpected concentrations of fluids or tissue and enables a diagnosis.

Superconducting magnets producing high magnetic fields are also used in magnetic research and in high energy physics research; other applications such as dc motors and generators, levitated trains, cables and ac switches have been explored but the complexity and high cost of providing the liquid helium environment prevented commercial development in most cases.

In late 1986 a ceramic material LaBaCuO was discovered to be superconducting at 35 K and in 1987 the material YBaCuO was found to have a critical temperature of 92 K. Since that time the critical temperatures of these new high temperature superconducting (HTS) materials has progressively increased to over 130 K.

Examples of these are BiSrCaCuO (with a Tc of 106 K), ThBaCaCuO (Tc of 125 K) and HgBaCaCuO (Tc of 133 K). The enormous significance of these discoveries is that these materials will be superconducting in liquid nitrogen, which has a boiling point of 77 K and is much easier and cheaper to provide than helium.

Much work has been directed towards finding materials with higher Tc values but this has remained at 133 K for some time. However, considerable effort with resulting success has been directed to the production of suitable HTS conductors. The HTS material is very brittle and it is deposited using laser deposition onto a suitable substrate tape.

The tape is 3 mm wide and cables of up to 600 m in length have been produced. There are many trials being made of the application of the HTS cables throughout the world including USA, Europe and Japan. There are prototypes of power transformers, underground power cables, large motors and generators, and fault current limiters in active development and in use.

The electricity supply of the City of Geneva in Switzerland is completely provided by power transformers wound with HTS conductors. Detroit is being re-equipped with HTS power cable for its transmission system and copper cables weighing over 7 tons are being replaced with HTS cables of less than 0.12 tons. These and other developments will help to establish the long-term feasibility of the HTS material. It is expected that there will be definite power saving from the use of HTS.

Small-scale applications which use HTS material include SQUIDS (Superconducting Quantum Interference Devices) which measure very low magnetic fields. They are applied in measurements in biomagnetism (investigations of electrical activity in the heart, brain and muscles) and in geophysics for the study of rock magnetism and anomalies in the earth’s surface.

MAGNET WIRE INSULATION BASICS AND TUTORIALS

What are magnet wire insulations?

The term magnet wire includes an extremely broad range of sizes of both round and rectangular conductors used in electrical apparatus. Common round-wire sizes for copper are AWG No. 42 (0.0025 in) to AWG No. 8 (0.1285 in).

A significant volume of aluminum magnet wire is produced in the size range of AWG No. 4 to AWG No. 26. Ultrafine sizes of round wire, used in very small devices, range as low as AWG No. 60 for copper and AWG No. 52 for aluminum.

Approximately 20 different “enamels” are used commercially at present in insulating magnet wire.

Magnet wire insulations are high in electrical, physical, and thermal performance and best in space factor. The most widely used polymers for film-insulated magnet wire are based on polyvinyl acetals, polyesters, polyamideimides, polyimides, polyamides, and polyurethanes.

Many magnet wire constructions use different layers of these polymer types to achieve the best combination of properties. The most commonly used magnet wire is NEMA MW-35C, Class 200,\ which is constructed with a polyester basecoat and a polyamideimide topcoat.

Polyurethanes are employed where ease of solderability without solvent or mechanical striping is required. The thermal class of polyurethane insulations has been increased up to Class 155 and even Class 180.

Magnet wire products also are produced with fabric layers (fiberglass or Dacron-fiberglass) served over bare or conventional film-insulated magnet wire. Self-bonding magnet wire is produced with a thermoplastic cement as the outer layer, which can be heat-activated to bond the wires together.

AWG (AMERICAN WIRE GAGE) CONDUCTOR SIZE DESIGNATION BASIC AND TUTORIALS

What is the American wire gage?

American wire gage, also known as the Brown & Sharpe gage, was devised in 1857 by J. R. Brown. It is usually abbreviated AWG.

This gage has the property, in common with a number of other gages, that its sizes represent approximately the successive steps in the process of wire drawing.

Also, like many other gages, its numbers are retrogressive, a larger number denoting a smaller wire, corresponding to the operations of drawing. These gage numbers are not arbitrarily chosen, as in many gages, but follow the mathematical law upon which the gage is founded.

Basis of the AWG is a simple mathematical law. The gage is formed by the specification of two diameters and the law that a given number of intermediate diameters are formed by geometric progression.

Thus, the diameter of No. 0000 is defined as 0.4600 in and of No. 36 as 0.0050 in. There are 38 sizes between these two; hence the ratio of any diameter to the diameter of the next greater number is given by this expression

The square of this ratio = 1.2610. The sixth power of the ratio, that is, the ratio of any diameter to the diameter of the sixth greater number, = 2.0050. The fact that this ratio is so nearly 2 is the basis of numerous useful relations or shortcuts in wire computations.

There are a number of approximate rules applicable to the AWG which are useful to remember:

1. An increase of three gage numbers (e.g., from No. 10 to 7) doubles the area and weight and consequently halves the dc resistance.

2. An increase of six gage numbers (e.g., from No. 10 to 4) doubles the diameter.

3. An increase of 10 gage numbers (e.g., from No. 10 to 1/0) multiplies the area and weight by 10 and divides the resistance by 10.

4. A No. 10 wire has a diameter of about 0.10 in, an area of about 10,000 cmils, and (for standard annealed copper at 20°C) a resistance of approximately 1.0 #/1000 ft.

5. The weight of No. 2 copper wire is very close to 200 lb/1000 ft (90 kg/304.8 m).