Title: Reading and Audio-Visual Assignments
Course: WGF 107 DEVELOPMENT > Section: Content

Image | Zero-center ammeter

Sub-Title | Low/High Amperage Multimeter Testing
Content | The ampere (SI unit symbol: A; SI dimension symbol: I), often shortened to amp,^[1] is the SI unit of electric current^[2]^[3] (quantity symbol: I, i)^[4] and is one of the seven^[5] SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time, with 6.241×10^18 electrons (or one coulomb) per second constituting one ampere.^[6] The practical definition may lead to confusion with the definition of the coulomb (i.e., 1 ampere-second) and the ampere-hour (A·h), but amperes can be viewed as a flow rate, i.e. number of (charged) particles transiting per unit time, and coulombs simply as the number of particles. In practical terms this means that measures of a constant current (e.g., the nominal flow rate of charge per second through a simple circuit) will be defined in amperes (e.g., "a 20 mA circuit"). But using coulombs for the current will require to be over a known period of time (e.g., "a variable-current circuit that flows a total of 10 coulombs over 5 seconds").
Image | Analogy between a hydraulic circuit (left) and an electronic circuit (right).
Content
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems.

Ampacity

Ampacity is defined as the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration. Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device or conductor can continuously carry while remaining within its temperature rating.

The ampacity of a conductor depends on:

- its insulation temperature rating;
- the electrical resistance of the conductor material;
- frequency of the current, in the case of alternating current;
- ability to dissipate heat, which depends on conductor geometry and its surroundings;
- ambient temperature.

All common electrical conductors have some resistance to the flow of electricity. Electric current flowing through them causes voltage drop and power dissipation, which heats conductors. Copper or aluminum can conduct a large amount of current without damage, but long before conductor damage, insulation would, typically, be damaged by the resultant heat.

The ampacity for a conductor is based on physical and electrical properties of the material and construction of the conductor and of its insulation, ambient temperature, and environmental conditions adjacent to the conductor. Having a large overall surface area can dissipate heat well if the environment can absorb the heat.

In cables different conditions govern, and installation regulations normally specify that the most severe condition along the run will govern each cable conductor's rating. Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation. Derating is necessary for multiple cables in close proximity. When multiple cables are in close proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. The overall ampacity of insulated cable conductors in a bundle of more than three cables must also be derated, whether in a raceway or cable. Usually the derating factor is tabulated in a nation's wiring regulations.

Depending on the type of insulating material, common maximum allowable temperatures at the surface of the conductor are 60, 75, and 90 °C, often with an ambient air temperature of 30 °C. In the United States, 105 °C is allowed with ambient of 40 °C, for larger power cables, especially those operating at more than 2 kV. Likewise, specific insulations are rated 150, 200, or 250 °C.

The allowed current in a conductor generally needs to be decreased (derated) when conductors are in a grouping or cable, enclosed in conduit, or an enclosure restricting heat dissipation. e.g. The United States National Electric Code, Table 310.15(B)(16), specifies that up to three 8 AWG copper wires having a common insulating material (THWN) in a raceway, cable, or direct burial has an ampacity of 50 A when the ambient air is 30°C, the conductor surface temperature allowed to be 75°C. A single insulated conductor in free air has 70 A rating.

Ampacity rating is normally for continuous current, and short periods of overcurrent occur without harm in most cabling systems. The acceptable magnitude and duration of overcurrent is a more complex topic than ampacity.

When designing an electrical system, one will normally need to know the current rating for the following:

- Wires
- Printed Circuit Board traces, where included
- Fuses
- Circuit breakers
- All or nearly all components used

Some devices are limited by power rating, and when this power rating occurs below their current limit, it is not necessary to know the current limit to design a system. A common example of this is lightbulb holders.
Links/Uploads
Wikipedia – The Free Internet Encyclopedia

Hydraulic Analogy  http://en.wikipedia.org/wiki/Hydraulic_analogy

Ampere                                    http://en.wikipedia.org/wiki/Ampere

Ampacity                               http://en.wikipedia.org/wiki/Ampacity

Ammeter                                http://en.wikipedia.org/wiki/Ammeter

Multimeter                           http://en.wikipedia.org/wiki/Multimeter
Attributions | Images and Content courtesy of Wikipedia – The Free Internet Encyclopedia