Measurement of wire sizes - Standard wire gauge- American Wire Gauge (AWG) - Outside micrometer .
Content :
- 👉Importance of measurement of wire
- 👉Standard Wire Gauge (SWG)
- 👉American Wire Gauge (AWG)
- 👉Outside micrometers
- 👉How to read a measurement with an outside micrometer?
- 👉Precautions to be followed while using a micrometer
Importance of measurement of wire
To execute a wiring job proper planning is necessary. After considering the requirements of the house owner, the electrician prepares a layout plan of the wiring and an estimate of the cost of the wiring materials and labour.
A proper estimate involves determination of current in different loads, correct selection of the type of cable, size of the cable and the required quantity.
Any error will result in defective wiring, fire accidents and bring unhappiness to both the house owner and the electrician.
So here we discus, How to do Measurement of wire sizes ? by - Standard wire gauge- American Wire Gauge (AWG) - Outside micrometer.
While selecting the cable sizes, the electrician has to take into consideration the proposed connected load, future changes in load, the length of the cable run and the permissible voltage drop in the cable.
A sound knowledge about the area of the cross-section of the core, the diameter of the single strand of the conductor and the number of conductors in each core of the stranded conductor is essential for a wireman to be successful in his career.
To measure the size of conductors, a electrician can use normally a standard wire gauge,American Wire Gauge (AWG) and Outside micrometer for more accurate results.
The size of wires are designed more carefully by the manufacturers. Though the Bureau of Indian Standards (BIS) specifies the cables by the area of the cross-section in square millimetres, the manufacturers still produce the cable with the diameter of each wire and number of wires in the stranded cables.
Sometimes the indicated size of cable by the manufacturer may not be correct and the electrician has to ascertain the size by measurement.
Standard Wire Gauge (SWG):
The size of the conductor is given by the standard wire gauge number. According to the standards each number has an assigned diameter in inch or mm.
The standard wire gauge, shown in Figure 1 could measure the wire size in SWG numbers from 0 to 36. It should be noted that the higher the number of wire gauge the smaller is the diameter of the wire.
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| Fig 1 |
For example, SWG No. 0 (zero) is equal to 0.324 inch or 8.23 mm in diameter whereas SWG No.36 is equal to 0.0076 inch or 0.19 mm in diameter.
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| Fig 2 |
While measuring the wire, the wire should be cleaned and then inserted into the slot of the wire gauge to determine the SWG number (Fig 2).
The slot in which the wire just slides in is the correct slot and the SWG number could be read in the gauge directly.
In most of the wire gauges to save the trouble of referring to the table, the wire diameter is inscribed on the reverse of the gauge.
American Wire Gauge (AWG)
The American wire gauge is different from the British standard wire gauge. In an American wire gauge (AWG) the diameter is represented in mils rather than inch or mm. One mil is one thousandth part of an inch. Please note there is no direct conversion from AWG to SWG.
Outside micrometers
A micrometer is a precision instrument used to measure a job, generally within an accuracy of 0.01 mm.
Micrometers used to take the outside measurements are known as outside micrometers. (Fig 1)
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| Fig 1 |
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The parts of a micrometer
Frame
The frame is made of drop-forged steel or malleable cast iron. All other parts of the micrometer are attached to this.
Barrel/sleeve
The barrel or sleeve is fixed to the frame. The datum line and graduations are marked on this.
Thimble
The thimble is attached to the spindle and on the bevelled surface of the thimble, the graduation is marked.
Spindle
One end of the spindle is the measuring face. The other end is threaded and passes through a nut. The threaded mechanism allows for the forward and backward movement of the spindle.
Anvil
The anvil is one of the measuring faces which is fitted on the micrometer frame. It is made of alloy steel and finished to a perfectly flat surface.
Spindle lock-nut
The spindle lock-nut is used to lock the spindle at a desired position.
Ratchet stop
The ratchet stop ensures a uniform pressure between the measuring surfaces.
Principle of the micrometer
The micrometer works on the principle of screw and nut. The longitudinal movement of the spindle during one rotation is equal to the pitch of the screw. The movement of the spindle to the distance of the pitch or its fractions can be accurately measured on the barrel and thimble.
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| Fig 2 |
Graduations (least count of the micrometer)
In metric micrometers the pitch of the spindle thread is 0.5 mm. Thereby, in one rotation of the thimble, the spindle advances by 0.5 mm.
In a 0-25 mm outside micrometer, on the barrel a 25 mm long datum line is marked. (Fig 2)
This line is further graduated in millimetres and half millimetres (ie. 1 mm & 0.5 mm).
The graduations are numbered as 0, 5, 10, 15, 20 & 25 mm on the barrel.
The circumference of the bevel edge of the thimble is graduated into 50 divisions and marked 0-5-10-15... 45-50 in a clockwise direction.
The distance moved by the spindle during one rotation of the thimble is 0.5 mm.
Movement of one division of the thimble = 0.5 x 1/50 = 0.01 mm.
This value is called the least count of the micrometer.
The accuracy or least count of a metric outside micrometer is 0.01 mm.
Outside micrometers are available in ranges of 0 to 25 mm, 25 to 50 mm, and so on. For electrician, to read the size of the wire 0 to 25 mm is only suitable.
Reading micrometer measurements
How to read a measurement with an outside micrometer?
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| Fig 3 |
a) Read on the barrel scale, the number of whole millimetres that are completely visible from the bevel edge of the thimble. It reads 4 mm. (Fig 3)
b) Add to this any half millimetre that is completely visible from the bevel edge of the thimble and away from the whole millimetre reading.
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| Fig 4 |
The figure reads one division (Fig 4) mm after the 4 mm mark. Hence 0.5 mm to be added to the previous reading
c) Add the thimble reading to the two earlier readings.
c) Add the thimble reading to the two earlier readings.
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| Fig 5 |
The figure shows the 5th division of the thimble is coinciding with the datum line of the barrel.
Therefore, the reading of the thimble is 5 x 0.01 mm = 0.05 mm. (Fig 5)
The total reading of the micrometer. a 4.00 mm b 0.50 mm c 0.05 mm.
Total reading = 4.55 mm
Precautions to be followed while using a micrometer
Before using the micrometer for measurement, it is necessary to ascertain that there is no error in the micrometer. To find the error, close the jaws of the measuring surfaces using the ratchet.
Read the micrometer. If the thimble zero is coincident with the datum line of the barrel, error is zero. If it reads higher value, the error is +ve; if it reads lesser value the difference between zero and the read value is -ve error.
If there is minus error it should be added to the total reading and if there is plus error the value should be subtracted from the total reading.
The faces of the anvil and spindle must be free from dust, dirt and grease. While reading the micrometer, the spindle must be locked with the reading.
Do not drop or handle the micrometer roughly.
This is my view after doing research and practical experience on Measurement of wire sizes - Standard wire gauge(SWG)- American Wire Gauge (AWG) - Outside micrometer | How to read a measurement with an outside micrometer? .
Thank you.
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