Chapter6 Flashcards
How many #1 THWN conductors can be installed inside: 1¼” rigid - schedule 40 - PVC?
Answer : 3
Toward the back of the Code book,
see Annex C table C.10 PVC Rigid
STEP 1
Find conductor type “THHN”, conductor AWG size “1”
STEP 2
Follow the row to the 1¼” column.
STEP 3
At the intersection of the row and the column (highlighted in yellow), notice 3.
This is how many #1 conductors, are permitted inside the Rigid PVC
What is the Electrical Code’s rule about rounding numbers: rounding either up or rounding down?
Answer : see “Electrical Code’s - Number Rounding Rule” explanation below.
Electrical Code’s - Number Rounding Rule.
The general rounding rule is: if the number to be rounded off is .5 or larger, round up to the next whole number.
If the number to be rounded off is less than .5, drop the decimal.
Okay, how do we round off 1.732 (√3) to one decimal point.
Here is how it is done: looking at 1.732 (√3) the “3” part of the number is less than “5”, so we drop the “32” of the number.
This rounds the number off to 1.7 (the nearest tenths - the first decimal point).
The reason we did this, is because the Code and most electrical exams, round off √3 to the first decimal, 1.7
As another example, let’s round off a random number: (417.081) to the second decimal point:
Since the third decimal place (thousandth) is 1, (drop the 1) which leaves 417.08
(8) #4/0 RHW aluminum service conductors are installed inside Rigid metal conduit . What is the minimum size (RMC)permitted?
Answer : 3 ½ “
See NEC Annex C.8
Mininum size RMC conduit for (8) 4/0 conductors 3 ½ “
At 3.98 KW - 3 phase a kitchen stove operates at 230 AC Volts at how much is the current?
Answer : 10.18 A
The circuit is 3 Phase (3 Ø), therefore the circuit’s voltage MUST be adjusted, by multiplying the circuit voltage times the square root of 3.
Rounded off, the adjustment factor is 1.7 (√3 = 1.732)
The correct formula and 3 Ø adjustment is as follows:
I = P / (E x 1.7)
I = 3980 W / (230V x 1.7)
I = 3980 W / 391V = 10.18 A
In the circuit below, what is the voltage drop across both R1 and R2?
Answer : 48 V
Earlier, in study EP 5, we learned the voltage drop across R3 is 72V (4A x 18 ohms).
Since both R1 and R2 are parallel resistors, the remaining voltage is dropped across each paralleled resistor, R1 and R2 is 48V
48V is the remaining voltage after the R3 voltage drop - (72V)
What does the winding ratio of a transformer refer to?
Answer 1.1
In the isolation/unity transformer above, the primary and the secondary, have exactly the same number of windings and the same voltage both “in” and “out.”
The winding ratio is 1 : 1
Transformers with the same number of windings on both the primary and secondary sides, are called “isolation” and/or “unity” transformers.
The purpose of isolation transformers, is to isolate the two voltage sources (windings) from each other. This is a common safety precaution used in health care.
Notice the voltage input (primary side), is the same as the voltage output (secondary side).
In the transformer below, why did the secondary voltage go down?
In transformer above, the secondary winding, has half the number of windings as the primary and half the voltage.
The secondary voltage will always do whatever the transformer winding ratio does.
In this case, the secondary windings are half the number of primary windings, so the voltage of the secondary is half of the primary voltage.
In the transformer below, why did the secondary voltage go up?
In the transformer above, the secondary winding, has four times the number of windings as the primary and four times the primary’s voltage.
The secondary voltage always does whatever the transformer winding ratio does.
In this case, the secondary windings are four times the number of primary windings, so the voltage output of the secondary winding is four times the primary’s voltage input.
How and where, are “autotransformers” used?
When we think of a transformer, we think of a primary winding and a secondary winding. Not so with a autotransformer.
“Auto” is a word from the Greek, which translated, means: “self, one’s own or by oneself.”
Therefore, a autotransformer is a transformer with one’s own self windings usually with multiple taps.
A automatic car transmission, is a (auto) self shifting transmission. When you hear the word auto being used, it is a derivation of the root word “auto,” meaning “one’s self.”
Autotransformers are the most used electrical transformer types on the planet.
Autotransformers are used in power supplies, signs, copiers . . . and billions are used in fluorescent light fixtures (ballasts).
With multiple winding taps, autotransformers can be wired, to transform normal system voltages to voltages high enough to ignite fluorescent lamps.
Keep in mind, autotransformers are permitted to be used in electrical branch circuits, as long as the circuit’s common (neutral) does NOT depend on the transform
