Working with Electricity Flashcards
Line loss, power factor
Paralleling
Increasing the number of conductors (usually 4/0)
cables must match in size, temperature rating, length
half-million run
DOubling the conductors
750 run
Tripling the run
million run
quadrupling the run
Three-phase run
Five pieces
Doubled run- nine-piece run (ground not doubled)
two factors in sizing feeder cable
- control of heat for safety
- perservation of voltage to allow for proper functioning of equipment
must be sized to provide sufficient capacity to carry the full load of the overcurrent decie protecting tha tcircuit upstream, asllowing for exisiting conditions
Existing conditions
temperature rating of the overcurrent device,
length of time
temperature conditions
amount of ventilation
runs longer than 150 ft
likely have unacceptable voltage drop if running near capacity
sizing neutral conductors
dependent on the type of load
when loads are resistive- same sizing as the phase wizes
when powering reactive loads- (magnetic ballasts, non PFC electronic ballasts, large numbers of non-PFC fluorescent or LED fixtures)
neutral must by 130% of ampacity of phase conductors
sizing of ground conductors
based on size of overcurrent protection
circuit protection up to 1600A per phase, single piece of 4/0
Sizing grounding electrode and bonding conductors
typically conductor is #2 AWG, would never need to be larger than a single-piece of 2/0.
multiple power sources used in proximity, or in same building
grounds must be bonded
eliminates voltage potential between ground of one power souce and ground of another
line loss
is the erosion of voltage over a long distance caused by the resistance of the feeder cables
severity of line loss increases with the amount of current carried by a particular conductor
can assume 4V per 100 ft when running at 80% capacity
assumed line loss at 80% capacity
4V per 100 ft
main causes of line loss
- length of the run
- cross-sectional area (large conductor, less the line loss)
- the load (large the amerage, larger the line loss)
(heat)
other possible causes of line loss (other than amperage, distance, improperly sized conductors)
resistance, heat, line loss accours when…
1. connection weak or loose
2.cable is frayed
3. connector only partially inserted
4. loaded beyond its capaicty
5. stacking cables closely together
6.making severe bends in cable creates hotspot
heat increases resistancce
7. circular coils in a single condcutor, current carying cable creates impedance, results in line loss and increased heating.
Allowable voltage drop
amount of voltage drop that still allows acceptable performance from the equipment and does not cause harm
NEC recommendation on acceptable voltage drop as percentage of rated voltage of load
For 120V, 208V, 240V, 480V
120V- 3.6V Feeders, 6V Overall
208V- 6.24V Feeders, 10.4V Overall
240V- 7.2V feeders, 12V Overall
480V- 14.4V feeders, 24V overall
Effect of line loss on tungsten lights
light output fallsoff geometrically as voltage decreases
2k lamp at 90% of rated voltage (108V), produces 68% of its normal light output
Kevlin temperature also decreases
Effect of line loss on Power
loss of voltage= loss of power
Power loss in a cable increases as the square of the amperage.
doubling amperage, doubles voltage drop, power less increase fourfold.
Performance of generator is reduced
Acceptable voltage range for electronic HMI ballasts
90-130V or 190-250V
result of voltage drop on constant power ballast
constant power electronic ballasts will draw more current if the line voltage decreases in order to maintain constant power to the lamp
ex: 4kw HMI is 19A at 240V
22A at 208V
24A at 190V
could overload the connectors on some ballasts
18kw ballast with 100A/240V Stage pin connectors at 240V, is 80A per phase at 208V, 93A at 190V, 102A- overheating connector most use camlok connectors
Mitigating line loss by increasing voltage
Drawbacks
increasing voltage at the source:
–transformer with tap switch- plus and minus 5% output in 2.5% increments
–generator- increasing field strength of alternator
larger adjustments can harm genny, can destabilize frequency control
HOWEVER- equipment powered upstream wil be over voltage.
voltage drop is porportional to amperage load, so when load is reduced, voltage must be turned down
power source still working harder, loss of efficiency, greater fuel consumption, reduction of maximum power available
must effectively reduce line loss by adding copper- reducing resistance
Formula to find Voltage Drop for single-phase loads
Vd= 2KIL/Cm
or Vd=21.6I*L/Cm
21.6 times current times length/cross-sectional area
K= specific resistance of material composing a conductor. for copper, 10.8 @25 degrees C I= current carried by the cable L= the length of the wire in feet. the one-way distance from source to load Cm= cross-sectional area of a wire in circular mils (cmil). These tend to be pretty big numbers. For example, 4/0 cable has a cross-sectional area of 211,600 cmil
1 mil in inches
1/1000 in
cmil
Cross-sectional area
(diameter in mils) squared
single-phase load
any load, regardless of voltage (120,208, 240) unless equipment requires all three phase wires to operate
4/0
Cross sectional area and ampacity
211,600 cmil
405 A
2/0
Cross sectional area and ampacity
133,100 cmil
300 A
#2 Cross sectional area and ampacity
66,360 cmil
190 A
#4 Cross sectional area and ampacity
41,740 cmil
140 A
#6 Cross sectional area and ampacity
26,240 cmil
105 A
#12 Cross sectional area and ampacity
6,530 cmil
20 A
4x 4/0
Cross sectional area and ampacity
846,400 cmil
1620 A
3x4/0
Cross sectional area and ampacity
634,800 cmil
1215 A
2x4/0
Cross sectional area and ampacity
423,200 cmil
810 A
Formula for cable gauge for given voltage drop
Cm= 2KIL/Vd
Cm=21.6I*L/Vd
finding Cm, using acceptable Vd, divide by cmil of a certain cable and you know how many piece of that cable you need for a given load
Formula for maximum current for a given voltage drop
I= VdCm/2KL
I=VdCm/21.6L
how many amps you can put through a given length and gauge of cable and remain within a specificed voltage drop
Three-phase loads
ex: Luminy’s soft sun
three-phase step-down transformer
three-phase xenon power supply
Formula, line loss for three-phase load
Vd=1.73KI*L/Cm
Simple line loss calculation- 4/0
1.02 V per 100A per 100 ft
Simple line loss calculation 2/0
1.62V per 100A per 100 ft
Simple line loss calulation #2
3.25V per 100 A per 100 ft
Weight lbs/ft of cable
4/0- .96 lb/ft 2/0 .68 lbs/ft 4-wire banded #2 1.33 lbs/ft 5-wire banded #2 1.70 lbs/ft 100A #4 .72 lbs/ft 60A #6 .55 lbs/ft multiconductor (Socapex) .75lbs/ft
Formula for weight of a waterfall
(number of cables)x(weight per foot)x drop in ft
Formula for maximum length given amperag eand voltage drop
L=VdCm/2K*I
L= VdCm/21.6I
Types of electronics that cause non-linear loads
DC rectifiers, tryristors (SCRs), high-freqency switching power supplies (IGBTs) with large capacitors
Issues caused by non-linear loads
overheating, failing equipment, efficiency losses, circuit breaker trips, excessive current on neutral wire, interference and isntability with generators, noisy or overheating transformers and service equipment, even loosened electrical equipment
inductance
created by coils in a transformer, motor, or magnetic ballast
as current increases and decreases in acoil carrying AC current, a magnetic field exxpands outward from the center and then collapses back inward
as current increases, the circuit stores energy in the magnetic field, then as the current decreases, the circuit gets the energy back. the energy drawn and released by the magnetic field does not accomplish any actual work, energy just keeps circulating back and forth between the coil and power source
when the lines of flu of the magnInductive reactanceetic field grow and colapse, they sel-induce a voltage in the coil, the counter-e;ectromotive force (counter EMF). has opposite polarity of the applied voltage.
unity power factor
100% power factor, 1.0 PF
inductive reactance
opposition to the flow of current, applied voltage must overcome the induced voltage before current can flow through the circuit
effect of inductive reactance on waveforms of voltage and current
inductive reactanc causes current to ag behind the coltage
the degree to which the two are put out of phase is expressed by the cosine of the phase angle between them. the phase angle depends ont eh relative amount of resistance and inductance offered by the load. the more they are out of phase, the lower the power factor.
capactive reactance
opposition to the flow of current cause by capacitance.
induces a power factor of less than 100%
causes current to lead voltage
power factor correction circuits
HMI ballasts have them to erstore the efficiency of a ballast.
power factor
the ratio of true power to apparent power. true power(W)/apparent power(VA)=power factor the feeder cable must be sufficient to supply the apparaent power, even thoguh only the true power does work
apparent power
measure the current and the voltage,multiply (voltamps)
the amount of power traveling back and forth in the cables
true power
the actual amount of energy being converted into real work bythe load- read with wattmeter
formula for amperage needed when PF is known
true power/E*pf=I true power- rated wattage of the equipment E- voltage pf-power factor I- current
current waveform made of
fundamental frequency 60 Hz, with varying amounts of harmonic frequencies, especially, third-order (180 Hz) and fifth order (300 Hz) harmonics
harmonics
multiples of the fundamental frequency
create heat in the cables and circuit and power source (transformer coils or generator windings) without doing any work
effect of harmonic currents on the neutral wire
when an inductive or capacitive load causes current and voltage to be out of phase, or harmonics present, pahse currents no longer cancel when they return on the neutral.
NEC requirements for non-linear loads
neutral to be able to carry at least 1.3 times the phase current.
Mitigating non-linear loads and harmonic distortion
over-size (derate) the generator or transformer or use an appropriate K-rated transformer
What to measure at the end of a run
-if circuit is live
-for proper polarity and ground
-check whether the system voltage is as expected (to be sure feeders connected properly)
check for voltage drop
CAT I meter
electronics within an appliance or device
CAT II
branch circuit receptables and commercial loads
CAT III
permanently installed loads like motors, AC distribution panels or commercial lighting
CAT IV
service entrance, main panel, and service meter
Hazards of using lower category meter
possible for meter to short the circuit, arc, or blow-up in the user’s hand
CAT IV protection
CAT IV meter has thicker insulation on test probes, bigger internal distances between electrical points, a fuse to protect the meter and the user, and a fuse to hel protect against high-energy transient voltages.
electrician required to wear face mask, electrical floves, welders jacket, and fire retardant clothing
true RMS vs average responding
average responding assumes current is a sine wave, true RMS is needed with non-linear loads
precautions when metering
hands, shoes, work areas dry
avoid metering in damp, humid conditions, or with dust or sawdust
place meter on stable surface or hook over stable vertical surface
make sure meter leads in good condition, connected to proper meter jack
select type of service and voltage range (start with highest and work down)
accidents caused when meter in amperage reading mode, or resistance or continuity mode when reading live circuit
touch probes to enutral terminal first, and phase terminal second.
Circuit tester
tests 1) circuit is live 2)polarity is correct 3)grounding wire is present
voltage sensor
senses magnetic field of electricity
checking if wire is live
prone to false positives
Frequency meter
especially useful with HMI magnetic ballasts
cinecheck meter reads freq optically, held up to HMI running on magnetic ballast
measuring amperage
with amp probe or clamp on ammeter
generally AC only
clamp meter measure magnetic field strength
with non-linear loads, need an AC-only RMS meter
Continuity testing
only when power is not connected to the circuit
can twist wires on one end, and read from only the other end
short circuit- testing
check continuity between wires
if you find continuity, there is a short, check each combination fo wires
remove bulb before testing for sorts on a light fixture
wattmeter/power meter
clamp-on device like an ammeter, and probes like a voltmeter
reads true-power in a circuit
can account for the phase diference between curretna dn voltage when there is inductive or capacitive reactance in teh circuit
some require you to turn off power when connecting and disconnect, turn on power to take readsings
reads amperage and voltage separately
